Gastrointestinal (GI) parasites are worms and protozoa (one-celled organisms) that take up residence in your dog's stomach and intestines. Depending on the dog's age, her level of immunity, and the number of parasites present, the infection can range from unnoticeable to life-threatening. Symptoms include diarrhea, vomiting, weight loss, and a failure to grow and thrive.
Roundworms, hookworms, tapeworms, and whipworms are the most common worm infections. Giardia and coccidia are the most common protozoal infections.
Risk factors
A dog's age, health, and exposure to parasites determine her risk of infection. Puppies are at high risk because their immune systems are immature, and they often live in a contaminated environment before weaning. Also, their small size means fewer parasites are needed to cause symptoms of disease.
Stresses, such as injuries, illnesses, and pregnancy, raise the risk of infection at any age. Adults develop some resistance to roundworms and hookworms but not to other parasites.
GI parasites thrive in warm, moist climates, because the parasite eggs and cysts can survive the winter and the larvae can live longer, giving them more opportunities to become infective. These climates are also perfect for fleas, which transmit tapeworms.
Because dogs transmit some parasites through their feces, areas such as parks, kennels, and breeding farms can be sources of infection. Dogs who roam outdoors freely are most likely to contract GI parasites. The most common parasites include:
Roundworms and hookworms. Pregnant dogs usually transmit roundworms across the placenta to their puppies. Nursing dogs transmit roundworms and hookworms to their offspring through their milk. The mother can pass on worms because she likely had the parasites when she was young. Some of the larvae migrated in her body and some developed into worms in her GI tract. As she matured and was dewormed, she cleared the worms in the GI tract, but the larvae that migrated stayed in her muscle tissue. Pregnancy may have activated the larvae, which she passed to her offspring. Puppies can develop some resistance to roundworms and hookworms as the larvae migrate through their tissues, activating the immune system. Juvenile and adult dogs can also contract infections by ingesting eggs or larvae from the environment or eating rodents infected with roundworms.
A large number of roundworms in the intestines will interfere with nutrient absorption and can lead to an unhealthy dog with a poor coat, abdominal enlargement, diarrhea, and occasional vomiting. The diarrhea and vomit may contain spaghetti-like roundworms. Infection also can cause coughing and pneumonia when roundworms and hookworms pass through the lungs during larval migration. In young animals, a large roundworm burden can create a life-threatening intestinal obstruction.
Hookworms cause intestinal damage by attaching to the lining and sucking blood. Once they detach and move to a new spot, the old wound still oozes blood and protein, which can cause anemia. Dogs with hookworms display many of the same signs as dogs with roundworms. Also look for anemia and black, tar-like stools, which result from digesting blood. Puppies who get hookworms from their mother's milk can die suddenly from anemia as early as five to 10 days after birth. And because hookworm larvae can burrow into the skin, dogs of any age can become infected.
Tapeworms. Tapeworms are common in dogs, but despite what you've heard, they don't cause severe weight loss. The infection actually causes little damage. You probably won't see the diarrhea, vomiting, unhealthy appearance, and weight loss seen with other GI parasites. Adult tapeworms live in the intestines and pass worm segments in the feces. This may cause anal itching, and your dog may scoot her rear across the floor (similar to anal gland impaction). You may notice 1/4-inch white worm segments on your dog's feces or on the fur around the anus. They look like tiny moving pasta segments or grains of rice.
Dogs can get tapeworms from eating infected fleas, rabbits, and rodents. Since larvae don't migrate throughout the body, dogs don't develop immunity against them. So dogs can become reinfected each time they eat a carrier.
Whipworms. Whipworms are most prevalent in the South, where the environment is moist and warm. Dogs become infected from ingesting the eggs in contaminated animal waste, soil, or grass. Whipworms live in the large intestines, causing diarrhea, protein loss, and occasionally anemia and weight loss. The worms are 2 to 3 inches long and resemble a whip, with one end thick like a handle. Dogs can become reinfected because whipworms don't migrate throughout the body, so the immune system isn't stimulated.
Giardia. This protozoan parasite lives in standing water, so dogs with access to streams and ditches—or the water collecting in your own yard—are at risk for infection. Giardia colonizes the small intestines of dogs. It causes foul-smelling diarrhea that can be watery, bloody, or the consistency of cow manure, and it occasionally causes weight loss.
Coccidia. This protozoan parasite commonly infects puppies when they ingest the cysts from their environment. A potentially debilitating infection, coccidia causes diarrhea (occasionally with blood), dehydration, and sometimes vomiting. It can mimic the signs of parvovirus infection in puppies.
Detection
Your veterinarian should thoroughly examine all dogs with signs of GI parasite infection to help rule out other causes of GI disease, such as canine parvovirus infectionand foreign body obstruction.
Your veterinarian will examine a fecal sample for parasite eggs or cysts. Tapeworm eggs don't appear on a fecal test unless a segment breaks open, releasing individual eggs. Sometimes the segments are passed in the feces, and they're visible to you and your veterinarian.
Your dog will pass whipworm eggs and Giardia only intermittently, making these infections particularly hard to diagnose. Your veterinarian may perform blood tests to detect a high number of eosinophils, a type of white blood cell that increases in dogs with parasitic infections. Blood tests may also reveal anemia and low protein levels, especially with hookworm infections.
Prevention and treatment
Preventing GI parasites is as simple as practicing good hygiene and using common sense—like cleaning up the feces in your yard and minimizing your dog's exposure to contaminated areas. Deworming pregnant dogs can reduce environmental transmission to newborns. Your veterinarian can discuss which products are safe for pregnant dogs. To decrease environmental contamination, puppies should be dewormed starting at 2 weeks of age, with additional treatments given at 4, 6, and 8 weeks of age.
Many heartworm preventives also control intestinal parasites, and flea control reduces the chance for tapeworm infection. Ask your veterinarian about the new Giardia vaccine and whether it's right for your dog.
Treatment varies, depending on the type of parasitic infection, so regular fecal examinations to identify the parasite are important. If the fecal test result is negative, but your veterinarian still suspects a particular infection, he or she may treat your dog anyway to see if the symptoms disappear. In general, over-the-counter medications are less effective and have more side effects than those prescribed by the doctor.
Some of the medications listed below are not FDA-approved for certain infections or a specific animal, so talk to your veterinarian if you have any questions. Several medications are given monthly for prevention and treatment, while others are given daily for a short period to treat an infection.
The most common preventive drugs (given orally once a month unless stated otherwise) include:
- Ivermectin plus pyrantel pamoate (Heartgard Plus). This medication prevents roundworms, hookworms, and heartworms.
- Milbemycin (Interceptor). This product prevents roundworms, hookworms, whipworms, and heartworms.
- Selamectin (Revolution). This topical medication prevents and roundworms in some dogs. Selamectin also protects against heartworms, fleas, ear mites, ticks, and mange.
- Diethylcarbamazine plus oxibendazole (Filaribits Plus) treats roundworms, hookworms, and whipworms, and is a daily heartworm preventive. Do not use this product in dogs with liver disease.
Make sure your dog tests negative for heartworms before administering products with heartworm preventives.
Your veterinarian can select from among several short-term oral treatments for existing infections:
- Pyrantel pamoate treats roundworms and hookworms.
- Febantel treats roundworms, hookworms, and whipworms.
- Fenbendazole treats Giardia, coccidia, roundworms, hookworms, whipworms, and tapeworms contracted from rabbits and rodents.
- Praziquantel treats tapeworms from fleas, rabbits, and rodents.
- Epsiprantel treats tapeworms from fleas, rabbits, and rodents.
- Metronidazole treats Giardia.
- Dichlorvos treats roundworms, hookworms, and whipworms. (Do not use in heartworm-positive animals or with other parasiticides.)
- Sulfadimethoxine treats coccidia.
Some of these drugs are combined in products to increase their effectiveness.
Public health concerns
Some GI parasites can be transmitted to people. Children can accidentally ingest roundworm eggs from playing in the yard or eating soil, resulting in larva migrans, an uncommon but serious infection. Since people are not the usual roundworm host, the larvae migrate in the body, particularly in the eyes and brain. This can cause serious problems, including blindness.
Hookworm larvae can penetrate the skin, causing a condition called cutaneous larva migrans (also known as creeping eruption). Itchy, red, raised tracks appear where the larvae tunnel under the skin. People can become infected with tapeworms from eating an infected flea or rabbit or from accidental ingestion of a tapeworm segment. Just like dogs, people can get Giardia from drinking infected water.
Though GI parasites are a common problem in dogs, they're easily treated. But it's better to prevent dogs from contaminating the environment and spreading infection by using dewormers and monthly preventives. This strategy will help protect you, your children, and your dog from infection.
