Washington State U. study details how parasite could threaten bighorn sheep

A recent study conducted by Washington State University researchers revealed that a parasite could threaten conservation efforts for bighorn sheep, according to a WSU Insider article published Jan. 24.

Toxoplasma gondii, a parasite believed to be present in more than 40 million people in the United States, is typically spread through contact with cat feces containing toxoplasma. The WSU study, published in the Journal of Wildlife Diseases, found that the parasite is causing abortions and neonatal deaths in bighorn sheep. While the researchers found five cases of the phenomenon in bighorn sheep, they believe that further studies are needed to determine the problem's severity.

“We have seen toxoplasma as a cause of fetal and neonate loss pretty commonly in domestic sheep but we hadn’t seen pregnancy loss due to toxoplasmosis yet in bighorn sheep,” said lead study author Elis Fisk. "Unfortunately, it does appear to be causing abortions and some level of death in young bighorn lambs."

Toxoplasma was first discovered in bighorn sheep from the Washington Animal Disease Diagnostic Laboratory's work with wildlife agencies and researchers to monitor the causes of death in the animals. The researchers found pregnancy losses due to toxoplasmosis while processing samples that were routinely submitted to them by wildlife agencies.

“Bighorn sheep are susceptible to disease from domestic sheep, so these agencies were routinely submitting samples to us to see how many they were losing due to other diseases,” Fisk said. “Pregnancy losses from toxoplasmosis were discovered while processing these samples.”

Following the discovery the researchers performed autopsies, microscopic examinations of tissue, and other tests to determine the cause of abortion or death in eight fatal and neonatal bighorn lamb cadavers collected between March 2019 and May 2021 from Idaho, Montana, Nebraska and Washington state. Five of the cadavers were found to be infected with toxoplasmosis.

“They were just riddled with these parasites, so we were pretty certain that is the reason behind their death given how extensive the lesions were,” Fisk said.

There is currently no available treatment for the parasite, and many observers are concerned that infected lambs that survive may be weak and more susceptible to disease and predation. 

The researchers suspect that bighorn sheep are contracting the parasite from domestic cats and wildcats such as bobcats and cougars, both of which have territories overlapping with those of bighorn sheep.

“It’s unclear at this point how widespread of an issue this is because we only detected five positive ones, which in the grand scheme of things is a pretty small sample,” Fisk said. “To learn more about how big of an issue this could be, or even where it’s coming from definitively, we really need more studies.”

Studies on lamb mortality causes and rates can be difficult, as related research has typically relied on visual observation. That can be complicated due to the rugged terrain where bighorn sheep give birth and raise their offspring, meaning that abortions and neonatal deaths can often go unnoticed.

WSU's study was conducted in collaboration with other agencies, including Idaho Fish and Game, the Nebraska Game and Parks Commission and the Montana Conservation Science Institute.