Roundworms are the most common intestinal parasite veterinarians see in dogs and cats, and if you have ever raised a puppy or kitten, there is a very good chance your pet has had them. These spaghetti-like worms live in the small intestine, where they feed on partially digested food and can grow several inches long. Most infected adult pets look perfectly healthy, which is exactly what makes roundworms easy to overlook — and because they can also infect people, they are one parasite every pet owner should understand. The good news: roundworms are easy to diagnose, inexpensive to treat, and almost entirely preventable with routine care.
What Roundworms Are and How Pets Get Them
The roundworms that matter most to pet owners are Toxocara canis in dogs and Toxocara cati in cats, along with a less common species, Toxascaris leonina, that infects both. According to the Merck Veterinary Manual, adult worms live free in the small intestine and shed enormous numbers of microscopic eggs into the feces — a single female can produce tens of thousands of eggs per day. Those eggs become infective in the environment after a few weeks and are remarkably tough, surviving in soil for months or even years.
Pets pick up roundworms in several ways. The most common route in adult animals is swallowing infective eggs from contaminated soil, grass, or feces — often just from grooming dirt off their own coat. Dogs and cats can also become infected by eating a rodent, bird, or other small animal carrying larvae in its tissues.
Puppies have an additional route that explains why infection is nearly universal in young dogs: before they are even born. In pregnant dogs, dormant Toxocara canis larvae encysted in the mother's tissues reactivate late in gestation and cross the placenta into the developing puppies. Larvae also pass through the mother's milk during nursing. Kittens are not infected in the womb, but they can acquire Toxocara cati through their mother's milk. This is why veterinarians assume essentially every puppy and most kittens are infected and deworm them proactively rather than waiting for a positive test.
Signs of Roundworm Infection
Many infected pets, especially adults, show no signs at all — the infection is discovered only on a routine fecal exam. When signs do appear, they are most dramatic in puppies and kittens, whose small bodies carry a proportionally heavier worm burden. Watch for:
- A pot-bellied appearance — the classic swollen, rounded abdomen of a wormy puppy or kitten.
- Poor growth and dull coat — worms steal nutrients, leaving youngsters thin over the spine despite a good appetite.
- Vomiting or diarrhea — sometimes with visible worms, which look like strands of wet spaghetti, in vomit or stool.
- Coughing — migrating larvae pass through the lungs, and heavy infections can cause a cough or even pneumonia in young animals.
- In severe cases, intestinal blockage — a massive worm burden can physically obstruct a small puppy's or kitten's intestine, which is a medical emergency.
Diagnosis is straightforward: your veterinarian examines a stool sample under the microscope for eggs. Because eggs are not shed continuously, a single negative test does not fully rule out infection, which is one reason annual (or more frequent) fecal checks are standard preventive care.
Treatment: Simple, Safe, and Effective
Roundworms are among the easiest parasites to treat. Common deworming medications — including pyrantel pamoate, fenbendazole, and milbemycin — kill adult worms safely and inexpensively. The catch is that most dewormers only kill worms living in the intestine, not larvae migrating through the body. That is why treatment is always repeated, typically every two to three weeks, so that larvae maturing into adults after the first dose are caught by the next one.
The Companion Animal Parasite Council recommends starting puppies on deworming at two weeks of age, repeating every two weeks until they begin a monthly parasite preventive, and starting kittens at three weeks of age on a similar schedule. Nursing mothers should be treated alongside their litters, since they often reactivate infections during lactation. For adult dogs and cats, many monthly heartworm preventives conveniently contain an ingredient that also controls roundworms — one of several good reasons to keep pets on year-round parasite prevention.
Do not be alarmed if your pet passes visible worms in the stool after treatment; that is the medication working. Your veterinarian will usually recommend a follow-up fecal exam to confirm the infection has cleared.
Why Roundworms Matter for People Too
Roundworms are zoonotic, meaning they can infect humans. People — most often young children — become infected by accidentally swallowing infective eggs from contaminated soil, sandboxes, or unwashed hands after contact with dirt. In humans the larvae cannot mature into adult worms, but they migrate through the body, causing a condition called toxocariasis. According to the CDC, most human infections cause no symptoms, but in rare cases migrating larvae damage organs (visceral larva migrans) or reach the eye (ocular larva migrans), where they can cause permanent vision loss.
This sounds frightening, but the risk is easy to manage with basic hygiene: wash hands after handling soil or pets, keep sandboxes covered, pick up dog feces promptly, clean litter boxes daily (fresh eggs need days to weeks in the environment before they become infective), and keep children from playing where animals defecate. Deworming your pets on schedule is the single most effective way to protect your family, because pets that do not shed eggs cannot contaminate your yard in the first place.
Preventing Reinfection
Because roundworm eggs are so durable — they resist freezing, drying, and most disinfectants — preventing infection is far easier than decontaminating an environment. A few habits make all the difference:
- Keep pets on year-round broad-spectrum parasite prevention prescribed by your veterinarian.
- Schedule regular fecal exams — at least annually for adults, and several times during a puppy's or kitten's first year.
- Pick up feces promptly, from the yard and on walks, before eggs have time to become infective.
- Discourage hunting where possible; rodents and birds are a common source of infection for outdoor cats.
- Deworm new arrivals — every new puppy, kitten, or rescue pet should be tested and dewormed before joining the household.
Roundworms have been living alongside dogs and cats for as long as we have kept them, and no amount of good housekeeping makes a pet immune to exposure. But with a simple routine of prevention, fecal checks, and prompt treatment, they are a problem you can keep permanently off your pet's radar — and out of your home.