Most worms cannot be seen from the outside. The tapeworm is the exception, and even it is usually found by its segments, not the worm. Everything else is found in a stool sample under a microscope, which is why the Companion Animal Parasite Council sets a schedule rather than telling owners what to look for.
The council's surveys find more than 30 percent of dogs under six months shedding roundworm eggs, and it says virtually all puppies are born infected, because the larvae cross to them from the mother before birth. A wormy puppy has a pot belly, does not gain weight, and has a poor coat. A heavily infected one may vomit up a mass of worms at four to six months.
The eggs need two to four weeks in the ground before they can infect anything, and then they stay infective for years and shrug off ordinary disinfectants. Picking up stool the same day, before the eggs mature, is the one thing that breaks that. Children who swallow soil with mature eggs in it can get larva migrans, in which the larvae travel through the organs, the nervous system or an eye.
Larvae in soil are swallowed or go straight through the skin, and puppies also get them through milk. The council calls the dog hookworm a voracious bloodsucker that can actually bleed a puppy to death. Pale gums and dark, tarry diarrhea are the signs. People get hookworm larvae through the skin from soil or sand, leaving intensely itchy winding tracks.
Hookworms resistant to several dewormers at once have now been confirmed in a number of dog breeds in the United States, not only in greyhounds. That is why the vet may want a second stool test after treatment rather than assuming it worked.
The common tapeworm is caught by swallowing a flea while grooming. What owners see is the segments, about half an inch long, like cucumber seeds or grains of rice, around the anus, in the bedding or in the stool, sometimes with scooting. Praziquantel clears it. The council says treatment must be combined with flea control, or reinfection is likely. Petipedia has a page on fleas that keep coming back.
Heartworm is spread by mosquitoes and has been diagnosed in all 50 states. The American Heartworm Society says it takes about six months from the bite to adult worms in the heart and lungs, and it asks owners to think 12: a test every 12 months, and prevention 12 months of the year. The early signs in a dog are a mild persistent cough, reluctance to exercise, tiredness after moderate activity, less appetite and weight loss. There is no approved drug that treats heartworm in cats, so for a cat, prevention is the only protection there is.
Which flea, tick and heartworm protection does my dog need? has which heartworm products also cover roundworms, hookworms, whipworms and tapeworms.
Write the date of each preventive dose and each test in the log on My Pet. The vet sheet prints it, and "since Tuesday" beats "recently".
Heartworm now has a page of its own: Does my dog or cat need heartworm protection?
Read 18 September 2026.