Of the four worms mentioned above; only one, the tapeworm, requires one deworming treatment*. Why? 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. Roundworms, hookworms and whipworms each have a slightly different life cycle, but they all produce migrating/developing larve 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.
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 (although, as noted in the previous Tapeworm FAQ, 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.
* = It is very important to note that the number of doses is highly dependent on the animal and the environment. Roundworm and whipworm eggs are notoriously long-lived in the environment. Hookworm larve live in sandy soil; a potential reinfection hazard for pets and a human health hazard for people who contact the infected soil. 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).
In other words, for all four of these parasites, reinfection is quite common. Some monthly heartworm preventives will take care of intestinal worms, too: please 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, environment, and on the pet. Please click here for more information on fleas.