Excerpted with permission from Root Kustritz MV. Feral cats: A review of concerns and control measures. Clinical Theriogenology
Cats (Felis catus) live outdoors on every continent except Antarctica and on islands of every size, either as natural or introduced species.
Because cats are loosely domesticated, with freedom to roam considered acceptable by cat owners in many countries, it is difficult
to classify cat populations as owned, stray, or feral. This classification becomes important when considering how best to
control the various populations, as one must consider the welfare of the individual cats, public attitudes, and legal implications
when designing control programs. A suggested scheme is the following: Companion or owned cats are those who live the majority
of the time with humans and are dependent on them for basic needs, stray cats are lost or abandoned companion cats that may
live outside but do so near human habitation and rely on humans to some extent, and feral cats are those that live outside
and are self-sustaining. Feral cats may have been companion cats once and may choose to live near human habitation but do
not rely on humans for shelter, food, or water. Another definition for a feral cat would be one that cannot be handled by
humans and would not be suitable for adoption into a home as a pet; this definition is complicated by lack of consistent guidelines
within or between facilities regarding methods used to identify whether or not a given cat is adoptable. A colony of feral
cats is defined as a group of three or more sexually intact cats living and feeding in close proximity.
Many papers cite the concerns associated with maintenance of large colonies of free-roaming cats. Some suggest benefits as
well. These include the social aspects of providing support for these cats and potential building of networks between caregivers.
Multiple stakeholders must be taken into consideration when evaluating control of feral cat colonies including humans who
consider cats a nuisance, who are at risk of their own health from diseases carried or transmitted by cats, or who are concerned
about welfare of wildlife or welfare of the cats, and the cats and other animal species themselves. Society at large also
has a stake with growing opposition to euthanasia and differing understanding of the role of humans in exerting dominion over
animals. Concerns about feral cat colonies include loss of wildlife from predation by feral cats, spread of disease from cats
to other animals, and potential spread of disease from cats to humans.
It is estimated that there as many feral cats as there are owned cats in the United States, with an estimated population of
73 million in the year 2000. Cats hunt and kill small mammals, reptiles, amphibians, and birds, both adults and nestlings,
with one estimate suggesting that over a billion small mammals and millions of birds are killed by cats each year. In Wisconsin
alone, it is estimated that 1-2 million free-roaming cats kill about 40 million birds annually. The large number of animals
killed is due in part to the density of cats tolerated in a feral colony. One study demonstrated 35 feral cats in the same
territory that would have supported only 1-2 pairs of larger natural predators, such as fox or coyotes. There is collateral
damage from cat predation of small animals to these larger predators, as cats decrease availability of natural food sources.
To be complete, one must also recognize that not all decline in prey availability is due to predation from cats and that cats
may help support prey populations in some instances by controlling intermediary predators, such as rats.
Intraspecific disease transfer
There are no reports in the literature of feral cat colonies specifically spreading disease to owned cats. However, because
feral cats do not receive regular veterinary care, including vaccinations, there is a greater risk of their contracting contagious
diseases such as feline leukemia and rabies, and transmitting them if they interact with owned cats. There are documented
reports of feral cats spreading disease to wild cat populations, including feline leukemia virus to mountain lions and panleukopenia
to Florida panthers.
Interspecific (zoonotic) disease transfer
Public health concerns center around spread from feral cats to humans of viral diseases, including rabies; protozoal infections,
such as Toxoplasma gondii; fungal infections, including dermatophytosis; and parasites. Some might argue that management of feral cat colonies increases
public health risk by exposing caregivers to disease and to direct injury; others suggest that risk is decreased by vaccination
of cats returned and removal of diseased cats from managed colonies.
Rabies is a disease of great significance in all species, including humans. It is fatal to humans unless they are treated
immediately after exposure. Rabies is maintained in wildlife and the variant of rabies contracted by domestic animals varies
by region, with raccoon, skunk, fox, and bat variants those most commonly seen in the United States. In the most recently
published survey of rabies surveillance in the United States, rabid animals were identified in 49 states and on Puerto Rico,
with 93% of cases occurring in wildlife and 7% in domestic animals; cats represented 4.3% of the total cases seen, a number
nearly 4 times that reported for dogs. In one survey of feral cats in Italy, two of eight rabid cats were from known feral
cat colonies that were managed by human caregivers. Concern exists even if feral cats are vaccinated; in one survey of 840
rabid cats in the United States, 22 (2.6%) had been previously vaccinated at some point in their lives, with three of those
animals described as being current on their vaccinations.
Toxoplasmosis is a protozoan organism carried asymptomatically by cats. In one survey of six feral cats trapped in Mexico
City, all were seropositive for Toxoplasma gondii. In larger studies, prevalence of seropositive cats varied from 73.9 - 84.7%. Because of constant exposure to Toxoplasma
through ingestion of prey species, Toxoplasma gondii always is present in feral cat colonies, although prevalence may vary over time and may be higher in adults than in juveniles.
Intestinal parasites most commonly identified in feral cat populations are roundworms (Toxocara cati), hookworms (Ancylostoma tubaeforme) and several species of tapeworm. Only a small percentage of cats were identified to be carrying those parasites most readily
transmitted to people, including Toxocari cati (0.8% of 658 cats) and Ancylostoma tubaeforme (14.7% of 658 cats). Reported percentages were higher in cats living on islands than on the mainland.
Passage of protozoa and parasites through feces are not the only public health concerns. One study estimated that in a single
community, the amount of feces deposited outdoors by the 2046 feral cats living there every year was about 29.5 tons. A study
tracking sources of E. coli in storm sewers feeding rivers and streams demonstrated that the highest percentage from any one source came from cats. It
may be that it is not exposure to the cats but rather exposure to their feces that is the greatest public health risk.