Haemoflagellates of the genus Trypanosoma are prevalent in freshwater fishes and are transmitted by leeches as vectors.
As demonstrated by sequence comparisons of nuclear small subunit rRNA genes, trypanosomes isolated from several 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-parasite immune response in specified
pathogen-free outbred carp. Infection leads to an initial rise in parasitaemia in the blood followed by a sharp decline in
all fish (acute phase). Thereafter, in some carp, parasites become undetectable 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 that 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. Passive immunization experiments with IgM purified
from serum of recovered carp indicate that the infection is controlled by antibodies. The anti-parasite antibody level in
recovered carp remains high for many months although the parasitaemia is controlled at very low levels and the half life
of IgM, t1/2=22·5 days, is comparatively short. The effective control of trypanosomes in laboratory infections is in contrast
to the high prevalence in natural and farmed freshwater fish populations.