Linking immune responses to disease dynamics in the wild: Bovine tuberculosis and brucellosis in African buffalo (Syncerus caffer)

Background/Question/Methods

Mounting evidence suggests that immune mediated interactions among co-infecting species can mediate disease progression and the dynamics of co-infecting pathogens, yet there is limited empirical evidence linking within host immunity to disease outcomes in the wild.  In this study, we test how immune-mediated interactions between an emerging bacterial disease, bovine tuberculosis (bTB) and a native bacterial disease, brucellosis, alter host susceptibility, disease progression, and the dynamics of infection in a free-ranging population of African buffalo.  We demonstrate that bTB and brucellosis are positively associated at the population level and use a combination of host immunity and disease modeling to test potential immune mechanisms that could result in a positive association.

Results/Conclusions

Our results show a pronounced decrease in bactericidal activity and increase in pro-inflammatory immunity (measured by the concentration of interferon gamma and acute phase proteins) throughout the course of infection.  Both infections and immune activity were additively associated with host mortality but infection with bTB was also associated with a 2.8-fold higher risk of brucellosis infection then uninfected buffalo.  We then scaled up these individual level effects to the population level using a disease dynamics model.  We parameterized the model based on bTB’s association with increased brucellosis transmission and increased mortality.  The results suggest that it is the relative magnitude of these two processes that are likely responsible for the positive association between bTB and brucellosis at the population level.