Intestinal Parasites


Internal parasites can be found in many different organs and tissues of an animal’s body. One area commonly targeted is the intestines. Parasite infections can occur through a number of possible routes, depending on the parasite species and the stage of the parasite’s life cycle playing a role in transmission. A primary route of infection is ingestion of infective eggs from the environment. Typically this happens when a dog or cat ingests fecal matter or soil that contains fecal matter. Another possible mode of infection includes ingesting an infected intermediate host; this will typically be a prey animal, such as a bird or mouse. Another mode of transmission is skin penetration; developmental stages of some parasites, such as hookworm, are capable of penetrating an animal’s skin. Kittens and puppies may also contract parasites while developing inside the mother or postnatally by ingesting the mother’s milk.

While there are others, three of the more common intestinal parasites that affect our companion animals are roundworms, hookworms, and tapeworms.


Roundworms are the most common of the parasitic worms found inside a cat or dog. Almost all cats and dogs become infected with them at some time in their lives, usually as kittens and puppies. The adult roundworm lives in the host’s intestines. Infections will become apparent when you observe adult spaghetti-like worms in the feces or vomit. The intestinal disruption may cause diarrhea, constipation, or vomiting. An animal suffering from a heavy roundworm infection may also exhibit signs of impaired growth and malnutrition. The hair coats of infected animals can often grow dull and unhealthy looking, due to the nutritional deficit being caused by the worms.


The hookworm attaches to the lining of the intestinal wall and feeds on the host’s blood. The worm also releases an anticoagulant at the site of attachment to increase blood flow. For hookworm infections, you may see adult worms in the feces or vomit. The signs can also reflect moderate to severe blood loss because hookworms will cause bleeding into the intestinal tract resulting in internal blood loss. Signs of intestinal blood loss can include black, tarry stool. These animals are also prone to anemia. Blood loss from severe hookworm infections may cause death in young kittens and puppies.


Tapeworms are long, flat worms that attach themselves to the host’s intestines. A tapeworm body consists of multiple parts, or segments, each with its own reproductive organs. These segments are called proglottids. Tapeworm infections are usually diagnosed by finding proglottids — which appear as small white worms that may look like grains of rice or cucumber seeds — on the rear end of the animal, in the feces, or where the animal lives and sleeps. A healthy animal with a moderate tapeworm infection may show no outward signs, but heavy or chronic infections can lead to a dull coat, decreased appetite, and weight loss. Several different species of tapeworms may infect our animals, each with different intermediate hosts. Because fleas are an intermediate host for the most common kind of tapeworm, if a dog or cat has fleas, there’s a good chance that it also has tapeworms.

Signs of a parasite infection may not always be outwardly apparent. That’s why it’s always a good idea to include a fecal examination as part of routine veterinary care. By examining the dog’s or cat’s feces under a microscope, a technician can detect the eggs of intestinal parasites. If your dog or cat has intestinal parasites, you’ll want to consult your veterinarian for safe, effective treatment options. Some over-the-counter products can cause cramping and other discomforts and can carry severe risk when used with animals that are too young, too old, or debilitated. The products available through veterinarians are safe and convenient and often protect against multiple types of parasites.

Keeping our animal’s surroundings clean and keeping them out of contaminated areas will go a long way in preventing parasites. Several convenient preventive medications are also available for intestinal worms. As with the treatment options, it is highly recommended to always use a product that is recommended by your veterinarian; again, many of these products also have the convenient feature of preventing more than one type of parasite.

Amy Fischer, Ph.D. – University of Illinois

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