Climate-Driven Increase in Transmission of a Wildlife Malaria Parasite Over the Last Quarter Century

Abstract

Climate warming is expected to influence the prevalence of vector-transmitted parasites. Understanding the extent to which this is ongoing, or has already occurred, requires empirical data from populations monitored over long periods of time, but these studies are sparse. Further, vector-disease research involving human health is often influenced by disease control efforts that supersede natural trends. By screening for malaria parasite infections in a wild population of blue tits (Cyanistes caeruleus) in Northern Europe, over a 26-year period, we tested whether prevalence and transmission changes were climate-driven. We found that all three malaria parasite genera occurring in blue tits (Haemoproteus, Plasmodium, and Leucocytozoon) have increased significantly in their prevalence and transmission over time. The most common parasite in the study, Haemoproteus majoris, increased in prevalence from 47% (1996) to 92% (2021), and this was a direct consequence of warmer temperatures elevating transmission. Climate window analyses revealed that elevated temperatures between May 9th and June 24th, a time period that overlaps with the host nestling period, were strongly positively correlated with H. majoris transmission in one-year-old birds. A warming climate during this narrow timeframe has had a demonstrable impact on parasite transmission, and this has favored an increase in the prevalence of parasites in wild birds in a temperate region of Europe. While more challenging to measure, similar implications of climate warming on human vector-disease systems might be occurring. It is therefore critical that we understand what specific aspects of malaria parasite development and transmission are most influenced by climate warming, for the benefit of human and wildlife health.