Antigenic variation of Trypanosoma evansi in rabbits.

In order to study the occurrence and sequential appearance of antigenic variation during the course of T. evansi-infection, rabbits were infected with a clone prepared from T. evansi, Java/55/PTV/1 (stock 1, clone 5). During the course of infection, trypanosome-populations were isolated from the cerebrospinal fluid and from each parasitaemia peak. Specific antisera against the clone, 6 parasitaemia-populations and two different liquor-populations (all isolated from 1 rabbit), were produced in rabbits. These antisera were used for the characterization and comparison of trypanosome populations with each other by means of immunolysis as well as neutralisation-infectivity-(NIF)-test. It was found that in all infected rabbits, the first parasitaemia contained trypanosomes, which were almost identical to those present in the clone. However, the clone contained already minor variant antigenic types ("minor-VATs"). All trypanosome-populations isolated from rabbits were heterogeneous. Heterogeneity was more pronounced in the later phases of the infection. After each fluctuating parasitaemia new dominant variants ("major VAT") were expressed. The sequential appearance of antigenic variation was similar among all infected animals. The effectivity of immunolysis was checked by the NIF-test. As it was expected, complete lysis of all trypanosomes of a population was not achieved, because some "minor VATs" could escape the influence of the antiserum. By this way, they formed the basis of new "major VATs" which dominated in the next fluctuating parasitaemia.