PREVALENCE OF DIROFILARIA MICROFILARIA IN AMERICAN BLACK BEARS (URSUS AMERICANUS) IN PENNSYLVANIA, 2018–2020

Dirofilaria ursi is a filarial nematode of American black bears (Ursus americanus) transmitted by black flies (Simuliidae). Overt clinical disease associated with D. ursi infection of bears has not been reported; however, adult or larval (i.e., microfilarial) stages of the parasite can be found incidentally in the subcutis, connective tissues of the abdomen and thorax, or on blood films. Over the last 60 yr, D. ursi has been reported in black bears throughout the northern United States and Canada. However, very few surveys have been conducted since 2000, particularly in the northeastern United States where black bear populations have expanded significantly over recent decades. During 2018–2020, blood films collected from 129 black bears from Pennsylvania during all seasons were examined for microfilariae. Microfilariae that were morphologically consistent with D. ursi were identified in 33 (25.6%) of the 129 sampled black bears. The average number of microfilariae per positive blood film was 27.4 (range: 1–282). Age, sex, and season did not have a significant effect on detection of Dirofilaria infection in black bears. Molecular identification of Dirofilaria species via polymerase chain reaction (PCR) and sequencing of the 18S rRNA gene region was conducted for 11 whole blood samples whose blood films contained microfilariae (average of 17.3 microfilariae/film; range: 1–55). Ten of the 11 whole blood samples were PCR positive for Dirofilaria, and the sequences were highly similar to sequences of Dirofilaria repens available in GenBank. The results of this study are consistent with those of previous D. ursi surveys in North American black bears and indicate that Dirofilaria infection is common in black bears in Pennsylvania but does not appear to be associated with overt disease.