In Praise Of Parasites

By some estimates, nearly half of the species in the animal kingdom are parasites. Most of them remain largely out of sight because they are small, even microscopic. Their ancestors didn’t always start with a parasitic lifestyle: Researchers have so far found 223 incidents where parasitic insects, worms, mollusks or protozoans evolved from non-parasitic predecessors. Some ate dead things. Others killed their prey and consumed it. Then their life strategy evolved because they proved more successful if they kept their prey alive, kept their victims close — so they could feed on them longer. It’s a strategy distinct from those of parasitoids, which outright kill their hosts, Lafferty explains, a glint of mischief in his eye. “Think about the movie Alien.  Remember when the alien sock puppet bursts its head out of John Hurt’s chest? That’s a classic parasitoid.”

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He is also a serious marine ecologist who holds passionately that parasites are worthy of study for how they influence ecological systems and how ecosystems influence them. For years, it was a fairly lonely position to take:  “Ecologists have built hundreds of food webs and they haven’t put parasites in them. And what we’ve lost from that is the ability to even think about parasites and their role in ecology,” Lafferty says. Ecology conferences used to struggle with where to place Lafferty’s talks in their schedules, but nowadays the meetings have dedicated sessions on wildlife infectious diseases. And ecologists, especially younger ones, are starting to recognize that they are missing part of the story if the food webs they model don’t include parasites that can influence predator-prey relationships and competition for resources. As illustrated by the trematode in the killifish, Lafferty says, “parasites are determining who lives and who dies in a way that benefits them.”

Moreover, parasites are a useful way to explore broader ecological questions: How does energy flow through those food webs? What forces maintain ecological stability and keep one species from overrunning all others? What are the implications of robust and healthy biodiversity on human health? Ecologists debate all sorts of competing theories, Lafferty says. What’s clear to him and other like-minded parasitologists: “We cannot answer these questions if we are going to ignore the parasite part of the equation.”

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It hit him that here was an opportunity to break new ground. “Although lots of people had studied parasites for their own sake, or as problems to be solved, it seemed like an open playing field to start asking how parasites fit into natural ecosystems,” he says. He spent the next two years cracking horn snails with a hammer to collect trematodes in estuaries from San Francisco to Baja. His work solidified how the parasites were affecting the snails’ abundance and evolution — finding, for example, that snails in areas with high infection rates have evolved to mature and reproduce early, before they get castrated.

- More here including Toxoplasma gondii. 

We know Toxoplasma causes "feline attraction" but it might also slow reaction times or diminish ones ability to focus, these may be why infected people have a nearly threefold higher chance of being involved in a car accident.