Yes, Toxocara canis is zoonotic. This common dog roundworm causes a human disease called toxocariasis, and it is one of the most widespread parasitic infections people acquire from animals. Roughly 19% of the global population, an estimated 1.4 billion people, shows antibody evidence of exposure to Toxocara.
How Humans Get Infected
Humans are accidental hosts. Adult Toxocara worms live in the intestines of dogs, especially puppies, and shed microscopic eggs into the environment through feces. Those eggs need time in soil to become infectious, typically around 12 days in warm, humid conditions. Once mature, the eggs are remarkably hardy. Research in natural soil found that more than 70% of infectious Toxocara eggs remained viable after four years buried just a few centimeters deep.
You get infected by swallowing these matured eggs. That can happen by touching contaminated soil and then your mouth, eating unwashed vegetables grown in contaminated ground, or handling a dog whose coat carries eggs from the environment. The eggs hatch in your intestine, but unlike in dogs, the larvae can never develop into adult worms in a human body. Instead, they migrate through tissues like the liver, lungs, muscles, eyes, and brain, causing damage along the way.
Who Is Most at Risk
Children face the greatest risk, largely because of behavior. Young children play in dirt, use sandboxes in public parks, and are more likely to put their hands in their mouths without washing. In one study, 80% of children who tested positive for Toxocara antibodies did not wash their hands before eating. Boys tend to have higher infection rates than girls, likely reflecting more outdoor soil contact.
Owning a puppy under one year old nearly doubles the odds of a child testing positive. That lines up with what’s known about infection rates in dogs: puppies younger than six months carry the parasite at rates around 44%, compared to roughly 24% in older dogs. Puppies can be born already infected, since Toxocara larvae pass from mother to pup during pregnancy and through nursing.
What Toxocariasis Does in Humans
The disease takes several forms depending on where the larvae end up and how many you’ve swallowed.
Visceral larva migrans (VLM) is the most dramatic form. Larvae invade major organs, most often the liver and lungs but sometimes the heart or skeletal muscle. Symptoms include fever, coughing, wheezing, fatigue, weight loss, rashes, and an enlarged liver. Blood tests typically show a sharp spike in a type of white blood cell called eosinophils. VLM occurs most often in preschool-age children.
Ocular larva migrans (OLM) happens when a larva reaches the eye. It usually affects only one eye and can cause inflammation, vision loss, or in severe cases permanent blindness. Larvae form small granulomas inside the eye that have sometimes been mistaken for retinoblastoma, a type of eye cancer. OLM tends to appear in older children and young adults, and it rarely overlaps with the organ involvement seen in VLM.
Covert toxocariasis is a milder, harder-to-pin-down syndrome. Symptoms are nonspecific: low-grade fever, abdominal pain, lethargy, headaches. Some evidence links it to allergic disorders and possible cognitive or developmental effects in children. Because the symptoms are so vague, covert toxocariasis often goes unrecognized.
How It’s Diagnosed
You can’t diagnose toxocariasis by finding the parasite itself. The larvae are too small and too scattered in tissue to reliably biopsy. Instead, doctors rely on a blood test that detects antibodies your immune system produces against Toxocara proteins. The standard test, an ELISA, has a sensitivity around 78% for visceral disease and 73% for ocular disease at the CDC. Commercial versions perform similarly, with roughly 91% sensitivity and 86% specificity.
For suspected eye involvement, the blood test is less reliable: only about 45% of patients with confirmed ocular toxocariasis test positive in their blood. In those cases, testing fluid from inside the eye itself can provide a clearer answer. Diagnosis also leans heavily on the clinical picture: a child with eosinophilia, a history of contact with puppies or soil, and compatible symptoms points strongly toward toxocariasis.
Treatment
Antiparasitic medication is the standard treatment for visceral toxocariasis, typically taken by mouth twice daily for five days. Anti-inflammatory drugs, including steroids, are sometimes added to control the immune reaction the dying larvae trigger. For ocular toxocariasis, the priority is protecting the eye from further inflammatory damage, so steroid treatment (applied directly to the eye or taken systemically) plays a central role alongside antiparasitic therapy.
Most people with mild or covert infections recover without lasting problems. Ocular cases carry the highest stakes, since inflammation inside the eye can cause irreversible vision loss if not caught early.
Reducing the Risk
Prevention centers on two things: keeping dogs dewormed and limiting your contact with contaminated soil.
Puppies should be dewormed starting at two weeks of age, then again at four, six, and eight weeks, followed by monthly preventive medication. Nursing mothers should be treated at the same time as their litters. Adult dogs benefit from year-round monthly parasite prevention, or at minimum four treatments per year combined with regular fecal testing. Treating pregnant dogs during pregnancy can reduce the number of larvae that pass to puppies before birth.
On the human side, washing hands after playing outside or handling dogs, rinsing produce thoroughly, covering sandboxes when not in use, and promptly picking up dog feces all reduce exposure. Toxocara eggs are not immediately infectious when passed in fresh stool, so daily cleanup of your yard prevents eggs from maturing to the dangerous stage.