Wandering domestic cats are allowed to transmit parasites and diseases to humans and wildlife. Credit: Shutterstock
For decades, scientists have warned that ecologically destructive activities increase the risk of disease spreading among wildlife and human populations. Examples of these factors are 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-moving pets, such as cats, can facilitate the spread and transfer of disease, affecting both humans and wildlife.
Infectious parasites
Free cats, including wild, stray, and domestic cats, have a particularly compelling case because of their large population size and their central role in the life cycle of a parasite called Toxoplasma gondii (T. gondii) that infects both wildlife and humans. . Most people may have only heard of toxoplasmosis from their doctors during pregnancy or in articles about “brain-altering” parasites.
However, T. gondii is one of the most common zoonotic parasites in the world and is estimated to affect between 30 and 50 percent of the world’s human population. T. gondii infections can have serious, life-threatening consequences; especially for immunocompromised people and infected babies during pregnancy.
Toxoplasma gondii forms a cyst of tissue resting permanently on a host’s muscle or nerve tissue, so that even infected healthy people are affected. Chronic toxoplasma infections have been linked to diseases such as degenerative neurological diseases, schizophrenia, and brain cancer.
Domestic cats or wild cats, such as lions, jaguars, or cougars, intermittently excrete millions of T. gondii eggs (called oocysts) into the environment through their feces. These oocysts persist in favorable conditions for years in water and soil, with long-range dispersal capacity.
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 water or contaminated food, or if they eat another animal that has already become infected.
Disease spread
Although both wild cats and domestic cats are sources of toxoplasma, domestic cats outnumber wild cats by several orders of magnitude. We recently tested whether mammals living in environments with higher densities of domestic cats would show higher rates of T. gondii infection.
Although there are no global data sets showing the densities of domestic cats, domestic cats are closely associated with humans, so human population density measurements can act as a substitute for the density of cats. which move freely. Using data from more than 200 studies, we showed that, in fact, wildlife living in densely populated areas had higher rates of T. gondii infection.
We concluded that this higher infection rate occurred due to a combination of two phenomena: the high densities of free-range 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 routes. Breaking the life cycle by preventing cats from hunting and restoring the landscape are key preventative measures.
If wildlife has an increased risk of exposure to T. gondii in certain areas, humans and livestock may also be unwanted targets. Public health researchers have repeatedly demonstrated this by sampling soil, orchards, and playgrounds.
Risk of anger
Rabies is another disease whose risk increases with cats moving free. In the United States, cats are the most common rabies-positive domestic species, with cats accounting for two and a half times the risk of rabies exposure compared to Pennsylvania bats. In Canada, we recently found similar public health concerns for cats released when we examined patterns of rabies shipments of bats in Canada.
In Canada, free cats were associated with 10 times more bats that were sent to rabies tests compared to indoor cats. In fact, in our dataset, there were five records of free cats carrying bats in the house that were later found to be positive for rabies. This cat hunting activity is obviously dangerous for people in the household, and is a very simple explanation for cases of cryptic rabies infections (cases of rabies without an identifiable source).
This risk is directly proportional to the frequency with which free cats kill bats, which is unfortunately common. Single cats have been known to kill a hundred bats in a week.
In our dataset, one free cat killed nine endangered brown bats in one month, with another record of one cat killing 14 bats in one night. Many bat populations have suffered severe declines, mainly due to an introduced fungal disease. Bats have a long life and low reproduction, so this additional source of mortality can severely affect bat populations.
Because cats only take home 20 percent of what they kill, prey returns, and rabies shipments only provide a glimpse of the actual predation rates of cats. Therefore, it is clear that while the natural prevalence of rabies in bats is low (less than one percent) in areas with cats that 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 the free roaming of domestic cats is detrimental to feline welfare, wildlife welfare, conservation, and human health. Wildlife has the same capacity for distress and pain as domestic animals, and performs irreplaceable ecosystem services with tangible economic benefits, making their predation unjustifiable from an ethical or economic perspective.
Free cats suffer an increase in mortality from traumatic injuries, illness, neglect and abandonment. This marginalization of cats should be replaced by resources of progressive enrichment and responsible management that does not foster inhuman and biased contempt for feline welfare standards, wildlife welfare, conservation, and human health.
Domestic cats promote the spread of the Toxoplasma parasite to wildlife provided by The Conversation
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