Experimental infections of farmed eels with different Trypanosoma granulosum life-cycle stages and investigation of pleomorphism

Trypanosoma granulosum, a flagellate protozoon commonly found in the blood of the European eel Anguilla anguilla, was injected experimentally into uni nfected eels purchased from a local farm. In order to investigate the infec tivity of different stages in the life cycle, trypanosomes from various sou rces were used for inoculation. Infectivity was greatly reduced in in vitro culture stages inoculated at 20 C. Isolated bloodstream stages injected in to soups of animals held at 12 and 20 C could be detected for over 70 days but did not appear to multiply. Naturally infected Hemiclepsis marginata, a piscivorous leech known to serve as vector, produced detectable, single-pe ak infections in eels held at 20 C. Infections were characterized by a prep atent stage and a phase of rising parasitemia. Peak infection intensities r anged between 1 and 7 x 10(4) trypanosomes/ml. Trypanosomes in the bloodstr eam of eels experimentally infected with leeches, divided at a very low rat e during the early stages of infection. Small morphs present during the ear ly phase of rising parasitemia were gradually replaced by larger trypanosom es. The overall length frequency distribution of trypanosomes was unimodal.