Primer On Intestinal Worms

Written and reviewed by John A. Bukowski, DVM, MPH, PhD
and Susan E. Aiello, DVM, ELS

Intestinal worms in dogs are parasites that live in the digestive tract, causing damage and robbing your dog of needed nutrients. 

The amount of damage they cause depends on both the type and number of worms involved. In general, the adult worms that infect dogs cannot live in people, but in rare cases immature forms of these parasites can migrate through human tissue, causing inflammation and potentially serious problems, especially in delicate tissues like the brain or eye.

The four common intestinal worms of dogs are roundworms, hookworms, whipworms, and tapeworms.

Roundworms are long, white worms that look like strands of spaghetti. 

Roundworm infections are most common in puppies because they are often infected directly from their mother (through the womb or mother's milk). Any age dog can become infected from eggs that are deposited with the feces. Adult worms live in the small intestine, where they steal nutrients and irritate the digestive tract. Large numbers of roundworms can even block the digestive tract. Sometimes, coiled-up white worms are seen in material that your dog vomits or in the feces, which may be diarrhea. Puppies  infected with large numbers of worms often have trouble gaining weight and may have "potbellies" and a dull hair coat.

Hookworms are small parasites that "chew" their way into the intestinal wall, where they attach and feed on blood. 

Infection is often passed directly from the mother to the puppies, but dogs can also be become infected by eating infested soil, or even by coming in skin contact with infested soil. Hookworms are very small, so you can't see them in fecal material, but infected dogs usually develop diarrhea that may be black and tarry from digested blood.

Whipworms get their name from their long, very slender bodies that are capped with a small club-like end. 

These worms lie coiled within the wall and lining of the large intestine. Dogs become infected from eggs that are deposited with the feces. In cases of light infection, dogs may not show any signs, but in heavier infections, dogs often develop diarrhea that may have red blood in it.

Tapeworms are long, segmented white worms that live within the digestive tract. 

The head of the worm embeds itself into the lining of the digestive tract, with the rest of the worm trailing downstream and absorbing nutrients that pass by. Segments containing egg packets break off the worm's body and are deposited outside with the feces. There are two common types of tapeworms: one has segments that are shaped like grains of rice or very small pumpkin seeds, and the other looks like a flat, narrow ribbon that is segmented into smaller squares or rectangles.

Dogs are commonly infected with the first type by eating fleas that contain immature tapeworms, so this type of tapeworm infection means that a flea infestation is also likely. Dogs can also become infected with the second type of tapeworm by eating infected rodents or other prey. Dried-up segments of either type of tapeworm can sometimes be found stuck to hairs around your dog's anal area. Many infected dogs show no signs, but vomiting and diarrhea can develop, and large numbers of tapeworms can rob your dog of the nutrients it needs.

Your veterinarian can examine a sample of your dog's feces under the microscope to look for eggs of the various intestinal worms. 

Often, multiple fecal samples collected on different days need to be examined to find the eggs. Many medications are available to treat the various types of worm infections. Depending on where the worms are in their life cycle, more than one treatment is often needed. Keeping your yard and your cat's litter boxes clear of feces helps limit the number of worm eggs in the environment that can reinfect your dog.


Visit WebVet for a wealth of information about the health and well-being of pets. All co