Worms steal nutrients, damage the intestinal lining, and in some cases cause serious blood loss or lung disease in cats. The specific harm depends on the type of worm, the number of parasites, and your cat’s age and overall health. Kittens and outdoor cats face the highest risks, but even indoor cats can pick up certain parasites.
Types of Worms That Infect Cats
Four main types of worms affect cats: roundworms, tapeworms, hookworms, and heartworms. Each lives in a different part of the body and causes distinct problems.
Roundworms are the most common intestinal parasite in cats. They live in the small intestine, where they feed on partially digested food. Cats pick them up by swallowing microscopic eggs from contaminated soil or by eating infected rodents. Kittens often get roundworms from their mother’s milk, since dormant larvae in the mother’s liver reactivate during pregnancy and migrate into the nursing kittens.
Tapeworms arrive through a more indirect route. Flea larvae swallow tapeworm eggs from the environment, and the parasite develops inside the flea as it matures. When your cat swallows an infected adult flea during grooming, the tapeworm is released into the intestine and anchors itself to the gut wall. Cats can also get tapeworms from eating rodents that carry the larvae.
Hookworms latch onto the intestinal lining and feed on blood. Heartworms, despite their name, primarily lodge in the blood vessels leading to the lungs and cause respiratory disease rather than heart problems in cats.
Digestive Problems and Nutrient Theft
Intestinal worms compete directly with your cat for food. Roundworms are especially greedy, consuming enough of the host’s nutrients to cause visible weight loss and a dull, rough coat. In kittens, this nutrient theft produces a classic potbellied appearance: the kitten’s body looks swollen at the abdomen even as the rest of it stays thin. The belly distension comes from a combination of worm mass, gas, and inflammation in the gut.
Vomiting and diarrhea are common with most intestinal worm infections. You might occasionally see live roundworms in vomit. They look like pale spaghetti strands, sometimes several inches long. Tapeworm segments, by contrast, show up around your cat’s anus or on the surface of fresh stool. Each segment is about the size of a grain of rice, and you can sometimes see them crawling. Once they dry out, they shrink to about 2 millimeters and turn hard and yellowish.
Blood Loss and Anemia
Hookworms cause the most direct physical damage to the intestinal wall. They bite into the lining, feed on blood, and then shift to a new spot, leaving behind small bleeding ulcers. A single hookworm can drain up to 0.1 milliliters of blood in 24 hours. That sounds tiny, but a heavy infection involving dozens or hundreds of worms adds up fast, especially in a small kitten.
The resulting anemia makes cats weak, lethargic, and pale around the gums. Even hookworm species that aren’t aggressive blood feeders still cause protein loss. Serum seeping from the damaged attachment sites can reduce blood protein levels by more than 10%, leading to fluid retention and poor wound healing.
Lung and Respiratory Damage
Heartworm disease in cats is fundamentally different from heartworm disease in dogs. When immature heartworms arrive in the small arteries of a cat’s lungs, they trigger a severe inflammatory response that damages the arterial walls, the small airways, and the tiny air sacs where oxygen exchange happens. Veterinarians call this heartworm-associated respiratory disease, or HARD.
Cats with HARD may cough, wheeze, and struggle to breathe. The symptoms can look a lot like feline asthma, which makes diagnosis tricky. In advanced infections, the worms physically block blood flow through the pulmonary arteries, forcing the heart to work harder to push blood through. Unlike dogs, cats cannot be treated with the standard heartworm-killing drugs because those medications are fatal to cats. Management focuses on controlling symptoms and waiting for the worms to die naturally, which can take two to three years.
Roundworm larvae also pass through the lungs during their migration cycle. After a cat swallows roundworm eggs, the larvae hatch in the gut, burrow into the bloodstream, travel to the lungs, get coughed up, and are swallowed again to mature in the intestine. This lung migration phase can cause coughing and mild respiratory irritation, particularly in kittens with heavy infections.
Why Kittens Are Hit Hardest
Kittens are more vulnerable to worms for several reasons. Their immune systems are immature and less equipped to keep parasite numbers in check. They’re also exposed early: most kittens contract roundworms through their mother’s milk in the first weeks of life, before they’ve had any chance to build defenses. Larvae that had been lying dormant in the mother’s liver reactivate during late pregnancy and time their migration perfectly to reach the nursing kittens.
Because kittens are small, even a moderate worm burden represents a larger proportion of their body weight and blood volume. A level of blood loss from hookworms that an adult cat could tolerate may cause life-threatening anemia in a kitten weighing just a few pounds. Severe roundworm infections can also cause intestinal blockages in very young cats, since the sheer volume of worms physically fills the small intestine.
Risks to Humans
Some cat worms can infect people, making this a household health issue. Cat roundworm eggs shed in feces and can survive in soil for months. If someone accidentally swallows contaminated dirt (children playing outside are the most common scenario), the larvae hatch and migrate through the body. They can’t complete their lifecycle in humans, but they don’t just die quietly. The wandering larvae cause a condition called visceral toxocariasis, with symptoms including fever, coughing, wheezing, belly pain, and an enlarged liver. If the larvae reach the eyes, they cause ocular toxocariasis, which can impair vision.
Tapeworms pose a lower risk to people because transmission requires swallowing an infected flea, not just contact with cat feces. It’s uncommon but not impossible, particularly for young children. Hookworm larvae can penetrate human skin on contact with contaminated soil, causing itchy, winding rashes on the feet or hands.
How Worm Infections Are Diagnosed
Your vet diagnoses most intestinal worms through a fecal flotation test. A small stool sample is mixed with a solution that causes parasite eggs to float to the surface, where they’re collected on a glass slide and examined under a microscope. Different worm species produce distinctly shaped eggs, so the test identifies exactly what your cat is carrying. No single stool sample catches every infection, since worms don’t shed eggs continuously. A negative result with ongoing symptoms usually means the test should be repeated.
Heartworm is harder to detect in cats. Blood tests used for dogs are less reliable in cats because cats typically harbor only one or two adult worms, which may not trigger a strong enough antibody response to register on standard screening. Diagnosis often requires a combination of blood work, chest X-rays, and ultrasound.
Treatment and Prevention
Intestinal worms are straightforward to treat. Deworming medications target specific parasite types. Tapeworm treatments dissolve the worm’s body so it’s digested in the gut, meaning you won’t see dead worms in the stool afterward. Roundworm and hookworm medications paralyze the worms, which are then passed out naturally. Most dewormers work within a few days, though a follow-up dose two to three weeks later is standard to catch any larvae that have matured since the first treatment.
Kittens should not be dewormed before six weeks of age. After that, most vets recommend deworming every two weeks until about three months old, then monthly until six months, because reinfection from the mother and environment is so common in young cats.
Prevention matters more than treatment in the long run. Monthly parasite preventives that cover multiple worm types are available as topical liquids or oral tablets. Keeping your cat on flea prevention is essential for breaking the tapeworm lifecycle, since fleas are the primary carrier. Indoor cats have lower risk overall, but they’re not immune. Fleas hitch rides on clothing and other pets, and heartworm-carrying mosquitoes easily get inside homes.