The lineage diversity, spatiotemporal distribution and pathological significance of Plasmodium and Haemoproteus spp. infection of wild birds in Great Britain

Avian haemosporidian parasites (AHPs), which include the genera <i>Plasmodium</i> and <i>Haemoproteus</i>, are protist parasites affecting at least 2000 species of birds with near global distribution. Outside of isolated, evolutionarily and immunologically naïve avian populations, the effects of AHPs on wild bird populations are poorly understood but have historically been considered benign. There is growing evidence to suggest, however, that high exoerythrocytic parasite burdens can cause disease and mortality in some host-parasite interactions, even in populations which have co-evolved alongside AHPs. Here, samples from 857 wild birds of 62 species, 27 families and eight orders were collected during post-mortem examinations over a 15-year period as part of a nationwide wildlife disease surveillance scheme and were screened by nested polymerase chain reaction (PCR) for the presence of <i>Plasmodium</i> and <i>Haemoproteus</i>. In total, liver and/or spleen tissues from 13.5 % of birds (n = 116) tested PCR-positive, comprising 8.9 % (n = 76) and 4.7 % (n = 40) infected with <i>Plasmodium</i> and <i>Haemoproteus</i> spp., respectively. The highest rates of <i>Plasmodium</i> infection were seen in the families Paridae (36.3 %; 4/11 birds examined) and Turdidae (34.5 %; 51/148), consistent with previous reports. Spatial analysis revealed a significant cluster of <i>Plasmodium</i>-positive cases in Southeast England with possible explanations including climatic effects on parasite development or spatial variation in vector abundance. A total of 30 AHP lineages (20 <i>Haemoproteus</i> spp. and 10 <i>Plasmodium</i> spp.) were detected, 23 of which have not previously been reported in Great Britain, with four being apparently novel. Tissue samples from a subset of 13 <i>Plasmodium</i>-positive Eurasian blackbirds (<i>Turdus merula</i>) underwent histopathological examination, which revealed evidence of exoerythrocytic parasites, or other lesions consistent with avian malaria, in four and five cases, respectively. These changes were considered of equivocal significance in four birds, with only one bird diagnosed with acute malaria as a contributory cause of death.