Toxocariasis is a parasitic infection caused by roundworms that normally live in the intestines of dogs and cats. When humans accidentally swallow the microscopic eggs of these parasites, the larvae hatch inside the body and migrate through organs and tissues, causing inflammation wherever they travel. About one in five children worldwide shows evidence of past exposure, making this one of the most common yet underrecognized parasitic infections globally.
How Humans Get Infected
Two species of roundworm cause toxocariasis: one found in dogs and one in cats. Adult worms live in the small intestine of these animals and shed eggs in their feces. A single infected dog can pass millions of eggs per day into the environment. Once in the soil, the eggs need a few weeks to mature into their infectious form, but after that they can survive for years. Soil in parks, playgrounds, sandboxes, and yards where pets defecate becomes a long-term reservoir.
You get infected by swallowing these eggs. This usually happens through contaminated dirt on unwashed hands, under fingernails, or on raw vegetables grown in contaminated soil. Young children are at the highest risk because they’re more likely to put their hands or objects in their mouths after playing in dirt. Eating raw or undercooked meat from animals that carry the larvae (like chicken or lamb) is another route. Drinking untreated water raises the risk as well.
Once swallowed, the eggs hatch in your intestine. But unlike in dogs and cats, the larvae can’t complete their life cycle in humans. Instead, they burrow through the intestinal wall and wander through the body, sometimes reaching the liver, lungs, eyes, or brain. They never mature into adult worms, so you can’t pass the infection to someone else.
Three Forms of the Disease
Visceral Larva Migrans
This is the most dramatic form and happens when large numbers of larvae migrate through internal organs, especially the liver and lungs. The immune system reacts intensely, surrounding each larva with clusters of inflammatory cells and forming tiny granulomas (nodules of inflamed tissue). Symptoms include fever, fatigue, coughing, wheezing, abdominal pain, and sometimes an enlarged liver. Most cases occur in children between ages one and four, particularly those with a habit of eating dirt or other non-food substances.
Ocular Larva Migrans
When even a single larva reaches the eye, it can cause serious damage. The larva triggers the formation of a granuloma on the retina, which can scar the tissue, distort vision, or cause retinal detachment. This form tends to appear in older children and typically affects only one eye. What makes it tricky is that there’s often no sign of infection elsewhere in the body, so it may initially be mistaken for other eye conditions or even a tumor.
Covert Toxocariasis
Many people with toxocariasis don’t develop the full-blown visceral or ocular syndromes. Instead, they experience subtler, chronic problems that are harder to pin down: persistent wheezing, reduced lung function, or recurrent respiratory symptoms that look like asthma. A large meta-analysis found that people with evidence of past Toxocara infection were over three times more likely to have asthma than those without. People with antibodies to the parasite also showed measurably reduced lung capacity compared to uninfected individuals, even after accounting for smoking, weight, and other factors. There’s also discussion in the medical literature about possible links to seizures and developmental delays in children, though that evidence is less definitive.
Who Is Most at Risk
A global analysis covering 40 years of data estimated that about 20.3% of children worldwide have antibodies to Toxocara, meaning they’ve been exposed at some point. The rates vary dramatically by region. In Southeast Asia, seroprevalence reaches roughly 37%, and in Africa about 32%. European and Eastern Mediterranean countries have lower rates, around 10 to 11%.
Several factors significantly increase a child’s risk. Living in a rural area roughly doubles the odds of infection. Close contact with dogs raises risk by about 80%, as does frequent contact with soil. Boys are slightly more likely to be infected than girls. Children in lower-income countries and communities with limited sanitation infrastructure face the highest burden, and the infection disproportionately affects populations where stray or untreated animals are common.
How Toxocariasis Is Diagnosed
There’s no simple stool test for toxocariasis because the larvae don’t mature or reproduce in humans, so there are no eggs to find. Instead, doctors rely on a blood test called an ELISA that detects antibodies your immune system produces against Toxocara larvae. The test uses proteins secreted by the larvae themselves, which helps distinguish a Toxocara infection from related parasites.
A high level of eosinophils (a type of white blood cell that responds to parasites) in the blood is a common clue that often prompts testing. However, ocular toxocariasis presents a diagnostic challenge: antibody levels in the blood can be low or even absent despite active disease in the eye. In those cases, fluid samples from inside the eye itself may reveal locally produced antibodies.
Treatment Options
Visceral toxocariasis is treated with antiparasitic medications, typically taken by mouth twice daily for five days. The same medications and dosing schedule apply to both adults and children. For severe cases with significant inflammation, anti-inflammatory treatment may be added to control the immune response, particularly when the lungs or other organs are heavily involved.
Ocular toxocariasis is more complicated. The same antiparasitic drugs may help with active disease, but managing the inflammation inside the eye is equally important to prevent permanent vision loss. Treatment decisions depend on where the granuloma is located and how much damage has already occurred.
The optimal duration of treatment isn’t firmly established. Five days is the standard recommendation, but some cases may require longer courses or repeated treatment.
Preventing Infection
Prevention starts with deworming pets. Puppies and kittens are especially likely to carry Toxocara and should be dewormed every two weeks starting at two weeks of age and continuing until about 16 weeks. Pregnant and nursing dogs and cats should also be dewormed regularly. Adult pets benefit from routine deworming at least once or twice after adoption or intake, with ongoing prevention as recommended by a veterinarian.
On the human side, the most effective steps are practical. Wash your hands thoroughly after touching soil, playing outside, or handling pets. Wash fruits and vegetables before eating them. Teach children not to eat dirt. Pick up pet feces from yards and public spaces promptly, before eggs have time to mature into their infectious form (which takes two to four weeks in the soil). Cover sandboxes when not in use.
Roundworm eggs are extraordinarily durable once they contaminate an environment. They resist most standard cleaning products and disinfectants, and they persist in soil for years. That’s why preventing contamination in the first place, through consistent pet deworming and feces removal, is far more effective than trying to clean up a contaminated area after the fact.