The epidemiology of a novel parasite in an endangered population of marbled murrelets () on the Oregon coast.

Parasitism is a known cause of morbidity and mortality in wildlife species and may exacerbate population declines in species threatened by changing landscapes. The marbled murrelet () is an unusual seabird that forages in the ocean but uses trees in old growth and late successional forests for nesting. Populations have declined in some areas due to both changing ocean conditions and reduction in breeding habitat, making it important to consider the impacts of other factors, such as parasites and pathogens, on murrelet health and reproduction. In this study we describe a novel blood parasite ( sp. nov. found in a murrelet population on the Oregon coast, USA and quantify its prevalence and burden in 374 individuals along the Oregon coast over a 6-year period. Genetic sequencing revealed that the species of we identified has a mitochondrial lineage most closely related to a found in yellow-eyed penguins () of New Zealand. The prevalence of in murrelets was 62 % (233/374) and within an infected individual the mean burden of parasitism was 7.1 parasites/100 white blood cells, with substantial variation between individuals (from 1 to 113 parasites/100 white blood cells). Both parasite prevalence and burden varied across years and were higher in years of poorer ocean conditions suggesting that birds experiencing poor conditions may have had reduced ability to fight infection. Male murrelets had significantly lower parasite burdens than females, which may be due to energetic constraints of egg production in breeding females. Importantly, murrelets that did not attempt a nest were associated with higher parasite burdens at the time of capture, perhaps due to correlations between parasitism and systemic health.