How does parasitism scale with group size?

Does living in a large social group make an organism more or less likely to be parasitized?  People have been thinking about this question for quite a while, and one proposed answer is: well, it might depend on the parasite.  For parasites/pathogens transmitted by close contact (and with density-dependent transmission), we expect transmission to increase with group size because contact rates among hosts increase with density.  (If the group increases in size but also takes up a larger area, that may not be the case!)  For “searching” parasites, parasitoids, and vectored parasites, we expect an individual’s risk of infection to decrease with group size if the number of parasites/parasitoids/vectors is constant among groups.  We call that decrease in an individual’s risk of infection encounter dilution, and it works a lot like the dilution effect works in communities.

Do we actually see those expected relationships in real systems?  Rifkin et al. (2012) performed a meta-analysis of 69 studies of host-parasite systems with transmission strategies that included close-contact transmission, environmental transmission, vector transmission, “searching” parasites, and parasitoids.  Contrary to what we expected, there was a positive relationship between group size and parasitism for all transmission modes!  (In the case of searching parasites, the relationship wasn’t significant.)  There was weak evidence for publication bias, where studies representing negative relationships between group size and parasitism were somewhat underrepresented, but even after correcting for that bias the overall relationship was still positive.

So, why didn’t they see evidence of encounter reduction?  If you remember back to my post about the dilution effect, the way that vector density scales with host density is really important.  Perhaps there tend to be more searching parasites/vectors/parasitoids in areas with larger host groups.  For instance, maybe searching parasites/vectors/parasitoids preferentially seek out larger host groups?  I wouldn’t blame them:

I really just wanted an excuse to make a wildebeest cartoon. I think my ungulates are improving?

Reference:

Rifkin, J.L., C.L. Nunn, and L.Z. Garamszegi.  2012. Do animals living in larger groups experience greater parasitism? A meta-analysis. Am Nat 180(1):  70-82.  (PDF link)