Detecting interactions between parasites in cross-sectional studies of wild rodent populations.

There is extensive and convincing evidence from experimental studies that interactions exist between helminths of different species during concurrent infections in laboratory rodents. Among the strongest interactions are those that arise from immune responses of the hosts. However, detecting comparable relationships in data acquired from wild rodent populations has not been easy. In general, helminth infections in naturally occurring rodent populations show highly predictable trends; seasonal, host age-dependent and spatial variation in the abundance of core species and in helminth species richness are regularly reported aspects of these host-parasite communities. Controlling for these strong effects is therefore extremely important, if interactions between species are to be detected. One such interaction, the positive relationship between Heligmosomoides polygyrus and species richness of other helminths in European wood mice, Apodemus sylvaticus, has been found in four totally independent data-sets, three drawn from the U.K. and one from Portugal. These analyses provide strong evidence that at the level of species richness a highly predictable element of co-infections in wood mice has now been defined.