A new Plasmodium Marchiafava & Celli, 1885 (Apicomplexa: Haemosporida) species in Cory's Shearwater (Calonectris borealis) [Cory]) (Aves: Procellariiformes) stranded in a coastal area in Brazil.

Cory's Shearwater (Calonectris borealis), a migratory seabird from the Procellariidae family, is widely distributed across the Atlantic Ocean. It breeds on islands of Azores, Madeira, Canary, and Berlengas, and winters in coastal areas of South America and Africa. This species' wide habitat range contributes to pathogen exchange across global regions. Plasmodium, a genus of apicomplexan parasites causing avian malaria, affects many wild bird species worldwide. Although rare in seabirds, Plasmodium infections can significantly impact these hosts. We identified and characterized a new Plasmodium species in an adult male C. borealis found live-stranded on the northern coast of São Paulo, Brazil, during the daily activities of the Santos Basin Beach Monitoring Project (PMP-BS). The bird was rescued and kept in rehabilitation for 61 days but was euthanized due to poor prognosis. Blood smears revealed parasitemia varying from 0.04% until 2.09%. Blood and tissue samples were tested using PCR targeting a cytochrome b (cytb) gene fragment. The obtained sequence was 100% similar to Plasmodium lineage pLK06. Phylogenetic analysis placed the parasite within the Novyella clade, with morphological features closely related to P. vaughani. Necropsy showed hepato- and splenomegaly, while histopathology confirmed hepatitis and pulmonary hemorrhage. These findings, combined with clinical relapse despite anti-malaria treatment, indicate the parasite's pathogenicity in C. borealis. This represents the first haemosporidian infection in this seabird and the first identification of the pLK06 lineage in Procellariiformes, resulting in the description of Plasmodium borealis sp. nov. These findings expand knowledge of parasite diversity in seabirds and highlight the need for continued surveillance in understudied avian groups.