Brainworm is only one of many diseases moose are susceptible too. We'll be adding other diseases and parasites to this section over time. Brainworm is probably the disease most people have heard about in moose. Abnormal behavior by moose affected by brainworm is one reason for this. Over 60% of the cases of sick moose reported by the public in NE Minnesota from 2002 to 2008 were probably suffering from a brainworm infection, according to Mike Schrage. Another misconception about brainworm is that it is always fatal to moose, which is not true according to Dr. Erika Butler, MN DNR Wildlife Veterinarian. There are also other diseases, nutritional deficiencies, and toxicities that have symptoms similar to those displayed by moose infected with brainworm.

Parelaphostrongylus tenuis is a nematode that naturally parasitizes white-tailed deer and several species of gastropods (snails and slugs) throughout central and eastern North America. Both deer and gastropods must be present for P. tenuis to complete its life cycle. In areas where deer, gastropods and moose overlap in range, moose can become infected with P. tenuis. White-tailed deer are the natural hosts of adult P. tenuis and survive infection with few ill effects. Moose however, are incidental hosts of the adult worms and infection can lead to death.

Cow with brainworm

Adult cow moose in the final stages of a brainworm infection near the Echo Trail in St. Louis County, Minnesota. Picture taken November 2008. This cow clearly showed weakness in the hindquarters and inability to stand. Clinical symptoms of brainworm infection in moose include circling, weakness in the hindquarters or inability to stand as well as turning of the neck and head to one side, lethargy, apparent blindness, loss of fear and rapid eye movement (Lankester and Samuel 1998). Caribou and elk are also affected by P. tenuis.

P. tenuis life cycle: Adult worms live in the cranium of deer for several years, during which time they pass numerous eggs into the blood stream. Once in the blood the eggs become lodged in the lungs where they develop into first stage larvae. Deer then cough up the larvae and swallow them. After being swallowed the larvae are shed in the deer’s feces. The eggs can survive for extended periods of time on the feces, in some cases over winter, and are eventually encountered by one of several species of gastropod. Once P. tenuis penetrates the gastropod it continues to develop into different larval stages. Moose and deer cannot become infected with the first stage larvae, therefore the gastropods must be present to allow for further development. Deer and moose forage in areas where gastropods are present and occasionally incidentally ingest the snails or slugs that have the second stage larvae. Once the infected gastropod is eaten the P. tenuis larvae are released, penetrate the stomach wall and migrate into the nervous system. In the nervous system of a deer the larvae develop into adults and begin to pass eggs. In moose however, the adult worms can cause the above mentioned symptoms, eventually leading to death. The time until clinical symptoms are first observed in moose can be as little as 60 days (Anderson 1964).

P tenuis in deerP tenuis in moose

On the left is a picture of a P. tenuis worm on the surface of a deer brain. P. tenuis are on the brain surface in deer, while in moose it is rare to find P. tenuis on the brain surface. Instead, the worms move to different parts of the brain and this is what causes the symptoms in moose.

Effect of P. tenuis on moose populations.

The effect of P. tenuis on individual moose is clear. Even with a historical record of individual moose dying from P. tenuis, there is little evidence that P. tenuis is a major cause of mortality in moose or that it was responsible for historic declines in moose populations. When white-tailed deer expanded their range northward into moose range following logging, they undoubtedly introduced P. tenuis to moose. While it may seem intuitive that higher deer numbers should translate into higher moose mortality, research has not corroborated this relationship. Rather, it has discovered that the transmission of P. tenuis between deer and moose is a complex relationship and after almost 45 years, this relationship is still poorly understood.

According to a recent review, reductions in deer density on moose range will likely have little effect on the population status of moose in Minnesota (Lenarz 2009). At the same time, historical records indicate that moose declines over the past century have often been associated with deer densities higher than 5 deer per square km (about 13 deer / square mile) (Whitlaw and Lankester 1994). The question of P. tenuis effects on moose is not clearcut.

Further Reading

Lankester et al. (2007) on diagnosing brainworm in moose from Minnesota and Ontario as pdf from Alces Journal website.

Lenarz (2009) review of P. tenuis and deer and moose populations for MN DNR Research Summaries