POPULATIONS AND SITE SELECTION OF DIROFILARIA URSI (NEMATODA: ONCHOCERCIDAE) IN AMERICAN BLACK BEARS (URSUS AMERICANUS)

Abstract: A total of 4,846 (84.5±87.15, 3–429: mean±SD, range) subadult or adult Dirofilaria ursi were recovered from 56 American black bears (Ursus americanus) in Ontario, 1975–77. Yearling bears had fewer worms than older bears; age classes greater than 2.5 yr had similar numbers of worms. Dead worms comprised 3.7% of all worms. There was one dead worm in total in 10 yearling bears and consistently more dead worms in older bears. The occurrence of worms within local sites in bears differed over time. Increasing numbers of worms appeared first in peritracheal sites (Site 1) following 1–3 seasons of transmission, in abundance in perirenal sites (Site 2) following the second season of transmission, and in lateral and ventral trunk and abdomen plus medio-proximal legs (Site 3) following three and four seasons of transmission. Few worms occupied other sites (Site 4). The proportion of worms recovered from the four sites was 46, 30, 19.8, and 3.9%, respectively. Dead worms comprised only 3.2% of worms in the first three definitive sites but 14% of worms in other sites. These data are consistent with primary, secondary, and tertiary site preferences for mature D. ursi in this species. Circulating microfilaremia in peripheral blood was periodic in 7/10 experiments and revealed a pattern of variable numbers during the day, highest in the evening and lowest during the night. The time of high density of microfilariae in peripheral blood was synchronous with the feeding cycle of Simulium venustum, the vector of D. ursi. Immediately postmortem, microfilariae were at highest densities in blood of lungs regardless of their relative density in peripheral blood. These data can direct diagnostic efforts to maximize detection of adult D. ursi in tissues and larvae in blood samples of naturally infected bears.