Field studies on premunition in guinea pigs and dogs
In contrast with the short-term resistance induced by nonreplicating immunogens, the TCC culture induced long-lasting resistance over a year after inoculation.80 TCC-induced protection was not restricted to laboratory mice and was also demonstrated in an ecologic field model of T. cruzi transmission consisting of standard corrals of loose bricks surrounded by mosquito net tents, where guinea pigs and Triatoma infestans shared an entomologically isolated, seminatural environment. Experiments involving large numbers of guinea pigs showed that TCC parasites did not propagate in a natural vector—host cycle. Moreover, TCC inoculations protected these domestic reservoir animals against naturally transmitted infection and the trafficking of parasites from hosts to vectors was significantly reduced, acting as a transmission-blocking vaccine.80,81
Measurement of the efficiency of insect vectors to transmit T. cruzi infection to mammalian hosts is cumbersome since variables such as parasite load and frequency of bites, and also behavioral and physiologic factors are involved. Nevertheless, the systematic measurement of several transmission variables in guinea pig yards, such as number of hosts, number of vectors, days of exposure, proportion of fed vectors and proportion of infected vectors, allowed to estimate the “number of bites necessary for infection” (NBNI).82 This estimation showed that vaccination with TCC parasites produced an average 4.28-fold increase in NBNI in several independent experiments.
Experiments with naturally infected dogs77 also indicated a transmissionblocking effect of the TCC live-attenuated vaccine. TCC-immunized dogs displayed a lower proportion of infected animals after 1 year of exposure to natural infection in an endemic area, as shown by serologic-xenodiagnostic parameters. A different parameter, the average percentage of bugs that became infected after feeding on vaccinated or control dogs exposed for 2 years to natural infection, also showed lower values in vaccinated dogs, indicating that vaccination apparently exerted a transmission-blocking effect: lower parasite load and reduced capacity to disseminate the parasite through vectors. A similar effect of vaccination has been confirmed by Basso et al.83 in dogs immunized with killed T. rangeli and challenged with T. cruzi.