| Mixed-species parasite infections in human malaria are common but only recently have studies explicitly examined disease in the context of the parasite community. This research suggests that immune response to one parasite species affects co-occurring parasites, called cross-species immunity. Analogous to competition, cross-species immunity could have consequences for parasite life-history evolution, disease severity and parasite invasion into new host populations. Because of constraints on long-term studies in human malaria, the nature and consequences of cross-species immunity remain poorly understood.; Here I examine malarial parasite communities in wild tropical birds, which are tractable host organisms for disease research and have high parasite species diversity. I used recently developed molecular techniques to identify parasite species in mixed infections and to examine the evolutionary relationships among parasites in a community.; In Chapter 1, I ask: "Are mixed infections in host individuals common and can I detect negative interactions between parasites?" Nearly all birds sampled were infected with multiple parasite species. Co-occurring parasites were distantly related to one another, each having closer relatives in other avian host species in the same community. This novel finding suggests that closely related parasites will not share host species, perhaps because antigenic proteins are similar and cross-species immunity will be prohibitively suppressive.; In Chapter 2, I investigate parasite/mosquito species associations and ask whether two distantly related parasite species, sharing an avian host, are transmitted by the same mosquito species. These two parasites did not share mosquito species and each was found co-occurring in mosquitoes with other, closely related parasites. Thus, related parasites have access to but do not infect the same avian host species.; Finally, in Chapter 3, I report on an eight-month study of the dynamics of mixed-species infections in a captive population of adult birds. I ask whether uninfected hosts are rare and thus a limiting resource, a requirement for competition. This study revealed that all adults were infected by both parasites and blood infection, even in individuals screened from mosquitoes, is chronic and fluctuating.; In conclusion, mixed-species infections are common in free-living tropical birds and uninfected adults are rare. Parasites appear to co-occur in hosts only with distant relatives. Thus, immune-mediated competition among parasites may be an important mechanism structuring malarial parasite communities. |
