Is defense independent of diet? Testing effects of deer mouse nutrition on resistance to the Rocky Mountain wood tick

Background/Question/Methods

The vertebrate immune system utilizes physiological resources to produce the diverse cells and proteins that provide defense against infectious organisms (parasites). Hypothetically, the resources allocated to the immune system must be traded against other physiological demands (e.g. growth). These internal trade-offs are believed to drive the variation in immune function often observed among individuals. These resource constraints begin with the diet itself, since nutrients and calories vary with food quantity and quality. Here we ask if diet manipulation affects immune function of a deer mouse (Peromyscus maniculatus) through its ability to gain resistance to an ectoparasite, the Rocky Mountain wood tick (Dermacentor andersoni). Four distinct diet groups were created using mixtures of arthropods and seeds. The range of diets included ratios of seeds versus arthropods at the limits of mouse homeostasis (i.e. further dietary skew produced rapid weight loss and mortality). Mice were fed the controlled diets before and during 2 tick infestation periods where each mouse was inoculated with 50 wood tick larvae. Engorged ticks were recovered and counted from each mouse during both infestations to determine the relative susceptibility/resistance to ticks among the mouse diet groups. 

Results/Conclusions

Mouse body weight was affected by diet and parasitism. In general mice lost weight during periods of infestation and regained weight during the time between infestations. Mice fed a diet skewed toward arthropods lost weight more rapidly during infestations than mice fed a diet skewed toward seeds. Within an infestation period the number of ticks that successfully obtained blood meals was unaffected by the variation in host diet. Significantly fewer ticks fed on mice during the second infestation compared to the first infestation, which indicated that mice acquired resistance to tick blood feeding during the initial exposure. This observation mirrors results of other experiments demonstrating deer mice will develop resistance to Rocky Mountain wood ticks. These results suggest diet affects body mass during parasitism, but immune defense remains intact regardless of dietary differences. This has implications for the interactions between host ecology and infectious disease. Though host condition (body weight) may be influenced by the interaction between food resources and infection, immune defense may remain intact even when dietary resources change over space and time. Ongoing work is also examining the effect of diet on immune function relative to parasitism by wood ticks through manipulation of food abundance.