Internal Parasites in Dogs

By Malcolm Weir, DVM, MSc, MPH; Ernest Ward, DVM; Catherine Barnette, DVM

This handout gives an overview of some of the internal parasites that can infect your dog. For more detailed information, refer to the separate handouts “Roundworm Infection in Dogs”, “Hookworm Infections in Dogs”, “Whipworm Infections in Dogs”, “Tapeworm Infections in Dogs”, and “Heartworm Infection in Dogs”.

Are there different types of internal parasites or worms?

There are several types of internal parasites that cause problems in dogs. These include roundworms (Toxocara cati, Toxascaris leonina), heartworms (Dirofilaria immitis), tapeworms (Dipylidium caninum, Taenia species, and Echinococcus species), hookworms (Ancylostoma), and whipworms (Trichuris vulpis).

Are these infections serious for dogs?

Intestinal worms can be a serious problem in young puppies. Hookworms can cause anemia and roundworms can lead to poor growth and development. In adult dogs, however, intestinal parasites are only occasionally life-threatening. Debilitated animals or those with a weakened immune system are more likely to experience severe intestinal parasitism and show clinical signs. Heartworm is a life-threatening parasite spread by mosquitoes and causes significant damage to the heart and lungs. Heartworm disease is one of the most serious conditions seen in small animal practice.

What will happen if my dog is infected by internal parasites?

If a growing puppy is infected with a large number of roundworms, the worms can stunt the puppy's growth, cause serious digestive upset, and result in excessive gas formation. These puppies often have a characteristic 'pot-bellied' appearance. Roundworms are free-living in the intestines. They do not require an intermediate host to spread from dog to dog but can be transmitted by ingestion of eggs that are shed in the feces of infected dogs.

Hookworms are one of the most significant intestinal parasites of dogs. The hookworm is approximately ½” to 1" (1-2 cm) long and attaches to the lining of the small intestines, where it feeds on blood. As a result of this bloodsucking, hookworms can cause severe anemia. The infective larvae enter the host either by mouth or through the skin, particularly the feet. Eczema and secondary bacterial infection can result due to irritation as they burrow through the skin.

Whipworms are small, usually only 1” to 2” (25-50 mm) long. They live in the large intestine where they cause irritation and inflammation. Symptoms of whipworm infection include chronic watery diarrhea, bloody diarrhea, and weight loss.

Tapeworms require an intermediate host, such as a flea, a bird, or certain species of rodents to complete their lifecycle. In other words, your dog cannot get tapeworms directly from another dog or cat.

Dipylidium caninum isthe most common tapeworm of the dog. It causes few problems in the adult dog but can result in digestive upset and stunting of growth in puppies. The intermediate host of Dipylidium is the flea. Dogs get this type of tapeworm from eating an infected flea.

Taenia species of tapeworms usually infect adult dogs and cause few problems. Puppies are occasionally infected and, depending on the type of worm involved, the large number of worms can cause intestinal blockage. The intermediate hosts for Taenia species are small mammals such as rodents and rabbits.

Echinococcus, another type of tapeworm, is important because it is zoonotic, meaning humans can be infected. The adult tapeworm is tiny, only about ¼" (5-6 mm) long. Echinococcus develops inside cysts in various organs in its intermediate hosts: sheep and humans. In humans, these cysts can involve the lungs or brain. Due to its association with sheep, Echinococcus is more common in dogs living close to sheep.

Heartworms are spread by mosquitoes. A mosquito, when feeding on a dog, may inject heartworm larvae into the bloodstream. These larvae mature and ultimately travel to the heart, residing in the major vessels of the heart and lungs. Typical clinical signs of heartworm infection are fatigue, coughing, and poor physical condition. Adult heartworms are large, reaching 6" to 14" (15-36 cm) long.

How are internal parasite infections diagnosed?

Hookworm, roundworm, and whipworm infections are generally diagnosed using a process called fecal flotation, where a small stool sample is mixed with a special solution that causes the eggs to float to the top of the solution. Tapeworm cannot always be detected by fecal flotation and diagnosis is typically made when segments (proglottids) are observed on the feces or around the dog’s anus. Heartworm is diagnosed using specific blood tests.

"...a fecal examination and heartworm test should be part of your dog’s routine annual health care program."

Sometimes, these internal parasites are not discovered until clinical signs become present. For this reason, a fecal examination and heartworm test should be part of your dog’s routine annual health care program.

How can I prevent or treat these parasites?

Prompt treatment for worms should be started when any intestinal parasites are detected. Periodic routine deworming may be appropriate for dogs that are at risk for re-infection. Controlling fleas will prevent infection of certain types of tapeworm.

You can prevent your dog from getting heartworm by using a heartworm preventive and minimizing their exposure to mosquitos by using an FDA- or EPA-approved insecticide designed for use on dogs. Many heartworm preventives also protect against certain intestinal worms. Speak with your veterinarian about the most appropriate parasite control program for your dog.

Are there other internal parasites that can affect my dog?

Other internal parasites that affect dogs include coccidia, giardia, and lungworm. See handouts “Coccidiosis in Dogs”, “Giardia in Dogs”, and “Lungworm Infections in Dogs” for more information on these topics.

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