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Historical review of piscine trypanosomiasis111host's B-lymphocytes into the blood plasma (Roitt, (van Duijn, 1973). Tryparsamide is a toxic organic1974) and may attack the parasites in various ways arsenical compound and is used under strictincluding complement mediated lysis (lysins), regulation. Fish intended for human or animalagglutination (agglutinins) and facilitating consumption should not be treated with arsenicals.phagocytosis (opsonins). Cottrell (1976) found thatplaice infected with T. platesae had elevated levels ofThe parasites can be controlled best by preventativeserum immunoglobulins possessing the betameasures. Piscivorous leeches must be restricted orelectrophoretic mobility which might be associatedeliminated from fish rearing areas and water supplieswith antibody. Goldfish experimentally infected withby screens and chemicals. Chemical removal requiresT. carassii, the immunity to super infection could lastthe addition of a soluble toxic compound, calculated toupto 350 days (Lom and Dykova, 1992). Probably thisattain a pre-determined dilution to the water untilwas due to non-sterile immunity as evidenced bydegradation or dissipation occurs. Freshwater fish-outbreaks of parasitaemia in some of the fishattached leeches can be removed by dipping fish into aapparently free from infection previously. Thiscommon salt or copper sulphate solution. Commercialappears to be temperature dependent as when fish arechemicals available for direct control of free-livingkept at a low sub-optimal temperature, the infectionleeches are Dylox, Baytex, and Baygon, butdoes not reach the last phase and a certain level ofeffectiveness varies among target species (Hoffmanparasitaemia is maintained. This may be due to otherand Meyer, 1974) and their toxicity to fish is as yetfactors as well- stress, nutritional deficiency andunknown.fishes showing low parasitaemia may be super REFERENCESinfected with the same trypanosome species by a leechAvise JC. 1994. Molecular Markers, Natural History andor by a syringe passage which may in some cases resultEvolution. Chapman and Hall. New York.in a high level of parasitaemia.The aim of the parasite is to persist in its host for aslong as possible at an optimum level in order to ensurea good chance of onwards transmission but lowenough to avoid undue risk of killing the host. Anunusual antibody, “ablastin” maybe directed againstcertain Trypanosoma species which interferes withparasite division but has no other harmful effect (D'Alesandro, 1975). The antibody “ablastin” is thusideal from the parasite's view point by preventingmultiplication, it reduces parasitaemia to a level whichdoes not seriously endanger the host and probablydoes not furnish too great an antigenic stimulus to thelatter. The adult parasites may thus be able to persistfor long periods in the blood of the vertebrates host,waiting to continue its life cycle in the vector.“Ablastic” antibodies have been found to occur inamphibians (Lom, 1969), however, their presence infish is uncertain. It maybe possible that ablastin orablastin-like antibodies are stimulated in fish hosts aswell and may account for the same phenomenon.Under certain conditions, fish may develop protective,nonsterile immunity to homologous challenge (Woo etal., 1983).TREATMENT AND CONTROLIt is difficult to eliminate trypanosomes from fish byany routine treatment. It is believed that tryparsamideadministered at low dosages in food might be effectiveBaker JR. 1960. Trypanosomes and Dactylosomes from theblood of fresh water fish in East Africa. Parasitology50:515-526.Baker JR. 1961. Trypanosomes of African fresh water fish, anaddendum. Parasitology 51:263.Baker JR. 1976. Blood fluids-protozoans. In: EcologicalAspects of Parasitology. Kennedy, CR (Ed.). North HolandPublishing Company, Amsterdam. pp 371-386.Barrow JH. 1954. Observations on some host specificity andimmunological reactions of trypanosomes infection in somefresh water fish of Europe. Anat Rec120:750-751.Bauer ON. 1984. Key to the determination of parasites of freshwater fishes of the USSR. Nauka, Leningrad. pp 431.Becker CD. 1967. Trypanosoma occidentalis sp n. fromfreshwater teleosts in Washington State. J Protozool14:153-156.Becker CD. 1970. Haematozoa of fishes with emphasis onNorth American records. In: A Symposium of Diseases ofFishes and Shell Fishes. Snieszko, SF (Ed.) Spec Publ AmerFish Soc Washington 5:82-100.Becker CD and Overstreet RN. 1979. Haematozoa of marinefishes from the northern Gulf of Mexico. J Fish Dis 2:469-479.Bennett G. 1989. The use of morphotaxonomy in describingnew species of Haematozoa. J Bengal Nat Hist Soc 8:3-4.Brumpt E. 1904. Contribution a l'etude de'l evolution deshemogregarines et des trypanosomes. Comptes rendus de laSociete de Biologie 57:165-167.