Toxocariasis - Causes of toxocariasis

Toxocariasis is caused by an infection of roundworm parasites.

Specifically, roundworms called Toxocara canis and Toxocara cati are responsible for most cases.

These parasites live inside the intestines of animals (often dogs, cats and foxes that appear healthy) and their eggs are passed in the faeces of these animals.

However, the eggs only become infectious after 10-21 days, so there is no immediate danger from fresh animal faeces. Once the eggs are passed into sand or soil they can survive for many months.

After the eggs become infectious, humans can become infected if contaminated soil gets into their mouth. Toxocariasis cannot be spread from person to person.

Once the eggs are inside the human body, they move into the bowel before hatching and releasing larvae (the earliest stage of development). These larvae can travel to most parts of the body. 

However, as humans aren't the normal host for these larvae, they can't develop beyond this stage and they don't produce eggs. This means the infection cannot spread between humans.