21 August 2009

That possum you just ran over? It might have saved you from Lyme disease

ResearchBlogging.orgGrowing up in suburban Pennsylvania, where the most hazardous wildlife not extirpated from our woods is the occasional crazed whitetail deer, there was really only one danger I associated with the outdoors -- ticks. Specifically, ticks carrying Lyme disease, a not-very-pleasant bacterial infection that attacks the joints, heart, and nervous system if left untreated. According to a paper released online early in Proceedings of the Royal Society B, my risk of picking up Lyme disease on an excursion into the woods behind my parents' house may have depended on the diversity of bird and mammal species in those woods [$-a].

Sure, it looks like a giant rat, but that opossum is a walking death trap for disease-carrying ticks. Photos by ricmcarthur and jkirkhart35.
In a way, the ticks that carry Lyme disease are a threat to humans precisely because they don't rely on us as a regular source for blood. Instead, they feed on a variety of mammals and birds, which allows them to maintain population densities high enough that a human wondering into a woodlot stands a good chance of picking up one or two of the little buggers.

But it turns out that not all of these non-human hosts are equally hospitable for ticks. The new paper's authors, Keesey et al., caught a range of tick hosts -- white-footed mice, eastern chipmunks, gray squirrels, opossums, veeries, and catbirds -- and experimentally infested them with ticks. They found a huge range of tick success across the six host species: almost half of all ticks introduced onto mice were able to feed, while only 3.5% of ticks introduced onto opossums were. Most ticks that failed to feed disappeared -- they were probably eaten when the host groomed itself.

The authors' field surveys of ticks carried by these animals in the wild make the difference even more pronounced. Wild-caught opossums carried an average of almost 200 ticks -- if that's 3.5% of the ticks that try to feed on a opossum, then that means each opossum had attracted, and eaten, up to 5,500 ticks!

But the real impact of this result comes into focus in a mathematical model the authors develop to determine the effects of removing each of the six hosts from a woodland ecosystem. Removing intermediately-useful hosts like veeries or catbirds doesn't have much effect on tick density. On the other hand, if you remove very tick-friendly hosts like the white-footed mice, tick populations plummet. And if you remove opossums, they increase dramatically. This is important because, the authors say, larger mammal species are the first to leave as patches of woodland are reduced to make way for human development -- so an early effect of woodland fragmentation may be to reduce or eliminate opossums in that woodland, and boost the density of disease-bearing ticks.

This result goes a long way to fulfilling a proposal the authors made in a 2006 review article, that the diversity of alternative hosts for disease vectors like mosquitoes and ticks may shape the risk they pose to human populations [$-a]. It shows that, even in the relatively tame landscapes of suburbia, the way we humans manage what wildlife remains may have real consequences for our own well-being.


Keesing, F., Holt, R., & Ostfeld, R. (2006). Effects of species diversity on disease risk Ecology Letters, 9 (4), 485-98 DOI: 10.1111/j.1461-0248.2006.00885.x

Keesing, F., Brunner, J., Duerr, S., Killilea, M., LoGiudice, K., Schmidt, K., Vuong, H., & Ostfeld, R. (2009). Hosts as ecological traps for the vector of Lyme disease Proc. R. Soc. B, (online early) DOI: 10.1098/rspb.2009.1159

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