Molecular investigation of an epizootic event in green sea turtles associated with coccidiosis
Date:
The spring months of 2014 saw mass mortalities among wild green sea turtle populations of south-east Queensland. Turtles were found stranded and moribund, often with neurological symptoms, and rarely recovered despite veterinary treatment. The suspected causative agent was Caryospora cheloniae, an eimeriid coccidian first identified in maricultured turtles from Grand Cayman, and implicated in an earlier epizootic in south east Queensland. 11 dead turtles, representing a subset of the total mortalities, were necropsied and samples taken for histopathology and molecular analyses. PCR of various tissues using apicomplexan specific primers returned positive results; infected organs included the brain, gastrointestinal tract, lung, kidney and thyroid. The brain was the most frequently infected site, giving positive results in 10 of the 11 tested, with granulomatous encephalitis consistently observed. Enteritis was also observed in several animals (4 of 9 samples examined), and, less frequently, thyroiditis and nephritis (2 of 9). No positive PCR results were obtained from heart, liver or reproductive tissues. Analyses of sequences obtained from tissues found that two distinct genotypes were present. These genotypes were associated with different sites within the host, with one found in the brain, gastrointestinal tract and lung, while the second was associated with the thyroid and kidney. Phylogenetic analyses revealed that the first genotype was most closely associated with the lankesterellid genus Schellackia, while the second was paraphyletic relative to the eimeriids. Infection of extra-intestinal tissues of the primary host is unusual among eimeriids, and requires further exploration. Additionally, the current placement of these species within the Eimeriidae may require review. This study is the first characterisation of these emerging pathogens in green sea turtles, and provides an important basis for life-cycle elucidation and future diagnostics.