What exactly are roundworms and how do dogs get them?

Roundworms (Toxocara canis and Toxascaris leonina) are large, cream-colored intestinal parasites that can grow 3 to 5 inches long and live coiled in the small intestine, where they feed on partially digested food rather than blood. The CDC reports that Toxocara eggs can persist in soil for years, remaining infectious even after the original stool has decomposed, which is why yards, parks, and sandboxes stay contaminated long after visible waste is gone (CDC, 2023). Dogs become infected by swallowing these eggs from contaminated soil, by eating an infected rodent, or through dormant larvae that reactivate in a pregnant female and cross the placenta or mammary tissue to her puppies. Because of this transmission route, the American Veterinary Medical Association notes that nearly all puppies are born with some degree of roundworm infection or acquire it within the first few weeks of nursing (AVMA, n.d.).

Once eggs are ingested, larvae hatch in the small intestine and, in young puppies, migrate through the liver and lungs before being coughed up and swallowed again, maturing into egg-laying adults in as little as four weeks. In adult dogs, the immune system often walls off migrating larvae in muscle or other tissue, where they stay dormant for years — explaining why a female dog with no visible signs of infection can still pass worms to a litter during pregnancy (Merck Veterinary Manual, 2022).

What symptoms should make an owner suspect roundworms?

The clearest visual sign is finding a spaghetti-like worm in vomit or stool, often noticed during a diaper change for young puppies or a walk cleanup for adult dogs. Beyond visible worms, Cornell University's Riney Canine Health Center notes that heavily infected puppies commonly develop a pot-bellied appearance, poor growth despite a normal appetite, a dull or rough coat, and intermittent vomiting or diarrhea that may contain mucus (Cornell Riney Canine Health Center, 2021).

Larval migration through the lungs can also trigger a soft cough or mild respiratory signs in puppies during the early weeks of life, sometimes mistaken for kennel cough. Severe infections in very young or malnourished puppies can cause intestinal blockage, since a large mass of worms can physically obstruct the small intestine, a complication that can turn into a surgical emergency if the puppy stops passing stool and starts vomiting repeatedly (Merck Veterinary Manual, 2022).

How do veterinarians confirm a roundworm diagnosis?

Veterinarians typically diagnose roundworms with a fecal flotation test, in which a small stool sample is mixed with a special solution so that lightweight parasite eggs float to the surface and can be identified under a microscope. The Companion Animal Parasite Council recommends fecal testing at every puppy wellness visit, generally every two to three weeks starting at 2 to 3 weeks of age, because a single negative test doesn't rule out infection if the dog isn't yet shedding eggs (CAPC, n.d.).

Because immature larvae migrating through the body don't yet produce eggs, a puppy can be actively infected and still test negative on a single fecal exam, which is part of why veterinary guidelines call for empirical deworming of puppies rather than waiting for a positive test before treating (CAPC, n.d.). In dogs with respiratory signs from larval migration, a veterinarian may also use bloodwork or chest imaging to rule out other causes of coughing before settling on a roundworm diagnosis.

What's the actual treatment for a dog with roundworms?

Roundworm treatment relies on oral dewormers such as pyrantel pamoate, fenbendazole, or milbemycin oxime, which paralyze or kill adult worms so they pass out in the stool. Because these medications only kill worms present in the intestine at the time of dosing — not larvae still migrating through tissue — the Companion Animal Parasite Council recommends a series of treatments, typically starting at 2 weeks of age and repeated every two weeks until 8 weeks old, then transitioning to a monthly broad-spectrum preventive (CAPC, n.d.).

Most healthy puppies tolerate deworming medication well, with mild vomiting or loose stool as the most common side effects reported. In cases of heavy intestinal worm burden causing a partial blockage, a veterinarian may recommend supportive fluids or, rarely, surgery if the worm mass can't be cleared with medication alone (Merck Veterinary Manual, 2022).

What does day-to-day home care look like while a puppy is being dewormed?

Between doses, owners should pick up stool from the yard daily rather than weekly, since Merck Veterinary Manual notes that Toxocara eggs need roughly two to four weeks in warm, moist soil before becoming infectious, so prompt cleanup breaks the cycle before eggs mature (Merck Veterinary Manual, 2022). Puppies recovering from a heavy worm burden often benefit from small, frequent meals rather than one or two large ones, since a gut still irritated from parasites and dewormer can be prone to loose stool if overloaded.

Owners should also wash bedding, food bowls, and any toys the puppy has had close contact with in hot water, and keep other pets in the household current on fecal testing, since a puppy shedding eggs can recontaminate a shared yard within days of finishing treatment. Handwashing after cleanup, especially for children who help with puppy care, is a simple habit that meaningfully reduces the household's exposure risk (CDC, 2023).

How can owners stop roundworms from coming back or spreading?

Because Toxocara eggs can survive in soil for months to years and resist most household disinfectants, preventing reinfection depends on prompt removal of feces before eggs have time to become infectious. Monthly heartworm preventives that also cover intestinal parasites, such as those containing milbemycin oxime or moxidectin, provide ongoing roundworm protection for dogs that have completed their initial deworming series.

Roundworms are also zoonotic, meaning eggs accidentally swallowed by people — most often young children playing in contaminated soil or sandboxes — can cause a condition called visceral or ocular larva migrans, in which migrating larvae damage tissue including, in rare cases, the retina. The CDC recommends washing hands after handling soil or stool, keeping sandboxes covered when not in use, and having pets tested and dewormed regularly to reduce this risk to families (CDC, 2023).

Can adult dogs get roundworms, and is the outlook different for them?

Adult dogs can absolutely become infected, usually by ingesting eggs from contaminated soil or by eating an infected rodent, though their more mature immune systems often keep worm burdens lower and symptoms milder than in puppies. Merck Veterinary Manual notes that adult dogs frequently harbor dormant, encysted larvae in body tissues for years without visible illness, and these larvae can reactivate during pregnancy regardless of the dam's current fecal test results (Merck Veterinary Manual, 2022).

With appropriate treatment, the prognosis for roundworm infection is excellent in the vast majority of dogs — most clear the infection fully after the recommended deworming series and show no lasting organ damage. The main long-term risk is reinfection in dogs that spend time in parks, dog runs, or yards with poor sanitation, which is why veterinary parasitologists frame prevention as an ongoing routine rather than a one-time treatment (CAPC, n.d.).