The prevalence of Dirofilaria immitis in domestic dogs was studied in a multicenter survey, which involved nearly 30,000 dogs from France, Italy, Spain, Portugal, Argentina, Brazil, Colombia, and Mexico. In most countries, the surveys were conducted in two phases. In the initialphase, ablood sample fromeach dog was examined by a concentration technique for detection of circulating microfilariae. In the second phase, blood samples were first examined by the concentration technique, then by an ELISA antigen detection test. This procedure provided for identification of dogs that were microfilaremic, as well as those with occult infections.

The two-phase investigation in France included 6,113 dogs, of which 47 were microfilaremic and 3 had occult infections. In Italy, 1,481 of 8311 dogs evaluated by the concentration technique were positive forcirculating microfilariae and 526 of 4,674 included in testing by the ELISA technique had occult infections. Blood samples from 100 of 3,406