TNR programs fail because they do not operate in a closed system and cannot spay or neuter a sufficient number of cats to affect feral cat numbers at the population level. Despite the good intentions of many involved in TNR programs, TNR has been found to be a waste of time, money, and resources.
The three most common protozoal diseases in cats and humans are cryptosporidiosis, giardiasis, and toxoplasmosis.
Cryptosporidiosis, a highly contagious disease, can cause diarrhea, vomiting, fever, abdominal cramps, and dehydration in both cats and people.
Giardiasis is an often waterborne intestinal parasite that has oocysts (eggs) that are difficult to kill and filter.
Toxoplasmosis is a disease for which the cat is an obligate host in conjunction with rodents. Infected rodents become attracted to cat urine, which makes them easy prey. Cat feces can contain millions of toxoplasma. Although cats show no symptoms, the disease can infect humans, causing miscarriage and other effects not yet fully understood.
Outdoor cats suffer a much higher incidence of injury, parasites, and disease than cats kept indoors. Although some diseases — such as Feline Immunodeficiency Virus and Feline Leukemia Virus — are specific to cats, others can afflict a wide variety of species, including people.
In the U.S., cats are more likely than dogs to be rabid. There is no known, effective treatment for rabies once symptoms of the disease appear. There are effective vaccines that provide immunity to rabies only when administered soon after exposure. They may also be used for protection before exposure for people such as veterinarians and animal handlers. “TNR” programs do not effectively prevent or control rabies in feral cat populations as originally posited.
Outdoor cats can get into fights; be exposed to deadly viruses, hit by cars, and attacked by wild animals; and become poisoned.