Parasite common in cats causes abortion in bighorn sheep

There is no treatment for the parasite.


Robust lamb recruitment is integral to bighorn sheep conservation; however, population restoration has been challenged by low lamb recruitment rates. Much literature on bighorn lamb mortality focuses on juveniles; little is known about abortion and neonatal death.

A recent study led by Washington State University researchers at the Washington Animal Disease Diagnostic Laboratory (WADDL) found that a parasite common in cats could hamper ongoing conservation efforts in bighorn sheep.

The parasite called Toxoplasma gondii infects most species of warm-blooded animals and causes the disease toxoplasmosis. This parasite is found to be a cause of abortions or pregnancy loss, as well as neonatal deaths in sheep. Unfortunately, it does appear to be causing abortions and some level of death in young, bighorn lambs.

“WADDL works with wildlife agencies and researchers to monitor causes of death in bighorn sheep, which led to the discovery of Toxoplasma in the species.”

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

Following the discovery, scientists conducted autopsies, microscopic inspections of tissue, and other tests on eight fetal and newborn bighorn lamb cadavers that were gathered from March 2019 to May 2021 from Idaho, Montana, Nebraska, and Washington. Five people were found to have toxoplasmosis.

Fisk said, “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.”

“There is no treatment for the parasite, and there is some concern, though not yet definitive evidence, that infected lambs that survive may be weak and more susceptible to disease and predation.”

“If you look at humans, for example, who have been infected with Toxoplasma in utero, sometimes they’ll have blindness or other issues. But we don’t know if bighorn sheep suffer ongoing effects from in-utero infections.”

The bighorn sheep are thought to catch the parasite via domestic cats or wild cats like cougars and bobcats, which share the bighorn sheep’s range.

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

For the study, WSU worked with several organizations, including Idaho Fish and Game, Nebraska Game and Parks Commission, Montana Conservation Science Institute, and others.

Journal Reference:

  1. Elis Fisk, Fraces Cassirer, et al. Abortion and neonatal mortality due to toxoplasma gondii in bighorn sheep (Ovis canadensis). Journal of Wildlife Diseases. DOI: 10.7589/JWD-D-22-00057
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