Evaluation of a polycarbonate filter for the detection of microfilaremia in dogs in central Michigan.

Several diagnostic techniques were used in determining the prevalence of microfilaremia in 479 dogs entering the holding facility for research animals at Michigan State University. The modified Knott test and cellulose filter procedures were initially applied, but persistent difficulties with the cellulose filter system prompted evaluation of the use of clear polycarbonate filters. This procedure was found to be equally as sensitive as the modified Knott test and more sensitive than the microhematocrit tube method. Microfilariae trapped on polycarbonate filters were easy to detect, and there were no problems with clogging of the filter pores. Losses of up to 3.5% of the microfilariae occurred with 8-micron pore size filters, but no more than 1% passed through 5-micron pore size filters. Identification of microfilariae was based on size and shape of the parasites. In questionable cases, the diagnosis was confirmed by means of the histochemical test for acid phosphatase distribution. Satisfactory results were obtained when the latter test was applied to microfilariae of Dirofilaria immitis trapped and fixed on filters. Twenty blood samples in the surgery (4.2%) contained microfilariae, all of which were classified as D imitis.