Intestinal Parasites in Dogs – Explained By a Veterinarian

As a veterinarian, I recommend fecal parasite examinations for my canine patients every single day. Every single day, at least one client responds “my dog can’t have worms – I don’t see any worms in his stool!”

If only it were that easy!

In most cases, intestinal parasites in dogs are not visually detectable in the stool. In fact, many dogs with intestinal parasites are completely asymptomatic. If dogs do show signs of intestinal parasites, the signs often include diarrhea or soft stools, vomiting, decreased appetite, and weight loss. In many dogs, however, intestinal parasites can only be diagnosed with a fecal parasite examination.

Diagnosing, treating, and preventing intestinal parasites is an important component of keeping your dog healthy. Performing fecal examinations every six to twelve months allows your vet to diagnose intestinal parasites and give the appropriate medications to treat infections. Taking measures to prevent infection will decrease the likelihood of your dog having repeated infections.

Intestinal parasites can be divided into two categories: worms and protozoa.

The Worms: Hookworms, Roundworms, Whipworms, and Tapeworms

Intestinal worms live within the intestine, feeding on either the host’s tissues or undigested food. These worms lay eggs that are shed in the feces; if another animal eats these eggs, that animal may become infected with worms.

Intestinal worms can be treated with a number of available dewormers, each of which is effective against different species of intestinal worms. Your veterinarian will determine the appropriate dewormer for your pet based on the types of worms that are seen on fecal examination. In most cases, dewormer must be repeated multiple times in order for it to be effective.

There are two primary ways to prevent intestinal worms. First, pick up feces in your yard. Parasite eggs that are shed in the feces can contaminate your yard, serving as a source of repeated infection. Second, give monthly heartworm prevention to prevent most (if not all) intestinal worms. All monthly heartworm preventives will prevent hookworms and roundworms; talk to your veterinarian if you would prefer a heartworm prevention that also prevents whipworms and/or tapeworms.


Roundworms are spaghetti-like worms that live in the intestine and feed on partially digested food. Most puppies are born with roundworms (which they acquire from their mother, across the placenta or through the milk). Adult dogs become infected if they ingest roundworm eggs (for example, by licking their paws after stepping in feces from an infected dog) or small animals (such as earthworms) that are infected with roundworms. Humans can also become infected with roundworms through ingestion.

Dogs with roundworms often develop vomiting and diarrhea. Worms may be seen in the vomit and feces. In young puppies, roundworms often cause malnourishment and a pot-bellied appearance. Less commonly, roundworms cause coughing as larvae migrate through the lungs.


Hookworms are small worms, less than one inch in length. They have sharp teeth, which they use to attach to their host’s intestine and feed on blood. Puppies can also become infected with hookworms via their mothers’ milk. Adult dogs acquire hookworms by eating an egg from the environment, eating infected rodents or insects, or by transmission across the skin. Hookworms can also be passed to humans, with infection occurring by ingestion or skin penetration.

Clinical signs of hookworms typically include vomiting, diarrhea, and weight loss. Additionally, because they feed on the dog’s blood, hookworms can also cause anemia. This anemia can result in pale gums, lethargy, and even death, especially in young puppies. Hookworm larvae can be found migrating through the lungs, sometimes leading to a dry cough.


Whipworms are small parasites (less than 1/2 inch in length) that take their name from their whip-like shape; one end of the parasite is thin (like a whip), and the other is thick (like a whip handle). Whipworm infections occur when a dog ingests a whipworm egg from the environment.

The signs of whipworm infection vary, depending on the severity of infection and in the individual patient. Some dogs experience no clinical signs. More commonly, however, affected dogs develop diarrhea or soft stools, blood or mucous in the stool, and straining to defecate.


If you have ever seen a small, rice-like worm around your pet’s rectum, that was a tapeworm. A live tapeworm in the intestine is composed of many of these rice-like segments; these segments contain tapeworm eggs and are the method by which the tapeworm eggs are shed into the environment.

Dogs typically become infected with tapeworms by ingesting a flea that contains tapeworm larvae. Therefore, the most effective means of preventing tapeworms is the use of effective monthly flea prevention. Tapeworms can also be acquired by eating an infested bird, rodent, or reptile, but this is much more common in cats than dogs. Tapeworms typically do not cause any clinical signs in affected dogs, but they are definitely disgusting for pet owners. Fortunately, they resolve quickly with a single treatment of dewormer.

Microscope view of an Intestinal Parasite.
Close up of a Giardia Parasite.

The Protozoa: Coccidia and Giardia

Protozoa are microscopic, single-celled organisms that are related to bacteria. These organisms grow within the intestines, causing significant inflammation. The organisms are shed in the feces of infected animals and can re-infect other animals in this manner. In addition, the organisms (or, in the case of giardia, environmentally-resistant cysts) can remain in the environment and re-infect dogs even after successful treatment. For this reason, the treatment of coccidia or giardia should always involve careful cleaning of the environment to prevent re-infection.


Dogs acquire coccidia when they come in contact with the parasite in the environment. For this reason, coccidia is often seen in puppies coming from animal shelters or breeders, where a large number of dogs are housed in one location.

Coccidia typically do not cause any clinical signs in adult dogs. In puppies and dogs with weakened immune systems, however, coccidian infection can cause severe watery diarrhea and vomiting. This vomiting and diarrhea can lead to dehydration and even (in rare cases) death.

In most cases, coccidia can be treated on an outpatient basis with a 10-day course of oral medication. Severely ill puppies may need to be hospitalized for intravenous (IV) fluids, but this is relatively uncommon.

In order to prevent re-infection, it is important to clean the environment during this treatment. Talk to your veterinarian about disinfecting options, but dilute bleach is the most common recommendation.


Giardia is another single-celled, microscopic parasite that can affect dogs. Dogs become infected when they ingest a Giardia cyst in the environment. These cysts are often found in contaminated water, though they can also be found in the soil. Giardia is zoonotic, meaning that it can also affect humans who ingest it through contaminated water or close contact with animals who are shedding the organism.

The signs of Giardia vary, based on the animal’s overall health and the severity of the infection. Healthy adult dogs may be asymptomatic for Giardia, or may experience only mild, intermittent soft stools. In more severe infections, however, dogs can develop soft to watery stool with large amounts of mucus. Signs may continue for weeks, leading to weight loss, loss of appetite, and/or vomiting.

Giardia is typically treated with a combination of drugs given over a 10-day period. Your dog may also be prescribed a bland diet to help decrease the diarrhea. Most dogs respond quickly to treatment. Giardia cysts can remain in the environment for prolonged periods of time, so cleaning the environment is essential to preventing reinfection. A variety of products are effective against Giardia, including bleach, Lysol, and other cleaners.


Both intestinal worms and protozoal parasites can have significant effects on your dog’s gastrointestinal tract and overall health. Follow your veterinarian’s recommendations for regular fecal parasite examinations, use parasite preventives as directed, and keep your immediate environment free of animal feces in order to keep your pet healthy and parasite-free.


Photo of Veterinarian Catherine Barnettte
Catherine Barnette

Catherine Barnette, DVM, is a veterinarian and freelance writer based in North Carolina.

Join the Discussion!

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s