Parelaphostrongylus tenuis

Parelaphostrongylus tenuis or brainworm is a small parasitic nematode that infects the brain of many ungulates. Its natural host, the white-tailed deer, is unaffected by its presence; other species, however, suffer severe neurological damage that eventually leads to death. This disease is known as moose sickness for its frequent occurrence in moose sharing territory with white-tailed deer.

P. tenuis was identified as the cause of moose sickness — a disease known since 1912 — by Canadian biologist Roy Anderson in 1967.

Life cycle
The life cycle of P. tenuis begins when female worms lay their eggs in blood vessels in the venous sinuses and subdural space of a white tailed deer's brain. The eggs are swept up in the blood circulation and they reach the lungs, where they hatch into first-stage larvae. Occasionally, the larvae hatch in the brain tissue and enter the blood supply as larvae. In either case, once the larvae are in the lungs, they enter the air passages. The deer coughs up and swallows the larvae, which pass through the digestive tract and are excreted in the deer's feces.

The first stage larvae emerge from the manure and infect snails and slugs that crawl over the deer droppings. These gastropods are infected through their foot. Once in the gastropod, the larvae develop into their second and third stage. When a deer feeds on vegetation, an infected snail or slug may be inadvertently ingested. The third-stage larvae emerge in the stomach of the deer, and migrate through the deer's stomach lining, along the outside curvature of the deer stomach (or abomasum) and through the abdomen until they reach the spinal cord. Once in the spine, they crawl through to the brain, where they grow and the life cycle begins anew. In ungulates other than deer (i.e. aberrant hosts), the mature worms do not produce eggs, and the infected animal becomes severely ill. Classic aberrant hosts are llamas, sheep, moose and some exotic antelope.

The slugs or snails that serve as intermediate hosts are very small, usually less than 1 cm, and have a predilection to live in the vegetation that deer like to eat. Although the accidental ingestion of an infected slug or snail suggests deer infection is a rare occurrence, especially since P. tenuis has been found to only infect roughly one out of 1,200 snails and slugs in the Grand Marais, Minnesota area, eighty percent of white-tailed deer in the region nonetheless become infected during their first year of life.

Timeline
The time between infection with third stage larvae and completion of the cycle by passing first stage larvae in the feces is 80-90 days. This is called the prepatent period.

It takes 3 to 4 weeks for the first stage larvae to develop into an infective-capable third stage larvae inside the snail or slug.