Worms/Internal Parasites

There are many different parasites that cats can come in contact with and contract disease from. This section will specifically discuss the internal parasites (worms and protozoans). If you need specific information on ear mitesfleasticks, or ringworm please visit those sections.

Internal parasites are those that inhabit the intestine and internal organs of the body. These are mostly the worm-type parasites: Roundworms, Hookworms, Tapeworms, Heartworm, Stomach Worms, Eyeworms and Whipworms. Other internal parasites are the protozoans: Coccidia, Toxoplasmosis, Giardia. Many animals, especially outdoor cats, will become infected with a parasite or two in their lifetime (and many of these infected animals show no symptoms of illness). Thus most vets recommend examining for parasites once a year (more often for an outdoor cat). This can be done by taking a stool sample to your vet. Also note that roundworms, hookworms, tapeworms, and whipworms are all contagious to humans so if you suspect your cat of having such parasites take him/her to the vet immediately to rid your cat of the problem as fast as possible and to protect your family and other pets from infection.

Roundworms are the most common type of parasite seen in felines. Many kittens contract roundworm at birth, either via the mother’s bloodstream in utero or via nursing. Older cats can be infected by ingesting eggs or eating rodents that are infected with roundworm larvae. Symptoms caused by roundworm are usually distended abdomen and diarrhea. In severe cases the parasite load can become so large that the intestines can actually become obstructed. Roundworms are 3-5 inches long, live in the intestine of your cat, and can usually be seen in your pet’s feces or vomit (they look like a little spaghetti). Treatment for roundworm usually requires several treatments before the parasite is fully extracted. Roundworms can also lay thousands of eggs which can be found in the litterbox and in your pet’s bedding or favorite areas. Thus you must clean your home to prevent reinfection. Note that this parasite can cause serious disease to humans especially children.

Hookworms are small blood-sucking worms that attach to the lining of the small intestine. These parasites can cause severe anemia due to their blood sucking ability. These worms are generally acquired by eating a rodent infected with hookworm larvae or ingesting hookworm larvae directly. These larvae can also penetrate the skin to enter the bloodstream and then travel to the intestines. Symptoms caused by hookworm are usually lethargy, diarrhea (or dark stools due to blood in the stool), weakness and vomiting. Hookworms are about 1/2 inch long, slim and thread-like. As they are very small one usually can not see them in the feces (however they are easily seen in a fecal exam). As larvae is usually spread through feces, prompt cleaning of litterboxes and removal of feces is key to stopping the spread. Hookworm larvae can also penetrate the skin of humans and cause a dermatitis (skin irritation).

Tapeworms are another common parasite. Tapeworms attach themselves to the intestinal wall and look like ribbons (long, flat and segmented). They are generally contracted through flea infestation in which a flea with a tapeworm is ingested by your pet (thus controlling flea infestation in your pet can also help control contracting tapeworm). Cats can also contract tapeworm by eating uncooked meat or prey (rodents). Symptoms of tapeworm are usually very mild in nature. Most often you will see a cats itching his/her anus or ‘scooting’ on a rug when the worms pass through the anus. If your cat has a tapeworm you may be able to see small eggs (they look like small seeds or white rice) moving in places that your cat likes to hang out such as his/her bed, in your cat’s feces, or actually on your cat’s anus. Although rare (as it requires the ingestion of an infected flea), children can get tapeworm.

Stomach Worms are generally contracted through eating bugs and other insects who are infected with stomach worms, eating a mouse or other prey who has eaten an infected insect, or by eating the infected vomit of other cats. Symptoms include chronic vomiting, loss of appetite, and weight loss/malnutrition. Note that some cats may show no symptoms at all. These worms usually resemble small roundworms, live in the stomach, and feed off of nutrients as they pass through the stomach.

Eyeworms are small pests (about 1/2 inch long) that infect tear ducts, the lining of the eye (conjunctiva), and under the eye lids. This is not a common disease in cats overall, and is more prevalent on the west coast than the east coast. Symptoms include excessive tearing, conjunctivitis, and even light sensitivity. Eyeworms are generally transmitted by flies and can be treated through topical anesthetic to the eye or even manual removal using tweezers (do not try this at home – this can and should be done by your vet).

Heartworm is a newer disease to cats, but a nasty disease none-the-less. Heartworm disease is usually contracted through a mosquito bite. Thus cats in hot, humid areas of the country where there are lots of mosquitos are at the highest risk. So how does a cat get heartworm? When a cat has heartworm the worms inside the cat’s body produce ‘babies’ called microfilarias. The microfilarias then circulate through the animal’s bloodstream and thus can be picked up by a mosquito feeding on that cat’s blood. Upon biting another cat that same mosquito can then transmit the microfilaria to a new cat. These heartworms then travel to the heart and pulmonary vessels where they then grow, mature, and live. It used to be thought that only outdoor cats could contract heartworm. However, as mosquitos can be found inside and outside of the home, we now know this not to be the case. Some studies also show that cats can acquire immunity to heartworm through successive mosquitos bites. Indoor cats, as they come in contact with mosquitos less often, usually have no such immunity. There currently is no treatment or cure for heartworm so the best thing you can do for your cat is prevention. There are many new medications available for heartworm. Talk to your vet about this if you think your pet may be as risk.

Whipworms live in the intestines and feed by sucking blood through the lining of the large intestine. Symptoms include chronic diarrhea, anemia and weight loss. Whipworm eggs can usually be seen in the feces and can live outside of the cat (in feces) for an extended period of time. Thus cleaning the litterbox and other areas of your home after an infestation is key. Whipworms are rarely seen in cats, usually they are only seen in dogs. It is controversial as to whether or not this type of worm can infect humans.

Protozoans are single celled organisms. The most common diseases caused by protozoans include Coccidia, Toxoplasmosis and Giardia.

Coccidia are protozoan parasites that live in the cells of the intestinal lining. Cats usually become infected with coccidia by encountering the stool of other infected cats. Such stool usually contains oocysts or immature coccidia. These cysts mature and if another cat ingests these cysts the new cat becomes infected. Symptoms of coccidia are usually diarrhea, dehydration, and vomiting. If your cat has/had coccidia you will also want to disinfect your home with a diluted bleach solution to kill any cysts that may still be present in the environment. You can also help stop the spread of coccidia by keeping a clean litterbox and removing any fecal material in the box promptly. Such cysts can survive in the environment and keep re-infecting your cats for a long time to come. The most common coccidia does not affect people. However there are some rare forms of coccidia which can infect humans, but these are rare.

Toxoplasmosis is caused by a parasitic protozoan called Toxoplasma gondii. Infection by Toxoplasma gondii is fairly common, however we rarely see disease caused by this parasite (generally disease is see in kittens, the old, and immuno-compromised animals). The parasite is seen in three different forms: cyst, oocyst or tachyzoite. Most felines get toxoplasmosis from meat that is raw or contaminated with any of the three stages of parasite. When a cat ingests meat (raw meat, a rodent, other prey) that is infected with the parasite, the parasite moves to the wall of the small intestine where it will grow and mature. At this point the cat will excrete oocysts (in high numbers) in his/her feces. These oocysts are infective to other animals and humans. Pregnant mothers need to be extra careful of contracting toxoplasmosis as it can cause birth defects in the unborn child – the best prevention is not to handle feces while pregnant. These oocysts are very resistant to environmental conditions and can survive in the environment for months. Thus proper cleaning and disinfection is important. Note that the parasite can also spread from the intestine to other areas of the body (liver, pancreas, nervous system etc.) In most cases the cat’s immune system fights off the parasite and thus no symptoms are seen. These cysts can simply remain dormant in the cat’s body for the lifespan of the cat (the cat does not shed oocysts into the environment at this time). If symptoms are see they can include lethargy, loss of appetite, fever, or respiratory distress.

Giardia is a parasitic infection of the small intestines. Most cats contract giardia by drinking contaminated water. This problem is most often seen in outdoor cats who drink ‘dirty water’. There are two forms of giardia, trophozoites and cysts. Trophozoites are the form that live in the intestines of the infected cat. While cysts are the form that is shed into the environment by the infected cat. These cysts are very resistent and can remain in the environment for months if affected areas are not appropriately cleaned (through disinfectants). Improper cleaning can be responsible for re-infection. Symptoms of giardia include diarrhea, stomach cramps, gas, bloating, lethargy, weight loss. It is unclear as to whether or not giardia can infect humans.

Symptoms to look for:

  • Diarrhea
  • Pot-belly appearance
  • Lethargy
  • Vomiting
  • Huge appetite/Poor appetite
  • Weight loss (despite large appetite)
  • Anemia
  • Poor coat appearance
  • Redness, discharge of the eye (for eyeworms only)


If you suspect that your cat has any sort of parasite take him/her to the vet. Your vet will perform blood tests, fecal examination and/or other diagnostic tests to determine what type of disease or parasite the cat has. Most parasites are fairly easily treatable with medications and drug therapy. And remember parasites can lay thousands of eggs in your home so make sure to thoroughly clean your house if your pet has been infected. This will protect your pet from getting reinfected.

Currently there is no known cure for heartworm. This is because when heartworms die they must pass through the pulmonary arteries which causes the possibility of arterial blockage and death. So the best course of action to date is prevention. If your cat does have heartworm the best thing to do is to treat the symptoms and to hope that the cat outlives the worm as killing the heartworm can be very dangerous to the feline.

Precautionary measures:

Many kittens are born with worms and thus may need several dewormings during the first months of life. If you have a cat who has had kittens or you have adopted abandoned kittens make sure to talk to your vet about deworming and other necessary vaccinations. For more information on kitten development and when kittens need dewormed click here.

For adults most internal parasites are contracted by eating uncooked food, infected food, dirty/contaminated water, or dirty litterbox conditions. Thus make sure your cat eats a healthy diet and eats out of clean water and food bowls at all times. Also keep your cat’s litter box clean and free from any potential bacteria or parasites, and dispose of excrements on a regular basis. If you have an outdoor cat you may want to talk to your vet about preventative steps that you can take to keep your cat healthy. And anytime you introduce a new cat, especially a kitten, into your home make sure to take him/her to the vet for a full examination first. This way the new cat will not unknowingly pass any diseases to your current pets.