Toxascaris leonina is a common parasitic roundworm found in dogs, cats, foxes, and related host species.

Toxascaris leonina differs from other Toxocara in that the larvae do not migrate through the lungs; but rather, the entire developmental cycle occurs in the gut.

Animals may not show any outward symptoms of roundworms at all, or in other, usually more severe cases, animals may have diarrhea, vomiting, loss of appetite, experience thinning, dull coats, and in puppies or kittens, can develop distended abdomens, or "pot-belly" appearance.

Unlike T. canis and T. Cati, T. leonina is nonmigratory with its life cycle affecting only one organ.

Roundworm infections are treated with medication, called "de-wormers", and includes such drugs as fenbendazole, pyrantel, milbemycin oxime, and piperazine.

To prevent reinfection of parasitic roundworms, it is recommended that anything that the animal has been in contact with should be cleaned thoroughly or replaced, including bedding and kennels.

It is also strongly recommended that outside areas where defecation may occur be cleaned, as well as all feces removed daily from outdoor pet runs, crates, and the yard.