Toxocara canis is a roundworm parasite that lives in the intestines of dogs. Adult females measure 6 to 10 cm long, while males are slightly smaller at 4 to 6 cm. The parasite is significant for human health because its microscopic eggs, shed in dog feces, can survive in soil for months or even years and cause a condition called toxocariasis if accidentally swallowed. Roughly 5% of the U.S. population tests positive for Toxocara antibodies, and global seroprevalence averages around 19%, making it one of the most common yet underrecognized parasitic infections worldwide.
How Dogs Become Infected
Dogs pick up Toxocara canis by swallowing embryonated eggs from contaminated soil or by eating smaller animals (rabbits, rodents, birds) that carry encysted larvae in their tissues. Once inside a dog’s gut, the eggs hatch and the larvae burrow through the intestinal wall. In puppies, the larvae take a roundabout path: they travel through the lungs, get coughed up, swallowed again, and finally mature into egg-producing adults in the small intestine. Older dogs can still harbor the parasite, but the larvae more often become dormant in body tissues rather than completing this full circuit.
The dormant larvae become a problem during pregnancy. In female dogs, late gestation reactivates arrested larvae, which cross the placenta and infect puppies before they are even born. This transplacental route is the major way newborn puppies acquire the parasite. A minor secondary route is through the mother’s milk. Because of these transmission pathways, virtually all puppies in endemic areas carry Toxocara canis unless they are dewormed early.
How Humans Get Infected
Humans are accidental hosts. The parasite cannot complete its lifecycle in a human body, but the larvae it produces still cause damage as they wander through tissues looking for a dead end. There are two main ways people become infected.
The most common route is swallowing embryonated eggs from contaminated soil. This happens most often with young children who play in parks, sandboxes, or yards where dogs have defecated. The eggs are invisible to the naked eye and stick to hands, toys, and surfaces. After being deposited in the environment, the eggs need 2 to 4 weeks to become infectious, but their tough outer shell lets them persist for months or years under favorable conditions. The second route is eating undercooked meat from animals that serve as intermediate hosts, such as rabbits or chickens carrying encysted larvae in their muscles.
What Happens Inside the Human Body
Once swallowed, the eggs hatch in the intestine and the tiny larvae (roughly 350 to 400 micrometers long) penetrate the gut wall. The bloodstream carries them to the liver, lungs, heart, brain, muscles, and eyes. Because humans are not the parasite’s natural host, the larvae cannot grow into adult worms. Instead, they migrate aimlessly through organs, triggering inflammation wherever they go.
Many infected people never develop noticeable symptoms. When illness does occur, it falls into three recognized patterns.
Visceral Toxocariasis
This form develops when larvae accumulate in major organs, especially the liver and lungs. Common symptoms include fever, coughing, wheezing, abdominal pain, and an enlarged liver. Blood tests typically show elevated levels of eosinophils, a type of white blood cell that responds to parasitic infections. Visceral toxocariasis is more common in young children, likely because they are more prone to ingesting contaminated soil.
Ocular Toxocariasis
When a larva reaches the eye, it can cause redness, irritation, seeing spots or flashes of light, an abnormally colored pupil, and vision loss. This form almost always affects only one eye. It tends to appear in slightly older children and young adults, and even a single larva can cause significant damage.
Covert Toxocariasis
A milder, harder-to-recognize form has been documented primarily in children. A study of pediatric patients found that cough, abdominal pain, headache, sleep disturbance, liver enlargement, and behavioral changes were all significantly associated with positive Toxocara antibody tests. The combination of abdominal pain, headache, and cough together was an even stronger signal than any single symptom alone. Eosinophil counts may or may not be elevated, which makes this form easy to miss. Researchers estimate that cognitive impairment in children accounts for over 50,000 disability-adjusted life years lost globally each year from toxocariasis, suggesting the subtle effects of infection may be more consequential than the dramatic organ-level disease.
How Toxocariasis Is Diagnosed
Doctors cannot simply look for eggs in a stool sample the way they would for other intestinal parasites, because Toxocara larvae do not mature or reproduce in humans. Instead, diagnosis relies on a blood test called an ELISA that detects antibodies your immune system produces in response to the larvae. The standard version of this test uses proteins secreted by Toxocara canis larvae and has a sensitivity of about 91% with a specificity of 86%. A positive result, combined with symptoms and a history of possible exposure (contact with dogs, soil ingestion, relevant travel), is usually enough to confirm the diagnosis.
Treatment
Visceral toxocariasis is treated with antiparasitic medications taken by mouth, typically for five days. These drugs work by killing migrating larvae. For ocular toxocariasis, the same antiparasitic approach may be used alongside treatments to manage inflammation in the eye, since the immune response to dying larvae can itself cause damage. The optimal treatment duration is not definitively established, and some cases require additional courses.
Many mild or covert infections resolve on their own as the larvae eventually die. Treatment decisions depend on symptom severity and which organs are involved.
Preventing Infection
Prevention starts with responsible pet care. The CDC recommends deworming puppies at 2, 4, 6, and 8 weeks of age, and treating nursing mothers every two weeks during the eight-week period after birth to eliminate reactivated larvae. Most veterinarians recommend an average of 3 to 4 deworming treatments during a puppy’s first 16 weeks, and about 65% advise prophylactic deworming of adult dogs roughly twice per year.
On the human side, the most effective measures are practical: washing hands after contact with soil or pets, covering sandboxes when not in use, picking up dog feces promptly (before eggs have time to become infectious), and cooking meat thoroughly. Because Toxocara eggs need at least two weeks in the environment to become capable of causing infection, prompt cleanup of dog waste is one of the simplest and most effective interventions.