It’s undeniable – Americans love their pets. The American Pet Products Association (APPA) reports that in 2015, pet owners spent more than $60 billion on their furry friends. And for many of us, our dogs and cats are more than just pets – they’re family members. According to a 2011 study , as many as 62 percent of dog and cat owners let their animals sleep on the bed with them.
But your pet can also bring some unwanted guests. Companion animals like cats and dogs are susceptible to a number of intestinal parasites, commonly known as worms. Some of these parasites are zoonotic, meaning they can be transferred from animals to humans.
Here are five of the most common GI parasites in dogs and cats.
- Hookworms. Hookworms are small, thin worms that attach to the wall of the small intestine and suck an animal’s blood. They are much more common in dogs than in cats. Dogs can contract hookworms in a number of ways – in utero, from the mother’s milk, from contact with feces contaminated with hookworm, or from ingesting hookworm eggs. Signs of hookworm infection include anemia (displayed by tiredness, reluctance to exercise, loss of appetite, or pale gums), diarrhea, weight loss, weakness, and blood in the feces. Hookworm is most life-threatening in puppies and young dogs, but chronic hookworm infestation is also very common in older dogs. Hookworms can also migrate to the lungs, causing fever, cough, and pneumonia-like symptoms.
Humans can contract hookworms through exposure (via bare feet or hands) to stool or ground where infective hookworm larvae are present. Hookworms can cause a skin reaction in people, but rarely migrate beyond the skin.
- Roundworms. Roundworms infect the intestinal tract of both dogs and cats; in fact, most puppies and kittens are born with roundworm larvae already in their system. The parasite can also be transferred through the mother’s milk and through contact with infected feces. Roundworms can often be seen with the naked eye in pets’ vomit or stool, and active roundworm infestations will often give your pet a pot-bellied appearance. Other symptoms include diarrhea and poor growth in young animals.
Like hookworms, roundworms are passed to humans through skin contact with infected feces or soil. However, roundworms can migrate beyond the skin in humans, causing damage to the liver, eyes, and central nervous system.
- Whipworms. Like hookworms, whipworms are more commonly found in dogs than in cats. They live in the animal’s large intestine and shed fewer eggs than other types of intestinal parasites so can be harder to detect from a stool sample. Dogs infected with whipworm often show no symptoms; however, animals with severe infestations suffer from chronic weight loss, diarrhea, and mucous-coated stool. Whipworms are extremely common for shelter dogs or dogs confined to kennels. The good news is that the risk of contracting whipworms from your dog or cat is very limited.
- Tapeworms. Tapeworms are transferred via the fecal-oral route; animals most often contract tapeworms from ingesting fleas infested with tapeworm eggs. Tapeworms can grow to up to six inches in the animal’s intestines. The parasite sheds the terminal end of its tail; these segments are detected in the animal’s stool or attached to the fur under the animal’s tail. Symptoms of tapeworm infestation are often heard to detect and include general itchiness around the anal area, butt scooting, weight loss without loss of appetite or increased appetite without weight gain, and a distended abdomen.
Since swallowing a tapeworm egg is needed to become infected, tapeworms are generally not transferred to humans. However, when human infection does occur, it is seen most often in young children.
- Giardiasis (Giardia). Giardia is different from the rest of the parasites on this list in that it is not a worm; rather, it’s a single-celled protozoan. The active form of the parasite lives in the intestine; the inactive form is encased in a hard shell (cysts) and can live outside of a host. Animals contract giardia by ingesting these cysts. Symptoms include vomiting, greasy or foul-smelling feces, and diarrhea. Young animals, animals living in crowded areas like shelters, and animals under stress are more vulnerable to infection.
In humans, giardia presents with diarrhea, nausea, and cramping. However, the risk of contracting giardia from your pet is small; the type of giardia parasite found in pets is typically not the same as the type that infects people.
Keeping your pet, and yourself and your family, safe from these parasites requires routine testing and preventative measures. If you suspect your pet may have contracted worms or any other intestinal parasite, take them to the vet immediately. Your vet will obtain a fecal sample to detect any infestation and prescribe the proper treatment (typically a week-long course of liquid de-worming medication that will kill the worms, larvae, and eggs).
In terms of prevention, animal feces should be removed from litter boxes and yards regularly, and pet owners should wash their hands after contact with their animal. Children should be supervised to ensure proper handwashing technique. In addition, be aware that areas with heavier pet populations, like dog parks, can be hotbeds of infestation; your pet should be monitored closely after visits to these areas. Finally, animals adopted from shelters will often need to undergo de-worming treatment right off the bat.
Adolph, C. 2015. “Top 5 GI Parasites in Companion Animal Practice.” Clinician’s Brief: April 2015.
Chomel, B.B. and B. Sun. 2011. “Zoonosis in the bedroom.” Emerg Infect Dis 17:167–172.
Pet MD.com. “Intestinal Worms in Dogs and Cats.” Accessed September 2026. Available at http://www.petmd.com/dog/general-health/evr_dg_intestinal_worms_in_dogs.