There are many types of intestinal parasites that can infest cats and dogs. Common intestinal worms of dogs and cats include tapeworms, roundworms, hookworms, and whipworms. Different areas of the world see different "common" regional parasites and some areas (notably the dry and arid climates) see few problems with intestinal parasites.
Is My Pet Infected?
In some cases, pets have no symptoms at all from intestinal worms. In other cases you might see diarrhea, weight loss, dry fur, or even vomiting. Roundworms can also affect humans, and, left untreated, can cause serious illness. Because worms are usually carried by fleas and ticks which carry other serious diseases, it's very important to address parasites at their source by removing fleas and ticks from your home.
Tapeworms Only Require One Deworming Treatment
Of the four worms mentioned above, only one, the tapeworm, generally requires one deworming treatment. This is because the roundworm, hookworm, and whipworm infections require one dose to kill the adult worms living in the intestine and a second dose to kill the migrating larval forms of the worm. The adult worms will often be seen in the pet's feces after a deworming.
The tapeworm does not have an intermediate internal migrating larval form like the round, hook, and whipworms do and thus does not require the second treatment. Tapeworm dewormer medication removes the tapeworm's protective coating and essentially dissolves the tapeworm and the eggs inside of it with one dose.
Tapeworms are not typically seen in the feces after a deworming, though tapeworm segments are often seen exiting the anus in pets that have tapeworms. Pets that have a large load of tapeworms may vomit up tapeworms.
Roundworms, Hookworms, and Whipworms Require Two Deworming Treatments
Roundworms, hookworms, and whipworms each have a slightly different life cycle, but they all produce migrating/developing larvae that require a second dose of dewormer to kill these new developing worms. For roundworms and hookworms, a second dose is needed in 3 weeks (and sometimes another dose in a few more weeks), for whipworms, the second dose is at 73-75 days.
Roundworm and whipworm eggs are notoriously long-lived in the environment. Hookworm larvae live in sandy soil, a potential reinfection hazard for pets and a human health hazard for people who contact the infected soil. Sandboxes are particularly problematic because pets and other animals tend to use them as "litter boxes;" it's a good idea to completely change the sand in a sand box and cover the box daily to avoid infection or reinfection. Tapeworms are transmitted most commonly by fleas (Dipylidium caninum) but can also be transmitted to dogs by rabbits (Taenia pisiformis) and to cats by rats and mice (Taenia taeniaeformis).
Some monthly heartworm preventives will take care of intestinal worms as well as heartworms, but it's important to consult your veterinarian for the best medication choice for your pet and your situation. Some veterinarians recommend a second tapeworm deworming, in cases of flea-transmitted tapeworm infection, to allow the owner time to address the flea problem in the home, in the outdoor environment, and on the pet.