Discussion

Many individual hosts are infected with multiple parasite species and this may increase or decrease the pathogenicity of the infections. This ‘heterologous reactivity’ is potentially an important determinant both of patterns of morbidity and mortality and of the impact of disease control measures at the population level. Using infections with Theileria parva in indigenous African cattle (where it causes East Coast fever, ECF) as a model system, we obtain the first quantitative estimate of the effects of heterologous reactivity for any parasitic disease. In individual calves, concurrent coinfection with less pathogenic species of Theileria resulted in an 89% reduction in mortality associated with T. parva infection. Across our study population, this corresponds to a net reduction in mortality due to ECF of over 40%. Using a mathematical model, we demonstrate that this degree of heterologous protection provides a unifying explanation for apparently disparate epidemiological patterns: variable disease-induced mortality rates, age-mortality profiles, endemic stability, and the poor efficacy of interventions that reduce exposure to multiple parasite species. These findings can be generalized to many other infectious diseases, including human malaria, and illustrate how coinfections can play a key role in determining population-level patterns of morbidity and mortality due to parasite infections.