Frogs' legs, anyone?

You may have heard of parasites influencing host behaviour, but they can go one step further, and even manipulate their bodies. In the mid-1990s there was a flurry of reports from the western USA of frogs being found with severely deformed limbs. Pieter Johnson, from the University Colorado, Boulder, was one of the scientists called upon to investigate, and he spoke to Chris Smith about what had caused this outbreak...

Pieter - One of the main causes turned out to be parasite infection. Essentially, there is this tiny flatworm parasite, no bigger than the period at the end of a sentence that would attack tadpoles while they were developing in ponds. This would lead to severe limb deformities. We would find frogs with 5 or 6 extra limbs, some frogs with no hind limbs at all, others with bizarre webbing. It sort of prevented their limbs from being able to extend. In some parts of the country, we would find 50, 60, or even 100% of the emerging frog showing this severe crippling deformities.

Chris - Why was the parasite doing this to the frogs?

Pieter - Great question! So, this trematoda, this flatworm is called Ribeiroia ondatrae and the main reason it would do this is to reproduce of course. The main reason most organisms do just about anything. It turns out that Ribeiroia has this complex lifecycle. So frogs are only one of the many host that it infects and other animals that it'll infect include freshwater snails that are in ponds. And finally, birds - they get infected once they actually eat the deformed frogs. And so, it turns out that by crippling the frogs, by inhibiting their primary mode of locomotion, you're making them easy prey for the birds. So the birds will swoop down, they'll eat the frogs, they'll get infected by the parasite and it's there that the parasite actually gets to reproduce sexually. And then you have this brilliant system of dispersal because birds fly across the landscape. Essentially, when they defecate, they're dropping out the eggs of the parasite all over the landscape.

Chris - And they then make their way back to the nearest pond and a snail that's in the pond, that will pick them up.

Pieter - That's exactly right.

Chris - How did you, as the scientist involved in investigating these flurries of deformed frogs, actually start piecing all these together to work out what was going on?

Pieter - Mostly by making a lot of mistakes I would say was the best strategy. So, we ruled a lot of things out. I started working on this when I was an undergraduate student. Essentially, what tried to do is combine careful field surveys of where these deformities were occurring, with laboratory examinations of the animals and then finally, experiments. What you find is that if you bring the water in from the ponds where these deformities are occurring, that water doesn't cause deformities in frogs in the lab. If you actually raise the offspring of deformed frogs, those too are not showing any deformity. So, we were zeroing in on something in the environment that was attacking them during early development. 

One of the key observations was that almost all of the ponds where we were seeing this really severe deformities supported this very particularl type of snail. It's called a ramshorn snail. It turns out that that's the key snail that this parasite needs to support its infection. So, once we'd started to make those connections, we actually brought the parasite and the snails into the laboratory and then you can expose tadpoles to even really small numbers of parasites - 3, 4, maybe as many as 10 of these tiny parasite and quickly, you see them attacking the limbs. They show a very specific distribution no matter where you put them on the tadpole that migrate all over the place until they find the limbs. And then they produce these very powerful enzymes that essentially allow them to burn their way into the tadpole's tissue, right where those limbs are trying to grow. Sure enough, if you do this in the laboratory, you get the animals with the extra limbs, with the missing limbs, with the severe skin webbings. You can essentially reproduce all of those same deformities that you see in nature.

Chris - How do you think parasites evolved to do that because it's such a complicated process of being able to target the snail then target the growing limbs of a tadpole, have the right enzymes to do it and then ultimately make your frog get eaten by a bird so that you can complete lifecycle. That's so many steps of evolution. How on Earth did that happen?

Pieter - Yeah, that's a great question. Most of the evidence suggests that if you look at these flatworm parasites that they probably started out as parasite of snails. And then they progressively added a host onto their lifecycle. Some of the reasons to do this are, maybe you end up adding the tadpole first. But it turns out that your frog gets eaten as it happens to many frogs and many tadpoles. Over time, the parasites have basically developed this ability to survive predation. That they are somewhat unkillable by those digestive enzymes produced by something like the bird. And so, this has  one for focusing on being a snail parasite, to being a snail plus frog and then maybe being a snail, frog as well as a bird parasite. 

The advantages are obvious in terms of that extra dispersal ability you have. You can suddenly crisscross the continent and travel thousands of miles inside a bird. And then in terms of the frog component, I think the big challenge faced there is that if your frog lives to ripe-old age and never gets eaten by a bird then the parasite dies with it, without ever getting to reproduce. So, you can imagine how that puts enormous selective pressure on the parasite to find ways, or to have selection operating, to find ways to make sure that frog dies and then it dies the way the parasite needs it to.