Parasite versus parasite

Parasite versus parasite McGill University

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McGill Reporter
March 21, 2002 - Volume 34 Number 13
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Home > McGill Reporter > Volume 34: 2001-2002 > March 21, 2002 > Parasite versus parasite

Parasite versus parasite

Natural resources science professor Manfred Rau got into parasitology "basically because of the complexity of the interactions."

Photo Natural resources science professor Manfred Rau
PHOTO: Owen Egan

It's a good thing he embraces such tangles, because your typical relationship between snails and parasites makes 21st century dating seems simple.

And, to complicate matters, Rau and his research team have shown that the existing pre-conceived rules of snail-parasite trysts may be wrong. The good news is this discovery could help control disease.

Rau has been following the rituals of Plagiorchis elegans, a parasite trematode that invades a compatible snail host, suppresses the snail's own reproduction while furiously multiplying itself. This spells bad news for the snail that is, in effect, castrated. But the parasite, it was thought, needed a compatible snail to accomplish a successful hijacking.

"Conventional wisdom says that if we get a parasite into the wrong snail host, an incompatible snail host, that's the end," says Rau. "Nothing happens. They disappear, they're eliminated."

What's interesting about Plagiorchis is that Rau and his research team have discovered it can suppress snail reproduction in more than one snail host, even in species previously believed to be incompatible.

The Plagiorchis develops only to the embryonic stage, but will still have a castrating effect and, furthermore, renders the snail unsuitable for other parasites.

Then Rau made a leap of logic that could help the world control schistosomiasis, a disease caused by another parasite whose life cycle takes it through the snail host wringer.

Schistosomiasis infects 200 million people in the tropics, and is on the rise due to water resource development. It's a blood fluke that lives as worms in the human host, then its eggs make their way outside, often through the bladder.

If it goes to a river, it infects the compatible snail that later releases schistosome larval-stage cercariae back into the water, where it easily infects humans again. "It penetrates through unbroken skin. You just have to get splashed by water containing cercariae.

"It debilitates people," says Rau of the nasty effects of schistosomiasis. "Productivity goes way down, people are sick, they're walking skeletons. It's severe. It causes a predisposition to cancer in some individuals."

Although there are drugs "that will kill the schistosome in the human, reinfection is almost inevitable. As soon as these people make contact with water again, within days they can have more parasites.

"We had a hunch, took a major step and infected the schistosome snails with a low dose of these parasites, Plagiorchis. The first thing we noticed was the snail's reproductive system shuts down. We expected that. Then we infected the snail with schistosomes. And we found a 91 percent reduction in the number of schistosome parasites produced. It was almost complete." The first parasite renders the host as an inhospitable habitat for the second, more dangerous parasite.

"Plus, if we infect snails that are already infected with the schistosomes, it has the same effect. It can be preventative and it can be curative."

Molluscicides are the usual means to try to control schistosomiasis, by killing the snails. Unfortunately "even the best of them tend to kill other organisms, such as fish and other aquatic organisms in rivers."

Rau has speculated about how Plagiorchis pulls a whammy on schistosomes. "Most parasites when they try to get in the snail, they Ôimmunosuppress' it. To become established, they have to knock out the internal defence mechanisms temporarily.

"Plagiorchis is very unusual. What it does is it actually stimulates the host's defence mechanism when it gets in. The host reacts and shoves in all kinds of cells trying to encapsulate the parasite, and it changes the defence cells into cells that actually nourish it.

"Plagiorchis has the ability to actually co-opt the responses of the host, to make use of them. Now imagine you shove this into a snail. The snail reacts by producing a non-specific internal defence response. And another parasite comes along and it's probably hit by it. So it may actually be hiking up the defence mechanisms of the snail against other invaders.

"When you look at the whole picture of snail biology, it was assumed that compatible parasites had an impact on snail reproduction. Now we're beginning to realize that a whole bunch of incompatible species may have a similar effect on reproduction."

Rau would like to make contacts with other researchers and take this to the river. Different species of snails yield different patterns of infection, and there are seasonal and occupational factors that can influence human infection. "When you do biological control you have to understand the system of where you're employing it. It's always dangerous to walk in when you don't know the system - you may miss things."

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