Internal Parasites in Cats

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

Are there different types of internal parasites or worms? 

There are several types of internal parasites that cause problems in cats. 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 cats? 

Intestinal worms can be a serious problem in young kittens. Hookworms can cause anemia (decreased red blood cells) and roundworms can lead to poor growth and development. Tapeworms can also accumulate in high numbers, leading to intestinal obstruction. In adult cats, however, intestinal parasites are only occasionally life-threatening. Debilitated animals or those that have a weakened immune system are more likely to experience severe intestinal parasitism and show clinical signs. Heartworm disease is a life-threatening problem in dogs and cats. Heartworm disease is transmitted by mosquitoes and can cause extensive damage to the heart and lungs. 

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

Different parasites will affect your cat differently.

Roundworms

If a growing kitten is infected with many roundworms, the worms can stunt the kitten's growth, cause serious digestive upset, and result in excessive gas formation. These kittens often have a characteristic “pot-bellied” appearance. Roundworms are free-living in the intestines. They do not require an intermediate host to spread from cat to cat but can be transmitted by ingesting the eggs passed in the feces of an infected cat.

Hookworms

Hookworms are one of the most significant intestinal parasites of cats. 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 cats. The infective larvae can 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. Whipworm infection in cats was rare in North America but cases appear to be rising. Infection is more common in tropical and sub-tropical areas.

Tapeworms

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

Dipylidium caninum, the most common tapeworm affecting cats, causes few problems in the adult cat, but can cause digestive upset and stunted growth in kittens. The intermediate host of Dipylidium is the flea; cats get this type of tapeworm by eating an infected flea. 

Taenia species of tapeworms usually infect adult cats and cause few problems. The intermediate hosts for Taenia species are small mammals such as rodents, rabbits, and some species of birds. Therefore, this parasite is most common in outdoor cats who hunt. Kittens are occasionally infected, mostly when they eat raw prey. In heavy infections, large numbers of tapeworms can cause intestinal blockage. 

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 cats that live near sheep. 

Heartworms

Heartworm disease is uncommon in cats but increasing in incidence, especially in certain areas of North America. Heartworms are spread by mosquitoes. A mosquito, when feeding on a cat, may inject heartworm larvae into the bloodstream. Most larvae are killed by the cat’s immune system and thus few survive to become adult worms. When a large number of larvae die, a severe inflammatory reaction occurs in the airways, which causes signs of coughing, rapid breathing, weight loss, and vomiting. The larvae that survive and mature ultimately travel to the heart, reside in the major vessels of the heart and lungs, then cause right-sided heart failure.

How are internal parasite infections diagnosed?

• Hookworm and roundworm infections are 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 can be difficult to diagnose in cats. No single blood test will detect heartworm infection so multiple tests may be recommended including an antibody test, which can detect early stages of heartworm infection, and an antigen test, which may detect the presence of adult worms (rare) later in the course of the disease. Occasionally, a cat infected with heartworms will die suddenly and the diagnosis will be made on a post-mortem examination.

Sometimes, these internal parasites are not discovered until clinical signs are present. For this reason, a fecal examination should be part of your cat’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 cats at risk of re-infection or with regular outdoor access. Controlling fleas will prevent infection with certain types of tapeworms.

There are excellent heartworm preventives now available for cats, making prevention of heartworm disease safe and easy. Many heartworm preventives also protect against certain intestinal worms. Speak with your veterinarian about the most appropriate parasite control program for your cat.

Are there other internal parasites that can affect my cat?

Other internal parasites that affect cats include whipworm, lungworm, coccidia, and giardia. See handouts “Whipworm Infection in Cats”, “Lungworm Infections in Cats”, “Coccidiosis in Cats”, and “Giardia in Cats” for more information.