A parasite infection is bad news, right? A fast track to anemia, brain damage, or death? Well, sometimes. More often than not, it isn’t nearly so bad. Lots of parasites are benign. And a paper in last week’s issue of the Journal of Experimental Biology shows that sometimes a host can just adapt.
Lisa Schwanz, working at the University of New Mexico, Albuquerque, studied the effect of the trematode parasite Schistosomatium douthitti on its natural host, the deer mouse. Like the schistosomes that affect humans, Schistosomatium has a complex life cycle that includes reproducing asexually inside a freshwater snail. The snail sheds infective larvae into the water, where they look for their final host. When they find one, they dig through its skin, migrate to its circulatory system, mature, and start laying eggs. Some of the eggs find their way back outside. But others get stuck inside the host’s tissues. This can be a problem. The eggs irritate the tissues, inflaming and damaging them.
This much was certainly true of Schwanz’s mice: infected mice had swollen livers, spleens, and intestines. But the tissue damage didn’t slow them down. Schwanz measured how much the infected mice ate and drank, how well they digested their food, and how fast they ran on their treadmills. They didn’t act sick. They didn’t lose weight, they didn’t eat more than usual, and they weren’t lethargic. They had parasites. The parasites were damaging their bodies. But the mice compensated for their parasite load so well that they behaved exactly like the healthy mice in the study.
Now, these mice were living a cushy life in a laboratory. They didn’t have to hide from predators and they got all the food they could eat. The next question is what happens to parasitized mice in the real world. The experiment suggests that if conditions are perfect, a parasite doesn’t use enough of a mouse’s internal resources to make it sick. If the mouse is instead cold, wet, half-starved and hiding from owls, is the damage the parasite inflicts enough to push the host over the edge into illness?
Schwanz, L. 2006. Schistosome infection in deer mice (Peromyscus maniculatus): impacts on host physiology, behavior, and energetics. J. Exp. Biol. 209: 5029-5030.
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