Of course, the most important reason to keep a cat in is for the animal's own safety. Cats like to go outside, but for their own good, they shouldn't be indulged. After all, young children might like to play outside unsupervised, but allowing them to do so is negligent. The same is true for allowing cats out.
Outside threats to cats are numerous and take their toll on cat's lives. According to Barbara L. Diamond's article "Bringing the Outdoors In" (Cat Fancy, April 1990), "While the average outdoor or indoor-outdoor cat lives two to three years, an indoor-only cat's average life span is 12 to 15 years or more." A look at just a few of the hazards facing outdoor cats explains why their lives can be so brief:
Disease. Rabies and other zoonotic diseases have already been mentioned as threats to people. More common are diseases that inflict cats only and that are spread through contact with other cats. Two diseases that kill large numbers of cats each year are feline leukemia and feline immunodeficiency virus. Both diseases are transmitted from cat to cat and, once contracted, result in the eventual death of the animal due to a compromised immune system. Keeping cats inside helps prevent the transmission of these killers.
Parasites. Outdoor cats inevitably pick up fleas and ticks and then bring these pests into the home with them. Fleas can cause anemia, skin irritations, and allergies in cats. These parasites also pose risks to humans since they can transmit disease through their bites. Ridding the pet and home of fleas and ticks is difficult and can expose the pet to harmful chemicals. Indoor cats aren't generally exposed to fleas, ticks, ear mites, or other parasites.
Poisoning. Poisons exist on chemically treated lawns, in bait left out to kill rats or mice, and in auto antifreeze drained from cars (a sweet substance cats love to lick, but which is deadly). Most cats love to chew on greens, but their fondness can be safely satisfied with grass grown in an indoor pot.
Other animals. Other cats, dogs, and wildlife are potential enemies of cats and often engage in fights that leave a cat injured. Outdoor cats can suffer torn ears, cut eyes, abscesses, and other injuries requiring expensive veterinary treatment.
Cruel people. All shelter workers can tell horror stories about cats that come in tarred and feathered, burned, or tortured in some other way by cruel kids or disturbed adults. A cat outside is a likely target for people who collect animals to sell to research laboratories. Outside pets are at the mercy of the people they encounter.
Traps. The HSUS speculates that over 100,000 cats are caught in traps each year. Those who aren't killed may suffer for days before being released and often lose limbs from the injuries.
Traffic. Most outdoor cats die from auto accidents. It is a myth that cats are "streetwise" about cars. Cats are intelligent and alert, but they stand very little chance against fast-moving vehicles.
Pet overpopulation. Anyone who's ever worked in a shelter knows that unaltered cats allowed to roam and mate at will account for millions of the cats euthanized each year. One female cat and her offspring can produce 420,000 cats in seven years. All pets, whether strictly indoor or indoor-outdoor, should be spayed or neutered. Pet owners who allow unaltered animals outside are irresponsible and at the root of the terrible pet overpopulation problem resulting in millions of animal deaths yearly.