Dog: Endoparasites

General information

Other common/scientific names: worms, parasites, intestinal parasites, nematodes, roundworms, hookworms, whipworms, tapeworms, Coccidia, coccidiosis, Giardia, giardiasis

Endoparasites are parasites that live in the internal organs of an animal. This article will discuss the four common intestinal endoparasites (Roundworms, Hookworms, Whipworm, Tapeworms) and the two common protozoal endoparasites (Coccidia, Giardia) which infect dogs. It is thought that 30-50% of dogs carry intestinal parasites. These parasites can pose health risks to both you and your dog.


Roundworms (Toxocara canis and Toxacaris leonine)

Roundworms are long, white thin worms resembling spaghetti which live free floating in the small intestine of the dog.

Abb. GSRBIPUV: Roundworm (Toxocara canis).

Life Cycle

Roundworm eggs are laid by the adult female and passed in the dog’s feces. These eggs must develop in the environment for about a month before they can infect a new dog. Therefore, fresh feces are not infectious. Roundworm eggs can weather harsh environment and remain infective for months to years. Dogs can acquire a round worm infection by four ways:

  • Consuming an infective worm egg from the soil
  • Nursing from an infected mother
  • Consuming a prey animal (rodent) that is carrying developing worms
  • During embryonic development (unborn puppies infected from their mother while in the womb)

Once this infective stage is acquired by a dog, the immature forms can migrate through the liver and lung, causing damage and inflammation. Heavy infections can cause pneumonia and intestinal obstruction. When the immature forms enter the upper airways, they are coughed up and swallowed, thus re-entering the intestinal track where they mature to adults and start the life cycle again. Occasionally, dogs will vomit these adult roundworms.

Hookworms (Ancylostoma caninum)

Hookworms are small reddish-brown worms which live in the small intestine of the dog. They hang onto the intestinal wall by using sharp teeth. The adult hookworm actually sucks blood from the dog’s intestine. Puppies with a heavy infection can lose a large quantity of blood becoming anemic resulting in lethargy, unthriftiness and possibly death.

Abb. GSRBK8A9: Hookworm, electron microscope.

Life Cycle

Hookworm eggs are laid by the adult female and passed in the dog’s feces. The egg hatches in the environment and develops into infective larvae. Dogs can acquire a hookworm infection by four ways:

  • Ingesting an infective larval form
  • Nursing from an infected mother
  • Penetration of the skin by an infective larval form
  • During embryonic development (unborn puppies infected from their mother while in the womb)

Once the larvae are inside the dog, they make their way to the intestine where they stay and mature into adults and start the life cycle again.

Whipworms (Trichuris vulpis)

Whipworms are small, dark worms which live in the large intestine of the dog. They are named because the head of the worm is skinny whereas their tail is stout, resembling a whip. The adult whipworm bites into the tissue of the intestine and sucks blood. Large numbers of whipworms can cause irritation and bloody diarrhea.

Life Cycle

Whipworm eggs are laid by the adult female and passed in the dog’s feces. These eggs must develop in the environment for 2-4 weeks before they can infect a new dog. Therefore, fresh feces are not infectious. Whipworm eggs can contaminate the soil for years making it almost impossible to remove or kill all the eggs. Once the infective egg is ingested, usually during grooming, the egg hatches in the small intestine and travels to the large intestine where it embeds into the tissue and matures into an adult. The female whipworm only lays eggs periodically (whereas other female worms lay eggs continuously). Because of this, diagnosing whipworm infection based on eggs in the stool can be challenging.

Tapeworms (Dipylidium caninum, Taenia hydatigena, Taenia pisiformis)

There are two types of tapeworms which can infect dogs. Both adult forms live in the small intestine. Tapeworms are flattened intestinal worms made up of many segments.

Dipylidium caninum (flea tapeworm) Life Cycle

The Dipylidium caninum or flea tapeworm sheds its segments which are full of eggs in the dog’s feces. These eggs are released into the environment and ingested by flea larvae. The dog ingests the fleas during grooming and the young tapeworm is released in the intestinal lumen where it matures into an adult. The head of this tapeworm hooks into the intestine lining with tiny teeth. This adult tapeworm can be 6-12 inches long. The segments of the Dipylidium tapeworm, which resemble grains of rice, are longer than they are wide. When the segments are shed in the dogs stool, they can be seen attached to the hair around the anal area or on the feces. These segments are able to move. While this type of tapeworm does not usually harm the dog, it can cause irritation around the dog’s anus causing the dog to scoot. They are also a cosmetic concern since most owners do not want to see live, wiggly tapeworm segments on their dogs.

Abb. GG0YQQ90
Abb. GG0YQQ90: Dipylidium caninum (flea tapeworm).
Photograph of a tapeworm segment which has the appearance of a grain of rice.

Taenia tapeworm Life Cycle

The Taenia tapeworm also sheds its segments which are full of eggs in the dog’s feces. These segments are then ingested by an intermediate host (mice, rabbit, deer) where they develop into immature tapeworms. Dogs acquire this infection by ingesting these intermediate hosts. The young tapeworm matures into an adult in the dog’s intestines and begins the life cycle again. The Taenia tapeworm segments are wider than they are long. These adult tapeworms are large and can cause intestinal irritation in heavy infections.

Coccidia (Isospora canis)

Coccidia are small protozoal or single-celled organisms that invade and infect the lining of the small intestine. There are many species of coccidia that infect animals. However, Isospora spp are the most common in dogs. These parasites can cause diarrhea which may be mild to severe depending on the level of infection. Young dogs with immature immune systems and dogs with weakened immune systems are most commonly affected.

Life Cycle

Coccidia eggs are called oocysts and are passed in the stool where they mature to infective stages in the environment. These infective stages are ingested and travel to the intestine. In some cases, mice swallow the infective oocysts and a dog becomes infected after eating the mouse. The time from exposure to the onset of signs is about 13 days. Pups can acquire the infective oocytes from their mother’s feces or exposure to any contaminated fecal material. Coccidia infection is common in shelters, rescue areas and kennels. Treatment and isolation of infected animals is important for control.

Giardia (Giardia canis)

Giardia are pear-shaped, single-celled (protozoal) organisms that infect the small intestine of humans, pets and wild animals all over the world. There are many species of Giardia. The life cycle of Giardia is not completely understood and the zoonotic potential is controversial among veterinary experts. Younger animals are more commonly affected by Giardia and show signs of pale, foul-smelling diarrhea. Chronic diarrhea can result causing weight loss and unthriftiness.

Life Cycle

The Giardia protozoa becomes a cyst in the intestinal tract and is passed in the feces. This infective stage can survive for many months in a cold, wet environment. It is ingested by the dog and the cyst shell dissolves, freeing the Giardia protozoa which attaches to the intestinal lining causing damage and preventing absorption of nutrients. Infection is more common in kennel situations where animals are housed in groups.

Cardinal symptom


In addition to the signs described above, diarrhea, vomiting and bloody stools can be seen with an intestinal parasite infection. Dogs with heavy parasite infections can suffer from weight loss and a dull haircoat due to a lack of nutrients which are being absorbed by the parasites. Puppies can have a pot bellied appearance. A parasite infection can weaken a dog’s immune system, making the dog more prone to other infections and disease.


Intestinal parasites are diagnosed by microscopic examination of a fecal sample for the parasitic eggs. This is commonly called a fecal or stool sample examination. Each parasite has a distinctive egg which allows for identification. Because some adult worms may shed eggs intermittently or the fecal sample obtained may not contain eggs, repeat samples may be necessary to diagnose the parasites. Tapeworms can be diagnosed by observing the segments in the dog’s stool or anal area.

Abb. GG0YYRR0: Coccidia.
This is a photograph of numerous Coccidia oocysts as seen under a microscope.

Diagnosis of Giardia using a fecal examination is difficult because the protozoa are small and not passed with every stool. A diagnostic test which identifies specific proteins found in the Giardia protozoa can be performed in your veterinarian’s office. For Giardia diagnosis, this test is much more accurate than a fecal examination.


There are several options for treating intestinal parasites. These antiparasitic medications come in the form of oral medications, injectable medications and topical (spot-on) medications. Your veterinarian will recommend a dewormer based on the parasite, your dog’s symptoms and his lifestyle. Coccidia are not susceptible to routine dewormers and are treated with sulfa-based antibiotics. Since most dewormers do not kill the immature forms of the endoparasite, treatment should be repeated once or twice to completely eliminate the parasite.

Many of the monthly medications used to prevent fleas, ticks and heartworm disease also treat intestinal endoparasites. Veterinarians recommend that most dogs be given these monthly medications. All dogs should have an annual fecal examination performed.

Puppies should be dewormed at two, four, six and eight weeks of age. Pregnant females should be dewormed after six weeks of pregnancy.


For healthy adult dogs, the prognosis is good for treating endoparasitic infections. However, in puppies or small, debilitated dogs, a heavy parasite infection can be deadly.


Regularly clean your yard and dog’s environment of dog feces to prevent contamination with parasite eggs. Support fecal removal and leash policies in pet exercise areas and public places. Do not allow your dog to eat uncooked meat. Keeping your home sanitized and preventing fecal contamination of food and water supplies is essential for preventing re-infection of coccidia and Giardia. Outbreaks of coccidiosis and giardiasis in kennels will require a multi-modal approach to decontaminate a facility.


Roundworms, hookworms and tapeworms are zoonotic and can cause infections in humans, especially children. If the infective roundworm eggs are swallowed by people, the larvae can migrate to various tissues and become encysted in the organs causing clinical disease. This includes infections in the eye causing blindness. Hookworm larvae can burrow into human skin when the skin comes into contact with contaminated soil. This can result from walking barefoot in contaminated areas or lying on contaminated beaches. These infections can cause the skin to become irritated and itchy. Tapeworms can be contracted by humans if they swallow a flea.

Update version: 4/24/2014, © Copyright by
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