Variation in pathogen load and the pathogen load–infectiousness relationship broaden avian malaria’s distribution

The infectiousness of host species and the feeding patterns of vectors influence pathogen distribution and transmission intensity. Two aspects of host infectiousness shape transmission but are underappreciated: the relationship between pathogen load and infectiousness and variability in pathogen load within species. We quantified the relationship between host parasitemia for avian malaria (Plasmodium relictum) and infectiousness for biting Culex quinquefasciatus mosquitoes using experimentally infected birds. We used this relationship and 4,218 samples from wild birds to estimate the relative importance of hosts’ acute and chronic stages of infection, the infectiousness of 17 bird species in Hawaiʻi, and mosquito feeding preferences. Infectiousness to mosquitoes increased with host parasitemia (and temperature and time since feeding), but the relationship’s gradual slope led to a wide range of parasitemias being partly infectious to mosquitoes. Chronic stage infections create far more infectious mosquitoes than the acute stage, and high within-species variability in parasitemia led to broad overlap in the infectiousness of most bird species. Disproportionate feeding patterns (inferred from relative infection prevalence) elevated the importance of a few bird species in transmission, but broad overlap in species infectiousness resulted in most host communities having similar total infectiousness. Similar community infectiousness helped explain the widespread distribution of avian malaria throughout Hawaiʻi despite highly variable species communities - it was detected at 63/64 moderately well-sampled sites. These results demonstrate the importance of the shape of the pathogen load-infectiousness relationship and within-species variability in pathogen load in influencing a pathogen’s host range, transmission intensity, and spatial distribution.

Methods
This dataset contains the data and code required to replicate analyses in Seidl et al. 2025, which examines the contribution of host community composition to pathogen transmission and distribution and how these are influenced by the relationship between pathogen load and infectiousness and variability in pathogen load within host species. The study system described by the data is avian malaria (Plasmodium relictum) in birds in the Hawaiian Islands, USA. Multiple files are included, containing data from field and laboratory studies, mathematical models, and published literature. Some of the files describe a series of experiments with mosquitoes, avian malaria, and domestic canaries. Other files report results from avian malaria sampling of wild birds in Hawaii that was conducted in 2015-2017 by the Hawai‘i Vertebrate Introductions and Novel Ecosystems project, in 2018-2021 by the U.S. Geological Survey’s Avian Malaria Genomic Research Project, and in 2019-2020 by author C.M. Seidl as part of her Ph.D. dissertation research. Point count sampling data and bird density estimates generated using distance sampling from sites on Hawaii Island are included. Finally, files containing data from published experimental infection studies of native Hawaiian birds and Culex quinquefasciatus mosquitoes are summarized.