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Sylvatic Plague in Grasslands National Park

In July of 2010, a black-tailed prairie dog (Cynomys ludovicianus) was found dead by researchers from the Calgary Zoo on a prairie dog colony located within Grasslands National Park near Val Marie, Saskatchewan. The dead prairie dog was submitted to the CCWHC Western and Northern Region for necropsy on July 19th, 2010. Lesions of a severe bacterial septicaemia were found on necropsy and tissues were submitted for bacterial culture to Prairie Diagnostic Services. The bacteria isolated on culture was consistent with Yersinia pestis and it was then forwarded to the Saskatchewan Public Health Laboratory and subsequently to the National Microbiology Laboratory in Winnipeg for confirmatory identification. The bacteria was subsequently confirmed as Y. pestis through PCR and biochemical testing.

At about the same time, other researchers conducting burrowing owl (Athene cunicularia) surveys on an existing prairie dog colony in a remote area of the park (over 20 kilometres from the colony where the dead prairie dog was found) noticed no signs of recent activity and it appeared the colony had mysteriously disappeared. Park staff confirmed this finding later in the week and swabbed several burrows to look for fleas that may indicate recent sylvatic plague activity, but no fleas or prairie dogs were found. The disappearance of this colony still has not confirmed as being due to plague, but it is certainly considered a high probability.

The finding of the Y. pestis in a prairie dog prompted the release of a local Information Bulletin on August 13th, alerting local and regional media to the confirmed presence of sylvatic plague in the park. This also began an important and ongoing dialogue between provincial public health authorities, local health authorities, and federal public health agencies and other federal government departments regarding the approach and management to this often misunderstood, often neglected zoonotic disease. Signage was erected on all accessible prairie dog towns within the park alerting visitors to the potential for plague, recommending against travel on prairie dog towns, prohibiting access of domestic dogs and cats and to take precautions to reduce potential transmission by fleas through the use of DEET-based insect repellents. Local veterinary clinics were also alerted to the possibility of plague in domestic pets and park visitors were informed of the potential for plague exposure. Subsequent intensive monitoring and surveillance of prairie dog colonies in the park revealed that densities have decreased significantly during the summer of 2010 to approximately 50%-70% of long-term average densities, but a large scale epizootic has not been observed to date. Much of this decrease could be attributed to decreased pup production resulting from a growing season drought which occurred in 2009 and significantly affected reproduction in the spring of 2010. A similar decrease in prairie dog density was observed on colonies in the summer of 2008, but it is difficult to determine whether these decreases were due to sylvatic plague or other environmental factors. Further research is being conducted in the park to better determine the specific causes of these declines over the coming years. Preliminary risk assessment and mitigation plans were previously developed in 2006 and a recent multi-jurisdictional plague response plan is currently being drafted to manage this disease in black-tailed prairie dogs and black-footed ferrets. As a precautionary measure, park staff will be applying DeltaDust®, a pyrethroid insecticide containing deltamethrin to prairie dog colonies within the park in an effort to reduce the potential impact of sylvatic plague on the remaining black-tailed prairie dog colonies in Canada. Large outbreaks of sylvatic plague with mortality rates exceeding 95% have significantly impacted prairie dog colonies as well as the black-footed ferrets (Mustela nigripes) whose primary prey are prairie dogs in the United States. DeltaDust® has been used successfully to reduce the acreage of prairie dog colonies as well as black-footed ferrets lost to sylvatic plague including some areas within US national Parks (e.g. Wind Cave National Park, South Dakota).

Previous research using domestic dogs and coyotes (Canis latrans) as sentinels (Leighton et al 2001, unpublished Parks Canada data) had indicated the presence of plague-specific antibodies at low levels in both southern Alberta and Saskatchewan, and in the area around Grasslands National Park, but a significant epizootic had never been observed in Canadian prairie dogs. To our knowledge, this is the first confirmed case of sylvatic plague in Canadian rodents since 1988, when it was isolated from two Bushy-tailed Woodrats (Neotoma cinerea) in southern British Columbia (Lewis 1989) and only the third report of plague in Canadian rodents. The last suspected human case in Canada was from a mink rancher in south-eastern Alberta near Hanna that became infected from skinning mink after being fed local ground squirrels (Ozburn 1944, Humphreys & Campbell 1947, Gibbons & Humphreys 1941) during an epizootic of plague in ground squirrels. It is an important reminder of the One World One Health concept being promoted by both wildlife and public health agencies around the world and the interconnectedness of wildlife, pathogens and humans. Veterinarians, physicians, biologists, conservation officers, hunters and trappers should be aware of the risks of infection with sylvatic plague through direct contact with infected carnivores or rodents and from their associated fleas, particularly in the southern regions of the prairie provinces.

Gibbons, RJ & Humphreys, FA. 1941. Plague surveys in western Canada. Canadian Public Health Journal, 32: 24-28.

Humphreys, FA & Campbell, AG. 1947. Plague, Rocky mountain spotted fevers, and tularaemia surveys in Canada. Canadian Journal of Public Health, 38(3): 124-130.

Leighton FA, Artsob HA, Chu MC, & Olson JG. 2001. A serological survey of rural dogs and cats on the southwestern Canadian prairie for zoonotic pathogens. Canadian Journal of Public Health, 92(1):67-71.

Lewis, RJ. 1989. Plague in bushy-tailed woodrats. Canadian Veterinary Journal, 30(7): 596-597.

Ozburn, RH. 1944. Problems of medical entomology of military importance in Canada. Journal of Economic Entomology, 37(4):455-459.

By: Todd Shury - Parks Canada