Moose Walking in Circles

Clumsiness and disorientation may be signs that a moose is suffering from a potentially fatal disease.

By Bill Peterson
Illustration by Jim Freitag

People usually think of moose as powerful monarchs of their domain, probably because of their impressive size and the ease with which they travel through water, deep muck, and wind-thrown trees. But moose do have their problems. One of the most serious is "moose sickness," a debilitating and often fatal disease, seen in Minnesota's moose since at least 1912.

Signs of moose sickness include aimless walking in circles, holding head and ears in abnormal positions, frequent stumbling, poor coordination or balance, and apparent weakness or paralysis of a leg. Sometimes I have walked up to an apparently sick moose and shoved it; other moose have charged at me from 20 yards away. Once I tried to shoot a videotape of a moose charging someone, but the moose kept coming after me even though the other person was closer.

Animals showing such severe signs of disease will eventually die. Conservation officers and wildlife managers are often called on to end the animal's suffering. Sick moose can be a safety hazard along roadways.

People sometimes see a sick moose but do not report the sighting because they do not realize the animal is sick. Some sick moose have been photographed by people who have assumed the animal was especially tolerant of people. People have reported leg or hip injuries in moose that, in fact, were suffering leg paralysis from the disease.

Moose Brainworm

The cause of this disease eluded detection until 1964, when Canadian biologist Roy Anderson discovered it was caused by Parelaphostrongylus tenuis, a parasitic nematode, or roundworm, commonly referred to as brainworm. The adult brainworm is about 0.01 inch in diameter, 2 to 3 inches long, and is found in or near the brain.

The white-tailed deer is the normal host and is unaffected by the parasite. Severe neurological damage occurs in several species, including moose, caribou, elk, mule deer, llamas, domestic sheep, and goats. Brainworm is found only in central and eastern North America. It has not spread to the western part of the continent, even though whitetails occur in much of the West.

Brainworm evolved with the white-tailed deer. Whitetails gradually expanded their range to the West, but the parasite apparently did not survive the prairie conditions. (One reason to regulate interstate shipment of animals is to prevent establishment of parasites or disease in new areas. If someone transported an infected deer to the West, the brainworm could become established there.)

Female worms in the deer's head deposit eggs into the blood vessels of the brain. The eggs hatch into first-stage larvae while being carried to the lungs, where they leave the bloodstream and enter the air passages. Then the deer cough up the larvae, swallow them, pass them through the digestive tract unharmed, and excrete them on droppings. View diagram

The first-stage larvae then infect various species of land snails and slugs that crawl over the deer droppings or soil that contains larvae. Some species of snails and slugs are more readily infected than others, even if exposed to similar numbers of larvae.

In snails and slugs, the larvae go through second and third stages. When a whitetail, moose, or other susceptible ungulate feeds on vegetation and inadvertently eats an infected snail or slug, the third-stage larvae migrate through its stomach wall, up to the spinal column, then up to the brain.

One might think relying on deer to accidentally eat snails and slugs is an inefficient way of transmitting this parasite, but nearly all deer in Minnesota eventually become infected.

Peppered With Snails

Over the last decade or so, I have worked on several studies of brainworm with Murray Lankester, a parasitologist at Lakehead University in Thunder Bay, Ontario. Of more than 12,000 snails and slugs we collected near Grand Marais during the summer and fall of 1994, only 10 contained the parasite. This suggests that a white-tailed deer or moose in that vicinity eats an average of 1,200 snails and slugs before encountering an infected one. We also found that 80 percent of the white-tailed deer in that area picked up the parasite during their first year of life. Obviously, snails and slugs frequently occur on deer foods.

Just because deer acquire the parasite readily does not mean they continue to become infected with more worms. Our studies found that adult deer had, on average, the same number of adult brainworms as did fawns. The number is probably limited by the deer's immune system, which produces antibodies that fight third-stage larvae as they enter the body. However, the deer's immune system does not usually kill established adult worms.

Does the immune system of a moose also react to the parasite? It seems so. Lankester, in a study that has not yet been published, infected four moose calves with three to five brainworm larvae -- the number believed typical of natural infections. Initially the calves exhibited slight signs of the disease, but later the signs disappeared. Upon examination of the heads, researchers recovered one adult worm from each of three of the four calves and no brainworms from the fourth calf. Apparently the calves' immune systems killed most, and in one case all, of the infective larvae. Calves infected with more larvae exhibited severe neurological signs typical for moose with this parasite.

Moose calves probably have more poorly developed immune systems than do adult moose. We rarely see this disease in moose older than 2 years, and we don't know if the few cases in moose over age 2 are new infections or merely old ones becoming active again. Determining the resistance of adults to the parasite would require further study.

Unknown Encounters

What proportion of our Minnesota moose herd encounters the parasite? That important fact is not yet known. However, researchers are trying to develop a blood test to determine if a moose has been exposed to brainworm. Blood serum samples from about 200 moose were collected in northeastern Minnesota and northwestern Ontario and placed in long-term storage, ready for examination if a suitable test is developed.

Moose in some areas might run a greater risk of getting brainworm than do moose in other areas. For example, 43 percent of the sick moose I have handled in Cook and Lake counties over the last few decades were within three-quarters of a mile of Lake Superior; yet the moose population along the lake is much lower than it is several miles inland (less than one moose per 4 square miles, compared with one moose per square mile farther inland). Perhaps moose along Lake Superior are more likely to contract the disease because white-tail deer congregate along the shore in winter. Higher deer densities result in more first-stage larvae from deer pellets, which usually lead to more infected snails and slugs. Indeed, we found infected gastropods were twice as numerous, per area sampled, in a wintering area along Lake Superior as they were only a few miles inland.

New Conclusions

The relationship between brainworm in moose, moose mortality, and whitetail numbers is unclear, though brainworm does not appear to have a major influence on Minnesota's moose herds. A recent evaluation of current data and the historical record suggests that moose population declines once believed to have been caused by brainworm occurred only when deer densities exceeded 12 per square mile. However, moose populations did not always decline when deer densities exceeded that level. And sometimes populations declined though no one had reported diseased moose.

Most areas in Minnesota's main moose ranges (in the northeast and northwest) have whitetail population densities of fewer than 10 deer per square mile, with only a few small areas of 12 deer per square mile. Moose populations have fluctuated considerably over the years, even in areas with low deer populations. The moose population in northeastern Minnesota has been about 4,000 to 5,000 for the last 20 years. Moose in northwestern Minnesota were equally abundant until recently; at this time they have declined to about 1,200.

A research study is examining the possible causes of the decline in northwestern Minnesota. A few of the sick or dead moose examined had brainworm, but it still isn't clear what killed the others. Unless brainworm causes subtle, hard-to-recognize changes in moose -- such as inappropriate courtship behavior or inadequate care of calves, which could result in decreased reproductive success -- it is unlikely that it has had a major impact on northwestern Minnesota's moose herd.

We still have much to learn about the brainworm: Do certain habitat conditions result in "hot spotsÓ of infected snails and slugs? Do infected gastropods behave differently from uninfected ones, resulting in their being more likely to be ingested by a deer or moose? Do infected moose whose signs disappear succumb to the parasite later? Does the parasite cause subtle changes in behavior that may influence survival or reproductive success in moose that show only slight signs of sickness?

What we do know can help wildlife managers evaluate an area's suitability for both deer and moose and determine how many of each is appropriate. To set wildlife management priorities and population goals, managers need to recognize habitat conditions or other ecological situations likely to result in frequent transmission of this parasite. This knowledge can also help landowners who raise sheep, goats, or llamas, which are also susceptible to brainworm infection.

Bill Peterson is the Area Wildlife Manager in Grand Marais, where he has studied white-tailed deer, moose, and other wildlife for 28 years.