There are millions of domestic cats in the U.S., many of which are outdoors for some or all of the day. Free-ranging cats feed on small mammals and birds, even when they are also fed by humans. In the contiguous U.S., free-ranging cats are estimated to kill 1 to 4 billion birds a year, with two-thirds of the wild animal deaths being caused by unowned cats. Estimates for mammal mortality are even higher. Collectively, free-ranging cats may take 6 to 22 billion small mammals per year. Free-ranging cats kill animals that are important prey for native species, such as raptors and snakes.
In rural areas, free-ranging cat populations may be large. For example, in central Illinois, 207 to 448 cats roamed an area of 20 square miles during a 1978-80 census. Despite being fed by humans, these cats also killed mice, rats, small birds, rabbits, and native rodents.
Some cats carry the parasite Toxoplasma gondii, which causes the disease toxoplasmosis in humans and other animals, including farm animals like pigs and chickens. Contact with rabid stray or feral cats is common in parts of the U.S., which is alarming because rabies is typically fatal to humans. Cats can harbor other pests, such as fleas and viruses like distemper and feline leukemia.