Parelaphostrongylus tenuis is newly endemic in white-tailed deer (Odocoileus virginianus) in central Saskatchewan.

Parelaphostrongylus tenuis, or meningeal worm, can be fatal and have detrimental effects at a population level in cervid species outside of the definitive host, the white-tailed deer (WTD) (Odocoileus virginianus), in which they cause little or no ill effects. Although well documented in Canada east of Saskatchewan, there is only one published case of P. tenuis in WTD in Saskatchewan from the 1990s, representing the northwestern distributional limit of this parasite in North America. We report 90 confirmed (47) or suspected (43) cases of P. tenuis diagnosed in natural mortalities in moose (Alces alces), elk (Cervus canadensis), and mule deer (Odocoileus hemionus) by the Western/Northern region of the Canadian Wildlife Health Cooperative between 2015 and 2024. To determine if this parasite was locally acquired, we examined meninges and associated venous sinuses of 330 WTD harvested in fall/winters of 2021-2024. We detected a mean of 2 (1-10) adult P. tenuis in 10.6 % of sampled WTD from 16 Wildlife Management Zones across central and northeastern SK, much further west than previously recorded. There was substantial overlap between endemic zones for the parasite in WTD and cases in aberrant hosts, suggesting local transmission. DNA was extracted from 4 nematodes from 4 separate infected WTD for each of the sampling years (16 worms) and assayed with PCR using primers targeting the Cytochrome C Oxidase Subunit I (COI) of mitochondrial DNA. Sequences for COI had 99.8 % similarity to sequences of P. tenuis in GenBank, and were most similar to sequences from Manitoba, Canada. Our results demonstrate establishment of viable populations of P. tenuis in WTD in east-central Saskatchewan, evidence of spillover into aberrant hosts, and the potential for both further westward and northward expansion, which could pose a risk to threatened boreal caribou (Rangifer tarandus) and moose populations. This work informs wildlife managers of a new threat cervid populations may face in western and northern North America.