The moose and the snail – an ecological wonder

Submitted by Dennis King

Early in the morning a couple of weeks ago, a moose behaving very strangely appeared on Mountainview Road, near McBride. Sometimes it stood still for long periods with its head up and tilted to one side. Sometimes it walked round and round in small circles stumbling over things in its path again and again. While circling, it scrambled over the top of a small flat-bed trailer several times. Then it went in a different direction and walked into an eight foot high pile of logs which it clambered over. The obvious conclusion was that the unfortunate animal was essentially blind. The other conclusion that could be drawn from its strange behaviour, especially the circle walking, was that it had inadvertently eaten a small snail while grazing sometime in the past.

“Are you serious? A small snail! Is this some silly folk lore or bush tale?”

I am indeed serious. It is all due to a very small creature which leads a complicated life and has a very long name: Parelaphostrongylus tenuis. Let’s make it easier and just call it the brain worm. The brain worm is a nematode or round worm and it is a parasite that normally spends part of its complicated life cycle living in a snail and part in a white-tailed deer.

“I thought you were talking about a moose.”

Be patient; I’ll get back to the moose soon. In the meantime, let’s follow P. tenuis a.k.a. the brain worm. The brain worm prefers to spend its adult life located on the meninges, the membranes covering the brain, of a white tailed deer. There it can feed by absorbing the body fluids of the deer while continuously laying eggs. A brain worm can live for several years in a white tailed deer and so can lay very many eggs over time. Each of the eggs hatches into a tiny larva which finds its way into the deer’s blood stream and so into the deer’s lungs. The deer coughs up the larva and then swallows it. The larva is resistant to digestion by the deer’s stomach and so finds itself deposited on the ground along with the deer’s faeces. Some snail species have quite a liking for feeding on the mucus covering of the deer’s dung and so, if the larva is lucky, it will have a chance to hop aboard a snail and burrow into its insides.

Once inside the snail, the brain worm larva matures through two more larval stages. When it reaches the final larval stage it is ready to move on to complete its life inside a white tailed deer. It will get a chance to do this if it is fortunate enough to be inside a snail which gets swallowed by a deer along with the plant material the deer is feeding on.

The deer’s digestion mushes up the snail and releases the larva. The larva burrows through the stomach wall of the deer and migrates to the covering membranes of the deer’s brain where it matures into an adult and lays eggs.

“So does this bother the deer?”

It doesn’t seem to bother the white tailed deer much at all which is perhaps not surprising as the adult worm is just sitting in a relatively harmless location between the brain and skull. Many parasites have evolved so that they do not harm their host since if the host dies, then the parasite dies too. It has been estimated that more than 80% of adult deer are infected with brain worm in some white tailed deer populations.

“Okay, so what about the moose?”

Well, it seems that once again it is we that have messed things up. What seems to have happened is that with deforestation, agriculture and global warming, the white tailed deer populations have been extending gradually northwards since the early 1900s and now overlap with moose and mule deer populations. Of course, snails infected with brain worm larvae can be swallowed by moose and mule deer in the same way as with white tailed deer. The problem is that, while the moose and mule deer stomachs are similar enough to that of a white tailed deer, the larva is triggered to migrate through the stomach wall and the larva is on unfamiliar territory from then on. The chemical signals or whatever it uses to find its way to the brain meninges in white tailed deer are not the same in the moose. The brain worm larva is somewhat lost. It usually finds its way to the brain and grows into an adult worm. But the disoriented worm keeps on migrating through the brain and central nervous system of the moose. The circle walking and the blindness seen in the Mountainview Road moose a couple of weeks ago are typical symptoms. Some moose lose the use of their hind legs too. Though the moose may temporarily recover somewhat as the worm moves around, the end result is almost always death. Occasionally mule deer can be similarly affected.

There is one little bit of good news in all of this. It is only brain worms living in white tailed deer that can pass on larvae.

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