No abstract

In the study area Triatoma guasayana Wygodzinsky & Abalos is the only wild triatomine found sympatric with Triatoma infestans (Klug) in peridomestic premises. The Trypanosoma cruzi Chagas wild cycle is centered around the same biotopes occupied by T. guasayana, which are also visited mainly by opossums with annual prevalences of 29-50%. Twelve hectares were sampled for 4 consecutive years during all seasons. During that time, 420 T. Guasayana individuals were collected in 11.3% of 1,188 biotopes of 4 types, which included quimiles (the cactus Opuntia quimilo), chaguares (several species of bromeliads), trees, and logs. Quimiles had the highest percentage of positive biotopes (31.5%) followed by chaguares (22.3%), whereas 5% of the logs were found infested. During all seasons, 9.6-15.2% of biotopes were found infested. Distance to artificial biotopes was not statistically significant when comparing the frequency of triatomine numbers per biotope in all biotope-season combinations. With the exception of quimiles in the fall, the mean number of triatomines was higher in chaguares during all seasons. Triatomine abundance by biotope and season strata showed a clumped distribution, except for the quimiles biotope during the summer. When pooling by seasons, the mean number of triatomines in chaguares and quimiles biotope was higher than in logs and trees, with all biotopes showing a strong clumped distribution. When pooling by biotopes, the mean number of T. guasayana was relatively similar for all seasons, with a strong clumped distribution. The strong contagious distribution of T. guasayana in the hardwood forest biotopes may explain the maintenance of the wild cycle of T. cruzi, despite the low number and the low prevalences of the insect vector.

This paper attempts to prove if a high Trypanosoma cruzi prevalence of opossums might be reached with few potential infective contacts. One non-infected Didelphis albiventris to T. cruzi and 10 infected nymphs of Triatoma infestans were left together during 23 hr in a device that simulated a natural opossum burrow. Twenty-six replicates were performed using marsupials and triatomines only once. Potentially infective contacts occurred in all the trials. From the 26 opossums used in trials, 54% did not eat any bug. Of the 260 bugs used, 21% were predated. In the 25 trials involving 205 surviving bugs, 36% of them did not feed. In 15/25 cases, > or = 60% of the triatomines were able to feed. The parasitological follow-up of 24 opossums showed that among 10 that had eaten bugs, 4 turned out infected and among the 14 that had not predate, 3 (21%) became positive. In sum, 7/24 (29%) of the marsupials acquired the infection after the experiment. This infection rate was similar to the prevalences found for the opossum population of Santiago del Estero, Argentina, suggesting that the prevalences observed in the field might be reached if each marsupial would encounter infected bugs just once in its lifetime.

Success in obtaining a blood meal and rapid access to hiding places after feeding are the principal requirements triatomines have as they colonize artificial ecotopes. Feeding success and postfeeding location of 3rd and 5th instars of Triatoma sordida (Stål), of T. guasayana Wygodzinsky & Abalos, and T. infestans (Klug) were studied in an experimental box in which triatomines and a chicken were left to interact overnight. The bird was enclosed in a glass cube, slightly raised to allow triatomines to get in and out, turning the space outside into an extensive refuge area. Four bunches of dry corn husks and a wooden panel were also offered as shelters. The number of live and dead insects and their locations at the end of the experiment were recorded. Predation--as the percentage of missing insects--and success--as the percentage of insects alive and fed--were calculated. Interactions with the host were different among species and sometimes also between individuals of different life stages of the same species. Predation was always > 20%. T. sordida was the best exploiter of the blood source, because 3rd instars were more successful and 5th instars were as successful as the corresponding T. infestans stages. Performance of T. guasayana was equivocal because survival and feeding success were different for both instars. T. infestans showed a pronounced tendency to climb, and 3rd instars of T. guasayana were equally distributed in the upper half of the box and in the corn husks, whereas the majority of T. sordida and 5th instars of T. guasayana sheltered in the husks.