Parelaphostrongylus Tenuis - Life Cycle

Life Cycle

The life cycle of Parelaphostrongylus 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. The first intermediate gastropod hosts include:

• Anguispira alternata
• Arion circumscriptus
• Discus cronkhitei
• Deroceras laeve - one of three most infected hosts
• Deroceras reticulatum
• Haplotrema concavum
• Mesodon thyroidus
• Stenotrema fraternum
• Triodopsis albolabris
• Triodopsis notata
• Zonitoides arboreus - one of three most infected hosts
• Zonitoides nitidus - one of three most infected hosts

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 Parelaphostrongylus 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.

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.

The worm is of no public health significance since it is not infective to humans, and meat of infected animals is safe for human consumption. The parasite may be of some importance to veterinarians since sheep and goats are susceptible.