Roundworms in Dogs and Cats: Symptoms, Treatment and Prevention

Roundworms are the most common intestinal parasite found in dogs and cats worldwide. Two species dominate: Toxocara canis in dogs and Toxocara cati in cats. These large, spaghetti-like worms can grow up to 18 centimetres long and live in the small intestine, where they feed on partially digested food and compete with your pet for essential nutrients. A single female roundworm can produce up to 200,000 eggs per day, which pass into the environment through faeces and can survive in soil for years.

"Roundworms are so prevalent that we assume nearly every puppy is born with them. Transplacental transmission means larvae cross from mother to puppies before birth, which is why deworming protocols start as early as two weeks of age." — Dr. Sarah Chen, DVM

What makes roundworms particularly successful as parasites is their complex lifecycle. In dogs, encysted larvae can lie dormant in muscle tissue for years, reactivating during pregnancy to infect puppies via the placenta (transplacental transmission) or through the mother's milk (transmammary transmission). In cats, transmammary transmission is the primary route. This means that even a well-treated mother can pass roundworms to her offspring — the encysted larvae are virtually impossible to eliminate completely from the mother's body.

Roundworm eggs are remarkably resilient. Once shed in faeces, they require two to four weeks in the environment to become infective (embryonate), but once they do, they can persist in soil, sandpits, and gardens for years. This environmental persistence is a major reason why roundworms remain so common despite routine deworming programmes. Understanding their lifecycle is the first step in protecting your pet from common parasites and keeping your family safe.

The symptoms of a roundworm infection vary significantly depending on the age and health of the animal and the severity of the worm burden. In adult dogs and cats with a mild infection, there may be no visible symptoms at all — the pet appears perfectly healthy while silently shedding thousands of eggs into the environment each day. This is one of the most insidious aspects of roundworms: by the time you notice signs, the infection is often well established.

In puppies and kittens, however, roundworms can cause dramatic and sometimes dangerous symptoms. The classic sign is a pot-bellied appearance — a visibly distended abdomen that seems disproportionate to the rest of the body. This swelling results from a combination of worm mass in the intestine and the gas and inflammation they produce. Affected puppies often have a dull, rough coat that lacks the healthy sheen you would expect.

Other common symptoms include intermittent vomiting (sometimes with visible worms — a startling sight for many owners), diarrhoea that may alternate with normal stools, weight loss or failure to gain weight despite a good appetite, and general lethargy. In heavy infections, you may notice whole worms passed in the faeces or vomit — they are white to pale yellow, round, and typically curled in a C-shape.

In severe cases, particularly in very young puppies, a massive roundworm burden can cause a potentially fatal intestinal blockage. Heavy infections can also trigger coughing as immature larvae migrate through the lungs during their lifecycle — a phase known as the pulmonary migration. If your puppy or kitten shows any combination of these signs, prompt veterinary attention is essential. Our new puppy checklist includes deworming as a day-one priority for exactly this reason.

Understanding how your pet acquires roundworms is essential to breaking the cycle of infection. There are four primary transmission routes, and most pets are exposed to at least one during their lifetime.

Mother to offspring. As mentioned, this is the most significant route in dogs. Encysted Toxocara larvae in the mother's tissues reactivate during pregnancy and cross the placenta into the developing puppies. Larvae also pass through the mammary glands into the milk. In cats, transmammary transmission is the dominant route. This is why virtually every puppy and many kittens are born already harbouring roundworm larvae, regardless of how well the mother has been dewormed.

Ingesting eggs from the environment. Roundworm eggs shed in an infected animal's faeces become infective after two to four weeks in the soil. Pets pick up these eggs by sniffing or licking contaminated ground, eating grass, or simply grooming their paws after walking through contaminated areas. Parks, gardens, and anywhere animals defecate are potential sources. The eggs are sticky and remarkably resistant to disinfectants, freeze-thaw cycles, and most environmental conditions.

Eating infected prey or transport hosts. Dogs and cats that hunt or scavenge can acquire roundworms by eating infected rodents, birds, rabbits, or earthworms that carry encysted larvae in their tissues. These animals serve as 'paratenic' or transport hosts — the larvae don't develop further in them but remain infective. This route is particularly important in outdoor cats and dogs with strong prey drives.

Contaminated surfaces. Although less common, eggs can be carried on shoes, garden tools, and other objects. Indoor-only cats are not completely immune — owners can inadvertently bring eggs into the home. Following a consistent preventative care schedule is the most reliable way to keep roundworm burdens under control, regardless of your pet's lifestyle.

Roundworms are not just a pet health issue — they pose a genuine risk to human health, particularly for young children. When humans accidentally ingest embryonated Toxocara eggs (typically from contaminated soil, unwashed vegetables, or hand-to-mouth contact after touching contaminated surfaces), the larvae hatch in the intestine and begin migrating through the body. Unlike in their natural hosts, the larvae cannot complete their lifecycle in humans, so they wander through tissues causing inflammation and damage — a condition known as toxocariasis.

There are two main forms of human toxocariasis. Visceral larva migrans (VLM) occurs when larvae migrate through major organs including the liver, lungs, and brain. Symptoms can include fever, fatigue, coughing, wheezing, abdominal pain, and enlarged liver. In most cases the infection is self-limiting, but in heavy exposures it can cause significant organ damage. Ocular larva migrans (OLM) is rarer but more serious — a single larva migrating into the eye can cause inflammation, retinal damage, and in severe cases, permanent vision loss in the affected eye.

Children aged one to five are at highest risk because they are more likely to play in soil, put dirty hands in their mouths, and have closer contact with pets. Studies have found Toxocara eggs in 10 to 30 percent of soil samples from public parks and playgrounds in the UK and US. The CDC estimates that approximately 5 percent of the US population has antibodies to Toxocara, indicating past exposure.

Prevention in humans centres on the same strategies that protect pets: regular deworming of dogs and cats to reduce environmental egg contamination, prompt faecal cleanup in gardens and public spaces, hand washing after handling soil or pets, covering sandpits when not in use, and teaching children good hygiene habits. Keeping your pets on a consistent deworming programme is one of the most effective public health measures a pet owner can take.

The good news is that roundworm infections are straightforward to treat with safe, well-established medications. Your veterinarian will typically prescribe one of three broad-spectrum anthelmintics (deworming drugs), depending on your pet's age, weight, species, and whether other parasites need to be addressed simultaneously.

Pyrantel pamoate is one of the most commonly used dewormers for roundworms. It works by paralysing the worms, which are then passed naturally in the faeces. Pyrantel is very safe, even for young puppies and kittens from two weeks of age, and is available as a palatable liquid or chewable tablet. It is effective against adult roundworms but does not kill migrating larvae, which is why repeat dosing is necessary.

Fenbendazole (brand name Panacur) is a broad-spectrum dewormer effective against roundworms, hookworms, whipworms, and certain tapeworms. It is typically administered for three to five consecutive days and is very well tolerated. Fenbendazole has the advantage of killing some larval stages as well as adults, making it particularly useful for heavy infections or when multiple parasite species are present.

Milbemycin oxime is found in many monthly heartworm preventatives (such as Sentinel and Interceptor). It kills roundworms along with hookworms and whipworms, providing ongoing protection as part of a monthly prevention regimen. This makes it an excellent choice for long-term roundworm control.

After treatment, you may see dead or dying worms in your pet's faeces — this is completely normal and a sign the medication is working. A follow-up faecal test two to four weeks after treatment confirms the infection has been cleared. Because medications kill adult worms but not all larval stages, repeat treatments at two to three week intervals are standard to catch larvae as they mature into adults.

Prevention is always preferable to treatment, and a consistent deworming schedule is the cornerstone of roundworm control. The recommended protocol varies by age and risk factors, but the following guidelines represent current veterinary best practice.

Puppies and kittens: Begin deworming at two weeks of age, then repeat every two weeks until eight weeks old. From eight weeks to six months, deworm monthly. After six months, transition to the adult schedule. This aggressive early protocol accounts for the near-certainty of congenital infection and the rapid lifecycle of roundworms in young animals.

Adult dogs and cats: Deworm at least every three months (four times per year) as a minimum. Dogs and cats with higher exposure risk — those that hunt, live in multi-pet households, have access to areas frequented by wildlife, or live with young children — may benefit from monthly deworming. Many modern monthly heartworm preventatives include a roundworm-active ingredient, providing continuous protection.

Breeding females: Deworm with fenbendazole daily from day 40 of pregnancy through to two days post-whelping. This protocol has been shown to significantly reduce the larval burden passed to puppies, though it cannot eliminate transmission entirely.

Environmental management is equally important. Pick up faeces from your garden promptly — ideally within 24 hours, before eggs have a chance to embryonate. Wash hands after gardening or handling soil. Keep sandpits covered. In multi-pet households, maintain all animals on a regular deworming programme to prevent re-infection cycles.

Annual or biannual faecal testing is recommended even for pets on regular preventatives, as no deworming programme is 100 percent effective. Early detection of breakthrough infections allows prompt treatment before significant environmental contamination occurs. Speak with your vet to tailor a deworming plan to your pet's specific lifestyle and risk factors.