Intestinal worms are parasites that live in the intestinal tract of dogs or cats. If left untreated, they can rob the animal of nutrients, cause severe gastrointestinal upset, and lead to secondary diseases. In addition, many of these worms can be communicated directly to humans. It is important, then, to control intestinal worms by having a pet annually examined by a veterinarian. Internal parasites are especially widespread in dogs. In fact, it is estimated that most puppies are born with some form of intestinal worms. Cats, too, are more susceptible to such worms when they are young. Both dogs and cats, then, should be frequently de-wormed during their first year. After that, they should have a stool examination as part of their annual checkup. Because many parasite eggs enter the environment through animal feces, regularly cleaning a dog's environment and a cat's litter box can help prevent the spread of these worms. For more information on preventing intestinal worms, consult your local veterinarian.