Because the Moose and the White tail deer historically occupied separate and distinct geographic regions(Moose more colder and Northern and Deer Southern and warmer) up until the last 300 years, the parasitic brainworm which is harmless to the deer can and does bring death to the Moose(which did not evolve with this parasite)..................We have previously reported that a combination of warming temps, increased Winter tick densities and deer brainworm are having devastaing impacts on Moose who reside in and around the Great Lakes...............For some reason, the returning New England and New York Moose population is expanding and seemingly getting past these dampening agents which are plaguing their Great Lakes cousins

Moose Suffers From Cousin's Parasite

by Li Shen |  

Our reaction to the word 'parasite' is usually one of disgust. After all, aren't parasites the creepy, revolting little creatures that burrow into animals' bodies and spread through unclean substances like blood, guts, and excrement? Don't parasites kill things? Not when they're on their home turf. A parasite's mission is to procreate without doing lethal damage to the host animal essential for its shelter and sustenance. In fact, most parasites have co-evolved with their hosts for millennia and have established a biological truce: you feed and house me, and I won't kill you. If this weren't true, either the host or parasitic species would have already died out.

It can be a different story when a parasite ends up inside a host animal where it didn't evolve. Parasites look for all sorts of clues inside their hosts to direct them to do the right thing at the right point in the parasite's life cycle. In the wrong host, when the clues aren't there, mayhem can ensue. Take, for instance, reports of moose behaving erratically, stumbling, walking in circles, or appearing either paralyzed or unusually tame. The cause of this moose disease was a mystery until a Canadian biologist discovered that the disease was caused by a parasitic worm (Parelaphostrongylus tenuis) more commonly found in white-tailed deer. The threadlike worms inhabit the space between the deer's brain and skull, where they live out their lives without producing obvious symptoms in the deer.

The brainworm has a convoluted life cycle. Female worms in the brain lay eggs that hatch into larvae that ride the blood stream to the lungs. The deer cough up the larvae into their mouths and then swallow them, where they travel through the digestive system and hit the ground in excrement. Then the larvae are eaten by small species of snails and slugs. Inside these secondary hosts, the larvae prepare for return to a deer. When a snail dining on foliage is inadvertently eaten by a deer, the larvae tunnel out of the deer's stomach, find the spinal cord, and travel back to the brain for another cycle. It takes about three months for the brainworm to complete its cycle inside the deer.

When a moose eats an infected snail, the parasite takes the same pathway from stomach to brain. However, the moose-brain environment isn't suitable for the worms to produce eggs. Instead, the moose becomes severely ill with neurological symptoms, and the worm eventually kills it. The same can happen with sheep, goats, and llamas, all of which are abnormal hosts for the moose brainworm. So why have moose never developed a tolerance for brainworm, as have deer? The answer lies in the fact that moose and deer haven't shared the same habitat for very long, in evolutionary terms.

The Latin name for white-tailed deer is Odocoileus virginianus, or Virginia deer. It is exclusively an American species that first appeared in the southernmost part of North America some four million years ago. As recently as 15,000 years ago, whitetails were confined to the southern end of their present range by the continental ice sheet and didn't return to northern areas until the glaciers receded.

Moose by contrast are a sub-arctic and boreal species with their origins in Asia. They are a relatively recent immigrant to North America, arriving during the same ice age, when a land bridge opened between Siberia and Alaska. Moose crossed east just before the time that deer started returning north.

In New England, the accounts of settlers show that moose were plentiful in the region in the 1600s. They were nearly extirpated, however, over the next two hundred years by excessive hunting and the clearing of forests for farmland. The latter circumstance favored the influx of deer, who were better adapted to the mixed landscape of forest and farm, though the deer, too, suffered from heavy hunting pressure and the loss of habitat. Finally in the twentieth century, with farm abandonment and reforestation across the region, moose have gradually returned, but to a patchwork landscape of fields and forests that supports more deer than were present in the 1600s. And where there are deer, there's brainworm.

Our understanding of the brainworm life cycle can help prevent infection of livestock. Excluding deer from pastures will keep the area clean of worm eggs. The snails that are the worm's intermediate hosts can be avoided by grazing livestock in or near wetlands only when snails are inactive – that is, after a killing frost or before the warm rains of spring. De-worming drugs can also help prevent infection.

In the long term, natural selection is at work, now that moose and brainworm have overlapping ranges. Perhaps there will come a day, thousands of generations in the future, when moose can tolerate the parasite the way their deer cousins already can.

Li Shen is an adjunct professor at the Dartmouth Medical School and the chair of the Thetford, Vermont, Conservation Commission.