MFR PAPER 1298

Parasitic Diseases ofFreshwater Fishes

W. A. ROGERS

ABSTRACT - The major parasites of freshwater fishes are briefly discussed.Ufe cycles, host-parasite relationships, pathogenicity, and possible preventionand control are given.

Parasitic diseases of fishes are usu­ally encountered more often then mi­crobial diseases. From 30 to 50 percentof the cases received at several fish dis­ease diagnostic laboratories involveparasites.

Host reaction to parasitic invasion ishighly variable. The severity of a para­site epizootic may be related to en­vironmental factors; host condition,age, and size; and population density.Some species that may be consideredcommensals become pathogenic undercertain conditions. However, most ob­ligate parasites are pathogenic andsome, such as the ciliated protozoanIchthyophthirius, cause great losses.Many helminths, such as the digeneangrubs, appear to be only slightly ormoderately pathogenic but destroy theaesthetic value of fish.

PROTOZOANSMany biologists consider the proto­

zoans to be the most important group ofparasites affecting fish. Fish culturiststhroughout the world report great lossescaused by protozoans. The most devas­tating fish parasite of all is lchthyoph­thirius. "leh" is an obligate parasiteinfecting the skin and gills of fish. Thelife cycle of "Ich" involves productionof many young individuals (tomites)produced from a single cell after the

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mature cell, the trophozoite, leaves thefish. The trophozoite settles on a sub­strate, secretes a cyst wall and under­goes multiple fission, giving rise to upto 1,000 tomites. The tomites thenleave the cyst, seek a host, bore into theepidermis or gill epithelium, and pro­duce a severe irritation accompanied byexcess mucus production and hyper­plasia of the epithelium. The cells ma­ture, creating small white spots that areeasily seen with the naked eye.

We were able to demonstrate that fishwill develop an immune response toIchthyophthirius infections. After fishwere experimentally infected, theywere treated with malachite green to ridthem of "Ich". Fifteen days later theywere challenged: All control fish de­veloped "leh" and died while previ­ously infected fish did not.

One researcher collected "Ich"trophozoites, ground them up using 1.0ml of trophozoites in 10 ml of 0.85percent saline, added Freund's adju­vant, and injected the fish intraperitone­ally. Fifteen days later upon challengethe controls developed' 'leh" and died,but all injected fish survived (Areerat,1974). A problem in developing a vac­cine for "leh" would be in-vitro cultur­ing of "leh." Many people have at­tempted in-vitro culture without suc­cess.

Several other ciliates such asChilodonella, Trichodina, Ambiphrya ,and Epistylis can cause mortality undercertain conditions. Flagellates such asCostia and Bodomonas often causemortality but are fairly easy to controlwith an external treatment.

Spore-forming Cnidospora includemembers of the genus Myxosoma thatcause whirling disease of salmonids inwhich young fish are infected and thecartilage destroyed prior to bone forma­tion. When the eroded cartilage os­sifies, the bone is distorted and the fishusually has severe curvature of thebackbone, resulting in the fish swim­ming in a circle thus giving the condi­tion the name "whirling disease."

Henneguya is another Cnidospori­dian commonly infecting channelcatfish, Ictalurus punctatus. Severaldifferent species occur in the gills andskin with an interlamallar form reportedto cause extensive mortality in veryyoung fish.

Several species of Pleistophora, amicrosporidian, occur in fish, one ofwhich occupies the ovary of goldenshiners, Notemigonus crysoleucas, andcan cause sterility. Older fish have amuch heavier infection rate and moreovary damage, but a technique of usingI-year-old brood fish has practicallyeliminated the problem of sterility.

W. A. Rogers is with the Southeast­ern Cooperative Fish Disease Proj­ect, Auburn University, Auburn, AL36830

Marine Fisheries Review

TREMATODES

Some genera of monogenetic tre­matodes can build up large populationsunder intensive culture situations butthey are not considered to be a seriousthreat to fish in North America.

Two groups of digenetic trematodesoccur in fish: adults normally living inthe gastrointestinal tract and larvalforms, metacercaria or grubs, living inthe tissues. The eye grub, Diplos­tomum, attacks the lens of the eye caus­ing blindness while white grubs, blackgrubs, blackspot, and yellow grubs af­fect mainly the aesthetic value of thefish. There are no controls known forthe tissue-inhabiting larval forms.

CESTODES

fish about every 3 years in an attempt tomanage around the problem by main­taining a low intensity of infection. Ear­lier attempts to rid fish hatcheries of thebass tapeworm by disinfecting theponds and restocking both brood andforage fish were unsuccessful as the fishwould show a reinfection within a yearafter restocking. Corallobothrium is atapeworm sometimes found in greatnumbers in the intestine of channelcatfish. Little apparent damage iscaused by this worm and it has beeneasily controlled using tin compoundsin the feed.

NEMATODES

Nematodes in the gastrointestinaltract are not considered detrimental, but

tissue inhabiting forms can seriouslyaffect the fish population. Philometra isa nematode commonly found behindthe eye of centrarchid fishes and up to40 percent of some reservoir popula­tions in the southeastern United Stateshave been affected.

CRUSTACEANS

Crustaceans are one of the maingroups of detrimental parasites, espe­cially the anchor worm that has bothadult and a parasitic larval stage thatfeeds on the fish.

LITERATURE CITEDAreerat, S. 1974. The immune response of chan­

nel catfish, Ictalurus punctatus (Raf.) toIchrhyophrhirius multijiliis. Masters Thesis,Auburn Univ., Auburn, Ala., 44 p.

Cestode pleurocercoids of the large­mouth bass, Micropterus salmoides,tapeworm, live in visceral organs ofbass and commonly migrate throughovaries causing sterility. One approachwe have taken is replacement of brood

March /978

MFR Papar 1298. From Marine Fisheries Review, Vol. 40, No.3, March 1978.Copies of this paper, in limited numbers, are available from 0822, User Ser­vices Branch, Environmental Science Information Center, NOAA, Rockville,MD 20852. Copies of Marine Fisheries Review are available from the Superin­tendent of Documents, U.S. Government Printing Office, Washington, DC20402 for $1.10 each.

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