Over-winter survival of first-stage larvae of Parelaphostrongylus tenuis (Nematoda: Protostrongylidae)

This study examined the ability of first-stage Parelnphostrongylus tenuis larvae to survive winter conditions at the northern limits of the white-tailed deer (Onocoileus virginianus) range. Fecal pellets freshly deposited by infected deer were collected at the beginning of each winter month (9 December 1996 to 12 March 1997) and placed out in a traditional deer wintering area located near Grand Marais, Minnesota, and at a site in Thunder Bay, Ontario. The temperatures experienced by larvae at different levels within the snow cover were monitored and survivorship was calculated each month. Overall larval survivorship from December to April was only 27%. Although temperatures experienced by larvae beneath the snow for 3 months at Grand Marais were greatly moderated (-0.2 to -2.5 degrees C) compared with air temperatures (6.5 to -24.0 degrees C), fewer larvae survived (16%) than at a lower, constant temperature (-14 degrees C) in the laboratory (76%). The mean numbers of first-stage larvae passed by deer (expressed per gram of dried feces (gd)) varied considerably over winter, from a low of 289/g(d) in December to a peak of 1127/g(d) in March. Considerable reproductive potential was lost because this spring (March) rise occurred before the end of winter. Nonetheless, the meningeal worm still successfully infects up to 82% of white-tailed deer in northern areas, and it is probably those larvae produced during the snow-fret period while deer an dispersed over their summer habitat that play the biggest role in transmission.