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Thursday, 01 December 2011 10:43

Zombie ants coming to a Cypress cattle herd near you

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By Jamie Woodford

There’s a new cattle parasite that lurks in Cypress Hills Park with an agenda straight out of a science fiction movie.

The lancet liver fluke lives in the bile ducts of an animal’s liver where it spends the rest of its life producing eggs. The eggs are defecated out in feces whereupon snails come and ingest them.

“Inside the snail the parasite reproduces like crazy. It castrates the snail for the snail’s life,” said Dr. Cam Goater, project leader on fluke research. The University of Lethbridge and Agriculture and Agri-Food Canada’s Lethbridge Research Station are working on the study.“Then the snail excretes these tiny, tiny little larva in what’s called a slime ball.”

Along come some ants that eat the slime balls for moisture and bring it back to the ant’s nest, exposing the rest of the nest.

Here’s where it gets weird. Between 50 and 100 parasites can infect just one ant. Most parasites will go to the ant’s abdomen, but one will go to the brain.

“The bizarre thing is ... that one goes

to a particular part of the brain that controls the action of the mandibles and other behaviours,” said Goater. “At dawn, when the rest of the ants are active, it’ll come out of the nest with all the rest of its buddies, but instead of foraging and doing what an ant does, it goes up to the top of the vegetation, usually it’s a flower head, and attaches, and it does that for a period of three or four hours depending on the temperature.”

The “zombie ant” stays attached to make itself more accessible to be eaten by the next host. The ant’s weird behaviour doesn’t stop there. Temperatures above 18 degrees would be lethal for the host and the parasite, so when it gets too warm the ant automatically detaches and goes back to the nest.

“They’re zombies in the sense that they’re doing something very different than an uninfected ant, but in many ways the most interesting thing is that they do this for the rest of their life, everyday,” said Goater. “They go down, they go up, they go down, they go up until they’re eaten, or they die for some other reason.”

Once the zombie ant is eaten, the parasite will make it’s way to the liver

of the animal to continue the cycle.

While an ant is sneaking away to sacrifice itself, the rest of the colony is oblivious to what’s going on.

“Nobody knows,” said Goater. “They don’t avoid their infected buddy. (The infected ants) seem to be really cryptic once they’re in the nest. So the question is how does the parasite make the ant do that? That’s what we don’t know, and that’s what we’re trying to find out.”

One theory is that the parasites are puppet masters.

“That they’re actually pulling strings to make the host do certain things for the benefit of the parasite.”

The amazing part of this cycle is that it works.

“The requirements for the transmission of the parasite are really, really rigid. This parasite, like lots of parasites have a complex life cycle, and that involves a snail, an ant, and something that eats ants.

So you need to have all three hosts together in one location for the life cycle to work,” Goater explained.

Cypress Hills, which appears to be the only place in Canada where the fluke is found, has the perfect conditions for that recipe.

“In Cypress Hills we’ve got cattle, deer and elk all grazing within a fairly narrow area, and those are probably ideal conditions for transmission.

Those conditions do occur elsewhere, so we think that it’s just a matter of time before it moves out of Cypress Hills and there’s lots of areas in Alberta where it’s almost certainly going to show up,” he said. Luckily humans are safe from becoming zombified by a fluke, as they are too big to control. “If we ate an (infected) ant, the parasite would go to our liver,” he said and added the parasite probably only lives for about two years.

The next step in researching the fluke phenomenon is figuring out exactly how the parasite is able to alter the behaviour of ants, particularly how the fluke knows the location of the nerves that affect mandible clamping.

“It’s one thing to explain how they go up the plant, and lots of parasites make their host do things like that, but it’s another thing to figure out how they can make them detach and do it repeatedly. That’s one thing that we don’t have a clue,” said Goater. “This business of doing it repeatedly and doing the zombie thing is truly odd and it means that we don’t understand the real basics of the full system.”

More importantly, Goater wants to know the effects of a fluke infection on cattle in Cypress Hills where it’s likely

90 per cent of cattle are infected.

Infection is rarely fatal in cattle, but heavily-infected animals can become anemic, emaciated and predisposed to other infections.  Economic losses come in the form of liver condemnations at packing plants, but usually the rest of the meat is fine, Goater said.

“The real issue is what sort of physiological effect they have on weight gain and feeding efficiency and that sort of thing.”

Another branch of fluke research is tracing the path of fluke’s 1950s invasion from Europe by comparing the genetic fingerprint of European worms to the ones in New York State, where the invasion originated, and in Canada to see if they’re related.

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