Infectivity to the vector Triatoma infestans of dogs infected with Trypanosoma cruzi in north-west Argentina.

The influence of reinfections on the infectivity to Triatoma infestans of dogs seropositive to Trypanosoma cruzi was investigated in Amamá, north-west Argentina, before (1984) and after (1986-1987) elimination of domestic bugs with residual insecticides in 1985. Infectivity was quantified by the percentage of infected bugs in each xenodiagnosis of a seropositive dog, i.e. the infective force (IF). Exposure to reinfection was represented by the domiciliary density of infected bugs and by an index of the potentially-infective contacts (IPIC) experienced by each dog (calculated as the product of the relative density of bugs per dog, the proportion of infected bugs, and the proportion of bugs taking blood meals from the dog). IF was unrelated to the dog's age or sex, or the level of specific antibodies to T. cruzi. Before vector elimination, the IF of seropositive dogs was significantly associated with both the density of infected bugs and the IPIC. After vector elimination, seropositive dogs resident at houses which had 10-70 infected bugs before spraying showed a higher median IF (67-75%) than those living in houses with zero to eight infected bugs before spraying (47-55%). The age-specific IF of seropositive dogs aged one year or older, recorded before (45-64%) and after (52-56%) the elimination of domestic bugs, did not differ significantly. Our data suggest that the infectivity of seropositive dogs to bugs is modified by previous or current exposure to bug-mediated reinfections, and that this relationship might be irreversible in the short term.