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The Skinny on Intestinal Worms

By Kim Everson, DVM

One day last summer my fearless sister-in-law plucked a “slug” from her cat Izzie’s bottom and sent me a video of the little bugger crawling around on her desktop. Sarah launched into a tangle of theories of where Izzie could have picked up a tiny slug. I suppressed a giggle. “Uh, Sarah, that’s not a slug. That is a tapeworm.”

Cats and dogs share our beds and kiss our babies. They also prey on rodents, scavenge dead animals, eat stool and lick their bottoms. Yep. It’s gross. Limit the grossness by regularly giving your pet a dewormer. There are more types of wiggly intestinal worms than you probably care to imagine with different types of medicines necessary for each. Here’s the low down!


  • Tapeworms are a common small intestinal parasite in cats and dogs.
  • Cats and dogs pick up tapeworms by eating infected fleas or rodents.
  • Signs of infection are seeing tapeworm segments (e.g., grains of rice, sesame seeds or slugs!) in the stool or around the anus. A fecal float is rarely helpful in diagnosis unless the technician finds tapeworm segments in the stool sample.
  • Some tapeworm species are contagious to people and can cause intestinal problems or dangerous neurological disease.
  • It may be necessary to treat for flea infestation at the time of tapeworm diagnosis.
  • Tapeworm dewormer is notoriously expensive and not included in most heartworm preventatives — make sure you ask your veterinarian for advice on treating tapeworms.  


  • Whipworms are a common cause of large bowel diarrhea in dogs.
  • A heavy infestation can cause bloody stool, weight loss, anemia and dehydration.
  • Whipworm infections are tough to diagnose on routine fecal floats because egg production is small, shedding is intermittent and the eggs don’t “float” well. Therefore, even with a negative fecal float, your veterinarian may still recommend aggressive deworming if suspicion is high.
  • Whipworm eggs can remain hidden in the soil for up to 7 years! That means you must keep your dog on a monthly whipworm dewormer for a very long time to avoid reinfection.
  • Whipworm dewormer is not found in most heartworm preventatives, but there are some that include it so ask your veterinarian.


  • Hookworms are commonly found in dogs and cats.
  • Hookworms feed on intestinal tissue causing blood loss and inflammation in the GI tract.
  • Severe infection may lead to anemia, debilitation and even death, especially in young animals.
  • Hookworms can infect people (Ewww!) by penetrating the skin or by being accidentally ingested. Infection in humans can cause an itchy rash or inflammation within internal organs.
  • A routine fecal float performed by your veterinarian is used to diagnose hookworm infections.
  • Many monthly heartworm preventatives also control hookworm infections, especially when used year-round. 


  • Roundworms are the most common intestinal parasite of dogs and cats. Assume every puppy and kitten has a roundworm infestation even though worms are rarely seen in the stool. Some animals with very bad infections will pass “spaghetti-like” worms in the stool or vomit. 
  • Adult worms live in the intestines depriving the host of nutrients. 
  • Common signs may include diarrhea, weight loss, swollen abdomen, vomiting or no signs at all.
  • Dogs and cats pick up roundworms from their mother or from infected feces. The potting soil in houseplants is a reported source of roundworm eggs for indoor-only felines! 
  • Humans can accidentally ingest roundworm eggs. Infection in humans can cause blindness or organ damage (yikes!).
  • Many heartworm preventatives are helpful in treating roundworms, but may need to be used year-round to be most effective.


  • Not an intestinal worm, I know! Heartworms live in the heart and lungs of infected dogs and cats.
  • Mosquitoes transmit immature heartworms from infected animals to healthy dogs and cats. Even “indoor only” dogs and cats are at risk…have you never been subjected to the whiny nighttime hum and nip of rogue indoor mosquitoes?
  • Untreated, heartworm disease is fatal and treatment itself can have dangerous side effects. 
  • In dogs, signs of infection may include coughing, difficulty breathing, sluggishness or no signs at all. Cats usually have signs of respiratory disease (similar to asthma), vomiting, lethargy or–you guessed it–no signs at all.
  • Heartworm infection can be diagnosed with a simple blood test.
  • Heartworm is easily prevented with a variety of safe medications that can be given orally or applied to the skin once a month. As discussed above, many heartworm preventatives do double duty as an intestinal dewormer and some also treat fleas! Your veterinarian can help you sort out the options.

The take home message is that parasites are almost never diagnosed by the naked eye. I hear it time and again: “My dog doesn’t have worms. His poop looks normal.” You will generally only see worms if your pet has tapeworms or has a really, really bad roundworm infestation! Next, almost all intestinal parasites require multiple courses of deworming medication to be effective. Like fleas, certain stages in the worm’s life cycle are resistant to treatment, so you need to deliver the punch repeatedly! Finally, some parasites are contagious to people (especially children and immune-suppressed individuals) making preventative deworming that much more important in certain households.

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