A third way exotic diseases of animals can be introduced is through introduction of infected animals. These new infected animals are then able to infect other animals in the same area through direct or indirect contact, infection of vectors native to the area, or by contamination of the environment. Introduction of infected animals is thought to have been responsible for bringing CSE to native Canadian caribou.

Cerebrospinal Elaphostrongylosis (CSE) is a severe neurological disease of cervids, sheep, and goats caused by the nematode Elaphostrongylus rangiferi.

This disease, which affects caribou and reindeer, was first recognized in Newfoundland in the 1970’s. E. rangiferi was introduced to Newfoundland in the early 1900’s when infected reindeer were imported from Norway to establish herds for food and draft animals. Once in Newfoundland, the reindeer traveled across native caribou range and some escaped, most likely commingling with caribou. At present, most of the caribou herds in Newfoundland are currently infected with E. rangiferi although mainland herds appear to remain free of infection.

Elaphostrongylus rangiferi has an indirect life cycle. The infected caribou or reindeer are definitive hosts that shed larvae in their feces. The larvae penetrate the footpad of a snail or slug intermediate host to develop into the infective stage. Caribou or reindeer then ingest infected snails or slugs while feeding. Once in the host, the parasite migrates through the spinal cord and brain before reaching the muscles of the shoulder and hindlimbs where they mature. Eggs passed by adults are carried via the bloodstream to the lungs where they hatch into larvae, cross the alveoli, travel up the respiratory tree, are swallowed, and are then excreted in the feces.