A new University of British Columbia study has found that free-roaming cats — both domestic and feral — are likely to blame for the spread of a potentially deadly parasite in densely populated urban areas.
The research found that Toxoplasma gondii — a common parasite that infects most warm-blooded animals, including humans, and causes the disease toxoplasmosis — is more likely to infect wildlife where human density is higher.
"As human density goes up, the population of cats is going to go up as well. And as the population of cats goes up, we expect the number of free-roaming cats to increase as well," said study lead Dr. Amy Wilson, an adjunct professor at UBC's Department of Forest and Conservation Sciences.
The study analyzed over 45,000 cases of toxoplasmosis in wild animals, using data from more than 200 global studies.
According to their findings, Wilson said domestic cats are "the most consequential host for toxoplasma," since infected cats can excrete the parasite's eggs in their feces.
"At the core of the toxoplasma life cycle are our cats — domestic cats and wild cats. They're the only ones that can secrete oocysts [eggs] into the environment," Wilson said.
"They're pushing infection rates within natural populations beyond what they normally are exposed to."
Toxoplasmosis, also known as kitty litter disease, can cause blindness and miscarriages in humans and can be fatal for those with weakened immune systems. The disease has also been linked to nervous system disorders, cancers and other debilitating chronic conditions.
In healthy animals and humans, the parasite can remain dormant and rarely causes direct harm.
Reduce outdoor access to curb spread
Wilson said responsible pet management could alleviate the spread of the parasite and, in turn, the risk to wildlife and humans.
"Outdoor cat owners need to understand what they're exposing their neighbours to," she said.
"I'm not in any way saying that cats are a risk factor. This is talking about reducing the risk and that's reducing outdoor access and preventing the hunting. That's what's causing this, the free roaming [and] defecating off property."
The study said one infected cat can excrete as many as 500 million toxoplasma eggs in just two weeks.
The eggs can then live for years in soil and water, with the potential to infect any bird or mammal, including humans, said David Lapen, a research scientist for Agriculture and Agri-Food Canada.
"There's contamination of soil and water. And [the parasite] moves around and gets ingested and fecal matter gets ingested or [infected] animals get ingested," he said.
"Getting exposed may not be as difficult as we might think."
Climate change may be a factor
Climate change might also be influencing the survival and movement of the parasite, Lapen said.
"Intense rainfall, for example, which might be expected to occur as a result of climate change in many regions of Canada — that could be a factor," he said.
"We could see that potentially facilitating transport pathways and exposure pathways [for the parasite] in environments that may not have been a problem before."
He also said warmer weather might mean people leave their cats outdoors for a larger portion of the year, increasing the chances of infection.
Wilson said the findings of the study also highlight how healthy ecosystems can protect against these types of parasites, because healthy forests, streams and wetlands can filter out dangerous pathogens like toxoplasma.
She said it points to the importance of conservation and the impact of human activity.
"I think our study is just highlighting one of the ways that we're actually affecting [wildlife] by increasing their disease incidence."