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Present and future possibilitiesfor the control of cowdriosis andanaplasmosisG. Uilenberg a ba Department of Tropical Diseases and Protozoology ,Veterinary Faculty , Utrecht, The Netherlandsb Institut d'Elevage ct de Medecine Veterinaire des PaysTropicaux , 10 rue Pierre Curie, Maisons‐Alfort, 94704,FrancePublished online: 01 Nov 2011.

To cite this article: G. Uilenberg (1990) Present and future possibilities for thecontrol of cowdriosis and anaplasmosis, Veterinary Quarterly, 12:1, 39-45, DOI:10.1080/01652176.1990.9694240

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REVIEW PAPERS

Present and future possibilities for thecontrol of cowdriosis and anaplasmosis1

G. Uilenberg2

SUMMARY Cowdriosis and anaplasmosis are most important tick-borne rickettsial diseasesof ruminants. After a short introduction, in particular of their aetiology, epidemiology anddiagnosis, present methods and future prospects for their control are briefly reviewed. Thevalue and disadvantages of the four possible approaches, chemotherapy, tick control, theutilisation of inverse age-resistance in order to attain endemic stability, and artificialimmunisation, are reviewed. Promising future developments in the field of immunisation areindicated.

INTRODUCTION

Cowdriosis (synonym 'heartwater') and anaplasmosis are among the mostimportant tick-borne rickettsial diseases of domestic ruminants, with Cowdriaruminantium and Anaplasma spp. as their respective causal agents.Cowdriosis is transmitted by several African three-host Amblyomma ticks.Transmission is transstadial, from larva to nymph, or from nymph to adult, andeven from larva through nymph to adult; transovarial transmission appears tooccur exceptionally (1), but is not thought to be an important factor in theepidemiology of the disease.Several genera and many species of ticks, three-, two-, as well as one-host species,act as vectors of anaplasmosis, while mechanical transmission by biting insectsalso plays a certain role. In spite of early reports of transovarial transmission,it appears that transmission by ticks is transstadial only. Although the cycles inthe tick of both micro-organisms have not yet been completely clarified, they showmany similarities (8, 9).Cowdriosis is limited in its geographical distribution to Africa south of the Saharaand neighbouring islands (Madagascar, La Reunion, Mauritius, Comores Islands,Sao Tome), as well as three islands in the Caribbean area (Guadeloupe, Marie-Galante and Antigua), where one of the African tick vectors has becomeestablished.Anaplasmosis is much more widely spread, being present almost everywhere inthe tropics and subtropics and occurring in many temperate regions as well.Although considerable diversity in pathogenesis, pathogenicity, host range andantigenic composition has been shown to occur, all isolates of Cowdria are so farconsidered to belong to one species, Cowdria ruminantium. It multiplies in theruminant host in endothelial cells, and has also been demonstrated in neutrophilicand eosinophilic granulocytes. Central nervous symptoms with a rapid fataloutcome are commonly seen in typical acute cases of cowdriosis.Four or five species of Anaplasma are so far generally recognised, on the basisof infectivity and pathogenicity for various ruminants, and the position taken up

I Invited paper in the colloquium 'Advances in Tropical Veterinary Medicine' of the EuropeanMulticolloquium of Parasitology (EMOP V), Budapest, September 1988.

2 Department of Tropical Diseases and Protozoology, Veterinary Faculty, Utrecht, The Netherlands.Present address: Institut d'Elevage et de Médecine Vétérinaire des Pays Tropicaux, 10 rue PierreCurie, 94704 Maisons-Alfort, France.

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in the erythrocyte, in which multiplication occurs, and which is the only host cellknown to be infected. Anaplasms associated with an appendage in the red cellhave been separated as a separate genus Paranaplasma, but since it has been shownthat the appendage is not parasite material but a function of the erythrocyte, thereasons for making a distinction at the genus level wouldappear to be insufficient.A severe anaemia is the predominant symptom in acute anaplasmosis.Cattle, domestic buffalo and small ruminants, as well as several wild membersof the families Bovidae and Cervidae, are susceptible to cowdriosis. Certainisolates of the casual rickettsia are also pathogenic for laboratory rodents, whileit has been shown recently that a bird (guinea-fowl) and a reptile (tortoise) canbe subclinically infected and even act as a source of infection for ticks (15).Anaplasma marginale and A. centrale occur in cattle, and the former species hasalso been found in domestic buffalo. A case could possibly be made for separatingA. caudatum as a distinct species from A. marginale, on the basis of its being ableto induce the formation of a crystalline appendage in the infected red cell, andon the basis of antigenic differences. Small ruminants are the hosts of A. ovis andA. mesaeterum. Wild Bovidae and Cervidae can also be infected with one or moreof these species, and may have natural Anaplasma infections. Reports of anaplasmsin animals other than ruminants have not been confirmed, and no laboratorymodels are available.The findings that the host range of Cowdria extends to rodents and even birdsand reptiles, together with the evidence for what appears to be exceptionallyoccurring transovarial transmission, may bring about considerable changes in ourconcepts of the epidemiology of cowdriosis. There are several other importantfactors in the epidemiology of the diseases. For instance the average low infectionrate of the tick population (because ofa very limited period during which infectedruminants are infective to ticks, and because many immature ticks feed on animalspecies which are not considered to be susceptible), together with the fact thatthe post-natal period during which young ruminants of susceptible breeds aretolerant to the disease and able to acquire immunity without significant clinicalsymptoms is very short, means that 'endemic stability' such as is known inbabesiosis, does not normally occur in cowdriosis. Additional important facts arethe great breed and population differences in susceptibility of domestic ruminants,and the antigenic differences between strains of Cowdria ruminantium. Generally,animals recovered from cowdriosis are solidly immune to homologous reinfectionsfor months, and an immunity sufficient to prevent clinical disease may persist foryears. However, because of antigenic differences between strains, heterologousprotection between different isolates is sometimes only partial or, more rarely,even virtually absent. Recent reviews of the epidemiology have been given in 1982,1983 and 1987 (3, 21, 23).As mentioned, the host range of the anaplasms appears to be more limited. Afurther difference is the fact that the blood of recovered animals remains infected,often indefinitely. It is believed that immunity to homologous reinfections persistsfor at least as long as the animal remains a carrier. The post-natal period oftolerance to the disease lasts for many months, so that endemically stablesituations are encountered ,wherever sufficient numbers of infected ticks occur.As in cowdriosis, this age-associated tolerance is independent of the immune statusof the dam. Breed differences in susceptibility to anaplasmosis do occur but arerelatively minor. Antigenic differences within the species A. marginale exists, buttheir importance in the epidemiology in the field is not really known; they mayhave practical implications where a certain type of dead vaccine is used (11). Thereare also strain differences in virulence. Recent reviews of anaplasmosis have forinstance been given in 1981 and 1986 (14, 22).

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The diagnosis of cowdriosis as well as of anaplasmosis on clinicalsymptoms and/or necropsy findings alone, can never be more than tentative.Acute anaplasmosis can easily be confirmed by microscopical examination ofGiemsa-stained blood-smears. However, direct microscopical demonstration ofcowdriosis in the live animal is possible only on smears of brain material obtainedby biopsy, not a suitable procedure for routine use. Another way of confirmingthe diagnosis is by the lengthy procedure of subinoculating and monitoring asusceptible ruminant. Recently, it was shown that the demonstration of C.ruminantium in one day old primary cultures of polymorphonuclear white bloodcells, has potential as an additional method of confirming the diagnosis (L. A.Wassink and F. Jongejan, unpublished data, 1987). After death, the microscopicaldiagnosis of cowdriosis can usually be made with certainty on smears of cerebralcortex, although a lengthy search of the capillaries may be necessary.Previous infections can be diagnosed in recovered animals by serological tests.Several tests have been developed for anaplasmosis, the most commonly used sofar being the complement fixation test, the indirect fluorescent antibody (IFA)test and the card agglutination test, each having advantages and disadvantages.The enzyme-linked immunosorbent assay may be used more extensively in future,with specific anaplasma surface proteins as antigen (17).There is as yet no really satisfactory serological test for cowdriosis. One methoduses antigen made from infected macrophages of mice inoculated with a peculiarmouse-pathogenic stock in the indirect fluorescent antibody test (4). It appearsto be difficult to prepare significant amounts of suitable antigen, and in view ofantigenic diversity within the species, there is no certainty that it is suitable todetect antibodies to all strains of C. ruminantium. It has also been used todemonstrate the presence of the infection in ticks by injecting mice withhomogenised ticks and testing the serum of the mice (5). More recently antigenconsisting of primary cultures of infected neutrophilic granulocytes has also beenused in the IFA test (13). However, in recent experiments in our laboratory distinctserological differences between stocks of Cowdria were found with this method(F. Jongejan, L. A. Wassink, M. J. C. Thielemans, N. M. Perié & G. Uilenberg,unpublished data, 1987-1988), and a certain degree of cross-reactivity withEhrlichia, as was first shown by Logan et al. (12), was confirmed. The value ofthis test in the field is therefore uncertain. It is hoped that purified Cowdriaorganisms will be available as antigen in the near future now that C. ruminantiumcan be grown in endothelial cells (2), and that from such material ultimately morerefined tests using single common antigens can be developed.

CONTROL

The control of both diseases is based on one or more of four approaches:(1) Individual chemotherapy;(2) Acaricidal tick control;(3) Achieving endemic stability;(4) Artificial immunisation.

Individual chemotherapy is of very limited value for cowdriosis. Even thoughtetracyline antibiotics do have an effect on C. ruminantium, and can cure infectedanimals if administered at an early stage, the disease is noticed only when obviousclinical signs appear, and treatment is usually too late to save the animal.Tetracyclines are the only drugs on the market known to be really useful.Anaplasmosis is more amenable to individual treatment. Both tetracyclines andimidocarb are effective; multiplication of the anaplasms is halted, and the host

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immune system is given the opportunity to dispose of infected red cells, so thatthe anaemia keeps increasing for the first few days. In very acute cases it maybe necessary to give a blood transfusion besides chemotherapy.Individual treatment in general is in any case of limited use in large, extensivelyranched herds, and the large doses of expensive drugs required for both diseasesare a heavy burden on developing countries.Acaricidal tick control is rarely 100% effective in preventing all disease transmis-sion. Complete success can only be obtained on premises protected against strayanimals and in the absence of wild hosts of the tick vector. A serious disadvantageis that the herd is not immune and vulnerable to the disease when tick controlbreaks down. This commonly occurs because of the nearly inevitable developmentof acaricide resistance, but also due to mechanical failure of tick control equipmentor interruption in the provision of acaricide, common in some developingcountries because of foreign exchange restrictions or political upheaval. Intensivetick control is also expensive, causes acaricide residues in meat and milk, andpollutes the environment, albeit to a relatively small extent.

There are very few circumstances indeed where it is realistic to consider completeeradication of the vector in a large region.Biting insects may cause limited outbreaks of anaplasmosis even where tick controlis good; for instance, if during the rainy season an animal with a high rickettsiaemiais exposed to a dense population of Tabanidae, a seasonal outbreak of anaplas-mosis may occur.In future, many aspects of tick control may change if present attempts atdeveloping vaccines against tick infestation are successful. Characterised antigenswill presumably be used. Recent reviews have been given in 1986 and 1987 (25,26).

Endemic stability is a situation in which all animals become infected with thedisease in question, and recover without becoming significantly ill. They aresubsequently immune. There is, ideally, a 100% infection without disease. Forthe realisation of this situation it is necessary that the initial infection occurs whenthe animal is resistant to it, either during the first few weeks of life because ofpassive immunity, or because of an as yet unexplained tolerance during the firstweeks or months, not associated with the immune status of the mother (inverseage-resistance). It is furthermore necessary that the inoculation rate of the diseaseis sufficiently high to infect all young animals during the period they are tolerant,that is to say that enough infected tick-bites occur to initiate early immunitywithout significant disease, and maintain the immune status during later life.An endemically stable situation is commonly encountered where anaplasmosis isconcerned. The inverse age-resistance diminishes slowly only towards the end ofthe first year of life. Although some calves may go through a period of someanaemia, serious symptoms are uncommon. Clinical anaplasmosis is a seriousproblem only where the animals are not exposed sufficiently to infected ticks,.eitherbecause of intensive acaricidal tick control, or because of management factors orclimatic conditions.The matter is different as regards cowdriosis. The normally low infection rate inthe tick population results in the majority of the animals not having been infectedduring the short post-natal period of inverse age-resistance, a few weeks only,unless tick numbers are intolerably high. The innate resistance of local populationsof ruminants in endemic areas may simulate an endemically stable situation, butthe true state of affairs becomes apparent when more susceptible exotic or cross-bred animals are involved.

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Artificial immunisation against cowdriosis and anaplasmosis is possible and usefulto guarantee against tick control failures, and generally to help to control thesediseases in situations which are not endemically stable. There are however seriousproblems.Immunisation against cowdriosis is so far only possible by a cumbersome infectionand treatment method, not suitable for large-scale application. Live virulentmaterial (infected blood or tick-derived material) is injected intravenously, thetemperature is monitored daily, and the reaction, if necessary, treated. It is notsuitable for application on a large scale. Even though isolates of moderatevirulence only should be used, the method entails a certain risk, as peracutereactions are fatal before treatment can be effected, and in a few cases no cleartemperature reaction occurs. Moreover, other pathogens have sometimes beentransmitted in donor blood. The result of immunisation is sometimes disappoint-ing, possibly because of the occurrence of considerable antigenic diversity in C.ruminantium in the field (7). Recent reviews have been given in 1984 and 1987(6, 16, 24). Since it has become possible to grow C. ruminantium in vitro (2), hopeshave risen that more purified immunising material can be obtained, and that inthe more distant future it may be possible to develop vaccines consisting ofcharacterised single antigens. Immunoblot techniques have recently been used inour laboratory to demonstrate the existence of a protein of 32 kilodaltons withepitopes common to immunologically different stocks of C. ruminantium (F.Jongejan & M. J. C. Thielemans, unpublished data, 1988), but it remains to beshown whether this antigen can induce protection.

Artificial immunisation against anaplasmosis is also in general carried out withinfected blood, containing live organisms, from a donor animal. The mostcommon 'vaccine' uses A. centrale, a much milder parasite than the average strainof A. marignale, and which confers a degree of protection against the latter. Itsuse has been reviewed in detail in 1984 (6). Although this procedure has beenapplied since the early years of this century, and the protection afforded is oftenregarded as fairly satisfactory, recent laboratory studies have thrown doubt onthis belief (20). Its pathogenicity is too high for use on lactating and pregnantanimals, and it is best used in young animals only. Another method ofimmunisation is the inoculation of young calves with blood from a donor animalcarrying A. marginale. A. marginale has also been used to reinforce the immunitycaused by A. centrale.

The age-resistance of the calves will in general prevent serious symptoms ofdisease, especially if care is taken to select as mild a stock of A. marginale aspossible. Nevertheless, some calves may temporarily be adversely affected by thisuncontrolled procedure. The incubation period of anaplasmosis is too long andvariable, and the first signs of the disease are not sufficiently characteristic, tomake an infection and treatment method feasible. Of course, any method usingfresh donor blood entails the danger of spreading other pathogens, and examplesof this are sufficiently numerous to show that this danger is real. A commercial'vaccine' in the USA uses an artificially attenuated stock of A. marginale whichappears to give a satisfactory protection in the field in Latin America (22), althoughthe possibility of a recurrence of virulence has been evoked (10). It also uses donorblood. There is also a dead commercial vaccine in the USA, which appears togive a certain protection to field challenge after 2 injections and needs an annualbooster unless the animals have become infected in the meantime. It has seriousdrawbacks, not only in causing significant numbers of cases of neonatalisohaemolytic anaemia (10), but also in not providing as good a protection toheterologous challenge than do live vaccines (11). The existence of antigenic

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diversity in A. marginate is to some extent in general a complicating factor of allthese methods of immunisation.The various disadvantages of the above methods have stimulated research on mormodern approaches. Using monoclonal antibodies, it has been possible todemonstrate the existence of strain-specific antigens, but also of antigens commonto isolates of A. marginate with geographically widely different origins and evenof epitopes common to the various isolates of A. marginate and A. centrale as well(18). Purified common antigens have been used experimentally to induce apromising degree of protection against homologous and heterologous challenge(19). It is hoped that appropriate antigens can be produced by recombinant DNAtechniques.Although the new developments that might lead to control of both diseases byimmunisation with single isolate-common antigens are exciting, only the futurewill show whether this approach can indeed significantly contribute to the well-being of livestock owners in the tropics. Even if and when all technical problemshave been solved, economical, logistical, and organisational factors will certainlyplay an exceedingly important role.

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by Amblyomma hebraeum. Onderstepoort J Vet Res 1986; 53: 31-4.2. Bezuidenhout JD, Paterson CL and Barnard BJH. In vitro cultivation ofCowdria ruminantium.

Onderstepoort J Vet Res 1985; 52: 113-20.3. Camus E and Barré N. La cowdriose (heartwater). Revue generale des connaissances. Institut

d'Elevage et de Médecine Vétérinaire des Pays Tropicaux, Maisons-Alfort, 1982.4. Du Plessis JL. The application of the indirect fluorescent antibody test to the serology of

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5. Du Plessis JL. A method for determining the Cowdria ruminantium infection rate of Amblyommahebraeum: effects in mice injected with tick homogenates. Onderstepoort J Vet Res 1985; 52: 55-61.

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8. Kocan KM. Development of Anaplasma marginale Theiler in ixodid ticks: coordinated develop-ment of a rickettsial organism and its tick host. In: Sauer JR and Hair JA. Morphology, physiologyand behavioral biology of ticks. Ellis Horwood, 1987; 472-505.

9. Kocan KM and Bezuidenhout JD. Morphology and development of Cowdria ruminantium inAmblyomma ticks. Onderstepoort J Vet Res 1987; 54: 177-82.

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11. Kuttler KL, Zaugg JL, and Johnson LW. Serologic and clinical responses of premunized,vaccinated, and previously infected cattle to challenge exposure by two different Anaplasmamarginale isolates. Am J Vet Res 1984; 45: 2223-6.

12. Logan LL, Holland CJ, Mebus CA, and Ristic M. Serological relationship between Cowdriaruminantium and certain ehrlichia. Vet Rec 1986; 119: 458-9.

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16. Oberem PT and Bezuidenhout JD. The production of heartwater vaccine. Onderstepoort J VetRes 1987; 54: 485-8.

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19. Palmer GH, Ober le SM, Barbet AF, Goff WL, Davis WC, and McGuire TC. Immunization ofcattle with a 36-kilodalton surface protein induces protection against homologous and hetero-logous Anaplasma marginale challenge. Infect Immun 1988; 56: 1526-31.

20. Potgieter FT and Van Rensburg L. Infectivity, virulence and immunogenicity of Anaplasmacentrale live blood vaccine. Onderstepoort J Vet Res 1983; 50: 29-31.

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22. Ristic M. Anaplasmosis. In: Ristic M and McIntyre I. Diseases of cattle in the tropics. MartinusNijhoff Publishers, 1981; 327-44.

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