Most cases of toxocariasis develop when someone handles contaminated soil and then transfers roundworm eggs into their mouth.
Soil becomes contaminated when an animal infected with the parasite (usually a dog, fox or cat) passes faeces that contain roundworm eggs into soil or sand. The eggs are produced in large numbers by the roundworm parasites that live in the animal's intestines.
The eggs have a tough outer shell and can survive for up to five years in sand or soil.
Once the eggs are inside the human body, they move into the person's bowel before hatching and releasing parasite larvae (the earliest stage of development). The larvae can travel to all parts of the body, but since humans aren't the normal host, the larvae don't develop into adults or invade the intestine.
