Avian malaria on Oahu: evidence of disease resistance in a native Hawaiian honeycreeper
The endemic birds of the Hawaiian Islands evolved in the absence of many diseases, with loss of immunity. Experiments on native Hawaiian forest birds from the Island of Hawaii have indicated high mortality rates when infected with introduced avian malaria (Plasmodium relictum). The Oahu amakihi (Hemignathus flavus), is resident and breeding in some low elevation forests on Oahu. The objective of this study was to experimentally infect Oahu amakihi with avian malaria, and track the disease pathology to determine if this species has evolved a resistance to avian malaria. Four Oahu amakihi were inoculated with malaria positive avian blood, while a control group (n=4) remained uninfected. Treatment group birds showed a peak parasitemia of 8% during 14-18 post-infection, then a decline to levels detected only by molecular methods by day 72. Infected birds showed a decrease in hematocrit and weight, then recovery by day 30 post-infection. Infected birds also showed a decrease in activity level during the pre-crisis and crisis periods, then returned to normal. Malaria antibody levels peaked in concordance with peak parasitemia, then slowly declined. Low parasitemia and morbidity combined with an efficient adaptive immune response and recovery indicate Oahu amakihi have evolved a resistance to this disease. The results provide valuable insights into the evolution of resistance to avian malaria in related species of native Hawaiian forest birds.