Invasion of the Brain Worm, Parelaphostrongylus tenuis, a threat to the survival of the Moose species in North America.

By Kaysey Ferris

Background: Parelaphostrongylus tenuis (Brian worm) is a species of parasitic roundworm that have a threadlike structure. These parasites are commonly found within white-tailed deer populations. However other species are susceptible including moose, elk, caribou, mule deer, fallow deer, bighorn sheep, pronghorns, domestic sheep, goats, llamas, camels, guinea pigs, and to a lesser extent domestic cattle. Humans are not at risk of contracting this parasite.

History: The effects of Brain worm in moose was first reported in Minnesota in 1912, followed by two major declines in the moose population, the first from 1925-27 and the second occurring in 1933-34. A meningeal worm was identified as the main contributor to the disease in 1963, and later classified as Parelaphostrongylus tenuis in 1971.

Distribution: Parelaphostrongylus tenuis are currently found within eastern and central Canada stretching into eastern and central North America. However as white-tailed deer populations continue to grow and move into new areas, the range of this parasite expands as well.

Key Words: L1-L4 larvae stages 1-4, Definitive host: supports the adult or sexually reproductive form of a parasite, Intermediate hosts: required by the parasite to undergo development to reach sexual maturity.

Transmission: The life cycle of P.tenuis takes place over 82 to 91 days. Within its definitive host a white-tailed deer, the life cycle begins when an infected intermediate gastropod host (snail or slug) is consumed during grazing. The intermediate hosts obtain the original infection when L1 larvae penetrate the foot of a gastropod. These larvae grow into their L3 stage within the gastropod over a three to four-week period but require a definitive host to complete their life cycle. During digestion the larva are released and will penetrate the abomasal wall of the stomach within 10 days after ingestion. They then travel from the peritoneal cavity along lumber nerves into the central nervous system. Once they enter the neural tissue, development into their L4 stage occurs within the dorsal horns of the spinal cord. After about 25 days the L4 larvae migrate from the neural tissue to the subdural space were the they develop into their adult stage.

As adults, P.tenuis will migrate to different areas of  the brain and spinal cord to deposit their eggs. This includes areas such as the venous sinuses within the cranium, the meningues, and the venous circulation (an interconnected system of veins and sinuses) where the eggs are moved to the heart and lungs. Eggs that are lodge into the lungs develop into fibrous nodules and embryonate into the first larvae stage, L1. The L1 larvae then move to the alveoli (tiny air sacs) in the lungs which allow for rapid gaseous exchange, where they are coughed up into the mouth and swallowed, and later released from the deer in the mucous coating of their feces. An image of the life cycle is proved in figure 1 below.

Figure 1 Life cycle of P.tenuis

Impact on Moose: Within an abnormal host such as a moose, they will become infected the same way as a white-tailed deer. However, the nematodes (roundworm) damage the nervous tissue through inflammation, manipulation, or mechanical destruction. This damage leads to a wide range of abnormal behaviors exhibited by the animal with a neurological disease. These symptoms include weakness, loss of coordination, head tilt, apparent blindness, circling, loss of fear, depression, inability to feed, weight loss, and finally paralysis and death. Moose populations in certain areas of North America have diminished by almost 50 percent. Brian worm is one factor believed to have contributed to this decline. This is due to the parasites ability to infect and kill moose of various ages of either sex.

Diagnosis: A definitive diagnosis cannot be determined from just witnessing the abnormal behaviors in an animal that is suspected of being infected. There also needs to be a necropsy (animal autopsy) were the presence of adult P.tenuis is confirmed within the spinal cord or the brain.

Importance: Even though P.tenuis causes little to no harm within white-tailed deer, it dose pose a danger to multiple species outside of their normal host. Due to their altered state of behavior, infected animals such as moose pose a danger to both themselves and humans. P.tenuis infections are also always fatal within abnormal hosts causing great damage to local populations that live alongside white-tailed deer.  

Treatment: Currently there is no form of drug treatment for P. tenuis once they have entered the central nervous system. Studies are currently underway involving the use of antihelmintics drugs (antiparasitic drugs) to manage infections in captive white-tailed deer.

Management:  According to researchers the white-tailed deer population has increased from about 500,000 in the early 1900s to about 30 million today. As this population continues to grow and spread into new areas it carries the parasite with it. Since there is no drug treatment or cure for P.tenuis the best way to manage infections in both normal and abnormal hosts is to control wild white-tailed deer populations and monitor their health.

References:

Murray W. Lankester. (2018). Considering Weather-Enhanced Transmission of Meningeal Worm, Parelaphostrongylus Tenuis, and Moose Declines. Alces : A Journal Devoted to the Biology and Management of Moose, 1.

Lankester, M. W. (2010). Understanding the Impact of Meningeal Worm, Parelaphostrongylus Tenuis, on Moose Populations. Alces, 46, 53–70.