By John S. Powers, Area Wildlife Biologist, Division of Wildlife and Freshwater Fisheries

Most of us are familiar with worms, or, at least, we think we are. As children, worms lived in the ground, and were what we used as bait when we went fishing. They were either “yucky and gross” or “neat and cool,” depending on our perspective. As we got older, we learned that worms were also something that sometimes affected our dogs. We could get them as well if we didn’t wash our hands and eat right. Many of us had even heard of brain worms, but those only existed in science fiction movies and scary comic books. Wrong! Brain worms are among us and more common than you might imagine.    

Brain worms, also known as meningeal worms, or technically, Parelaphostrongylus tenuis, are long, threadlike, parasitic nematodes that commonly infect white-tailed deer. Historically, brain worms have occurred throughout eastern North America wherever the white-tailed deer has been found. While clearly a parasite of the deer, brain worms and white-tails have apparently coexisted long enough that a relationship has evolved that rarely causes significant harm to the deer host.

The white-tailed deer’s tolerance to infection by brain worms is beneficial to the parasite in that white-tails are the only species that allows the worms to complete their life cycle on a regular basis. Most, if not all, ungulates occurring in North American (moose, elk, mule deer, black-tailed deer, reindeer, goats, sheep, cattle, and even llamas and alpacas) may be infected by brain worms, but all are considered unsuitable hosts. Infection by brain worms in species other than white-tailed deer causes varying degrees damage to the host’s central nervous system (spinal cord and brain). The damage caused by the worms produces a variety of symptoms ranging from apparent lameness in one or more limbs, to blindness, paralysis and death. While, some animals other than white-tailed deer do survive infection, brain worms do not regularly complete their life cycle and successfully reproduce.

The brain worm’s life cycle, like that of many parasites, is more complicated than simply male plus female yields offspring. First, both male and female are not necessarily required. Female brain worms are capable of asexual reproduction in the event no males are present. Moreover, the life cycle cannot be completed wholly within the confines of the deer host. Adult brain worms live in the spaces between the deer’s brain and its skull and sometimes in the large veins nearby. Eggs are laid in these locations or are dumped directly into the blood stream. Eggs laid near the brain hatch and the larvae move into the circulatory system and travel to the lungs where they begin to mature. Eggs laid in the blood are also deposited in the lungs where they form cysts (lumps) until they hatch and begin the maturation process. Larvae from the lungs move up the windpipe to the mouth where they are swallowed, pass through the digestive system, and are passed from the body.

Once out of the deer, the larvae pass into body of one of many species of land snails and slugs that serve as intermediate hosts while the larvae mature further. Nearly mature brain worm larvae return once more to a white-tailed deer (ideally) when an infected snail or slug is swallowed by a feeding deer. Swallowed larvae pass through the lining of the deer’s digestive tract, along the surface of the internal organs, and into the spaces surrounding the deer’s spinal cord. Here, brain worm larvae complete maturation into adults, travel up the spine to the brain, and begin the cycle all over again.

While brain worm infection rarely results in serious harm to their white-tailed deer host, the negative effect they have on other ungulate species causes or may cause various problems. Infection of domestic livestock results in economic loss, sometimes significant, in some parts of the country. This problem may be minimized by refraining from pasturing animals in low, wet areas having higher snail/slug populations, treatment of pastures to reduce populations of these intermediate hosts, and by treating at risk animals to reduce or prevent infection.

The presence of brain worms also presents problems for other native deer species. It is believed, but not certain, that the elk and moose, which once lived in the eastern United States, must have had a greater tolerance to brainworm infection than do northern and western populations today. Consequently, brainworms can present problems when reintroduction of moose and elk in the eastern portions of their historic range are attempted. The incidence of this parasite has been implicated in the failure of several such attempts and is believed to have significantly hindered others.

Of potentially greater concern is the fact that the range of the brainworm has been expanding with the range of the highly adaptable white-tailed deer. This parasite was once confined primarily to the eastern half of the United States by unsuitable white-tailed deer habitat to the north (dense, unbroken coniferous forest) and unsuitable snail habitat (arid plains and desert) to the west. Human activities, primarily in the interest of agriculture and the timber industry have opened the northern forest, allowing white-tailed deer--and hence the brainworm--to expand their range. This expansion has been associated with reduction in moose and elk populations in these areas. More troubling is the likelihood that both white-tailed deer and brainworms will eventually move into the region west of the Great Plains. This is an area which is home to a diverse array of susceptible species which occur in habitat which will likely prove ideal for the invading white-tails as well. The eventual effects of such range expansion by the brainworm and its host are currently unknown.

The Alabama Department of Conservation and Natural Resources promotes wise stewardship, management and enjoyment of Alabama’s natural resources through five divisions: Marine Patrol, Marine Resources, State Parks, StateLands, and Wildlife and Freshwater Fisheries.