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Parasites in dogs take many forms, but they all have one thing in common: sooner or later their presence will almost always have an impact on your pet’s health or comfort. Parasites can cause anything from mild irritation to serious illness.

To cover all of the parasites in detail would (and does) take up a book. So here is an overview of the most common internal parasites dogs can get, how they work, and the problems they may cause. For information about external parasites, such as fleas, ticks, and mites, check here.

First, let’s define what a parasite is. It’s actually pretty simple. The Centers for Disease Control and Prevention sums it up:

“A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host.”

According to the Companion Animal Parasite Council (CAPC), many dogs will be infected with parasites at some point in their lifetime.


Heartworms enter a dog’s bloodstream from the bite of an infected mosquito. The worms mature in the dog’s heart (they can grow up to an amazing one foot in length), and then mate and produce offspring. Adult heartworms live in the heart, lungs, and associated blood vessels of an infected dog.

Inflammation in the dog’s arterial wall disrupts blood flow, making the heart have to work harder. Once blood flow slows sufficiently, a heartworm-infested dog develops a mild, persistent cough, may become fatigued after only mild exercise, and suffers from a reduced appetite. If left untreated, heartworm disease may cause damage to heart, lungs, liver, and kidneys. The end result can be heart failure and death.

Though veterinarians look for these typical signs, most dogs harboring this parasite do not have clinical symptoms prior to the worms being detected via screening tests. The American Heartworm Society recommends that blood tests be conducted annually. The test can detect a single worm in a dog’s body. However, it can only detect the presence of adult heartworms, so timing is very important. There are other tests to determine the presence of heartworms, and your vet can walk you through them.

According to the U.S. Food and Drug Administration, heartworm disease is most common along the Atlantic and Gulf Coasts from the Gulf of Mexico to New Jersey and along the Mississippi River and its major tributaries, but it has been reported in dogs in all 50 states.

Treatment for heartworm is expensive and hard on the dog and owner, and must be administered by a veterinarian. In rare cases, surgery will be required to remove the worms. Luckily, there are many effective options for heartworm prevention. Most veterinarians prescribe monthly chewables, topical treatment, or an annual injection. Collies and certain other herding breeds have a sensitivity to specific heartworm preventives that is genetically based, and your vet should test for the condition before prescribing a heartworm preventative.

Welsh Springer Spaniel puppy laying down in the grass.
animalinfo/Getty Images Plus

Intestinal Dog Parasites

Hookworms live inside a dog’s digestive system and are acquired by puppies from their mother (through the placenta or when nursing), or by adult dogs swallowing the parasite’s eggs or having the hookworm burrow into the skin. Hookworm larvae live in soil for weeks, and can be ingested when the dog comes in contact via eating infected soil, sniffing dog feces, lying on infected ground, or by licking their feet.

Although difficult to see with the naked eye, these tiny worms can cause major damage. After attaching to the lining of the intestinal wall, the hookworm feeds on the dog’s blood, which can cause anemia and inflammation of the intestine, especially dangerous for puppies. Your veterinarian detects hookworms by examining a stool sample under a microscope.

As with a number of intestinal parasites in dogs, diarrhea and weight loss are common symptoms of infection, along with pale gums, fatigue, weight loss, and dry coat. Usually oral drugs are administered twice, several weeks apart, to effectively kill the adult worms. Keeping your dog’s environment clean can prevent infection.

Ringworm is actually a fungus, not a worm, which causes an infection of the skin, hair, or claws. Because of their still-developing immune system, puppies are more susceptible to ringworm. Adult dogs who are malnourished or stressed or whose immune system is diminished are also at risk, and Yorkshire Terriers appear to be more susceptible than other breeds.

Most cases of ringworm are spread by contact with infected animals or contaminated objects, such as furniture or grooming tools, according to the Merck Veterinary Manual. People can also be easily infected. An infected dog will develop lesions on his head, ears, paws, and forelimbs — causing circular bald spots that sometimes look red in the center. In mild cases, a dog might suffer only a few broken hairs. In severe cases, the infection can spread over most of the dog’s body.

Treatment depends on the severity of the infection. Veterinarians typically prescribe a medicated shampoo or ointment to kill the fungus in mild cases. Severe cases may need oral medications, in addition to clipping the fur.

Roundworms are an extremely common parasite, and puppies are most at risk from being infected while in utero or nursing. The worms look like white, firm, rounded strips of spaghetti, one-to-three inches long, and can sometimes be seen in the stool. Symptoms include coughing, vomiting, diarrhea, or malnourishment.

The easiest way to prevent roundworms from also infecting humans is to deworm puppies before the worms become adults and produce eggs. Eggs are extremely sticky and can survive for years. Children are especially vulnerable if they put a dirty hand in their mouths or play in an infected sandbox. Carefully cleaning up dog feces and covering sandboxes helps prevent the worms from spreading. Veterinarians can administer effective deworming drugs and conduct annual fecal exams.

Tapeworms are ingested via a host that is harboring a tapeworm egg. This usually happens when a dog bites at an infected flea. The dog may lose weight and have occasional diarrhea, and may scoot his bottom on the ground. You can see flat segments of the worms that look like grains of rice around the dog’s anus or in his stool. Puppies may become anemic or have a GI blockage.

Your veterinarian will administer medication by injection or orally. The medication is highly effective. The best protection against tapeworms is to the keep your dog free of fleas and away from dead animals and garbage.

Whipworms are acquired when dogs lick or sniff contaminated ground or dog feces. The eggs can live in the ground for several years. An adult whipworm is often less than one-inch long and looks like a fine piece of thread. They live in the dog’s large intestine, but unlike other parasitic worms they are very difficult to spot in a stool sample. A telltale sign, though, is stool covered in mucous, usually at the tip.

Weight loss is the chief symptom of a whipworm infestation. Severe infections cause bloody diarrhea. Though whipworms are rarely a cause of death, an afflicted dog will need to be treated with a dewormer. People can’t get whipworm.

Coccidia, Giardia, and Spirochetes are invasive, non-worm parasites that live in a dog’s intestinal tract. What makes them particularly dangerous is that they can infect a dog before he appears sick. It may not be clear that the dog is carrying the parasites until stress or another immune-compromising factor arises. Diligent sanitation practices are important to stave off these parasites.

Coccidia are single-celled and found more frequently in puppies because they may acquire it from their littermates or mother. Older dogs may also be susceptible by ingesting infected soil or feces. Common signs include diarrhea, loss of weight, and dehydration. Fecal exams are used to detect infection.

Giardia is found throughout the U.S. and is a pervasive protozoon. Transmission can come from infected soil, water, feces, food, or other animals. Symptoms include diarrhea, stomach pain, nausea, and vomiting, but infected dogs might not display any symptoms. Giardia is sometimes difficult to diagnose based on stool samples. Although rare, it can pass from dogs to humans.

Spirochetes are spiral-shaped bacteria that can live in the bloodstream, as well as in the intestine. Different kinds of spirochetes cause leptospirosis or Lyme disease in dogs. Leptospirosis bacteria live in wet and stagnant areas. Lyme disease bacteria live in the blacklegged or deer tick. Puppies and dogs with compromised immune systems are especially at risk. Dogs can be infected with these bacteria and be free of the associated illnesses, however, they can also develop serious symptoms.

How to Prevent Dog Parasites

  1. Medicate with preventatives. Your veterinarian can prescribe safe, effective treatments.
  2. Monitor your pet with annual screening tests administered by your veterinarian and watch for changes in your dog.
  3. Maintain a clean environment, including washing bedding and water dishes; keep your dog away from garbage, dead animals, and infected dogs or cats; remove dog feces as soon as possible.
Related article: External Dog Parasites: Fleas, Ticks, Lice, Mites
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