This handout provides 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 roundworm (Toxocara cati, Toxascaris leonina), hookworm (Ancylostoma), whipworm (Trichuris vulpis), tapeworm (Dipylidium caninum, Taenia species, and Echinococcus species), and heartworm (Dirofilaria immitis). 

Are these parasites dangerous to dogs? 

Intestinal worms can be a serious problem in young puppies. Hookworms can cause anemia (decreased red blood cells) 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?

Different parasites will affect your dog's body differently.

Roundworms

If a growing puppy is infected with many 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 ingesting the eggs passed in the feces of infected dogs.

Hookworms

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

Whipworms

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

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

Dipylidium caninum, the most common tapeworm affecting dogs, causes few problems in the adult dog, but can result in digestive upset and stunted 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 it can infect humans. The adult tapeworm is tiny, only about ¼" (5-6 mm) long. Sheep and humans can act as the intermediate hosts for Echinococcus, with the immature forms of Echinococcus developing inside various organs of these species. Echinococcus is a risk to dogs that live near sheep. 

Heartworms

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. Adult heartworms are large, reaching 6" to 14" (15-36 cm) long. Typical clinical signs of heartworm infection are fatigue, coughing, and poor physical condition. In advanced cases, congestive heart failure occurs and dogs may suddenly die during exercise or excitement. Prognosis is poor without treatment.

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 can be detected by observing segments (proglottids) on the feces or around the cat’s anus.
• Heartworm is diagnosed using specific blood tests.

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

How can I prevent or treat these parasites?

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

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.