This infection is caused by ingestion of eggs from the dog and cat parasites called Toxocara canis and Toxocara cati, respectively. These worms inhabit the intestinal tract of infected dogs and cats, and their eggs appear in abundance in the feces of these animals. However, the parasites are not infectious until three weeks after they have been excreted -- so the major risk factor for this infection is not the animal feces but the soil into which the eggs have passed. Humans may ingest the eggs by eating unwashed raw vegetables. Young children with pica (eating inedible things such as dirt) are at highest risk, but this infection can also occur in adults. People can also become infected by eating raw liver. After ingestion, the eggs hatch into larvae in the gastrointestinal tract and are carried into the circulation and to various tissues -- primarily the lung, liver, and eyes. The brain, heart, and other organs can also be affected.
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