Indoor cats most commonly get roundworms from their mother’s milk as kittens, from eating insects or rodents that carry larvae, or from roundworm eggs tracked into the home on shoes and clothing. Even a cat that never steps outside has several realistic exposure routes.
Passed From Mother to Kitten
The single most common way an indoor cat ends up with roundworms is by getting them before it ever comes home. The primary feline roundworm, Toxocara cati, can pass from a mother cat to her kittens through her milk. If a queen picks up an infection during late pregnancy, larvae migrate into her mammary tissue and transfer to nursing kittens. By the time a kitten is weaned and adopted into an indoor-only home, it may already be carrying a growing roundworm population.
This matters because many cat owners assume their kitten is parasite-free since it came from a clean breeder or foster home. The reality is that transmission through nursing happens before anyone has a chance to intervene, and a kitten can shed roundworm eggs in its stool without showing obvious symptoms for weeks.
Eating Insects and Rodents Indoors
Cats become infected with Toxocara cati by ingesting rodents or other small animals that carry larvae in their tissues. These animals are called transport hosts: the roundworm doesn’t mature inside them, but the larvae sit dormant in their muscles and organs, waiting to be eaten by a cat. A mouse that slips into your home through a gap in the foundation can deliver roundworm larvae straight to your cat’s digestive tract if your cat catches and eats it.
Cockroaches and beetles can also carry roundworm larvae after contact with contaminated soil or feces outdoors. Indoor cats are natural hunters and will eat insects without hesitation. One swallowed cockroach carrying Toxocara larvae is enough to start an infection. A second feline roundworm species, Toxascaris leonina, spreads the same way, through ingesting eggs from the environment or larvae from rodent tissues.
Eggs Tracked in on Shoes
Roundworm eggs are remarkably durable. After being shed in animal feces outdoors, eggs need two to four weeks in the environment to become infectious. Once they reach that stage, their tough protective shell allows them to survive for months or even years under the right conditions. You can carry these microscopic eggs into your home on the soles of your shoes after walking through contaminated soil, parks, or sidewalks.
The CDC specifically recommends covering shoes or wearing rubber boots that can be scrubbed when working in areas where animal feces may be present, precisely because eggs cling to footwear and transfer to indoor surfaces. Once on your floor, an indoor cat that grooms its paws after walking on those surfaces can ingest the eggs. Clothing, gardening tools, and even your hands after outdoor yard work are other potential carriers.
Houseplants and Potting Soil
Cats that dig in indoor plant pots face a less obvious risk. While commercial potting soil is generally processed, outdoor garden soil brought inside or soil amendments containing animal byproducts can harbor roundworm eggs. Roundworm eggs remain viable in soil for up to a year. If your cat likes to dig in houseplant containers, especially those filled with soil from your yard, it can pick up eggs on its paws and ingest them during grooming.
Two Roundworm Species, Different Risks
Cats can host three roundworm species, but two are relevant in most of the world: Toxocara cati and Toxascaris leonina. T. cati is the more important one. It is the most common, it poses the greatest health risk to kittens, and it can also infect humans. T. leonina follows a simpler lifecycle and tends to cause less severe disease, but indoor cats can pick it up through the same routes: contaminated environments and infected prey.
T. cati is classified as zoonotic, meaning it can spread from cats to people. Humans become infected by accidentally swallowing eggs from contaminated surfaces or unwashed hands. Inside the human body, the larvae don’t mature into adult worms the way they do in cats. Instead, they migrate through organs and can cause two forms of disease. Visceral toxocariasis occurs when larvae travel to the liver or central nervous system, causing fever, coughing, wheezing, and abdominal pain. Ocular toxocariasis happens when a larva reaches the eye, potentially causing inflammation, retinal damage, or vision loss, typically in just one eye. Many infected people never develop symptoms, but the possibility makes regular deworming of indoor cats important for the whole household.
Why Symptoms Are Easy to Miss
Adult cats with roundworms often look perfectly healthy. Kittens are more likely to show signs: a pot-bellied appearance, dull coat, vomiting, diarrhea, or visible worms in their stool. Adult cats with small worm burdens may shed eggs in their feces for months without any outward change in behavior or appetite. This is why routine fecal testing catches infections that owners would never suspect based on how their cat looks or acts.
Because roundworm eggs take two to four weeks to become infectious after being shed, keeping the litter box clean on a daily basis dramatically reduces the chance of reinfection and limits the risk to other pets and people in the home. Scooping every day removes eggs before they have time to mature, breaking the cycle even if your cat is actively shedding them.
Practical Prevention for Indoor Cats
Regular deworming is the most effective prevention, especially for kittens. Most veterinary protocols call for deworming kittens multiple times during their first few months, then following up with periodic fecal exams or preventive treatments throughout adulthood. Many monthly parasite preventives cover roundworms alongside fleas and heartworm.
Beyond medication, a few habits reduce exposure. Removing shoes at the door keeps contaminated soil off your floors. Sealing gaps where mice or insects enter eliminates the transport hosts your cat would otherwise hunt and eat. Washing your hands after gardening or handling outdoor soil prevents you from inadvertently transferring eggs to surfaces your cat contacts. If your cat digs in houseplants, switching to decorative stones or covered pots removes that exposure entirely.