Cats spread parasites (and destroy wildlife.)
Danger from gut parasites has been recognised now for a long time, with dog faeces and wild rodents a common cause for several common health problems and some fatalities. The relationship between wild and domestic stock is studied in the mutual badger/cattle transmission of tuberculosis, and now in feral cats, which normally host the parasite concerned, and culled white-tailed deer.
It would seem unlikely that small felines, Felis catus, would interact with white-tailed deer, Odocoileus virginianus. but Gregory A. Ballash and colleagues have investigated the situation thoroughly in Ohio, USA. The urban situation is vastly different from any natural environment for these 2 species, at least. They published in the journal
The urban and semi-urban cats contaminate their wild environment (and any domestic areas they inhabit) with the oocysts of Toxoplasma gondii. Cat infection is linked to the amount of time they spend outside and the amount of hunting they engage in, especially of rodents. 200 cats and 444 deer were sampled and the antibodies for the parasite found present in 51.5% of cats and 58.8% of deer. Older deer (over 1-year-old) were more likely to have the Toxoplasma but no age difference was detected among cats. Urban cats were more infected than the suburban sampled specimens, however.
The deer seem to pick up the ingested oocysts from contaminated environments (where cats are present), without transmitting the
Protist within their population. The cats serve as unique hosts for the sexual reproduction of the organism. Their faeces are likely to be the source of deer infection, just as changing your cats litter ray is a risk for humans who are susceptible to the disease toxoplasmosis. 30% of the human population worldwide have this infection, but most show no symptoms, apart from a suspected neurological link. That link has already been proved to cause rodents to become easier for cats to catch. The world is much stranger than once thought, for hidden factors such as these zoonoses (infections by other animals parasites and diseases.) Like Toxopasma, many protists such as Sarcocystis in SE Asia, can be transmitted from rats or even less obvious animals.
Almost any warm-blooded animal can pick up Toxoplasma infection. Sea otters, bobcats, beluga whales and even birds such as woodchuck and many others have been noted with these infections recently. The cats presence of course is necessary, so urban situations are the source in almost all cases. To avoid a baby or any other person likely to be susceptible being infected, all meat should be thoroughly cooked, milk must be pasteurised, vegetables must be washed efficiently and any soil must be avoided. Children and others obviously might disregard such advice, but the infection seems
reasonably safe for all except those whose health is weakened.