SEASONAL AND ANNUAL CHANGES IN SHEDDING OF PARELAPHOSTRONGYLUS TENUIS LARVAE BY WHITE-TAILED DEER IN NORTHEASTERN MINNESOTA

Abstract

Changes in prevalence and intensity of Parelaphostrongylus tenuis larvae in white-tailed deer (Odocoileus virginianus) feces were studied over a 9 year period from October 1986 to May 1995. Overall, first-stage larvae occurred in 46% of fecal samples from 1480 known-age deer killed by vehicles in northern Minnesota. Prevalence in fawns rose steadily from October to January and remained fairly constant from February to May (47%) as the animals approached one year of age. The overall prevalence was 59% in deer older than one year and did not vary with increasing age. Prevalence in fawns during February to May varied annually. Number of deer-herd days on winter range with <20cm of snow before December 31 was the most important predictor of prevalence. Prevalence was also positively correlated with deer population density, mean May-October temperature, and number of days in September with minimum temperature >14℃; but it was negatively correlated with total May-October precipitation. Mean number of larvae/g of feces (overall mean = 49.1 ±3.43SE; range = 0.1-1250) was negatively correlated with deer age. More larvae were shed by deer of all ages during February-May than at other times of the year. Mean numbers of larvae in feces of fawns during February-May varied among years and was significantly correlated with deer population density.

Prevalence is an easily measured parameter that may reflect the multifactorial influence of host habitat on the transmission of P. tenuis, but standardized sampling of deer populations is required to reliably estimate it. Because the parasite is long-lived, only prevalence in the fawn cohort will reflect year to year changes in rates of transmission but infections will only be patent in fawns older than about 8 months.