Tuesday, 22 August 2006

Diet of worms

In one of yesterday's posts I mentioned that Angiostronyglus cantonensis, the rat lungworm, was found in Australia and that infections occur in humans from time to time. (I didn't say it in those words exactly but that was the gist.)

I'm a fan of parasites. They're marvellous animals, much maligned by us. If I had to make a list of my favourites, the flukes would be at the top and the nematodes down towards the bottom. But they'd definitely be on the list. Even if their life cycles are a touch on the dull side, comparatively speaking.

Angiostrongylus is a nematode. Adult worms live in the pulmonary arteries of rats. Their larvae migrate through the lungs and are swallowed. They pass out in poo, which is grazed on by snails and slugs—and crustaceans, if it drops into water. The juvenile worms develop in this intermediate host but do not reach adulthood until they're in the definitive (final stage) host, usually a rat.

Once in the rat, they burrow through the gut wall and worm their way through tissues until they arrive at the spinal cord. From there, they wriggle along to the brain. On reaching the brain, they hang around for a while, living it up among the grey matter. Then they mosey on down to the arteries of the lung. And the cycle begins again.

Although they infect humans, the worms are not well adapted to life in us. They often don’t get very far and are cleaned up by the immune system before they can do much damage. Even if ensconced in the brain, infection frequently passes unnoticed. Eventually, the worms die. End of story.

In other cases, the worms cause eosinophilic meningitis. A very small proportion of those infections may be fatal.

Although the parasite is widely distributed in South East Asia and the Pacific Islands, it has also been spread to other parts of the world. In Australia, human infections have been reported from Queensland and New South Wales.

A range of snail species harbour the larval stage, including the giant African snail (Achatina) and many slugs. In a 2001 case in Sydney, a young man ended up in hospital after eating a couple of slugs for a dare. It took five weeks before the nematodes got the better of him. But when they did, he was in trouble.

Eating intermediate hosts is a great way to get a case of Angiostrongylus. That was the method of infection in the Beijing case. And it doesn't have to be snails or slugs. Raw prawns are just as good.

And the moral of this tale? If you’re going to eat a slug for a dare, cook the bloody thing first.

DISCLAIMER: I am neither a medical practitioner nor a particularly good cook.

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