Does variation in host immune function explain differences in reservoir competence among small mammals?

Background/Question/Methods

Differences in reservoir competence of vertebrate hosts (the ability of hosts to support replication and transmission of pathogens) may be related to the immunological strategies that they use. We predicted that the most competent reservoir species would favor cheap, broad immune defenses that pathogens may be able to circumvent, whereas species with a low reservoir competence may be more effective at preventing infection by favoring specific but expensive immune defenses. Previous work has suggested that these patterns may be related to host life history strategy.  We tested this hypothesis by comparing immune functions among a group of rodents frequently exposed to the Lyme disease bacterium, Borrelia burgdorferi.  The most competent reservoir, the white-footed mouse, is a short-lived and ubiquitous species whereas other species with lower reservoir competence, such as eastern chipmunks and gray squirrels, have a slower pace of life and occur at lower densities. To assess immunological strategies we captured wild individuals, held them in captivity temporarily and measured bacterial killing capacity of blood in vitro. We also injected the animals with lipopolysaccharide (LPS), a component of Gram-negative bacteria, and compared the concentration of antibodies in plasma as well as changes in body mass during the week in captivity.

Results/Conclusions Our preliminary results suggest that white-footed mice have a greater bacterial killing capacity against E. coli than sciurid species do. Male white-footed mice showed the smallest increase in antibody production to LPS from day 0 to day 7, whereas the largest change was observed in female gray squirrels. In fact, during the 4 days following the immunological challenge, mice gained on average 10% of their initial body mass, whereas sciurids gained only 1%.  The data provide preliminary support for our hypothesis, reflected by the greater bacterial killing capacity of mouse blood and the low antibody production at least in males. Interspecific variation in reservoir competence for zoonotic pathogens may be caused by variation in immunological strategies, which in turn may be determined by life history strategy (e.g., pace of life) of hosts.  If so, highly resilient species with high reproductive potential (e.g. mice) might generally be expected to be highly competent reservoirs for zoonotic pathogens. Such a relationship would provide a mechanism by which the loss of biological diversity should exacerbate disease risk. A larger sample size will be needed to verify the relationships described here.