Antigenic variation in a strain of Trypanosoma brucei transmitted by Glossina morsitans and G. palpalis

SUMMARY Antigenic variation was studied in a strain of Trypanosoma brucei transmitted by Glossina morsitans and G. palpalis. In goats and rabbits infected by tsetse flies, the antigenic character of the strain did not change until the 7th day of infection; thereafter new antigens developed at 2-to 3- day intervals until the infected animal died. The antigens of the T. brucei strain developed in a similar sequence in the early stages of infections induced by different tsetse flies; in later stages of the infections, the sequences in which antigens developed varied, but many of those produced in different hosts were similar. A common antigen, provisionally called the basic strain antigen, occurred in all substrains of the strain isolated during the first 7 days of infection from animals infected by different tsetse flies. This basic strain antigen was relatively stable and, when present in try-panosomes ingested by tsetse flies, it persisted throughout the period required for cyclical development and for the remainder of the life of the infective fly. It also tended to displace variant antigens when trypanosomes with such antigens multiplied in environments free from antibody. Tsetse flies which ingested trypanosomes with variant antigens transmitted trypanosomes with either the basic strain antigen only or with a mixture of the ingested variant antigen and the basic strain antigen. The basic strain antigen also developed at an early stage of infection when nonimmune animals were infected with variants of the strain transmitted by syringe. These findings are discussed in relation to the serological classification of brucei subgroup trypanosomes and the immunization of animals against trypanosomiasis.