Parasites That Cause Diarrhea in Dogs

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Parasites That Cause Diarrhea in Dogs

Parasites, by definition, are organisms that live on a host organism at the expense of the host’s health. They can take many forms, from visible insect parasites to microscopic organisms in the dog’s blood. But one thing all parasites have in common is that they cause a varying degree of health problems depending on the severity of the infestation. Intestinal parasites are those that invade and attach along the intestinal wall, causing gastrointestinal (GI) signs such as vomiting and diarrhea. Keep reading to learn more about the parasites that might be causing your dog’s diarrhea!

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Types of Intestinal Parasites in Dogs

Intestinal parasites can be divided into different types:

  • Intestinal worms or helminthic parasites
  • Protozoal parasites

Helminthic Parasites

Intestinal worms, or helminths, are a type of intestinal parasitic worm that invades and lives along the mucosal lining of the intestinal tract. They survive by getting their nourishment from their host through their attachment. This causes an inflammatory response along the lining of the intestinal wall, leading to diarrhea. Intestinal worms also cause poor nutrient absorption which can result in malnutrition and weight loss if left untreated.

Below is a list of helminthic parasites known to cause diarrhea and other health concerns in dogs:


Also known as ascarids, this is one of the most common intestinal parasites found in dogs. Roundworms affect all ages but are commonly seen in younger puppies. Unlike other intestinal parasites, ascarids can be passed on from a pregnant dog to her puppies through the placenta. This unique mode of transmission is the reason why ascarids are more commonly seen in young puppies compared to other intestinal parasites.


There are different species of hookworms in dogs and the species will vary depending on the geographical location. Hookworms are blood-sucking intestinal parasites that cause anemia and bloody diarrhea in dogs. In severe cases, hookworm infestation can be fatal, especially if not treated appropriately.


Whipworms get their name because of their physical appearance. They have a very slender and pointed anterior end and a wide posterior end, giving the appearance of a bullwhip. In mild infestations, symptoms are rarely seen. But as the number of worms increases, dogs start to get diarrhea and may develop anemia as well.


Of all the intestinal worms, tapeworms have the most unique physical appearance. Unlike other worms where they have one singular body, tapeworms have segmented bodies, and each segment functions as a means for tapeworms to propagate and spread.

This segmented physical appearance makes tapeworms more difficult to treat. Continuous medication is needed to make sure that the entire length of the tapeworm is killed.

Protozoal Parasites

Protozoal parasites are single-celled organisms that infect the intestines of dogs. Most protozoal parasites in dogs are considered opportunistic pathogens. This means that the dog can get infected by it and not show any clinical signs unless the dog becomes immunocompromised from stress or a different health condition.

When an infected dog becomes immunocompromised, dormant protozoal parasites will proliferate rapidly and cause severe diarrhea. Examples of protozoal parasites in dogs are Giardia and coccidial parasites like Isospora.

How do dogs get intestinal parasites?

The most common route of infection of intestinal parasites is fecal-oral. Infected animals shed infectious stages of the parasites through their feces and ingestion of food or water contaminated by these feces leads to infection of new individuals.

Some parasites, like roundworms, can also be transferred from a pregnant female dog to her pups through the placenta. Unlike other intestinal parasites, certain species of tapeworms will require the presence and ingestion of an intermediate host, often a flea, to successfully infect another dog.

How are intestinal parasite infections diagnosed?

When your dog starts to have diarrhea, it’s best to bring them to a vet for a proper diagnosis. In mild and moderate cases, there would be no apparent signs that will point to intestinal parasite infection as the cause of diarrhea. Your vet will obtain fecal samples from your dog and look at them under the microscope.

Most intestinal parasites can easily be identified and diagnosed through a fecal smear. Protozoal parasites are more difficult to see under the microscope but may still be detected. The presence of helminth eggs can also be observed from a fecal sample under a microscope.

In severe intestinal worm infestation, you might see actual worms from your dog’s feces, a definitive sign of intestinal parasitism.

How are intestinal worms treated in dogs?

Most intestinal parasites respond well to medication and resolve easily with proper treatment. Most intestinal worms can be treated with deworming medications such as pyrantel or praziquantel. Tapeworms, because of their segmented bodies, will require continuous treatment before they’re completely eradicated. Since some tapeworms rely on the presence of fleas for proper transmission to other animals, external parasite control is often a part of treatment for tapeworm infestation.

Protozoal parasites are treated with anti-protozoal medications such as metronidazole or sulfadimethoxine. Most protozoal parasites respond well to treatment but accompanying clinical signs such as vomiting must be addressed as well.

Prevention of Intestinal worms in Dogs

Since most intestinal parasites are transmitted via the fecal-oral route, appropriate cleanliness and hygiene are often the most effective way to prevent intestinal parasitism in dogs. Making sure that drinking water and food are always fresh is often sufficient enough to prevent the spread of intestinal parasites.

In cases of tapeworms, flea control through proper tick and flea preventive medications can help break the transmission cycle and help prevent the spread of tapeworms in dogs. Regular deworming doses, usually done every 3 months, can help prevent ingested helminth eggs from developing into an adult intestinal worm, and thus help prevent it from causing GI signs.

Read more:

Deworming Your Dog - Q&A

Everything You Need to Know About Diarrhea in Dogs

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Published: 7/12/2021

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