Entamoeba invadens infection contributes to juvenile mortality in captive sea turtles

Sea turtles are globally threatened marine reptiles, and infectious diseases are increasingly recognized as important causes of juvenile mortality. However, the etiology and pathogenesis of many diseases, including amoebiasis, remain poorly understood. Systemic amoebic lesions have been reported in sea turtles, yet the causative Entamoeba species has never been identified at the species level using molecular methods. We therefore investigated the etiology and pathogenesis of fatal amoebiasis using carcasses of 15 captive juvenile sea turtles (three loggerhead, Caretta caretta, and 12 green turtles, Chelonia mydas), integrating histopathological and molecular approaches. Histopathology revealed severe necrotizing enteritis, hepatitis, and pneumonia with abundant amoebic trophozoites and cysts. Polymerase chain reaction, Sanger sequencing, and in situ hybridization identified Entamoeba invadens in the intestine, liver, and lungs; 18S rRNA amplicon sequencing also detected E. invadens in fecal and intestinal samples. Phylogenetic analysis showed that the sea turtle-derived sequence clustered with E. invadens sequences from other reptile hosts. Bacterial and fungal aggregates were also observed within necrotic lesions, and 16S rRNA amplicon sequencing detected diverse bacterial communities without a consistently dominant taxon, consistent with secondary microbial translocation following intestinal barrier disruption rather than primary bacterial infection. These findings provide the first evidence that E. invadens causes fatal systemic amoebiasis in juvenile sea turtles and are consistent with polymicrobial invasion following parasite-induced intestinal damage. This study establishes a pathological and molecular basis for diagnosing systemic amoebiasis in sea turtles and has important implications for wildlife disease ecology and sea turtle conservation.