  Co-infecting parasites and pathogens remain a leading challenge for global
public health due to their consequences for individual-level infection risk and
disease progression. However, a clear understanding of the population-level
consequences of co-infection is lacking. Here, we constructed a model that
includes three individual-level effects of co-infection: mortality, fecundity,
and transmission. We used the model to investigate how these individual-level
consequences of co-infection scale up to produce population-level infection
patterns. To parameterize this model, we conducted a four-year cohort study in
African buffalo to estimate the individual-level effects of co-infection with
two bacterial pathogens, bovine tuberculosis (BTB) and brucellosis, across a
range of demographic and environmental contexts. At the individual-level, our
empirical results identified BTB as a risk factor for acquiring brucellosis,
but we found no association between brucellosis and the risk of acquiring BTB.
Both infections were associated with reductions in survival and neither
infection was associated with reductions in fecundity. Results of the model
reproduce co-infection patterns in the data and predict opposite impacts of
co-infection at individual and population scales: whereas BTB facilitated
brucellosis infection at the individual-level, our model predicts the presence
of brucellosis to have a strong negative impact on BTB at the population-level.
In modeled populations where brucellosis is present, the endemic prevalence and
basic reproduction number (Ro) of BTB were lower than in populations without
brucellosis. Therefore, these results provide a data-driven example of
competition between co-infecting pathogens that occurs when one pathogen
facilitates secondary infections at the individual level.
