Morphological and molecular evidence of Peircei-group Babesia in brown boobies (Sula leucogaster) from the Brazilian coast

Piroplasmids of the genus Babesia are globally distributed intraerythrocytic parasites transmitted by ticks and are recognized as significant pathogens in domestic mammals. In contrast, avian Babesia spp. remain poorly characterized, particularly in seabirds from the South Atlantic. This study describes the occurrence of Babesia sp. in brown boobies (Sula leucogaster) rescued along the Brazilian coast. Two individuals were analyzed: a juvenile from Espírito Santo and an archival sample from Santa Catarina. The 18S rRNA and ITS1 sequences were obtained and compared with reference datasets through maximum likelihood phylogenetics. Both samples exhibited characteristic forms of avian piroplasms, including rounded trophozoites, amoeboid stages, schizonts with residual bodies, and merozoites with variable chromatin positioning. Phylogenetic analyses placed the parasites within the "Peircei group", showing closest affinity to Babesia poelea and Babesia peircei. The recurrence of cases across a ten-year interval suggests persistent circulation in South Atlantic seabirds. Our findings expand the known geographic distribution of Peircei-group Babesia into the Brazilian coast and underscore the importance of rehabilitation centers as sentinel environments for detecting latent or emerging hemoparasites. The results also highlight the need for integrative taxonomic revision of avian Babesia spp. and investigation of potential tick vectors in marine ecosystems.