PROBABILITY OF INFECTION WITH TRYPANOSOMA-CRUZI OF THE VECTOR TRIATOMA-INFESTANS FED ON INFECTED HUMANS AND DOGS IN NORTHWEST ARGENTINA

The probability that an uninfected Triatoma infestans would become inf ected with Trypanosoma cruzi after a single feed on people or dogs ser opositive for T. cruzi was estimated in Amama, a rural village in nort hwest Argentina where transmission had resurged four years earlier. Th e prevalence of seropositivity for T. cruzi was 34.2% among 225 people tested, and 65.1% among 83 dogs tested. Parasitemia was detected by x enodiagnosis in 29.3% of 41 seropositive persons and in 85.3% of 34 se ropositive dogs. Parasitemia decreased with age more sharply in seropo sitive people than in seropositive dogs. Seropositive humans infected 2.6% (95% confidence interval = 1.6%-3.6%) of 963 third or fourth inst ar nymphs fed once on them, whereas dogs infected 48.7% (44.7%-52.7%) of 610 nymphs. The probability of bug infection increased significantl y with instar and was positively related to molting success. The infec tivity to bugs of seropositive dogs was 12 times higher than that of s eropositive children, and 100 times higher than that of seropositive a dults. The weighted probability of infection of an uninfected bug fed randomly on any dog (0.3082) was about 50 times higher than that of bu gs fed on any human (0.0062). Such differences in relative infectivity , combined with the relative host-feeding preference of domiciliary Tr iatoma infestans for dogs, reinforces the important role of domestic d ogs as a risk factor for the domestic transmission of T. cruz.