Probability of Trypanosoma cruzi transmission by Triatoma infestans (Hemiptera : Reduviidae) to the opossum didelphis albiventris (Marsupialia : Didelphidae)

The probability of Trypanosoma cruzi transmission to opossums by independen t events of predation and fecal contamination during feeding ("biting") wit h positive Triatoma infestans was estimated. Negative female opossums were challenged for 23 hr with 10 infected third and fourth instars of T. infest ans, and tests for positivity for T. cruzi by xenodiagnosis were performed at 30, 60, and 90 days. From these data, seven probability parameters were estimated by maximum likelihood, and likelihood ratio statistics confidence intervals were calculated. Simultaneous estimation of p(1) (probability th at a "bite" will infect an opossum), p(3) (probability that a bug that has been eaten by an opossum will infect it), and p(6) (probability that the op ossum will become infected if faced with an infected triatomine), resulted in (p) over cap (1) = 0.06, (p) over cap (3) = 0.075, and (p) over cap (6) = 0.059. On average, each opossum should be exposed to an average of 700 en counters with bugs during its life, resulting in about eight potentially in fective contacts, to produce the 35% opossum prevalence found in the field.