Cats get roundworms in three main ways: swallowing microscopic eggs from contaminated soil or surfaces, eating infected prey like rodents, or nursing from an infected mother. Of these, environmental egg ingestion and hunting are the most common routes for adult cats, while kittens typically pick up the parasite through their mother’s milk. Roundworms are the most frequently diagnosed intestinal parasite in cats, found in roughly 14% of owned and shelter cats in recent large-scale surveys.
Swallowing Eggs From the Environment
The most straightforward route is also the easiest to overlook. An infected cat sheds roundworm eggs in its feces, sometimes millions per day. Those eggs land in soil, grass, litter boxes, or any surface that contacts contaminated waste. In moist, shaded soil at cool temperatures, the eggs can survive for several years. Extreme heat, direct sunlight, and thorough drying will kill them, but a typical garden or sandbox offers near-ideal conditions for long-term survival.
Here’s the important detail: freshly shed eggs aren’t immediately dangerous. They need two to four weeks in the environment to develop into an infectious stage. Once mature, the eggs are sticky and remarkably tough. A cat picks them up by grooming dirt off its paws, sniffing contaminated ground, or eating grass where infected feces once sat. Even indoor cats can encounter eggs tracked inside on shoes or carried in on potting soil.
After a cat swallows the eggs, they hatch in the gut. The tiny larvae then burrow through the intestinal wall and begin a winding journey through the body. They pass through the liver, travel to the lungs, get coughed up, and are swallowed back down into the stomach. Once they return to the small intestine, they mature into adult worms and begin producing eggs of their own. The whole cycle from ingestion to new egg production takes roughly four to five weeks.
Eating Infected Prey
Hunting is the second major route, and it’s especially relevant for outdoor cats. When a mouse, rat, bird, or other small animal swallows roundworm eggs from the environment, the larvae hatch and migrate into the animal’s tissues, where they become dormant inside tiny cysts. These animals are called transport hosts because they carry the larvae but don’t develop an active infection themselves.
When a cat catches and eats one of these animals, the dormant larvae reactivate in the cat’s digestive system. They develop into adult worms directly in the small intestine without needing to migrate through the lungs first. This shortcut makes prey-based infection efficient. Any cat that hunts, even occasionally, faces ongoing exposure to roundworms regardless of prior deworming.
Transmission Through Mother’s Milk
Kittens are especially vulnerable because they can become infected before they ever set foot outside. If a mother cat (queen) carries roundworm larvae in her body, those larvae can migrate to her mammary glands during late pregnancy. Larvae that had been lying dormant in her liver reawaken during gestation, timing their migration so they reach the milk supply right as the kittens begin nursing.
This transmammary route means kittens can harbor developing worms within their first days of life. By four to five weeks of age, those worms may already be producing eggs. This is why veterinary guidelines from the Companion Animal Parasite Council recommend starting deworming treatment at just two weeks of age, repeated every two weeks until the kitten is two months old, then monthly until six months.
Two Roundworm Species Affect Cats
Cats can host two different roundworm species. Toxocara cati is by far the more common and the one responsible for mother-to-kitten transmission through milk. It’s the species most frequently found in domestic cats. Toxascaris leonina is less common in house cats but shows up more often in zoo and wild felines. One survey of zoo cats found Toxascaris leonina in about 24% of animals tested, compared to 9% for Toxocara cati.
The practical difference for pet owners is minimal since both species cause similar symptoms and respond to the same treatments. But Toxascaris leonina does not migrate through the lungs or pass through mother’s milk, so its transmission relies entirely on egg ingestion and prey consumption.
Signs of a Roundworm Infection
Many adult cats with roundworms show no obvious symptoms, which is part of why the parasite spreads so easily. The cat feels fine, keeps eating, and quietly sheds eggs into the environment for weeks or months before anyone notices a problem.
When symptoms do appear, they tend to be more dramatic in kittens and young cats. A distended, pot-bellied appearance is classic, caused by a heavy worm burden in the small intestine. Vomiting is common, and sometimes whole worms are visible in the vomit. They look like pale, rubbery spaghetti strands, typically 3 to 5 inches long. Diarrhea, dull coat, slow growth, and general poor condition all point toward a significant infection. During the larval migration phase, some cats develop a mild cough as the larvae pass through the lungs.
Treatment and Prevention
Roundworm infections respond well to deworming medications. Several effective options exist for cats, and most work by paralyzing or killing the adult worms in the intestine so they’re passed in the stool. The key limitation is that most of these treatments only kill adult worms, not migrating larvae. That’s why a single dose isn’t enough. Two treatments spaced 10 to 14 days apart are standard, giving time for any larvae still migrating through the body to mature into adults that the second dose can target.
For ongoing protection, year-round broad-spectrum parasite prevention is the current recommendation. Adult cats not on continuous prevention should be dewormed at least four times a year. Outdoor cats and active hunters may need more frequent treatment given their constant re-exposure through prey.
Keeping litter boxes clean helps break the cycle at home. Since eggs need two to four weeks in the environment to become infectious, scooping litter daily removes them before they pose a risk. For outdoor areas, removing feces promptly and keeping sandboxes covered reduces contamination in soil where eggs would otherwise persist for years.
Risks to Humans
Roundworm eggs from cats can infect people, a condition called toxocariasis. Humans aren’t a natural host, so the larvae can’t complete their lifecycle. Instead, they wander through tissues and sometimes trigger inflammatory reactions. In most cases this causes no symptoms at all. In more significant infections, it can lead to fever, cough, abdominal pain, or an enlarged liver. Rarely, a larva reaches the eye, where it can cause vision problems.
The typical route of human infection is accidentally swallowing contaminated soil, which is why young children who play in dirt or sandboxes are at highest risk. The eggs don’t become infectious for at least two weeks after being shed, so fresh cat feces aren’t an immediate threat. Toxocariasis doesn’t spread from person to person. Consistent deworming of cats, prompt waste removal, and basic hand hygiene after gardening or handling soil are the most effective ways to prevent human exposure.