Seasonal prevalence of third-stage larvae of Dirofilaria immitis in mosquitoes from Florida and Louisiana

Heads of 109,597 mosquitoes collected during 1996 and 1997 from Gainesville . Florida (1996, n = 39,131: 1997, n = 34,209). Bartow, Florida (1996, n = 12,000: 1997. n = 12,000), and Baton Rouge, Louisiana (1996, n = 12,257) we re tested by a polymerase chain reaction and Southern hybridization-based t est for the presence of third-stage larvae of the canine heartworm Dirofila ria immitis. Mosquito heads were pooled (1-200 heads) by month, locality, a nd species for testing. The test used was species specific for D. immitis a nd was capable of detecting DNA from a single larva in a pool of 200 mosqui to heads. Specificity for the third larval stage was achieved by probing on ly mosquito heads. One or more D. immitis-infected mosquito heads were dete cted in each month of the year from Bartow in both 1996 and 1997. No infect ed mosquito heads were detected from Gainesville or Baton Rouge in December , January. February, or March. These results are in general agreement with previous sentinel dog and model prediction studies that showed heartworm tr ansmission in the warm temperate Gulf coast region of the United States to be seasonal rather than continuous as previously believed.