Toxoplasma gondii, a common parasite in domestic and wild cats, causes abortions in wild bighorn sheep, suggesting it is an underappreciated cause of reproductive loss
A recent study published by a team of researchers at the Washington Animal Disease Diagnostic Laboratory (WADDL) at Washington State University (WSU) found that a common parasite spread by domestic house cats as well as by wild cats is causing abortions in wild bighorn sheep. The parasite, Toxoplasma gondii, can infect most warm-blooded animals, including birds, rodents and even humans.
It was previously known that the parasite causes abortions (pregnancy loss) and increased mortality amongst newborn and young lambs in domestic sheep. But in a recent study, a team of researchers documented five cases in wild bighorn sheep.
“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 author of the study, veterinarian Elis Fisk, a doctoral researcher and anatomic pathology resident in WSU’s school of veterinary medicine. “Unfortunately, it does appear to be causing abortions and some level of death in young bighorn lambs.”
Detecting this infection in bighorn sheep, Ovis canadensis canadensis, is very rare, having only recently been reported in 2000 in a juvenile from the Hells Canyon area of Washington state (ref).
It is believed that more than 40 million Americans, and at least one-third of all humans worldwide, are infected with Toxoplasma gondii. People with this parasitic infection are typically is asymptomatic, although it can cause serious health problems for pregnant people and for those with compromised immune systems. It is spread in a variety of ways, but most commonly by consuming food or water contaminated by cat feces.
One unique characteristic of Toxoplasma is that in some hosts, including humans and rodents, the parasite establishes a chronic infection as latent tissue cysts in the brain, heart and skeletal muscle. But unlike other CNS pathogens, this lifelong persistent brain infection does not seem to be detrimental to the life of the host — as long as their immune system functions normally. (However, toxoplasmosis has been shown to alter human brain chemistry, see here: ref)
State wildlife agencies ordinarily work with academic researchers to monitor causes of death in bighorn sheep, which is how the infection was discovered.
“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”, Dr Fisk said in a statement. “Pregnancy losses from toxoplasmosis were discovered while processing these samples.”
The researchers performed autopsies, microscopic examinations of tissues and other tests to determine the causes of abortion or death in eight fetal or newborn bighorn lambs collected in Idaho, Montana, Nebraska and Washington states between March 2019 and May 2021. They found that five died from 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”, Dr Fisk explained.
Unfortunately, toxoplasmosis is incurable. Further, there is concern that infected lambs that survive the initial onslaught can still be weak and more susceptible to other diseases, as seen previously in the Hells Canyon juvenile (ref), and to predation.
“If you look at humans, for example, who have been infected with Toxoplasma in utero, sometimes they’ll have blindness or other issues”, Dr Fisk mentioned. “But we don’t know yet if bighorn sheep suffer ongoing effects from in utero infections.”
But where are bighorn sheep getting this infection?
Considering that bobcats and cougars share their ranges with bighorn sheep, and that the only known definitive hosts for Toxoplasma gondii are domestic cats and their relatives, it is possible that these wild cats may be spreading the parasite to bighorn sheep. However, domestic house cats are also a possible source of toxoplasmosis in 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”, Dr 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.”
That said, studies of lamb mortality in free-ranging bighorn sheep can be quite difficult due to the rugged terrain where wild bighorn sheep give birth and raise their young lambs. As a result, neonatal lamb deaths are easy to miss.
Elis A. Fisk, E. Frances Cassirer, Katey S. Huggler, Allan P. Pessier, Laura A. White, Joshua D. Ramsay, Elizabeth W. Goldsmith, Holly R. Drankhan, Rebecca M. Wolking, Kezia R. Manlove, Todd Nordeen, John T. Hogg, and Kyle R. Taylor (2023). Abortion and neonatal mortality due to Toxoplasma gondii in bighorn sheep (Ovis canadensis), Journal of Wildlife Diseases | doi:10.7589/JWD-D-22-00057