Common Internal Dog Parasites

What are the most common internal dog parasites?

Internal parasites are common in dogs and can lead to serious health issues for them. In this article, we will discuss the most common internal dog parasites: roundworms, tapeworms, hookworms, and heartworms.


Roundworm is a broad description of a type of worm that affects the intestines and lungs of dogs and many other mammals. It is contracted by encountering the feces of infected dogs. The roundworms live in the intestines of infected dogs and shed eggs which are passed through their feces. Other dogs can pick up these eggs on their paws and fur and ingest them while grooming. These eggs hatch and grow into adult worms that continue the cycle. Puppies can also contract roundworms from their mother’s milk (transmammary) or be born with them (transplacental). Some types of roundworms can also be passed to humans if they are.


Tapeworms are a common parasite that can infect dogs. They are typically transmitted to the dog when they eat a flea or the meat of an infected animal. There are several different types of tapeworms that can affect dogs and each one has its own symptoms. The two most common in our area are Dipylidium and Taenia.

Dipylidium. Also called the flea tapeworm, infection occurs when a flea containing tapeworm larvae is ingested. The flea bites the pet causing irritation, and the pet bites at the flea and accidentally swallows it. Digestion releases the eggs into the gastrointestinal system.

Taenia. These flat tapeworms use their scolex (head) and three pairs of hooklets (fun fact, the anatomy that holds these hooks is called the rostellum – don’t include that in the article, I just think it’s a fun word) to attach to the intestinal wall. Rabbits and pigs become infected by ingesting tapeworm larvae that encyst within the muscle. When the meat is eaten, digestion releases the eggs into the gastrointestinal system. Some types of Taenia can also infect humans.


Hookworms are parasitic worms that can be found in dogs. They’re typically found in the small intestine, but they may also reside in other areas such as the lungs and stomach if there is a large worm burden. The worms are about an inch long and have a curved shape with a pointed tip. Their scientific name, “Ancylostoma,” means “hooked mouth” in Latin, referring to the “hook” they use to bite onto the intestinal wall. As opposed to tapeworms and roundworms which passively absorb nutrients of the food ingested by the pet, they feed on blood.


Heartworms are spread by mosquitoes. A mosquito, when feeding on a dog, may pass heartworm larvae into the bloodstream. These larvae then mature within the blood vessels, and ultimately make their way to the heart. Large worm burdens can cause blockages and prevent blood and oxygen from properly circulating. Heartworm prevention medications affect the larval stage of heartworm so it is unable to grow into an adult and reproduce. Heartworm treatment is not cheap – there is only one medication that can treat heartworm, and pets require multiple visits to the vet for treatment. Cage rest is extremely important during treatment as the worms are killed and then broken down and expelled by the body; activity can cause circulation of these worm bits which can cause damage and even become life-threatening.


Internal parasites are a common problem in dogs. Parasites can be transmitted through contact with infected stool or by ingesting infected insects, dirt, or food. Parasitic infection of meat is one of the concerns associated with feeding raw diets.

Roundworms in dogs are a common problem. The best way to prevent roundworms is to use a monthly preventive prescribed by your veterinarian and do regular stool testing (yearly) to screen for parasite eggs. Do not let your dog sniff or walk through areas where the feces of other animals are present. Talk to your veterinarian about what is right for your dog.

Tapeworms can be contracted by both dogs and humans. Fleas and infected meats are the main culprits of tapeworm, so ensure you have your pet on a monthly preventive. Different types of worms may require different preventives or treatments, so develop a deworming program for your dog with the help of your veterinarian. Don’t allow your dog to eat uncooked meat or catch and ingest wildlife.

Hookworms are one of the most common parasites found in puppies. To prevent hookworms, a regular monthly parasite preventive is the most effective means of protection. Picking up immediately after passing stool is also important in keeping the environment clean. Wiping your puppy’s paws after they go outside will also help.

When it comes to Heartworm, it is critical to talk with your vet and ask about the preventative medicine that will ward off and kill the heartworm larvae if it ends up infecting your dog. There are various types of this preventive medicine, and your vet will know which one is best for your dog.


Internal parasites are some of the most common infectious agents found in your dog and can cause serious health problems if left untreated. The most seen signs are vomiting, diarrhea, and a general unwell feeling.

Symptoms of intestinal worm infestation in dogs is pretty similar regardless of type, and include:

  • Weight loss
  • lack of appetite
  • pot-belly appearance
  • loose stool or diarrhea
  • tender abdomen
  • lethargy
  • abdominal tenderness
  • nausea
  • vomiting
  • the passing of worms in stool or vomitus

Heartworm infestations are different as they affect the circulatory system, not the digestive system. Symptoms include:

  • fatigue
  • coughing
  • exercise intolerance

It’s important to note that some pets with active infections do not show any symptoms at all, which is why regular screening is so critical as part of keeping your pet safe.



One of the most common treatments for canine roundworms is a medication called ivermectin, administered to dogs orally or by injection and is available in different strengths depending on the size of the dog. This medication was originally developed to treat parasites in cattle, and the FDA approved it for use in animals in 1982. Studies have shown that ivermectin is safe for use in all sizes of dogs, but veterinarians may not recommend it for those pregnant or nursing.


There are several treatments for tapeworms in dogs, but the most common is a pill called Praziquantel. The treatment is safe for pregnant or nursing dogs and works quickly to get rid of the infection. Other medications used to treat tapeworms are Fenbendazole, Pyrantel Pamoate, and Levamisole.


Praziquantel is the most common treatment for hookworms. the FDA approved it for animal use in 1988 and is not associated with any serious side effects. It is a broad-spectrum antiparasitic drug used to treat infections caused by worms of the genus Ancylostoma.


Heartworm, as discussed before, is a bit more complicated due to its involvement in the circulatory system. Treatment consists of a combination of antibiotics, steroids, and several injection treatments, during which time your pet will need to be hospitalized for observation. Luckily, gastrointestinal parasites are typically very easy to get rid of once the infection is confirmed. A poop sample will be sent to the laboratory to look for worm eggs and determine which type of worm your pet has. Once known, the appropriate treatment can begin! A prescription dewormer in liquid or tablet form for a few weeks is usually all it takes to get rid of the worm's burden.


As pet owners, the best course of action is always prevention. Keeping a clean environment and properly disposing of dog waste, and consulting with your veterinarian about using preventative treatments can help you avoid major problems. The purpose of this article is to educate you about parasites in dogs and is not intended to replace medical diagnosis or your veterinarian’s opinion. If you suspect your dog may have a parasite, make an appointment with your veterinarian immediately. Prompt treatment will help reduce the health risks associated with infection, as well as the risk of spreading to other pets and people.

We hope this article has proved informative! Please contact the team at Wellesley if you have any further questions or concerns about pet parasites. We recommend you take the time to talk in detail with one of our licensed veterinarians. They will provide the best suggestions and strategies for your pet. For an appointment, please contact us at 416-966-1830.