In an increasingly globalized society, pathogen interactivity, coinfection events are likely to be the norm rather than the exception. Interactions between pathogens within a host may drastically alter the dynamics of infection and disease outcomes by affecting disease severity, duration, and transmission. We experimentally determined the effects of infections and coinfections on the fecundity and mortality rates of flies. We could determine the infection state of flies only at the end of the experiment; therefore we used latent variable models to model disease prevalence and influence on vital rates showing how to determine the effects of coinfection without the need for a longitudinal study.
Finally, we used the estimates of individual fecundity and survival to illustrate a procedure for testing the additivity of infections on individual fitness. This test provides a null model of coinfection virulence that compares the fitness of a coinfected individual to a hypothetical individual that is infected by each virus independently. This null model allowed us to determine whether the fitness effects of coinfection are consistent with the additivity of disease effects on fitness.
Results/Conclusions
We find that the fecundity of coinfected flies could be predicted by knowledge of each diseases effect on fecundity independently. However, the average lifetime of coinfected flies was longer than we would expect based on the effect of each disease independently, suggesting a potentially competitive interaction between these two diseases. Finally, we show how to determine the fitness of a coinfected fly from these two pieces of information.