Kittens get roundworms primarily by swallowing microscopic eggs from contaminated soil, surfaces, or feces, and less commonly through their mother’s milk. Roundworms are the most common intestinal parasite in cats, with an estimated prevalence of 25% to 75% in the general cat population and even higher rates in kittens. Understanding the specific routes of infection helps explain why nearly every kitten is assumed to need deworming.
Swallowing Eggs From the Environment
The most common way kittens pick up roundworms is by ingesting eggs shed in the feces of infected cats. An infected cat can pass millions of roundworm eggs in its stool. These eggs don’t become infectious right away. They need one to four weeks in the environment to mature to a stage capable of causing infection. Once mature, the eggs are extremely hardy and can survive in soil for years.
Kittens don’t need to eat feces directly. They can swallow eggs while grooming their paws after walking on contaminated ground, sniffing around litter boxes, or exploring outdoor areas where cats have defecated. Indoor kittens can be exposed if eggs are tracked inside on shoes or if they share space with an infected adult cat. Because the eggs are microscopic, there’s no way to see them on surfaces or in soil.
Eating Prey Animals
Kittens and cats that hunt can also get roundworms by eating infected prey. Mice, birds, insects, and other small animals can harbor dormant roundworm larvae in their tissues after accidentally ingesting eggs from the environment. These animals act as transport hosts: the larvae sit in their bodies without developing further, waiting to be consumed by a cat. When a kitten eats an infected mouse or bird, the larvae reactivate and mature into adult worms in the kitten’s intestine. This route becomes more relevant as kittens grow old enough to hunt, but even very young kittens may catch and eat insects.
Transmission Through Mother’s Milk
Unlike puppies, kittens do not get roundworms before birth. Transplacental transmission, where larvae cross from the mother’s bloodstream into the developing fetuses, does not occur with the cat roundworm. This is a key difference from dogs, where prenatal transfer is the primary infection route.
Kittens can, however, pick up larvae through their mother’s milk. This happens when a nursing queen becomes newly infected during late pregnancy. The larvae pass into her milk and are swallowed by nursing kittens. Interestingly, when kittens are infected this way, the larvae mature directly in the intestine without migrating through the lungs and liver first. That said, this route plays a relatively minor role overall. Most kitten infections come from environmental exposure rather than nursing.
What Happens Inside the Kitten’s Body
Once a kitten swallows infectious eggs, the larvae hatch in the intestine and begin a migration through the body. They burrow through the intestinal wall, travel to the liver, then move into the lungs. In the lungs, they’re coughed up and swallowed again, finally returning to the small intestine where they grow into adult worms. This loop, sometimes called tracheal migration, continues to occur in cats of all ages, unlike in dogs where it stops after about two months of age.
Adult roundworms live in the small intestine, feeding on the kitten’s partially digested food. They can grow several inches long. A mature female worm produces enormous numbers of eggs daily, which pass out in the kitten’s stool and start the cycle over again.
Signs of Roundworm Infection
Many kittens with light infections show no symptoms at all. Heavier burdens are where problems show up. The classic sign is a potbellied appearance in a kitten that otherwise seems thin or isn’t gaining weight well. The worms are consuming nutrients the kitten needs, leading to poor growth and a dull coat.
Diarrhea and vomiting are common. You might occasionally see adult worms in the vomit or stool. They look like pale, rubbery spaghetti strands. Very heavy infections can cause a serious pneumonia as large numbers of larvae migrate through the lungs simultaneously. In extreme cases, a mass of worms can physically block the intestine, though this is less common and less dangerous in cats than in dogs.
Deworming Timeline for Kittens
Because roundworm infection is so widespread and often invisible in the early stages, veterinary guidelines recommend deworming all kittens on a set schedule rather than waiting for symptoms or a positive test. The Companion Animal Parasite Council recommends starting deworming at just 2 weeks of age, repeating every 2 weeks until 2 months old, then monthly until 6 months, and quarterly after that.
This aggressive early schedule exists because deworming medications only kill adult worms in the intestine. They don’t reach larvae still migrating through the liver and lungs. Repeated doses catch new batches of worms as they arrive in the gut and mature. By the time a kitten is 6 months old, the cycle is typically broken.
Risk to Humans
Roundworm eggs shed by kittens can also infect people, particularly young children who play in soil or sandboxes and put their hands in their mouths. Humans are a dead-end host: the larvae hatch and migrate but can’t complete their life cycle. Instead, they wander through tissues like the liver, lungs, or eyes, causing a condition called larva migrans. Because the eggs need weeks to mature before they become infectious, fresh cat feces aren’t an immediate risk. The danger comes from eggs that have been sitting in soil or litter for weeks or longer. Regular litter box cleaning, prompt disposal of feces, and routine kitten deworming all reduce the chance of human exposure significantly.