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Worms 101: All You Need to Know About Internal Parasites


When it comes to internal parasites, or worms, it’s important to know the facts. Whether you’re looking for symptoms of infection or curious how certain internal parasites affect your pet, learn more about each worm from the Companion Animal Parasite Council.


While heartworms are one of the most damaging parasites in dogs, they are nearly 100 percent preventable. Dogs (and cats) throughout the United States can be infected with heartworms. Transmitted by mosquitoes, mature heartworms live in the heart and large blood vessels of the lungs (pulmonary arteries). Adult heartworms can measure over one foot in length!

How can heartworms affect your dog? Once in the pulmonary arteries, the worms can affect oxygenation of blood and make it more difficult for the heart to pump blood throughout the body. This can result in damage to the heart itself, the lungs, kidneys and liver, in addition to simply sapping energy from your beloved pet. Symptoms might include shortness of breath or coughing, although other signs can be present. Untreated, heartworm infection can be fatal.

Fortunately, with medication, heartworm infection is almost always preventable. Ask your veterinarian about heartworm prevention and mosquito repellence.


Hookworms are intestinal parasites that live in the digestive system of your dog or cat. They feeds on your pet’s blood after attaching to the lining of the intestinal wall, and their eggs are passed into the environment through your pet’s feces. Because they feed on the pet’s blood, hookworms can cause internal blood loss, a serious threat to dogs and cats.

Signs of infection might be diarrhea and weight loss. If you think your dog or cat is infected with hookworms, call your veterinarian to schedule an appointment for evaluation, diagnosis, and safe, effective treatment.


Almost all dogs become infected with roundworms at some point in their lives, usually as puppies. As such, roundworms are the most common of all parasitic worms found inside dogs. Oftentimes, mothers will pass the worms to the puppy while it’s still in the womb. They can also develop in a puppy after it’s born if the puppy happens to eat eggs from the environment or drink larvae found in the mother’s milk.

Similar to hookworms, adult roundworms live in the dog’s intestines. While there aren’t many signs of infection, you might notice white or light brown roundworms, several inches long, in your dog’s feces or vomit. Protecting your dog from roundworms includes keeping your dog’s living area clean and feces disposed of regularly. If you think your dog or cat is infected with roundworms, call your veterinarian to schedule an appointment for evaluation, diagnosis and safe, effective treatment.


There are several kinds of tapeworms that can infect your dog. Tapeworms are flat worms that attach to your dog’s (or cat’s) intestines, and are usually diagnosed by finding segments of the worm on the rear of your dog, in the dog’s feces or where he lives and sleeps.

Signs of tapeworms in your dog are generally minimal. Dogs with tapeworm infections are typically not sick and do not lose weight from the worms. Keeping your dog from coming in contact with intermediate hosts of tapeworm, such as fleas, will help reduce occurrence of infection. If you think your dog is infected with tapeworms, call your veterinarian for an appointment to get an accurate diagnosis and safe, effective treatment options.