In the 1800s there were about two million bighorn sheep in North America. Due to disease, unregulated hunting, and competition for forage with domestic animals, bighorn sheep numbers dropped drastically by the 1950s; approximately 25,000 sheep remained in all of North America. By far the biggest contributor to their die-off was disease.
There is a strain of pneumonia-causing bacteria which has had, and continues to have, devastating effects on bighorn sheep. It is spread to wild sheep from domestic sheep and goats that harbour the bacteria; however, the domestic animals rarely show symptoms themselves. Since their grazing and roaming areas often overlap with wild sheep, and since goats are used increasingly for weed control, domestic carriers of this disease can spread it far and wide.
The infection most commonly ends in death in wild sheep after they battle symptoms like coughing and bloody nasal discharge. According to the Wildlife Society, there is a 90 per cent mortality rate in wild sheep within two months of exposure to the disease.
A lone wild sheep can come in contact with the bacteria and then when it returns to the herd or joins a different herd can easily spread it to the rest of the herd. It causes total herd die-offs in many instances. Where there are survivors, the bacteria can have an effect for many years after an outbreak – a surviving ewe will now be immune, but her lambs will not be and often die soon after birth.
What is being done to combat this sickness? While research into the effectiveness of vaccination for this bacterium is ongoing, prevention is key! Separation of domestic and wild sheep is the most obvious way to prevent spreading the bacteria; however, this strategy relies on participation and co-operation from sheep owners.
Building wildlife-friendly fences to prevent domestic sheep from interacting with bighorn sheep is helpful, as is having domestic sheep tested for the bacteria. If a farmer knows that it owns some individuals that are carriers of the disease and is situated on land near known bighorn sheep habitat, they are encouraged to graze their herd elsewhere or relocate their carriers to reduce the risk of infecting the wild herd.
While bighorn sheep numbers in western North America are still starkly lower than they used to be, they have increased since the 1950s to more than 80,000 individuals. Hunting regulations, habitat enhancement, and the separation of wild and domestic animals have helped bighorn sheep come back from their severe die-off. Many organizations, such as public and Crown land management agencies, fish and game departments, and foundations like the Wild Sheep Foundation, have been working to increase the population and conservation of these sheep.
The WSF has contributed over $100 million to conservation since their founding in 1977. Most of their funding comes from hunters. Since the opportunity to hunt bighorn sheep is won through a competitive lottery system in most places, the lottery can raise substantial monies that eventually find its way back into sheep conservation.
There is still a long way to go in the recovery of bighorn sheep. Although their numbers may never climb back up to two million, there is some comfort in knowing that there has been improvement and that land and wildlife managers are continuing to research this issue. Hopefully more effort can be put toward the health and vitality of bighorn sheep now that we know what is causing this sickness and can continue working to prevent it.
Miranda Hark is a second year recreation, fish and wildlife student at Selkirk College in Castlegar