Roundworms in Dogs: Can They Infect Humans?

By Sarah Bennett, Certified Animal Nutritionist

Warning: Roundworms Are a Public Health Risk — Especially for Children

Toxocara canis roundworm eggs shed in dog faeces can survive in soil for years. Children playing in contaminated sandpits or gardens can accidentally ingest eggs, leading to a condition called visceral larva migrans — and in rare but devastating cases, ocular larva migrans, which can cause permanent partial or total blindness. This is not a theoretical risk: it happens every year.

Roundworms are among the most common parasites found in dogs worldwide, particularly in puppies. Most pet owners know their dog needs worming — but far fewer understand why it also matters for their own health and the health of their children. Toxocara canis, the dog roundworm, is a genuine zoonotic parasite with the potential to cause serious human disease. Understanding it is essential for any responsible dog owner.

What Are Roundworms?

Toxocara canis are large nematode worms that can grow to 10–18 cm long. They live in the small intestine of dogs, where they absorb nutrients directly from the host's gut. Adult females produce hundreds of thousands of eggs per day, which are shed in faeces and develop into infectious larvae in the soil over two to four weeks. Under the right conditions — warm, moist soil — these eggs can remain viable and infectious for up to ten years.

Almost all puppies are born infected. Toxocara larvae can cross the placenta during pregnancy and are also transmitted through the mother's milk. This is why routine worming of puppies beginning at two weeks of age, and continuing every two weeks until 12 weeks, is a standard veterinary recommendation.

Symptoms of Roundworm Infection in Dogs

In adult dogs with a healthy immune system, roundworm infections are often subclinical — meaning the dog shows no obvious signs. However, in puppies and immunocompromised dogs, the signs can be significant:

The pot-bellied appearance is classic in heavily infected puppies. The abdomen becomes visibly distended, giving the puppy a rounded, barrel-shaped look. This is caused by the worm burden combined with gut inflammation and gas.

Other signs include vomiting (sometimes bringing up intact worms that look like spaghetti), diarrhoea, slow growth, poor coat condition, and coughing. The cough occurs because roundworm larvae undergo a migration through the lungs — a stage called pulmonary migration — before returning to the intestines via the trachea and being swallowed. In heavy infections, this can cause pneumonia-like symptoms.

In severe cases, roundworm masses can cause intestinal blockage, which is a veterinary emergency requiring surgery.

Diagnosis and Treatment

Diagnosis is typically confirmed by faecal flotation — a lab test that identifies the characteristic eggs under a microscope. In puppies with visible worms in vomit or stool, diagnosis is immediate and obvious.

Treatment is straightforward. Fenbendazole (Panacur), pyrantel pamoate, and milbemycin oxime are all highly effective against adult roundworms. Most broad-spectrum dewormers available for dogs will cover Toxocara canis. Treatment typically needs to be repeated two to four weeks later to catch newly matured worms from larvae that were not killed in the first dose.

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Can Humans Get Roundworms from Dogs?

Yes — and this is where roundworm becomes a serious public health issue, not just a pet health concern.

Humans most commonly become infected by accidentally ingesting embryonated Toxocara eggs from contaminated soil, sandpits, or unwashed vegetables grown in contaminated ground. Young children are at highest risk because of hand-to-mouth behaviour during play.

Visceral larva migrans (VLM) occurs when ingested larvae hatch and migrate through human organs. In humans, Toxocara cannot complete its normal life cycle — instead, the larvae wander through the liver, lungs, brain, and other organs, causing inflammation and granuloma formation. Symptoms include fever, coughing, abdominal pain, and fatigue. Most cases resolve with antiparasitic treatment, but some cause lasting organ damage.

Ocular larva migrans (OLM) is a rarer but potentially devastating complication. Larvae that migrate into the eye can cause severe inflammatory reactions, retinal damage, and permanent vision loss — including complete blindness in the affected eye. OLM occurs in approximately 1 in 1,000 Toxocara infections in humans but represents one of the most preventable causes of childhood blindness in developed countries.

Hygiene Measures to Protect Your Family

The good news is that simple hygiene practices dramatically reduce the risk of human infection. Always wash hands thoroughly after handling dogs, cleaning up faeces, or gardening. Teach children never to eat sand or soil. Dispose of dog faeces immediately and properly — eggs are not immediately infectious when passed, but become so within days in warm conditions. Cover sandpits when not in use to prevent cats and dogs from defecating in them. Worm your dog regularly — at least every three months for adult dogs, more frequently for dogs with outdoor access or contact with children.

Key Takeaways

Roundworms (Toxocara canis) are extremely common in dogs, especially puppies, who are often born infected.
Symptoms in dogs include pot belly, vomiting worms, diarrhoea, coughing, and poor growth.
Roundworm eggs shed in dog faeces can survive in soil for years and infect humans who accidentally ingest them.
Children are at highest risk — ocular larva migrans can cause permanent blindness.
Regular worming of dogs and strict hand hygiene are the most effective ways to protect the whole family.

References

Overgaauw PA, van Knapen F. "Veterinary and public health aspects of Toxocara spp." Vet Parasitol. 2013;193(4):398-403. PMID: 23490540
Despommier D. "Toxocariasis: clinical aspects, epidemiology, medical ecology, and molecular aspects." Clin Microbiol Rev. 2003;16(2):265-272. PMID: 12692098

Written by Sarah Bennett, Certified Animal Nutritionist. This article is for informational purposes only and does not substitute professional veterinary advice.