What exactly are roundworms and why are they so common in cats?

Toxocara cati and, less often, Toxascaris leonina are the two roundworm species that infect domestic cats, and they are considered the most prevalent intestinal parasite in the species worldwide, with prevalence estimates in some kitten populations exceeding 25% (Companion Animal Parasite Council, 2023). Adult worms live free-floating in the small intestine, can reach 3 to 5 inches long, and feed on partially digested food rather than blood, which is part of why chronic infections in adult cats can go unnoticed for months. The Cornell Feline Health Center notes that a heavily infected cat can pass thousands of eggs per gram of feces daily, which is what makes environmental contamination such a persistent problem in multi-cat households and outdoor colonies (Cornell Feline Health Center, 2021).

Eggs shed in feces are not immediately infective; they need roughly two to four weeks in soil to develop into an infective larval stage, after which they can remain viable in the environment for years because of a resistant outer shell (Merck Veterinary Manual, 2022). This environmental durability explains why roundworms show up even in cats that never leave a house with a sandbox-style litter substrate previously used outdoors, and why simply deworming once rarely solves an ongoing problem in a contaminated environment.

How do cats actually catch roundworms in the first place?

Kittens most often become infected transmammary, meaning dormant larvae stored in the queen's tissues reactivate during late pregnancy and pass to nursing kittens through her milk, which is why kittens can test positive as early as three to four weeks of age even if they've never eaten solid food (American Association of Feline Practitioners, 2020). Adult cats are typically infected by ingesting infective eggs from contaminated soil or litter, or by eating a paratenic host such as a mouse, bird, or earthworm that is carrying dormant larvae in its tissues.

Outdoor and hunting cats carry substantially higher infection risk for this reason, since rodents and birds act as a bridge that concentrates larvae from the environment into a single meal (CDC, 2023). Because eggs are sticky and can cling to paws, bedding, and litter scoops, indoor multi-cat households can also sustain low-level transmission for a long time without anyone realizing an infected cat is reseeding the environment.

What symptoms do owners actually notice at home?

In kittens, roundworm burden can be severe enough to cause a pot-bellied appearance, poor weight gain despite a normal appetite, dull coat, and intermittent diarrhea or vomiting, and the Cornell Feline Health Center lists failure to thrive as a hallmark finding in heavily parasitized litters (Cornell Feline Health Center, 2021). Occasionally a kitten or cat will vomit up an intact adult worm, which is often the single event that prompts an owner to call the veterinary clinic.

Adult cats usually tolerate lower worm burdens with few or no visible signs, but a heavy infection can still cause soft stool, weight loss despite normal eating, and occasional coughing if migrating larvae pass through the lungs during their developmental cycle. Because clinical signs are often mild or absent in adults, the American Association of Feline Practitioners recommends fecal screening rather than waiting for symptoms to appear before assuming a cat is worm-free (American Association of Feline Practitioners, 2020).

How does a veterinarian confirm it's roundworms and not something else?

Diagnosis is typically made through a fecal flotation test, in which a stool sample is mixed with a dense solution that causes the lightweight roundworm eggs to float to the surface where they're visible under a microscope; this remains the standard, low-cost screening method recommended for every kitten visit and at least annually for adult cats (Companion Animal Parasite Council, 2023). Because a fecal flotation only detects eggs, not larvae or immature worms, a single negative test in a young kitten doesn't fully rule out infection, and vets will often repeat testing or treat empirically based on age and exposure risk.

Occasionally an adult worm is identified visually in vomit or stool, which gives an immediate diagnosis without waiting on lab results. Veterinarians also factor in the cat's history — nursing status, outdoor access, and hunting behavior — since these details shape both diagnostic suspicion and how aggressively to recheck after treatment.

What treatment actually gets rid of roundworms?

Roundworms are treated with oral anthelmintic drugs such as pyrantel pamoate, fenbendazole, or milbemycin oxime, all of which are effective against adult worms in the intestine but do not kill immature larvae encysted in tissue, which is why a single dose is never sufficient (Merck Veterinary Manual, 2022). Standard protocol for kittens calls for deworming starting at two to three weeks of age and repeating every two to three weeks until they are old enough for monthly preventive parasite control, since new larvae continue maturing into egg-laying adults between doses (Companion Animal Parasite Council, 2023).

Many monthly heartworm preventives, including those containing milbemycin oxime or selamectin, also control roundworms as a secondary benefit, which is one reason veterinarians push year-round parasite prevention rather than symptomatic deworming alone. Reinfection is common in cats with continued outdoor access or hunting behavior, so follow-up fecal testing four to six weeks after treatment is the only way to confirm the worms are actually cleared rather than just reduced.

What can owners do at home to stop reinfection from happening again?

Daily litter box scooping is one of the most effective home interventions because it removes eggs before they've had the one-to-four-week window needed to become infective, cutting down environmental buildup significantly (Cornell Feline Health Center, 2021). Washing litter boxes with hot, soapy water and avoiding letting cats hunt or scavenge dead rodents further reduces the paratenic-host route of infection.

For households with kittens or immunocompromised cats, the CDC recommends handwashing after litter box cleaning and keeping sandboxes covered when not in use, since Toxocara cati eggs are also zoonotic and can cause visceral or ocular larva migrans in people, particularly young children who play in contaminated soil (CDC, 2023). Routine, veterinarian-guided deworming on a set schedule, rather than reactive treatment only when worms are seen, is the most reliable long-term strategy.

Can roundworms in cats make people sick too?

Yes — Toxocara cati is zoonotic, and the CDC identifies toxocariasis as one of the five neglected parasitic infections it prioritizes for public health action in the United States, with infection occurring when a person accidentally ingests infective eggs from contaminated soil, sandboxes, or unwashed hands (CDC, 2023). In most healthy adults, human infection is mild or asymptomatic, but larvae migrating through tissue can occasionally cause organ or eye damage, which is why pediatricians and pest-control guidance both flag outdoor sandboxes and gardens as risk areas in homes with cats.

Regular deworming of household cats, prompt disposal of feces, and basic hygiene after handling litter meaningfully reduce this risk, and the CDC specifically calls out covering sandboxes and children's play areas as a practical prevention step for households with outdoor cats (CDC, 2023).