Feeling Feverish? Sit Still

Plodia interpunctella moth larva. Photo credit: Lyle Buss, University of Florida

Thursday, March 1st, 2007--

Viruses evolve differently depending on how free their hosts are to move and mingle. A study using caterpillars, published this week in the journal Science, showed that a virus' ability to infect is related to the mobility of its host.

The caterpillars, larvae of the moth Plodia interpunctella, were housed in vats of food—a mixture of "organic harvest" breakfast cereal, yeast, honey, and a few preservatives. To limit caterpillar motion, the researchers moistened their cereal surroundings. The food is normally loose and easy to inch through, but a little distilled water thickens it up, making it harder to for caterpillars to move around in it.

Limiting the caterpillars' motion also limited their interactions with neighbors, and their exposure to the virus. (The caterpillars contracted the virus mostly by eating other infected caterpillars.) This study showed that the viruses living in slow food changed to be less infective than the viruses living in fast food. The virus evolved, in other words, to make itself worse at getting caterpillars sick.

But here's the paradox: a less infective virus can infect more hosts in the long run. And this is why: If a virus has a limited number of hosts to infect—because, for example, the host is living a relatively solitary life in extra-sticky food—the virus benefits from not killing off every other viable host too quickly. Angus Buckling, a professor in the Department of Zoology at the University of Oxford in the U.K., calls this viral adaptation: prudence.

A non-prudent virus runs the risk of "self-shading." Michael Boots, professor at the University of Sheffield in the U.K. and an author of the study, explains self-shading: "when a parasite transmits rapidly locally, inevitably all the infected individuals are surrounded with other infected ones." Self-shading is bad for viruses because no hosts means no propagation.

What does it mean for us? Buckling, who published a corresponding article in Science this week, says the study is likely to have broad implications. "Any increase in movement—I guess that's the real issue—is going to increase transmission opportunities of parasites," Buckling says. As we humans live in a more globally mixed world, our viruses are exposed to more hosts—and that means viruses can afford to be less prudent.

What did you think of the story? Send us some feedback.

--Flora Lichtman