Parasites: here, there, and everywhere

by Cheyenne Tait – I think that parasites must hold some kind of deep fascination for biologists. Either that or I just attract strange colleagues. I don’t know what else can explain how often I’ve been drawn into conversations, usually with people I’ve just met at workshops or conferences, about the most gruesome parasites we can collectively think of.

For instance, how about those grasshoppers that throw themselves into bodies of water, committing suicide, only to spool out a fine, wriggling worm (horsehair worm) which happily swims off to infest more grasshoppers? There are truly awful videos of this on Youtube, I don’t suggest you go see them if you’re squeamish.

And of course these conversations usually happen at lunch. It must be some kind of unwritten rite of passage for a biologist – can you hold in your lunch while you talk merrily about what else might be inside you… also eating your lunch?

In fact, I recently read the book “Parasite Rex” by Carl Zimmer. If you want to be as freaked out, and fascinated, as I am by colleagues’ stories at lunch, and/or gather your own stories to tell, it’s a good book to read. Some of the more interesting examples of parasitism?

There’s the gruesome – Leishmania brasiliensis, a microorganism parasite that, if left untreated too long, is said to render its victims faceless. Similarly gruesome are the multitude of human parasites mentioned in Carl Zimmer’s book. Guinea worm, exploding from a cyst in a person’s leg, is a particularly nasty image. But luckily Guinea worm is almost eradicated.

The parasite behind malaria, plasmodium, however, is another story entirely in that it is not on the wane, and it’s deadly. Its effects on mankind have been powerful, with evolutionary consequences (sickle-cell disease in Africa, for instance). But in terms of gruesome, I’d say that the mosquito has it worse.  Plasmodium expertly manipulates the mosquito’s behavior, first making it more likely to avoid humans (and avoid being killed) as the parasite is growing by having the mosquito not eat. Then, once in adulthood, plasmodium shifts and somehow makes the mosquito more ravenous, more likely to bite human hosts, so it can be passed on.

Actually, a lot has been studied on parasites that somehow possess “mind control” powers over their hosts. Flukes that make ants climb to the top of a blade of grass so that they can be eaten more easily by their parasite’s next host. Parasitic wasp pupae that somehow gain active protection from the half-eaten, half-alive caterpillar they emerged from, with the poor insect thrashing about in response to disturbances and only getting to die once the wasps emerge from their pupae. Toxoplasmosis influencing rats to be less afraid of cats. It’s all about chemicals, often inside the nervous system, and it’s all because the parasite involved needs to get to its next life stage. When it’s ready to go, the current host better watch out!

But the book “Parasite Rex” isn’t out to show us how disgusting and deadly parasites are, with mind control powers that make them perfect stars for movies like “Alien”. The book wants to show how far-reaching their effects can be. How powerful they really are – they’re the “Rex”, the king! It turns out that parasites not only manipulate single hosts, their presence impacts entire food webs. The book mentions deer that carry a parasite which devastates moose but is relatively benign inside the deer. Thereby, the parasite keeps the moose population down and allows the deer to co-exist with moose, which should be the stronger competitor for food and drive out the deer. Instead the parasite balances out their relationship.

Another example of the ecological involvement of parasites, tiny and unseen, centers on the killifish. One of its particular parasites affects its nervous system, making the fish more likely to be near the surface of the water, where birds, the parasite’s next host, are more likely to capture and eat it (Lafferty and Morris 1996). Parasitized fish are apparently 30 times more likely to be taken by birds than non-parasitized fish. This, of course, means the birds are parasitized on a massive scale, and only works out if the food resource (the fish) is worth more to the birds’ fitness than the negative impact of the parasite feeding off the bird. Thus parasites are involved in complex ecological interactions, manipulating the biological fitness “calculus” not just from the bottom up or the top down, but from every angle in between.

With such far reaching ecological effects, parasites which have an effect on evolution should not be surprising. In fact, parasites are prominent in the history my own study species, Rhagoletis pomonella. The apple maggot fly is infested by its unique group of Braconid wasps. In fact, they are one hypothesis for what drove the apple maggot fly to speciate, moving from its original host fruit (hawthorn, where it is parasitized at very high levels by wasps), to a new host fruit (apple, less parasitism): the enemy free space hypothesis (Feder 1995). Flies switched hosts to escape their enemy, the wasp. However, given time, the wasp of course followed the fly. Thus the fly’s speciation event, driven possibly in part by the wasps, led to a cascading speciation event (Hood et al. 2015), wherein the wasps followed the flies to the new host fruit and diverged from the other wasps, each on their own way to becoming distinct species.

Braconid wasps emerging from apple fly pupae

The interplay of ecology, evolution, and parasitism is a complicated dance that scientists are only just starting to get a hang of. Whether epiphanies occur over lunch or not, well, that depends on the strength of the individual scientist’s stomach.

Works Cited

Feder, Jeffrey L. “The effects of parasitoids on sympatric host races of Rhagoletis pomonella (Diptera: Tephritidae).” Ecology 76.3 (1995): 801-813.

Hood, Glen R., et al. “Sequential divergence and the multiplicative origin of community diversity.” Proceedings of the National Academy of Sciences 112.44 (2015): E5980-E5989.

Lafferty, Kevin D., and A. Kimo Morris. “Altered behavior of parasitized killifish increases susceptibility to predation by bird final hosts.” Ecology 77.5 (1996): 1390-1397.

Zimmer, Carl. Parasite rex: inside the bizarre world of nature’s most dangerous creatures. Simon and Schuster, 2000.

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