UC Santa Cruz

Anthropogenic activities alter the host communities and environmental conditions that determine vector-borne pathogen transmission. Avian malaria (Plasmodium relictum) is a multi-host vector-borne parasite that infects numerous bird species. Avian malaria was recently introduced to Hawai‘i, where it is responsible for significant declines in native bird populations and is increasing in distribution with warming global temperatures. Our understanding of how anthropogenic changes to host communities and environmental conditions affect avian malaria transmission is limited because the roles of most host species are poorly understood, and the thermal responses of Hawaiian mosquito populations are undefined. My dissertation combines fieldwork and laboratory studies to quantify the infectiousness of different host parasite loads, or parasitemias, to mosquitoes, and it describes the influence of temperature on mosquito life history traits important for transmission. In my first chapter, I examined the effect of temperature on traits that determine population size for the primary vector in Hawai‘i, Culex quinquefasciatus. I described population-specific thermal response curves for larval development time, larval survival, and adult longevity. I used these data to predict adult mosquito populations as a function of temperature across time and space. My results indicate Cx. quinquefasciatus populations in Hawai‘i have increased over the last decade and will continue to increase as global temperatures warm. In my second chapter, I quantify the relationship between two methods of parasitemia measurement: microscopy of blood smears and qPCR. I then examined the extent to which microscopy underestimates infection prevalence in Hawaiian bird species and found that underestimation differed among species due to differences in species’ average parasitemias. In my third chapter, I quantified the effect of host parasitemia, day since mosquito feeding, and temperature on Cx. quinquefasciatus infectiousness, or the fraction of mosquitoes likely to transmit malaria. I found mosquito infectiousness increased gradually with increasing parasitemia, temperature, and the time since feeding. With this relationship, I estimated the infectiousness of parasitemias of wild-caught birds in Hawai‘i. The gradual shape of the relationship and considerable within-species variation in parasitemias led to many parasitemias, species, and communities creating infectious mosquitoes. As a result, most bird communities in Hawai‘i can sustain avian malaria transmission. This work provides a framework for determining the roles of host species and environmental conditions in avian malaria transmission and underscores the importance of using laboratory studies to quantify these relationships.