Dog roundworm (Toxocara canis), computer illustration. This is a dog parasite whose eggs are found in dog faeces. Humans, often children, can become infected with the eggs (which is known as toxocariasis) when coming into contact with an infested dog or contaminated soil. The swallowed eggs hatch in the intestines and the larvae then migrate to organs such as the liver, lungs, brain and eyes. Symptoms include asthma, seizures, pneumonia and blindness.