Dirofilaria immitis infection in dogs from underserved, Native American reservations in the United States.

Blood samples from 1,822 dogs residing on Native American reservations in 10 states were collected between February 2004 and August 2007. Samples were examined using a commercial enzyme-linked immunosorbent assay (ELISA) antigen capture test. All ELISA-positive samples were subsequently examined for microfilaremia. All dogs were asymptomatic for Dirofilaria immitis infection, resided outside (except for winter in the Northern latitudes), not on prophylaxis for D. immitis, and had not traveled off of the reservation. Reservations were characterized by degrees of north latitude, true prevalence of D. immitis, and a reproductive index calculated by multiplying the number of months of the year in which infective stage larvae (L(3)) could develop in 30 days or less for each reservation by the 30 yr mean annual rainfall in centimeters recorded for that community. The prevalence of heartworm was directly related to the reproductive index on each reservation.