Field trial of vaccination against American trypanosomiasis (Chagas' disease) in domestic guinea pigs.

Domestically bred South American guinea pigs received 3 to 5 immunizing intradermal inocula of 28 X 10(6) live attenuated Trypanosoma cruzi epimastigotes (TCC strain) per kg. These inocula were unable to produce patent infections or to propagate through vectors. Groups of experimental and control guinea pigs were exposed to natural T. cruzi infection in a field yard for periods of up to 551 days. Xenodiagnoses were applied periodically to all animals. This showed that the incidence of natural T. cruzi infection was significantly lowered at various periods post-exposure. The final proportion of infected animals was 39% (20/51) among vaccinees vs. 63% (32/51) among controls (P less than 0.02). The protective effect was exerted particularly upon males and lasted for over a year in one experimental series (infection in 1/7 vaccinees vs. 6/7 controls, P = 0.014). Vaccination reduced vector transmission rates from 38% to 18% (P less than 0.001). These results agree with previous laboratory experiments in showing a partial resistance which does not eliminate residual T. cruzi infection. However, the field work indicates that even this kind of resistance may have epidemiological impact, reducing both the number of reservoirs spreading the disease and the rate of vector transmission.