Freshwater fish trypanosomes: definition of two types, host control by antibodies and lack of antigenic variation

Haemoflagellates of the genus Trypanosoma are prevalent in freshwater fishe s and are transmitted by leeches as vectors. As demonstrated by sequence co mparisons of nuclear small subunit rRNA genes, trypanosomes isolated from s everal fish species at different localities can be divided into at least 2 closely related types, designated Type A and Type B. A clone derived from a Type A isolate from carp (Cyprinus carpio) was used to study the anti-para site immune response in specified pathogen-free outbred carp. Infection lea ds to an initial rise in parasitaemia in the blood followed by a sharp decl ine in all fish (acute phase). Thereafter, in some carp, parasites become u ndetectable both in the blood and in internal organs while, in others, low numbers can be found in the blood for up to 1 year (chronic phase). Fish th at have controlled an acute infection with the clone are not only protected against an homologous challenge infection, but also against the infection with parasite lines derived from carp in the chronic phase of infection. Pa ssive immunization experiments with IgM purified from serum of recovered ca rp indicate that the infection is controlled by antibodies. The anti-parasi te antibody level in recovered carp remains high for many months although t he parasitaemia is controlled at very low levels and the half life of IgM, t(1/2) = 22.5 days, is comparatively short. The effective control of trypan osomes in laboratory infections is in contrast to the high prevalence in na tural and farmed freshwater fish populations.