There are so many reasons to teach young children to keep their fingers out of their mouths and not to eat dirt. Add the fact that there are so many dogs and puppies using our yard for a bathroom; your own, a neighbors, even a stray. Check https://www.familyfriendlyworking.co.uk/2013/07/20/prevent-your-children-picking-up-toxocara/.
In the United States, most puppies and many adult dogs are infected with the roundworm, Toxocara canis.
Infection of the adult dog with this parasite happens when they ingest eggs from the contaminated environment. The puppies usually get this parasite from the mother before birth through her milk. The puppies will start passing eggs in their feces by 3 weeks of age.
This parasite can be spread to humans by ingestions of eggs from the soil or from contaminated hands. Most at risk are young children under 5 years of age, especially those with a history of eating dirt and exposure to puppies.
One study showed that 20-60% of soil samples from backyard residences, public parks and children’s sandboxes were contaminated with Toxocara canis eggs. Though it is found throughout the U.S., diagnosis of disease in children is made most frequently in the south.
Light infections in humans are usually asymptomatic, and probably occur quite frequently.
Human infection with Toxocara usually takes one of two forms; visceral larval migrans (VLM), this is where the larvae migrate aimlessly. They usually end up migrating to the liver, but almost any tissue can be invaded.
The other more rare disease is ocular larval migrans (OLM), where the larvae enter the eye where permanent vision loss is possible.
The diagnosis of Toxocara is usually based on clinical signs, history of exposure to puppies, laboratory findings (including eosinophilia), and the detection of antibodies to Toxocara. Stool samples are not useful for examination since the larvae don’t mature to adults in humans. Anti-parasitic drugs are available for treating VLM. You cannot treat OLM with antiparasitics and is usually left to trying to prevent more damage to the eye.