In February 2011 and 2012, two emaciated bighorn sheep rams with generalized skin disease were reported to, and euthanized by, government staff near Olalla in BC’s Similkameen valley. Lab diagnostics confirmed psoroptic mange – also known as sheep mange or sheep scab – caused by the ectoparasitic mite Psoroptes ovis. This mite is transmitted through direct contact among sheep. Although endemic in some US wild sheep populations, “this is the first reported case in bighorn sheep in Canada and the first since eradication from Canadian domestic sheep herds in 1924” (Scott 2013).
Similkameen bighorn subpopulations are part of the South Okanagan (SOK) metapopulation, with known linkages to bighorn herds in northern Washington. Since 2011, evidence of psoroptic mange has been documented in all herds on the west side of the SOK valley south of Penticton, and in a herd approximately 15 km south of the US border at Palmer Lake, Washington. The potential for Psoroptes transmission to subpopulations on the east side of the valley is of significant management concern because those populations are still recovering from a pneumonia die-off in 1999/2000 (Reid 2013b), they are an iconic SOK species that is highly visible to the public, and they support significant hunting opportunities.
The challenge for management of psoroptic mange is that no practical and effective treatment exists to eradicate the mite in wild sheep populations (Scott 2013). Recommendations for managing the disease are limited to euthanization of the most severely affected individuals, habitat alteration to improve the nutritional quality of the herd’s home range (e.g., burning, livestock stocking levels) or herd augmentation by translocation from outside populations. Population-level intervention through herd augmentation is not recommended until movement patterns of infested bighorn herds are well understood (Scott 2013).
Further, we don’t know if mange is responsible for persistent declines and poor lamb recruitment in some infested herds. The Ashnola bighorn sheep herd was once the largest in the Okanagan, and offered the best opportunity for bighorn viewing and hunting in alpine wilderness. Recent annual surveys found a significant decline in the Crater Mtn herd, from estimates of >300 sheep in 2006 to as few as 83 (Reid 2013a). Lower Similkameen bighorns (MU 8-07 and 8-02) appear to be stable despite widespread prevalence of Psoroptes, but population numbers are depressed and near the minimum huntable size threshold (Reid 2013).
Addressing the risk of Psoroptes transmission among herds and managing the effects of psoroptic mange in Similkameen bighorns is currently the BC Ministry of Forests, Lands and Natural Resource Operation’s highest wildlife management priority in the Okanagan region. Given that the SOK bighorn sheep metapopulation occurs across a complex landscape of private, municipal, provincial, federal, Aboriginal, and protected lands, this is a transboundary management challenge best met through a collaborative approach, shared knowledge, and unified commitment to sustaining sheep population health in the South Okanagan Similkameen.
The South Okanagan Similkameen Bighorn Sheep Psoroptes Project (SOKS Bighorn Project) is a collaborative effort focused on active monitoring and management to assess the risk of Psoroptes transmission and mitigate its effects on infested wild sheep. We are focusing initially (2015 and 2016) on determining bighorn sheep movements and dispersal, and the prevalence of mange. On-going monitoring of collared individuals will help to evaluate the influence of mange on individual recruitment (ewes) and mortality (ewes and rams), and overall herd dynamics. Beginning in Yr 3 (2017), we will implement and test the effectiveness of targeted management actions for declining herds.