Humans could be contributing to the spread of a cat-loving, mind-altering parasite called Toxoplasma gondii, new research this week suggests. The study found that domesticated and wild cats were more likely to carry the parasite in areas densely packed with humans.
T. gondii is a single-celled protozoan parasite with a complex and notorious life cycle. Cats are its only primary host, but it takes the scenic route to infect them. To end up inside a cat, the parasite will infect and manipulate the behaviour of certain intermediate hosts — particularly rodents — and make them more reckless and vulnerable to getting eaten by cats. Once inside, T. gondii will reach full maturity and birth the next generation of eggs, which are then pooped out by the cat into the surrounding environment, allowing the gruesome chain of events to start again.
While T. gondii might prefer to get swallowed by rodents or other animals commonly preyed upon by cats, it can infect just about every warm blooded animal, including humans. These unintended hosts are a dead end for the parasite, but it can still stir up trouble for those unlucky enough to get them. In humans, it can cause an usually benign but sometimes life-threatening acute infection called toxoplasmosis. And studies have suggested that chronic T. gondii infection can influence the behaviour and brain health of humans and other large animals, if on a more subtle level than with rodents.
The far-reaching effects of T. gondii have made it an important topic of research, but according to the authors of this new study, published Wednesday in PLOS-One, there hasn’t been too much work looking at the human-led, or anthropomorphic, factors that might be affecting the spread of the parasite.
The authors reviewed dozens of past studies conducted across the globe that tracked how often domesticated and wild cats shed T. gondii eggs in their poop, using that as a proxy for the environmental prevalence of the parasite. Then they analysed the characteristics of the places where the cats pooped.
Overall, the team found a clear association between greater rates of T. gondii in the environment and cats living in areas of higher human population density.
Cats might have been doing fine before they befriended (or simply tolerated) humans, but our partnership over the past ten thousand years or so has undoubtedly made them even more plentiful. So it makes sense that, as cats expanded their territory, so too has the parasite they often carry. But there are likely other things that are making T. gondii more successful in human-dense areas, the authors say.
Our cities are probably a safer place for free-roaming or wild cats to live in than the wild, for instance. Additionally, cities could contribute to the growth of larger rodent populations, providing more opportunities for the parasite to infect and ultimately reach cats. And even our roads or other architecture might allow surface water runoff to transport T. gondii eggs more widely and effectively than usual.
Another factor might be climate change. The authors didn’t find a clear link between warmer temperatures and greater T. gondii prevalence in this study, but they did find one between larger average fluctuations in daily temperature and its presence. Other research has suggested that warmer temperatures could increase the risk of T. gondii infection in humans, but at this point, the authors say more research will be done to validate a possible relationship between climate change and the parasite.
In any case, the findings seem to provide yet another reason for keeping our domestic cats indoors and doing more to trim down the feral cat population in cities.
“Management of free-ranging domestic cats could lower the burden of environmental oocysts due to their large population sizes and affinity with human settlements,” the authors wrote.