Intestinal Worms in Dogs

Updated on January 7, 2020
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Eirevet is a veterinarian specialized in canine and feline internal medicine who owns a small animal veterinary hospital in Ireland.

How Common Are Worms in Dogs?

Every dog born outside of a strictly controlled setting is born with worms. While the puppy is still within the womb, worms which may have laid dormant in the mother's body for years become active and migrate across the placenta to parasitise the unborn fetus. This staggering opportunism demonstrates just how highly evolved the relationship between host and parasite has become over millennia.

Due to their sophisticated transmission methods, dog worms have proven impossible to eliminate completely, and responsible dog ownership must involve regular deworming of pets to control the population of worms within the pet. By ensuring that your dog's number of worms does not get out of control, you can ensure that your dog never has to develop signs of disease associated with these parasites.

Of all worms in dogs, this is the parasite most commonly seen in most areas of the world (Toxocara canis).
Of all worms in dogs, this is the parasite most commonly seen in most areas of the world (Toxocara canis).

Roundworms in Dogs (Toxocara canis)

Toxocara species are the most common roundworms found in pet dogs worldwide. These large, white worms are the ones usually associated with the transplacental infestation mentioned above. As well as puppies acquiring these worms in the womb, they are also passed through the bitch's milk in the first few days of life.

After ingestion, the worm larvae migrate through the pup's intestinal wall via the liver to the lungs. They are then coughed up, swallowed, and develop to adults in the puppy's digestive tract. Disease may develop as a result of parasite migration or due to large numbers of adult worms in the intestine. Clinical signs relate to the site of the problem and may include:

  • pneumonia
  • liver damage
  • bowel obstruction
  • bowel perforation

Adult dogs that ingest worm larvae will usually develop an immune response to the parasite, resulting in the formation of cysts containing larvae in the liver, lungs, muscle and other tissues. The larvae may escape these cysts if the pet becomes stressed, immunosuppressed, or pregnant at a later date.

Toxocara is a zoonotic parasite, meaning that it can also infest humans. Children are particularly vulnerable. When canine roundworms infest species other than dogs, they may lose their normal ability to navigate between intestines and lungs, and their abnormal migration can result in the syndrome of visceral larval migrans. One of the more severe forms of this disorder occurs when the parasites inadvertently enter the eye, where they may cause a severe reaction in the retina, which is sometimes mistaken for a form of cancer (retinoblastoma).

The cause of hookworm disease in warmer parts of the world. Check out those teeth! (Ancylostoma caninum).
The cause of hookworm disease in warmer parts of the world. Check out those teeth! (Ancylostoma caninum).

Hookworms in Dogs (Ancylostoma caninum)

Hookworm infestations are most commonly caused by Ancylostoma caninum in warmer parts of the world, for example in the mid- and southern United States, whereas Uncinaria stenocephala (sometimes referred to as the fox hookworm) is more common north of these latitudes. As you can see from the magnified picture on the right, this family of dog worms is equipped with a fearsome set of teeth, which are responsible for the clinical signs seen in dogs and puppies with this infestation.

Young pups become infected by the larvae of these worms directly from their mother's colostrum (milk) within the first day of life. The larvae may also be ingested from the environment, and can even burrow through the skin and travel through the unfortunate animal's bloodstream. The larvae again migrate to the lungs, where they are coughed up and swallowed before maturing in the gut.

The main problem caused by these worms in dogs is anemia due to blood loss. The adult worms bite through the mucosa lining the dog's gut in order to feed, and because of their sharp mouthparts they leave a bleeding surface behind when they move to a new feeding site. Digested blood may be seen in your dog's feces, but in some puppies a sudden, severe, and sometimes fatal anemia is the first warning sign that the parasite may be present.

Eggs found in a fecal sample from an infested dog (Trichuris vulpis).
Eggs found in a fecal sample from an infested dog (Trichuris vulpis).

Whipworms in Dogs (Trichuris vulpis)

Of all the worms in dogs which we commonly see, whipworm infestations are usually the least severe. The adult worms of Trichuris vulpis reside in the rectum and large intestine of dogs. They can take up to 3 months to mature, and can be very long-lived. The main problems they can cause relate to irritation and damage to the rectal wall, so straining to pass feces which may contain fresh blood is the most common combination of signs.

One of the tapeworms capable of infecting humans
One of the tapeworms capable of infecting humans

Tapeworms in Dogs (Echinococcus multilocularis)

The tapeworm family contains a great many species which may be found in pet dogs. Most of these parasites are acquired through feeding raw meat or hunting, with the notable exception being Dipylidium caninum. This worm is actually passed between pets by fleas. The infective stage of this parasite is called the scolex, which is a small white organism about the same size as a grain of rice which may be seen crawling from your dog's rectum.

While tapeworms rarely cause serious disease in our pets, they can present a very serious health risk to humans. Members of the Taenia and Echinococcus families can infect humans, resulting in the formation of massive cysts and causing damage to internal organs, particularly the liver, lung, and brain.

Diagnosing Worm Infestations in Dogs

A heavy worm burden should be suspected in any puppy displaying signs of a potbelly, poor weight gain, poor coat quality, vomiting, diarrhea or anemia. Some pups may pass live worms in their feces, although again this is usually only seen in dogs with a large number of the parasites. In other pets it will be necessary for your veterinarian or a veterinary reference laboratory to perform investigations on a fecal sample from your dog.

Microscopic examination of a fresh fecal smear or a fecal flotation sample will reveal the presence of worm larvae or worm eggs depending on the types of worms present in your dog's gastrointestinal tract. For most of the common worms seen in dogs these tests are very sensitive, and are a useful part of routine health screening in any pet.

Prevention and Treatment of Worms in Dogs

While animals showing signs of disease relating to these parasites should be examined by a veterinarian and have fecal and other investigations performed, those without obvious signs can and should be treated regularly with effective anti-parasitic products.

The American Animal Hospital Association recommends that puppies should be treated as follows:

  • every 2 weeks from 3 weeks to 9 weeks of age
  • every month from 3 to six months of age

The British Small Animal Veterinary Association recommends that adult dogs should be treated at least once every 3 months. I have displayed suitable products for these age groups below. These are safe and effective drugs which are routinely used by veterinarians for the treatment and prevention of intestinal and other worms in dogs.

Durvet 12-Pack Triple Wormer Tablets for Medium and Large Dogs
Durvet 12-Pack Triple Wormer Tablets for Medium and Large Dogs
For the regular treatment of dogs over 3 months of age, this product contains pyrantel and praziquantel for treatment and prevention of roundworms, hookworms, and tapeworms.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.


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    • profile image


      5 years ago

      Might be blood- take your dogs to the vet!!

    • profile image

      verlane mcdonald 

      5 years ago

      My dogs were eating perina wet and dry dig food in i noticed that there poop turned black so i switched t home made dog food two weeks latter there stools are still black but lumpy in breaks when i pick it up what could this be


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