This work was carried out with a strain of trypanosomes isolated by inoculation into domestic rabbits of fleas naturally infected from wild rabbits in Hertfordshire.
The course of infection was studied in experimentally infected normal and splenectomized rabbits, in which the incubation period varied from 5 to 12 days. After recovery from the infection, rabbits became immune to re-infection. Immune rabbit serum had no protective action against Trypanosoma lewisi.
Attempts to infect other animals with the rabbit trypanosome produced negative results.
Young rabbits proved to be more susceptible to infection with this trypanosome, and their parasitaemia lasted longer than in adult rabbits, but the parasite was found to be non-pathogenic to these animals.
It was demonstrated that this trypanosome multiplies in the spleen, where the stages of division are confined to the capillaries. Multiplication follows the inoculation of metacyclic trypanosomes from infected fleas or blood forms. It proceeds by division of the nucleus and kinetoplast into four, followed by fission of the cytoplasm, giving rise to four small daughter individuals. These are transformed into trypanosome forms, which are shed into the peripheral blood, where they change into the typical adult trypanosomes.
The development in the intermediate host takes place in the gut of the rabbit flea, Spilopsyllus cuniculi, with the production of metacyclic trypanosomes in the hind-gut (posterior station).
The nomenclature and synonymy of the rabbit trypanosome are discussed, and it is concluded that its valid name is Trypanosoma nabiasi Railliet, 1895.