Light micrograph transverse section through muscle of a polar bear infected with Trichinella spirals. Trichina worm is a viviparous nematode parasite that occurs in rodents, pigs, bears and humans and causes the disease trichinosis. It is sometimes referred to as 'pork worm' as it is often found in undercooked pork. The photograph shows how the larvae encyst in a type of capsule in the muscle.