Toxascaris leonina is a common parasitic roundworm found in dogs, cats, foxes, and related host species. Toxascaris leonina, or T. leonina, is an ascarid nematode, a worldwide distributed helminth parasite which is in a division of eukaryotic parasites that, unlike external parasites such as lice and fleas, live inside their host. The definitive hosts of T. leonina include canids (dogs, foxes, etc.) and felines (cats), while the intermediate hosts are usually rodents, such as mice or rats. Infection occurs in the definitive host when the animal eats an infected rodent. While T. leonina can occur in either dogs or cats, it is far more frequent in cats.