Many puppies and kittens are born with an intestinal parasite known as roundworm. Roundworms are transmitted from an infected mother during the development of the embryo, or by the mother’s milk. Roundworms can also be transmitted through digestion of eggs from infective soil, generally by normal grooming.
Other common intestinal parasites that effect cats and dogs of all ages include, hookworms and whipworms, (both more common in puppies than kittens), tapeworms, toxoplasma gondii, and coccidiosis. Some of these parasites if left untreated can cause severe illness, and in some cases can be fatal.