Avian malaria on Oahu : disease ecology, population genetics, and the evolution of resistance in Oahu amakihi

Abstract

Introduced avian malaria is considered one of the greatest threats to the survival of the endemic Hawaiian honeycreepers. This study investigated avian malaria in Oahu forest bird communities, and modeled quantifiable variables to determine what factors of the host and vector could be used to predict the prevalence of the pathogen. I also examined the relationship between avian malaria and the Oahu amakihi (Hemignathus flavus), a native species that is not currently endangered, by experimental infection with the pathogen, as well as population genetic analyses to look for evidence of a bottleneck event in the evolutionary past of this bird. I found a high prevalence of avian malaria in Oahu forest birds, which demonstrated a considerable parasite reservoir in avian hosts and a vector population that facilitates frequent, year-round transmission of malaria. Host community composition, particularly the proportion of house finches in a community, had a strong influence in the model of malaria prevalence, as did species and evidence of pox-like lesions, which may indicate dual transmission of both diseases by the mosquito vector. The low malaria prevalence found in the native Oahu amakihi suggests the potential for the evolution of resistance in this species. Low parasitemia and mild morbidity as measured by hematocrit, mass, nectar consumption, and behavior, combined with adaptive immune response and recovery, indicate that Oahu amakihi may have evolved a resistance to avian malaria. Oahu amakihi seem to survive infection by mitigating parasite load even during the crisis stage. This study reports the lowest peak parasitemia ever recorded in Hawaiian honeycreepers experimentally infected with avian malaria Oahu amakihi are currently found throughout the Koolau mountains, and in the southern Waianae mountains, all that remains of a distribution that was once island-wide. Analyses of the mitochondrial control region showed low diversity with only three haplotypes, including a common haplotype that was found in 95% of the samples. Analysis of nuclear microsatellite loci revealed high genetic diversity, and low but significant genetic structure, with the population fragmented into two genetic clusters by mountain range, with evidence of a population bottleneck.