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661© CAB INTERNATIONAL 1999. Fish Diseases and Disorders, Volume 3:Viral, Bacterial and Fungal Infections (eds P.T.K. Woo and D.W. Bruno)

18Ichthyophonus and Related Organisms

A.H. McVicar

FRS Marine Laboratory, PO Box 101, Victoria Road, Aberdeen AB11 9DB,UK.

INTRODUCTION

Ichthyophoniasis, due to infection with Ichthyophonus hoferi Plehn and Mulsow(1911), has been known in fish since the end of the last century (von Hofer,1893). The disease is recognized to be of economic significance, in both fishcultivation and wild fisheries, and to have a wide host and geographicaldistribution. It has been the subject of several comprehensive reviews(Reichenbach-Klinke, 1954–5; Dorier and Degrange, 1960; McVicar, 1982;Rand, 1990; Sindermann, 1990). Within the last few years, there have beenfurther and particularly significant developments in knowledge of the disease.Major new epizootics have occurred in the North Sea and western Baltic Sea andthese have stimulated new research into the biology and epizootiology of thedisease (Anon, 1991, 1993; Rahimian, 1994; Rahimian and Thulin, 1996;Spanggaard, 1996; Mellergaard and Spanggaard, 1997). Uncertainties about thetaxonomic position of I. hoferi have led researchers to place the parasite with theprotozoans and more recently with the fungi. The application of moleculardeoxyribonucleic acid (DNA) technology has clarified the phylogenetic positionof the infective agent. Spanggaard et al. (1996) and Ragan et al. (1996) nowconsider its proper position is as a member of a lower protistan group, closelyrelated to the Dermocystidium group of organisms. Although this chapter onichthyophoniasis concentrates on I. hoferi, it is appropriate that someinformation on Dermocystidium is also considered, in recognition of this newdevelopment.

Because of its perceived fungal associations, the terminology normally usedby researchers to describe the various development stages of Ichthyophonus hasnaturally closely followed that of the mycologist. However, some reappraisalwould now seem appropriate.

Nomenclature is often a controversial area in descriptive biology, often withcomplex and highly specific language being developed and defended byspecialists in their subject areas. However, as most researchers only need to be

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able to identify the structures being discussed in a report, the use of taxon-specific terminology is avoided in this review. Where possible, straightforwarddescriptive terms are used, although it sometimes proves necessary to crossreference these with established terms, to avoid further confusion.

ICHTHYOPHONUS

Cause of ichthyophoniasis

Riddell and Alexander (1911) suggested that Ichthyophonus may be a secondaryinvader, masking a possible primary bacterial agent that was responsible for thedebilitating condition in plaice. Since then, no evidence to support thishypothesis has been provided. The role that any given pathogen has in a diseaseprocess will seldom be possible to determine accurately, as disease-determinantfactors are typically multifactorial and usually significantly interact in acomplex manner. It therefore becomes academic whether or not an infectionsuch as I. hoferi initiates the disease process. In the applied scientific field, it ismore important to establish if a particular infection significantly contributes tomortality. The widespread occurrence of I. hoferi in diseased fish, the ease ofestablishing experimental infections and of sequentially maintaining these indifferent species of fish, the isolation of infective stages and the development oftypical pathology in each pass (e.g. by McVicar and McLay, 1985) suggest amajor role of this infective agent in the disease process.

Host range

The wide range of host types from which Ichthyophonus has been reported raisesthe question whether only one species of an organism is involved in all hosts andwhether the identification has been accurate. Included as hosts have beenvarious marine and freshwater crustaceans, elasmobranch and teleost fish,amphibians, reptiles and piscivorous birds (Reichenbach-Klinke, 1954–1955,1957; McVicar, 1982). Humans or other mammals are apparently not at riskfrom infection, as Spanggaard (1996) and Spanggaard and Huss (1996) reportedthat infection trials with mice have shown no indication of toxicity or patho-genicity and that no cases of human infection have been reported. Spanggaardand Huss (1996) found survival of the parasite to be less than 3 min at 40°C.

Fish predominate as hosts, and Reichenbach-Klinke and Elkan (1965)reported 35 marine fish species and 48 freshwater species to have had infectionrecorded. Since then, new host species have continued to be added. Recently,Spanggaard et al. (1994) noted more than 80 fish species as infected. Theliterature therefore indicates a low parasite–host specificity in fish (McVicar,1982). Consequently, new host records and a host list are probably of littlescientific significance and they may largely reflect whether a particular speciesof fish has been sufficiently and appropriately examined. It is noteworthy thatrecords from elasmobranchs are rare (Reichenbach-Klinke, 1957).


In view of the strong teleost link, the records of Ichthyophonus orIchthyophonus-like infections from various other groups of hosts, such ascopepods (Jeeps, 1937), amphibians and reptiles (Chauvier, 1979; Herman,1984) and even from a fish-eating bird (Chauvier and Mortier-Gabet, 1984),should be treated with some caution. Verification is required that the organism isthe same as in fish (for example, by molecular genetics) and that the occurrenceis a true infection and is not due to contamination from infected fish tissues.Also, the possibility that these ‘infections’ may represent temporary transport,paratenic or dead-end hosts should not be ignored. Ichthyophonus typicallyelicits a granulomatous response in its host, and some records may probablyhave arisen through researchers observing such lesions and attributing them tothe disease without investigating the actual cause. As many infectious organismsand non-infectious conditions may elicit a similar granulomatous host reaction,there are risks in assuming that such types of lesions are indicative ofIchthyophonus infection without sufficient evidence that it has been the actualcause of the reaction. Without a thorough reappraisal of the original material or areassessment of new material from the same source as the host and geographicalrecords, determining the extent of such errors is not possible.

Geographical distribution

Ichthyophonus has been recorded from many temperate and some tropicalwaters throughout the world, both north and south of the equator (McVicar,1982). The low number of records from tropical waters, compared with the manyreports from the North Atlantic (east and west) and the Sea of Japan, probablyreflects the research effort on fish diseases in these areas, rather than a truedifference in the pattern of distribution. It can now be safely concluded that thedisease has a global occurrence in sea water. Consequently, the recording of newgeographical records of the infection is unlikely to be of scientific significance.

There is now a consensus in the literature that Ichthyophonus is primarily ofmarine origin. Most reports are from the open sea and estuaries. In addition,many freshwater isolations have been directly linked to the sea, as they occurredin fish farms using fresh marine fish as feed for farm stocks (Neresheimer andClodi, 1914; Rucker and Gustafson, 1953; Dorier and Degrange, 1960; Fijan andMaran, 1976; Munday, 1976; Miyazaki and Kubota, 1977a). An exception is thereport by Reichenbach-Klinke and Elkan (1965), who listed 48 freshwater fishspecies among the 83 species reported as infected. It is essential that theidentification of these latter records is verified again. With the disease being sowidespread in coastal marine waters, the lack of evidence of natural endemics ofIchthyophonus in freshwater environments is difficult to explain. The organismis capable of horizontal transmission between fish in fresh water by contact orfeeding on infected dead fish (Rucker and Gustafson, 1953; Gustafson andRucker, 1956; McVicar and McLay, 1985), but there is no evidence of itbecoming established near freshwater fish farms that have become infected orfrom infected wild fish. Migratory salmonids are known to harbourIchthyophonus, the infection being found in wild sea trout (Robertson, 1909) and

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regularly in Atlantic salmon, Salmo salar, returning to rivers in Scotland since atleast early this century (McVicar, 1982). There is no evidence from extensivedisease studies in Scotland of Ichthyophonus infection being present in salmonidfreshwater populations.

Diagnosis from gross observations of clinical signs

The ability of observers to detect Ichthyophonus from gross signs is closelylinked to the level of infection and pathogenicity recorded. As this substantiallydiffers between different host species, obvious signs of disease mayconsiderably differ between different fish. As with most disease conditions whenpathogenicity is high, several non-specific signs may accompany the advancedstages of infection with Ichthyophonus. These include behavioural changes andchanges associated with organ failure, such as lethargy, emaciation, colourabnormalities, fluid accumulation, nervous disorders and an increase inmortality. Such changes are evident in groups of captive fish but are alsodiscernible in wild populations. McVicar (1979, 1981) showed that infectedplaice, Pleuronectes platessa, could be selected from commercial catches byweight loss. External signs, including skin roughening (‘sandpaper effect’) andoccasional ulceration, permitted Sindermann and Scattergood (1954) todistinguish infected herring, Clupea harengus, in the western North Atlantic.Rahimian and Thulin (1996) and Hodneland et al. (1997) also used similar signsto detect infected herring in the Kattegat and Norwegian Sea areas. Todifferentiate Ichthyophonus from other granuloma-inducing conditions, it isrecommended that the field diagnosis be supported by some evidence of thecharacteristic germination of the parasite after the death of the host (McVicar,1982).

When infected fish are examined internally, it is common for gross white orcream-coloured nodular lesions 1–5 mm in size to be visible throughout mosttissues, although, for unknown reasons, the organs most heavily infected may bedifferent in different species of fish (McVicar and McLay, 1985). For example,in haddock, Melanogrammus aeglefinus, the most obvious lesions occur in thewhite muscle, in herring in the heart and in plaice in the liver and kidney. It isnecessary to take account of such differences when determining the prevalenceor the effect of infection. These lesions usually consist largely of thegranulomatous reaction surrounding single ‘spores’ 10–250 µm in diameter orgroups of spores. Where the host cellular reaction is weak, as in plaice, thenodules may comprise almost pure Ichthyophonus tissue and, in cases of heavyinfections, most of the normal organ tissues may be replaced by the parasite.

Following two special workshops on Ichthyophonus in European herringunder the auspices of the International Council for the Exploration of the Seas(ICES) in 1991 and 1993 (Anon., 1991, 1993), it was noted that it was possibleto obtain an estimate of the prevalence of infection in a herring populationthrough observation of hearts showing gross lesions. In recent infections or insmall fish, the microscopic examination of tissues was also considered desirable.Without doubt, some infected fish are missed if only gross signs are relied on as a


sign of infection. Holst (1994) observed a 10% higher detection level of infectedherring by microscopic examination of squash preparations of heart tissue thanby macroscopic examination, while Rahimian and Thulin (1996) found fourtimes as many infected herring when using microscopic examination. However,when the association between gross pathological changes and infection asdetermined by the most sensitive detection method has been properly verified,the visible signs of abnormality may be safely used for rapidly screening largenumbers of fish in infected populations. In studies on the prevalence of a disease,such as Ichthyophonus, it has to be accepted that the earliest stages of infectionwill be difficult, or, in practice, impossible, to accurately detect. Variability inthe accuracy of detection can be reduced by selecting a cut-off point where theextent of infection is within the limits of practical detection, such as grosslyvisible heart lesions. Establishing the relationship between the selected indexand the most accurate detection method will always be desirable. Hodneland etal. (1997) undertook such a systematic study in Norwegian herring, invest-igating the infection associated with skin papules, blebs, erosion, ulceration andhaemorrhage. They demonstrated a close correlation with the presence ofIchthyophonus infection detectable by microscopic examination.

Microscopic diagnosis

The ability to identify Ichthyophonus from the morphological characteristics ofthe parasite in the tissues of infected hosts is restricted by lack of obviouslydifferent development stages, such as a sexual phase, the absence of rigid partsand the variation in the size and shape of the same development stages. Themorphological variations found in different hosts, or even in different parts ofthe same host, and in cultures are reflections of interspecific host effects and thenutritional and physiological factors being encountered. Spanggaard et al.(1995) and Spanggaard and Huss (1996) showed that variable pH, carbondioxide, glucose availability and salinity affected the growth of the parasite.

Fresh squash preparations of infected organs typically reveal the presence ofthe normally spherical resting stage (‘spore’) which varies in size from 10 to 250µm in diameter. These are usually surrounded by varying amounts of hostgranulomatous reaction tissues, and the use of phase contrast microscopy isadvisable to facilitate distinction of them from other similar structures present inthe body of fish. However, because of the lack of specific distinguishing featuresin the spherical bodies (‘resting spores’) of Ichthyophonus, caution should beexercised in diagnosing the disease solely from the observation of these,particularly when there are limited numbers present in the body of the fish.Difficulty is also encountered with hosts that are more resistant to the diseasewhen there are no traces of the parasite in granuloma. Overall, it is advisable thatadditional features which are specifically characteristic of Ichthyophonus arefound before a final diagnosis is made. McVicar (1982) recommended that thecharacteristic parasite germination, which occurs after the death of the host, maybe used as a specific diagnostic index. Spanggaard and Huss (1996) noted that arise in the carbon dioxide levels as the host dies stimulates Ichthyophonus cells

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to germinate and that growth was unaffected by temperature up to 25°C. At a lowpH, Spanggaard et al. (1995) showed that germination occurred within4–5 h. Consequently, the re-examination of suspect squash preparations onmicroscope slides after a period of more than 5 h at room temperature shouldprovide convincing evidence for or against the presence of Ichthyophonus.

Histology of infected tissues provides useful data for the diagnosis ofIchthyophonus. Spherical bodies typically predominate. Sections from newlydead fish do not normally show germination of the resting cells and theoccurrence of development (‘hyphae’) in the illustrations shown in publishedreports on Ichthyophonus probably indicates that the tissues were not fixedimmediately after the fish was killed. Cells of the parasite occurred singly or,more commonly, in groups, typically surrounded by host reaction. Cells in thesegroups were usually of varying size, indicating a lack of synchrony in growth ordivision of the parasite. The smallest cells are bound by unilaminate walls ofparasitic origin, but in larger cells the wall becomes multilaminate. Thecytoplasm of the individual Ichthyophonus cells is normally lightly staining withhaematoxylin and eosin (H & E) and structurally uniform in appearance, with anirregular network of cords and fine granular material. The smallest cells foundcontained a single centrally located nucleus (McVicar and McLay, 1985), butlarger cells have varying numbers of nuclei, scattered throughout the cytoplasm.The nuclei have a characteristic orange tincture in H & E staining, are pyriformin outline, have a central prominent nucleolus and occasionally have chromatinon the periphery. The outer walls of Ichthyophonus cells have a strong periodicacid-Schiff (PAS)-positive staining reaction, which is a useful additional aid indetecting low levels of infection. Transmission and scanning electron-microscope studies of Ichthyophonus (McVicar and McLay, 1985; Paperna,1986; Rand, 1990; Spanggaard et al., 1996) have shown many characteristicfeatures of the parasite useful for specific identification in individual cases.However, the complexity of the preparation and the limited amounts of materialwhich can be examined offer no significant advantages in diagnosis.

If culture conditions are suitable, prolific growth of Ichthyophonus can beobtained on minimum eagle’s medium (MEM) (McVicar, 1982; Spanggaard etal., 1994). This has revealed details of the life cycle and how these may beinfluenced by surrounding environmental conditions. However, as a primarymethod of diagnosing ichthyophoniasis, culture techniques are largelyimpractical and unnecessary with most material, because of the clear gross andmicroscopic features.

Sampling methods

The distribution of Ichthyophonus is not uniform, either between areas or evenwithin populations of fish, and there are serious difficulties in determining thetrue prevalence of infection in natural populations. In designing a samplingprogramme to determine the prevalence of the infection, these factors, whichinfluence sampling variation, should be taken into account and the approachstandardized where possible. Mellergaard and Spanggaard (1997) suggested that


the high degree of temporal and spatial variations in the prevalence of thedisease in herring was partly due to the migration patterns of different hoststocks, while McVicar (1979) also noted local variations in infection levels inhaddock, linked to differences in the origins of stocks. A positive correlationwith host age was noted in plaice (McVicar, 1979) and in herring (Mellergaardand Spanggaard, 1997), possibly due to the higher feed consumption in thelarger fish. Holst (1994) found that Ichthyophonus-infected herring were over-represented on the margins of schools and in scattered layers of herring, possiblybecause of decreased swimming ability, both in burst speed and sustainedswimming speed. Predation of infected herring may be increased by theimpaired condition of herring, non-shoaling and other behavioural changes(Hjeltness and Skagen, 1992). Mellergaard and Spanggaard (1997) found theprevalence of Ichthyophonus to be significantly higher (1.8 times) in research-vessel catches from prefixed fishing positions, compared with catches fromcommercial fishing boats taken mainly from dense concentrations of fish.Hodneland et al. (1997) found that the relationship between external signs andinfection detected by other methods may vary between samples, probablyinfluenced by different selectivity of fishing gear, herring school size, infectiondynamics of the disease organism and the skill of the observer.

The disease

The most likely route of infection of a fish with Ichthyophonus, confirmed byexperimental transmission, is through the intestine (McVicar, 1982). Althoughother routes, such as through the gills or skin lesions, as suggested by Riddelland Alexander (1911) and Sproston (1944), still cannot be completelydiscounted, they are generally considered to be of lower significance. As withmost diseases, it is likely that the success of the pathogen in infecting a new host,its subsequent spread, its persistence within the body and its pathogenicity areinextricably linked with the general stress level and health status of the fish.Such factors may, in part, account for the noted discrepancies in host records ofthe disease.

Once within the fish body, Ichthyophonus shows all of the characteristics ofa typical systemic invader. The infective stages become spread throughout thebody through the blood or lymphatic systems and become lodged in capillarybeds. Blood-rich organs therefore typically become a primary location ofinfection and, in these areas, proliferation of the parasite occurs, leading both toa secondary invasive spread in the immediate vicinity and to a release of furtherinfective stages into the circulatory system.

Epizootic disease is common in fish farms, but rarely found in wild fishpopulations, because of the level of adaptation between host and parasitenormally established in the natural environment through long associationbetween the two organisms. Ichthyophonus is therefore unusual among fishparasitic diseases as there is a long history of epizootics due to this infection.Cox (1916), Daniel (1933), Sindermann and Scattergood (1954) andSindermann (1956, 1958, 1963, 1990) reported serious epizootics in herring in

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the Gulf of St Lawrence and the Gulf of Maine off the eastern coast of NorthAmerica until the mid-1950s. These epizootics led to mass mortality, with anestimated 50% of mature herring destroyed during the period 1954–1956(Sindermann 1958; Tibbo and Graham, 1963). A significant reduction in the sizeof herring stocks in these areas was directly attributed to the infection(Sindermann, 1990). The absence of records of later epizootics has been linkedto the smaller size of the herring stocks after that time (Sindermann andChenoweth, 1993).

Ichthyophonus has long been known in various species of marine fish inother parts of the world, and especially in European waters, since at least thebeginning of this century, although serious epizootics were not reported in thatregion until the 1990s. This may partly be attributed to the lack ofepizootiological research, but is probably due to the difficulty in establishing theextent of the effect of the disease in the affected populations. There is theevidence that Ichthyophonus has been regularly recognized in haddock inScottish waters, with infected fish often being rejected by fish processors overthe last 100 years (McVicar, 1982), but without reports of serious disease-induced mortality at sea. In a study of the relationship between prevalence andincidence of Ichthyophonus infection and mortality rate in plaice, McVicar(1990) used the period taken for the fish to produce antibodies to the parasite asa time marker in the development of the disease, to calculate the disease rate. Aninfection prevalence of less than 10% was found to cause an annual mortality ofover 50% in a wild plaice population north of Scotland. This can be consideredan epizootic of significant proportions, but, again, no dead fish were observed byresearch vessels or fishermen in the area. One reason may be that the predationpressures in the area were sufficiently high to rapidly remove moribund andnewly dead fish.

The European situation changed in 1991, when mass mortality of herringwas observed for the first time in the Kattegat, with dead fish floating on thesurface, on the sea bottom, resulting in clogging of trawls, and washing up onbeaches along the Swedish west coast and in the Sound between Denmark andSweden. Further studies revealed high prevalence, up to 100% in certain areas ofthe northern North Sea, and also that the infection was widespread in theNorwegian Sea, Skagerrak, Kattegat and western Baltic Sea. The factors whichled to the development of an epizootic of Ichthyophonus in herring of the easternNorth Atlantic region are unknown. As indicated above, the parasite has beenpresent in the area over an extended period, even leading to significant mortalityin species such as plaice (McVicar, 1981). It may be significant that the periodbetween the late 1980s and early 1990s was characterized by high populationnumbers of herring in these areas and the possibility of changes in thedistribution of herring populations over the same period was also considered asimportant (Anon., 1993). Sindermann and Scattergood (1954) suggested thattemperature or salinity changes in the western North Atlantic could beinfluencing factors. The occurrence of the European outbreak has led to aresurgence of studies on the parasite through the auspices of ICES (Anon., 1991,1993) and independently by researchers in Denmark, Norway, Russia, Scotlandand Sweden (Hjeltnes and Skagen, 1992; Rahimian and Thulin, 1996;


Hodneland et al., 1997; Mellergaard and Spanggaard, 1997). It has been thusdetermined that a major epizootic of Ichthyophonus occurred in Europeanherring stocks in the early 1990s, covering an extensive area of the westernBaltic to the north of Norway. From a prevalence of 3.7–10.6% of herring stocksin commercial and research-vessel catches in the North Sea in 1991, Mellergaardand Spanggaard (1997) calculated the total Ichthyophonus-induced mortality ofherring to be between 12.8 and 36.8% of the North Sea herring population andreferred to data indicating a 25% reduction in the herring spawning-stockbiomass over the same period. These authors also referred to a report (ICES,1996, cited by Mellergaard and Spanggaard, 1997) showing a reduction in theherring spawning-stock biomass from 1.1 million tonnes in 1991 to 0.5 milliontonnes, in 1995, but suggested that this reduction may be due to a combination ofincreased fishing intensity and the effect of Ichthyophonus. The prevalence levelof Ichthyophonus showed a decreasing trend from 1991 to 1993 to a level below1% in most areas (Mellergaard and Spanggaard, 1997). It is now considered thatthe infection is at background non-epizootic levels, although Ichthyophonus isstill present in herring in several European herring stocks (Anon., 1997).Although other fish species (flounder, sprat) were also found concurrentlyaffected in the area (Rahimian, 1994), no similar mortality has been shown inthese populations during the epizootic in herring.

Although records of infection may suggest an infectious agent with a highlevel of transmission and infection success and a high pathogenicity, it cannot beassumed that challenge with Ichthyophonus will inevitably lead to a systemicinfection, overt disease or the beginning of an epizootic. There are many recordsof the parasite occurring without accompanying evidence of seriousconsequences to individual fish or to populations. McVicar and McLay (1985)gave evidence that different species of fish, or even different individuals of thesame species, may show a range of effective resistance or tolerance of theinfection. Evidence has been presented that species such as plaice and herringare highly susceptible and species such as haddock and rainbow trout(Oncorhynchus mykiss) may show an intermediate tolerance, while other speciesmay show a high level of resistance. Some species of fish, such as cod, Gadusmorhua, may be more refractory than others (McVicar, 1982; McVicar andMcLay, 1985). Others, such as goldfish, Carassius auratus, guppy, Lebistesreticulatus, squawfish, Ptychocheilus oregonensis, and catfish, Ameiurusnebulosus, are resistant in experimental challenge (Gustafson and Rucker,1956). Even in known susceptible species, infection of a new host is notnecessarily a simple process of a susceptible host coming into contact with aninfective stage of the parasite. Sindermann (1965) reported that multiple,massive exposures on successive days with Ichthyophonus were necessary toobtain infection of herring, although high levels of natural and experimentalinfection can be shown and are considered terminal to this species.

During an epidemic of the disease in the Gulf of St Lawrence, which wascausing high mortality in populations of herring, alewives (Alosapseudoharengus) and mackerel (Scomber scombrus), Sindermann (1966) notedthat cod benefited from the presence of the disease and showed an increasedgrowth rate. This was probably related to the abundance of feed in the form of

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diseased, dying and dead fish in the area. There is a contradiction in the literatureregarding the effect of the disease on cod in European waters. McVicar (1982)showed a strong host cellular-containment reaction to the infection in thisspecies. In cod populations off the Scottish coast, while infection could bedetected in some fish, none showed an extensive invasion (A.H. McVicar,unpublished observations). In contrast, Möller (1974) found 15.2% of cod in thewestern Baltic Sea infected with Ichthyosporidium (Ichthyophonus) hoferi.Kock (1975) suggested that most of the cod mortality in a cage experiment hadan associated Ichthyophonus infection and by implication suggested that thiswas the main cause of death. To clarify this, further studies are necessary in cod,particularly in view of the interest in the culture of this species in fish farms. Inplaice, the disease is considered lethal in approximately 2 months (McVicar,1990). Because of this high mortality rate, the prevalence found in the infectedpopulation was consistently low (less than 10%). A similarly rapid removal ofinfected fish from the area may explain why Rahimian (1994) did not findinfected plaice in the Kattegat during the major epizootic of the disease inherring during the early 1990s. In herring, the disease may progress eitherthrough an acute course, with death occurring within 1 month of exposure, orthrough a chronic course, with some fish surviving for up to 18 months(Sindermann, 1956, 1958). Introduction of Ichthyophonus into a freshwaterrainbow trout farm can have both serious consequences, as Rucker andGustafson (1953) recorded the death of 50–90% of stocks on one farm in theUSA, or limited effect, as noted by McVicar (1982) on a rainbow trout farm inScotland. Aspects of the host’s response, particularly the cellular reaction, havebeen implicated in the intra- and interspecific variations in fish susceptibility toIchthyophonus. Because little is known in detail of the factors influencing fishcellular-defence mechanisms, it will be difficult to predict the effects of anoutbreak of the disease in a population of farmed or wild fish.

The microorganism

The taxonomic position of Ichthyophonus has had a chequered past. Researchinterest on fish diseases considerably increased around the turn of the twentiethcentury, and diseases with clearly visible gross signs, such as ichthyophoniasis,frequently became the subject of study. The standard of observation in theseearly studies was exceptionally high and they gave sufficient details in text andaccompanying drawings for later comparative studies to be possible. However,communication was difficult during that period and the publication of the studiesoften in journals with limited circulation tended to lead to a lack of cross-referencing. Much of the subsequent confusion regarding the nomenclature usedfor the infective agent originates from this period and may, at least in part, beattributed to these difficulties.

The disease appears to have been first reported by von Hofer (1893) asdizziness disease (Taumelkrankheit) in trout and salmon in Germany. Theseearly studies described and illustrated the morphology and some stages of thedevelopment of the infective organism in sufficient detail to demonstrate that the


same or closely related organisms were clearly being examined. Johnstone(1905) described an epizootic in a tank population of plaice, in the Isle of Man,and, on the basis of morphology, tentatively linked the infective agent with fungiof the Entomophthorineae. Robertson (1909) reported what appears to havebeen the same disease from sea trout, Salmo trutta, flounder, Platichthys flesus,and haddock, and, because it was thought that the parasite had some similaritiesto a protozoan described by Caullery and Mesnil (1905), described the parasiteas Ichthyosporidium gastrophilum. Laveran and Pettit (1910) recorded the samedisease causing high mortality in a fish farm in France. They noted that thenormal means of propagation of the parasite might be by subdivision of primaryprotoplasmic bodies into secondary cysts and believed that it had some affinitieswith the protozoa, especially the Haplosporidia, and considered that there weresome plant-like structures present. From material obtained from two infectedrainbow trout, Plehn and Mulsow (1911) finally described the organism causingthe disease as I. hoferi, classing it in the Phycomycetes close to the Chytridinae.Almost simultaneously, in a study of diseased haddock (known as ‘spottedhaddock’, ‘greasers’ or ‘smelly haddock’) caught off the west of Scotland in1911, Williamson (1913) clearly illustrated the same disease signs andmorphological features, and considered that the parasite was undescribed. Somepoints of resemblance to stages in certain Myxo-sporidia (sic) were found andthe name Dokus adus proposed. However, under rules of nomenclature, thisname was a junior synonym and became redundant. Pettit (1913) recognized thatthe first use of the generic name in relation to the disease agent had been byCaullery and Mesnil (1905) as Ichthyosporidium and, also according tonomenclature rules, transferred Ichthyophonus hoferi to Ichthyosporidiumhoferi. This was accepted by some but not all researchers and the diseaseregularly appeared in the subsequent literature under both generic names,creating considerable confusion. However, in a review of the taxonomy ofIchthyosporidium, Sprague (1965) concluded that this genus should be reservedfor a protozoan group, with Ichthyosporidium giganteum as the type species ofthe genus. As Ichthyosporidium hoferi was considered clearly not to be related tothat type species, Sprague reverted the name back to that of the first valid nameddescription of the organism, namely Ichthyophonus hoferi Plehn and Mulsow(1911). Ichthyophonus is now generally considered the valid generic name of theparasite.

The confusion regarding the taxonomy of the causative agent ofichthyophoniasis is not restricted to the genus level. The original description ofI. hoferi by Plehn and Mulsow (1911) was incomplete and consequently leads todifficulties when direct comparisons are attempted. In addition, only vegetativegrowth stages have been observed, both in vitro and in vivo (Spanggaard et al.,1995), and as these are highly malleable in their morphological appearancedepending on host and substrate conditions (McVicar, 1982), the stagesavailable for specific comparison and identification are limited and variable.Spanggaard et al. (1995) and Spanggaard and Huss (1996) demonstrated thatdifferent types of development of Ichthyophonus could be triggered by changesin pH, temperature, salinity and carbon dioxide tension. Rand (1994) alsodescribed an unusual form of Ichthyophonus in yellowtail flounder (Limanda

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ferruginea), which bore similarities to atypical features noted in other isolations(Jeeps, 1937; Hendricks, 1972; Gartner and Zwerner, 1988). He raised thepossibility that these could be a separate ‘entity’ from the ‘species’ I. hoferi ascommonly described, but cautioned against the erection of new species, due tothe current lack of detailed taxonomic knowledge of I. hoferi. Alderman (1976,1982) suggested that I. hoferi had been used to describe a complex of organismsand has become a ‘waste-basket’ taxon, with poorly defined species limits. It isunlikely that the specific or strain relationships among all the different isolatesof Ichthyophonus will be resolved until samples from the original sources arestudied with new approaches that combine structural, biochemical andmolecular data, such as that used by Spanggaard et al. (1996) and Ragan et al.(1996).

The lower fungi, lower algae and lower protozoans show many features ofaffinity with each other and, not surprisingly, the taxonomy of organisms inthese groupings has been the subject of controversy (Alderman, 1976). As manyresearchers who have worked on these organisms have been pathologists anddiagnosticians and not specialist taxonomists, it is not surprising that there havebeen few in-depth assessments of the taxonomy. Many researchers either havechosen to follow the most recent trend in publications or have grouped theorganism on the basis of the most obvious morphological features of thespecimens they were studying. Consequently, Ichthyophonus, Dermocystidiumand similar organisms have largely remained as taxonomic orphans, which havebeen variously assigned to the protozoans and fungi, because of their superficialmorphological similarities to these groups, without strong conviction beingshown by most authors. The absence of rigid body parts, which could be usefulin diagnosis, and the typically high level of variability in the structure of the life-cycle stages available for study, with the absence of definitive sexual stages,have also undoubtedly contributed to the practice of using genera or even speciesas a convenient ‘waste-basket’ for partially observed organisms (Alderman,1982). Several workers (Alderman et al., 1974; Corliss, 1987) have attempted toresolve the difficulties in the placing of these organisms through the use ofultrastructural features, but, lacking definitive stages, serious questions remainas to their exact relationships. Latterly, there would appear to have been ageneral acceptance that, until more definitive evidence was available, thesuggestion made by Alderman (1982) to place Ichthyophonus in the Fungiincertae sedis should be followed.

With the recent application of molecular biological techniques to thediagnosis and differentiation of fish diseases, the opportunity has now beentaken to re-evaluate the position of Ichthyophonus, Dermocystidium and similarorganisms. Spanggaard et al. (1996) and Ragan et al. (1996) used amplificationand sequencing of small subunit ribosomal ribonucleic acid (RNA) in these andsimilar organisms (‘rosette agent’). These two groups of researchers independ-ently reached the conclusion that Ichthyophonus and the Dermocystidium/rosette agent grouping are a closely related protistan clade situated near to thechoanoflagellates and are not members of the Fungi. Although someinconsistencies in this grouping have been noted, particularly when usingultrastructural and biochemical information (Spanggaard et al., 1996), this


clarification of the taxonomy of these organisms is welcomed and should formthe basis for future research.

Life cycle

There have been many investigations on parts of the life cycle of Ichthyophonus,through studies on various fish species with natural and experimental infectionsand through in vitro culture. Only Sproston (1944) described a range of complexstructures and an intricate life cycle, including sexual reproduction. Theuniqueness of this report among the many studies on the disease places somedoubt on its validity, and it is possible that a mixed infection of more than oneorganism was present. As a whole, other studies have shown a limited range ofmorphological structures, with there being general agreement that the life cycleis relatively simple (Daniel, 1933; Fish, 1934; Sindermann and Scattergood,1954; Dorier and Degrange, 1960; Miyazaki and Kubota, 1977b; Chien et al.,1979a,b; McVicar, 1982; Okamoto et al., 1985; Spanggaard et al., 1995).

The most commonly observed stage of the parasite is the spherical cell(often called the ‘resting spore’) (Fig. 18.1), characterized by a thick wall,surrounded to a greater or lesser extent by a capsule of host reaction tissue. Thereare many light-microscopic descriptions of this stage in the literature, forexample, by Sproston (1944), Sindermann and Scattergood (1954), McVicar(1982) and Rand (1990). Although some variations have been noted in the size ofthe parasite in different organs and different host species, there is a remarkableconsistency in the structure reported in all these. When the spherical cell ofIchthyophonus has been subjected to detailed electron-microscopic study(McVicar and McLay, 1985; Paperna, 1986; Rand, 1990; Spanggaard et al.,1995), there has again been general agreement among the findings. These haveshown a cell wall, of varying thickness, which is fibrillar and densely laminatedin structure, a fine granular cytoplasm packed with ribosomes, scattered vesicles,some with lipid-like contents, mitochondria with tubular cristae and a varyingnumber of nuclei. Despite the large number of nuclei in many cells, there was noevidence of zonation of the cytoplasm into functional areas or partitioning of thecytoplasm between nuclei, which could suggest cellular division. The nuclei werespherical, with a prominent central spherical nucleolus.

The initial infective challenge is generally considered to be through theingestion of thick-walled resting spores with infected feed (especially fish) ordirectly from water. As infection has been recorded in copepods by Jeeps (1937),and Calanus finmarchicus could be experimentally infected (Sindermann andScattergood, 1954), it is possible that these may act as paratenic hosts, whichaccumulate or harbour temporary infections, so concentrating infective stagesfor plankton-feeding fish. However, several studies (Sindermann andScattergood, 1954; Sindermann, 1958) have found that a large challenge withinfective material or a repeated challenge may be necessary for infection tobecome established. Dorier and Degrange (1960) suggested that this may berelated to the time taken for spores to develop in relation to the time taken formeals of different sizes to pass through the fish intestine, although this

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interpretation has been subsequently challenged, as the parasite has been shownto rapidly pass through its development stages in 3–5 h under conditions ofvarying pH (Spanggaard et al., 1994, 1995).

Germination of ingested spores in the host intestine typically involves thedevelopment of tubular protrusions (germination tubes or ‘hyphae’) (Figs 18.2and 18.3), which usually become branched (Fig. 18.4). These are similar to the

Fig. 18.1. Squash preparation of plaice liver infected with Ichthyophonus. Scale bar = 100 µm.Figs 18.2 and 18.3. Beginning of germination of Ichthyophonus spherical bodies in theintestine of an experimentally challenged rainbow trout 4–6 h after feeding. Scale bar = 100 µm.Fig. 18.4. Development of branched germination tubes (‘hyphae’) from spherical bodies ofIchthyophonus in the intestine of an experimentally infected rainbow trout. Scale bar = 100 µm.Fig. 18.5. Internal subdivision of the cytoplasmic contents of a spherical body produced at thetip of a germination tube of Ichthyophonus. Scale bar = 100 µm.Fig. 18.6. Spherical body with uninucleate motile bodies in the intestine of a rainbow troutexperimentally challenged with Ichthyophonus approximately 5 h after feeding. Scale bar =100 µm.


development observed in tissues when the host dies (McVicar, 1982;Spanggaard et al., 1994, 1995). Although such developments are structurallysimilar, there may be subtle differences in the nature of such hyphae underdifferent conditions of growth. Within the fish stomach, the stimulation of‘resting spores’ to develop may be related especially to the low or changing pHand to other factors in the environment, such as glucose (Spanggaard et al.,1994, 1995). Other factors were thought to be involved in the post-mortemdevelopment in dead fish tissues, as the pH there does not become sufficientlylow to trigger hyphal development. Spanggaard et al. (1995) used scanningelectron microscopy to visualize the morphology of the developmental stages ofIchthyophonus cultured in vitro and this showed a generally smooth wall to thesurface of the spherical cell and developing germination tube. An indication of alongitudinal ‘keel’ along the length of the latter was also shown in one instance.

Development of the parasite typically proceeds by the cytoplasmic contentsof the spore flowing up the germination tubes (‘hyphae’) to accumulate in themultiple tips, with subsequent rounding up and walling off to form thin-walledspherical structures. These are of varying size, frequently detach from theirparent structures and may be individually scattered around the original infectionsite or more distally throughout the body. The outcome of this proliferation in thetissues of a dead host, or in the gut of a potential new host which has eaten liveinfected prey, would, in both cases, result in a large increase in the number ofpotentially infective units. There are significant advantages to survival of aspecies from such a proliferation of potentially invasive stages at the point wheretransmission to new hosts is occurring.

Further proliferation of the infection occurs through the subdivision of thecontents of each of the spherical bodies (Figs 18.5 and 18.6), ultimately toproduce small uninucleate stages (Dorier and Degrange, 1960; McVicar, 1982;Okamoto et al., 1985; Spanggaard et al., 1995). These may show intermittentrotational movements while enclosed within the wall of the spherical body, and,when released, they had the power of movement (probably amoeboid) awayfrom the point of their release (McVicar, 1982). Various studies have suggestedthat these amoeboid bodies could survive from 1 to 5 days after release, and ithas been suggested that these bodies (‘endospores’) could be associated with theinfective stage of Ichthyophonus. Because of their small size, they will bedifficult to detect in living fish, particularly in natural infections, and this has ledto speculation on the precise route of infection. However, in experimentalsituations with exceptionally large infective doses, transmission electronmicroscopy has provided some clarification (A.H. McVicar and J. Gilmour,unpublished results). Small membrane bound bodies with cytoplasmic contentstypical of Ichthyophonus were observed adhering to the glycocalyx layer of thebrush border of the intestinal mucosal cells. Slender ‘hyphae’ were foundpenetrating between the microvilli of the brush border, and finally these hyphaewere detected penetrating through the cytoplasm of the mucosal cell (Fig. 18.7).No penetration was observed through the intercellular spaces between mucosalcells, presumably due to the difficulty in breaking through the tight junctionbetween the cells, whereas the surface membranes of the cells offered an easierroute. Similar ‘hyphae’ were detected deep in the cytoplasm of mucosal cells,

676 A.H. McVicar

and, although none were found penetrating through the basement membrane, itis likely that this would not be an impenetrable barrier for further passage to theintercellular lymphatic spaces of the gut wall.

Once inside the fluid spaces of the host tissues, the subsequent passivespread of small infective units throughout the body via the circulatory systemmay then occur. The predilection of infection for blood- rich tissues in most fishinfections indicates the importance of the host circulatory system in thedissemination of the infection within the host. Rapid growth and nuclearproliferation of the invasive stage to form typical spherical bodies in the tissuesof fish have been reported (Dorier and Degrange, 1960), with similar rapidgrowth also found in culture conditions (McVicar, 1982; Spanggaard et al.,1994). In each case, this leads ultimately to the further subdivision of the sporesand the formation of new infective elements. Dorier and Degrange (1960)suggested that a secondary invasion may occur within about 8 days of a primary

Fig. 18.7. Electron-microscope (EM) section through the microvillous border of the intestine ofa rainbow trout experimentally challenged with Ichthyophonus. Ichthyophonus-like bodies in thegut contents are applied to the tips of the microvilli and a germination tube (‘hypha’) ispenetrating through a mucosal cell (arrowed). Scale bar = 2 µm.


infection. The information currently available indicates that germination-tubeproduction, such as is observed in the tissues of a dead host or in the intestinalcontents after an infective meal, does not normally occur in the tissues of theliving host. In the latter, the infective stages are released into the circulatorysystem or into adjacent tissues by the rupture of multinuclear bodies. Suchdifferences may be related to pH and availability of nutrients and could be anadaptation to facilitate infection, as suggested by Spanggaard et al. (1994).

With the possible exception of the study by Sproston (1944), which, asindicated above, has some unresolved uncertainties about it, no research onIchthyophonus has reported a sexual or conjugative phase as part of the lifecycle. This paucity of information would suggest either that such events do notoccur as a regular event in the life cycle, the stages found being part of a purelyvegetative proliferation, or that conjugation occurs in a form which is rapid andnot obvious. More detailed study of the small and motile ‘amoeboid’ stagesseems to be justified, as these characteristics are common for the gamete stagesin many other Protista.

Release of infective stages has been noted through the gills and intestinalmucosa (Dorier and Degrange, 1960), through necrotic areas of the skin(Sindermann and Scattergood, 1954) or through kidney tubules (McVicar,1982), but it is probable that the main route of transmission of infection betweenhosts is through susceptible potential hosts feeding on moribund or dead hosts.The possibility of paratenic or transport hosts, such as copepods, being involvedhas been raised by various authors (Jeeps, 1937; Sindermann and Scattergood,1954), particularly for pelagic species, such as herring, where the main dietarycomponent is plankton.

Control and treatment

The widespread natural reservoir of infection in marine and estuarine waters andthe relative ease of horizontal experimental transmission of the disease betweenmost species of fish suggest that wild and farmed fish stocks in these areasshould be at risk from the infection. However, in practice, there have been norecords of farmed stocks becoming infected because of the proximity of infectedwild stocks, despite the size of the Atlantic salmon sea cage-farming industry inEurope and the yellowtail industry in Japan in regions known to harbour highlevels of Ichthyophonus infection. Sindermann and Scattergood (1954) foundthat repeated exposure of herring to large doses of Ichthyophonus was necessaryto establish experimental infection, and it is possible that similar difficulties mayalso occur in nature.

An assessment of the literature indicates that, for most fish, infection isacute, with associated high pathogenicity, and several researchers haveconsidered the disease to be inevitably terminal. Van Duijn (1956) noted that itwas impossible to heal fish infected with Ichthyophonus, but suggested thatfungicidal drugs, such as phoxethol, could be partially effective against earlystages of the infection. Although ichthyophoniasis may often have majoreconomic consequences in fish farms, since that report there have been no

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significant research efforts on chemotherapy of the disease. With this absence ofgenerally recommended treatment methods, efforts should be focused onprevention of the disease rather than its cure.

Infections in both freshwater and marine farms have been commonlyassociated with the use by fish farmers of fresh marine fish as feed (Pettit, 1913;Sindermann and Scattergood, 1954; McVicar, 1979; Okamoto et al., 1985). Aparticular feature of the life cycle of Ichthyophonus is the massive replication ofthe parasite after the death of the host, possibly stimulated by the change in pH,change in carbon dioxide tension (Spanggaard and Huss, 1996) or loss of theintegrity of the host cellular defences (McVicar and McLay, 1985). Theconsequence is a major increase in the number of infective stages beingavailable, a mechanism probably evolved to enhance the chance of transmissionto a new host. Using material from Ichthyophonus-infected fish which have beendead for a prolonged period as feed will therefore pose a particularly high risk.Particular attention should be paid to avoiding this source of infection in fishfarms. In view of the length of time this association has been known (e.g. byPlehn and Mulsow, 1911; Pettit, 1913; Rucker and Gustafson, 1953), it isdisappointing that infections still regularly occur in fish farms from suchpractices. The spherical bodies (‘resting spores’) of Ichthyophonus are resilientto a variety of physical conditions, a feature which could pose particularproblems in avoidance, prevention and treatment of the disease in cultured oraquarium fish. Spores have been shown to survive for almost 2 years in sea water(Spanggaard and Huss, 1996) and to be fully viable after 6 months in sterile seawater (Sindermann and Scattergood, 1954). Spanggaard and Huss (1996) alsoshowed that the parasite can grow well in the temperature range 0–25°C andbetween pH 3 and 7 (3–9 was reported by Okamoto et al., 1985) but was not ableto tolerate salinities above 4% sodium chloride (NaCl). Most agents infectious tofish can be destroyed by treatment of infected tissues with high or lowtemperatures. Slocombe (1980) found Ichthyophonus infection to persist after 2days at –8°C, but Athanassopoulou (1992) found that the parasite was killed at –20°C. Gustafson and Rucker (1956) suggested that freezing may be a suitablemethod of killing infection in wild fish being used as a feed source in fish farms.Spanggaard and Huss (1996) experimentally demonstrated a maximum survivaltemperature of 40°C for a period of 3 min. As, increasingly, low temperatures arebeing used in the processing of compounded foods for use in aquacultureindustries, some caution should be exercised when marine fish are being used asa source of raw material. However, the absence of any infection so far associatedwith this feed currently indicates a low risk from this area.

Host response

Ichthyophonus is highly antigenic and elicits a strong host cellular reaction and,at least in some species of fish, a marked humoral response (McVicar, 1982).

Using an indirect fluorescent antibody test on germinating spores in culture(incubated with serum from an infected fish, rabbit antibody against antibody ofthat fish species and goat antirabbit antibody), it has been shown that the general


surface of the parasite shows a positive reaction. A particularly strong reactionoccurs in a cap surrounding the tips of the developing germination tubes (Fig.18.8). The nature of the antigen in this area has not been characterized, but maybe an enzymatic system associated with the ability of the hyphae to penetratethrough host tissue and, in particular, the surrounding capsule of host reactionafter the death of the fish. However, Spanggaard (1996) found no or only slightenzyme activity associated with the growth of Ichthyophonus in culture, asurprising result in view of the characteristic softness and slimy texture ofinfected herring fillets. The development of a strong precipitating antibodyreaction in infected fish (plaice, turbot, Scophthalmus maximus, and rainbowtrout) was demonstrated by McVicar (1982) in natural and experimentalinfections of Ichthyophonus. The development of antibody in all infected plaiceand the absence of antibody in uninfected plaice in the wild indicated that

Fig. 18.8. Germinating Ichthyophonus spherical cell in MEM culture tested by an indirectfluorescent antibody test (IFAT) for the presence of host stimulating antigen. The exterior surfaceof the tips of the growing gemination tubes show a particularly strong reaction.

680 A.H. McVicar

antibody production was always a consequence of infection in this species(McVicar, 1982, 1990). However, not all naturally infected plaice had detectableantibody to Ichthyophonus. This result was attributed to the duration of theinfection being too short for the fish to respond to the infection, a proposalsupported by results from experimental infections (McVicar, 1990). Despite thestrong antibody reaction against Ichthyophonus, there was no evidence that thisconferred any protection on plaice and it has been concluded that the infection isinvariably lethal. A similar conclusion that the disease is progressive andterminal to herring has been made by Hodneland et al. (1997).

The degree of cellular response of fish hosts to the infection is highlyvariable in different host species and even within one host species. A light- andelectron-microscopic study of the tissue response in three host species – plaice,haddock and rainbow trout – was undertaken by McVicar and McLay (1985).Infection in all three species elicited a chronic inflammatory response, which,depending on the duration of infection in the tissue and, to some extent, the hostspecies, consisted variably of lymphocytes, macrophages, epithelioid cells, giantcells, fibrocytes and eosinophilic granular cells. The resulting granuloma wasfocal around individual Ichthyophonus bodies or obliterative around groups ofparasitic bodies.

Most or all of the pathogenesis of Ichthyophonus can be directly linked tothe replacement, disruption and atrophy of infected tissues by the proliferationof the parasite, with, in extreme cases, the normal tissues of organs being almostcompletely replaced. The degree of directly lytic activity associated with thenutrition of the parasite is uncertain in living fish, but this is evident in newlydead fish, as the germination tubes of the developing spores cause extensivedestruction of surrounding tissues. As suggested by McVicar and McLay (1985),the success of the fish host in preventing invasion or in containing infection,once present, probably greatly influences the level of pathogenicity ofIchthyophonus. Aspects of the host’s response have been implicated in the intra-and interspecific variations in fish susceptibility to Ichthyophonus.

Feed-quality aspects

The spoilage characteristics of fish flesh as feed by Ichthyophonus have beenwell known for many years, with Williamson (1913) using the terms ‘spottedhaddock’, ‘greasers’ and ‘smelly haddock’ to describe haddock infected withIchthyophonus caught off western Scotland. As pointed out by McVicar (1982),at least one of these terms has persisted in common usage to the present day,indicating the continual concern about this condition in the fish processingindustry over that period. Sindermann (1958) reported that acute infections inherring caused degeneration and necrosis of the body muscles and that such fishwere poor for smoking and pickling. The fillets of infected herring from bothNorth American and European waters are characterized by being very soft andslimy, with strong off odours and often with pigment deposition around sites ofinfection. To determine if the characteristic germination of the parasite after thedeath of its host led to increased spoilage, it was important to understand the


factors influencing growth in fish products. Spanggaard (1996) and Spanggaardand Huss (1996) carried out detailed studies of the growth conditions ofIchthyophonus in North Sea herring products. Temperatures between 0 and 25°Cand pH between 3 and 7 did not affect the growth ability of the parasite, but thesignificant effect of increasing concentrations of NaCl showed that it wasunlikely that Ichthyophonus would develop and spoil processed products, suchas pickled or salted herring, by continued growth. Both heating at 40°C andfreezing at –20°C killed the parasite.

DERMOCYSTIDIUM

Dermocystidium has been described as a parasitic genus since early this century.As with Ichthyophonus, there has been similar uncertainty about the precisetaxonomic position of these organisms and their proper affinity, particularlywhether they should be linked to protozoans or to the lower fungi (Lom andDykova, 1992). It is probable that they have become a ‘waste-basket’ forpossibly unrelated, but morphologically similar organisms. Some early reportsinclude species of Dermocystidium as parasites of oysters (Mackin et al., 1950),many species of freshwater fish and some amphibians (Reichenbach-Klinke andElkan, 1965). However, Perkins (1976) demonstrated that the oyster pathogenwas in fact an apicomplexan protozoan, and Levine (1978) erected a new genusand class to distinguish it from the amphibian and fish parasites. The studies bySpanggaard et al. (1996) and Ragan et al. (1996) on the molecular genetics ofDermocystidium show that the parasites have close affinities with Ichthyo-phonus, and these authors place both groups as lower protistan parasites near theanimal–fungal divergence.

Dermocystidium from fish and amphibians usually form subcutaneous orgill cysts (Reichenbach-Klinke and Elkan, 1965). Severe gill pathology andmortality have been reported in prespawning adults and emergent fry of chinooksalmon (Oncorhynchus tschawytscha) in the USA (Pauley, 1967; Allen et al.,1968). Heavy gill infections of Dermocystidium infection have been reported bySpangenberg (1975) to result in reduced growth in eels, Anguilla anguilla, but,in contrast, Wootten and McVicar (1982) found no evidence of major effectswith a Dermocystidium infection, again of gills of eels, reared in a recirculationsystem. They found that the infection was resolved by the host withoutintervention by the fish farmer. Infections of Atlantic salmon with anunidentified species of Dermocystidium occurring as a visceral parasite of the fatbody had serious effects on individual fish, but, as a relatively small proportionof fish on the farm were affected, the economic significance was not clear(McVicar and Wootten, 1980). Höglund et al. (1997) described Dermocystidiumfrom the mucus of the epidermis of gills and fins, associated with fin thickeningand often occurring in combination with fin erosion.

Two groups of Dermocystidium were identified by Höglund et al. (1997),based on their position of infection on the host, namely those contained withinmacroscopically visible cutaneous cysts and those present as chronic internalsystemic infections. The scarcity of data on early development and modes of

682 A.H. McVicar

transmission has led to difficulties in the close comparison of the differentnamed species. Different isolates of Dermocystidium show a commondevelopment stage with cells containing a single large cytoplasmic vacuole(signet-ring stage) and a prominent vacuoplast (Fig. 18.9). These features areremarkable similar to those found in Perkinsus from oysters and the possibilityof there being a relationship between the two groups should still not bediscounted. Despite the demonstrated genetic relationship between Ichthyo-phonus and Dermocystidium by Spanggaard et al. (1996) and Ragan et al.(1996), there are major histological and ultrastructural differences in the twotypes of organisms.

Peroral infection of Atlantic salmon with Dermocystidium and horizontal

Fig. 18.9. EM section of Dermocystidium sp. from the visceral fat body of Atlantic salmonshowing the characteristic ‘signet ring’ and other developmental stages. Scale bar = 2 µm.


transmission have been demonstrated experimentally by McVicar and Wootten(1980), but a rapid spread of the parasite from fish to fish in individual fish farmpopulations was not found. This parasite was particularly amenable to in vitroculture in thioglycollate medium and MEM, and many developmental stageswere clarified. Cells produced by cleavage of spherical mother cells werereleased through an elongate tubular extension of the cell (McVicar andWootten, 1980), a development bearing little resemblance to that observed withIchthyophonus. Further studies are apparently required to clarify the closeness ofthe relationship between the two groups of organisms.

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Alderman, D.J. (1982) Fungal disease of aquatic animals. In: Roberts, R.J. (ed.)Microbial Diseases of Fish. Academic Press, London, pp. 189–242.

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