Domestic cats are allowed to roam and can transmit parasites and diseases to humans and wildlife. Credit: Shutterstock
For decades, scientists have warned that environmentally destructive activities increase the risk of disease spreading among wildlife and human populations. Examples of these engines include climate change, habitat loss, wildlife trafficking, environmental pollution, the expansion of anthropocentric activities and the introduction of invasive species.
Pets also contribute to the movement of diseases between species. Free-roaming pets, such as cats, can facilitate the spread and transmission of diseases that affect both humans and wildlife.
Infectious parasites
Free-range cats – which include wild, stray and domestic cats – are particularly convincing due to their large populations and their central role in the life cycle of a parasite called Toxoplasma gondii (T. gondii), which infects both wildlife and humans. Most people may have heard of toxoplasmosis only from their doctors during pregnancy or in articles about parasites that “change the brain.”
However, T. gondii is one of the most common zoonotic parasites worldwide and is thought to affect about 30 to 50 percent of the world’s human population. T. gondii infections can have severe and life-threatening consequences; especially for people with weakened immunity and babies infected during pregnancy.
Toxoplasma gondii forms a permanent tissue cyst at rest in the muscle or nerve tissue of the host, so that even healthy infected people are affected. Chronic toxoplasmosis infections are associated with diseases, including degenerative neurological diseases, schizophrenia, and brain cancer.
Domestic cats or feral cats – such as lions, jaguars or cougars – periodically release millions of T. gondii eggs (called oocysts) into the environment through their feces. These oocysts continue to exist under favorable conditions for years in water and soil, with the ability to spread over long distances.
If a warm-blooded animal ingests an oocyst, it can become infected with T. gondii. This can happen if a person or animal ingests oocysts in contaminated water or food, or by eating another animal that has already become infected.
Spread of diseases
Although both feral cats and domestic cats are sources of Toxoplasma, domestic cats are several orders of magnitude superior to feral cats. We recently tested whether mammals living in higher-density domestic cats would show a higher rate of T. gondii infection.
Although there are no global data sets showing the density of domestic cats, domestic cats are closely related to humans, and therefore measurements of human population density can act as a substitute for the density of stray cats. Using data from more than 200 studies, we have demonstrated that indeed wildlife living in areas with higher human densities has a higher rate of T. gondii infection.
We concluded that this higher rate of infection was due to a combination of two phenomena: the high density of stray domestic cats producing infected feces and the loss of natural habitats. Natural ecosystems play an important role in filtering, sequestering and removing T. gondii and other pathogens from human, livestock and wildlife exposure routes. Interrupting the life cycle by preventing cat hunting and restoring the landscape are key preventive measures.
If wildlife has an increased risk of exposure to T. gondii in certain areas, then humans and livestock may also be unintended targets. Researchers in the field of public health have proven this many times by taking samples from soil, vegetable gardens and playgrounds.
Risk of rabies
Rabies is another disease whose risk is increased by stray cats. In the United States, cats are the most common rabies-positive domestic species, with cats two and a half times more likely to be exposed to rabies than bats in Pennsylvania. In Canada, we recently discovered similar public health concerns about stray cats when we studied patterns of bat rabies in Canada.
In Canada, stray cats are associated with 10 times more bats sent for rabies testing than indoor cats. In fact, there were five records in our dataset of free-roaming cats bringing bats into the house, which were later found to be rabies-positive. This hunting activity by cats is obviously dangerous for people in the household and is a very simple explanation for cases of mysterious rabies infections (cases of rabies without an identified source).
This risk is directly proportional to the frequency of stray cats killing bats, which is unfortunately common. Single cats are known to kill a hundred bats in one week.
In our dataset, a stray cat killed nine endangered small brown bats in one month, with another record of a cat that killed 14 bats in one night. Many bat populations have declined sharply, especially due to the introduction of a fungal disease. Bats are long-lived with low reproduction, so this additional source of mortality can seriously affect bat populations.
Because cats bring home only 20 percent of what they kill, returning prey and rabies requests give little idea of the true levels of predation in cats. It is therefore clear that although the natural prevalence of rabies in bats is low – less than one percent – in areas where cats kill large numbers of bats, the risks of rabies exposure will increase.
Protection of health and wildlife
There is a broad consensus among veterinarians, environmentalists, public health experts and animal rights activists that free roaming of domestic cats is detrimental to cat welfare, wildlife welfare, conservation and human health. Wild animals have the same capacity for suffering and pain as domestic animals and provide indispensable ecosystem services with tangible economic benefits, making their predation ethically or economically unjustified.
Free-roaming cats suffer from increased mortality due to traumatic injuries, illness, neglect and abandonment. This marginalization of cats must be replaced by progressive resources for enrichment and responsible management, which does not encourage inhumane and biased disregard for the standards of cat welfare, wildlife welfare, conservation and human health.
Domestic cats stimulate the spread of the parasite Toxoplasma in the wild Provided by The Conversation
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