Coccidiosis causing mass mortality of wild green sea turtles in south east Queensland, Australia: Molecular investigations of an emerging disease.

The spring months of 2014 saw mass mortalities among wild green sea turtle populations of south-east Queensland. Turtles were found stranded and moribund and rarely recovered despite veterinary intervention. The suspected causative agent was Caryospora cheloniae, an unsequenced eimeriid coccidian implicated in earlier epizootics.

Eleven turtles underwent post-mortem examination. PCR testing of various tissues with apicomplexan specific primers was followed by sequencing. Histology was performed to detect parasites and characterise associated pathology. A quantitative real-time PCR was developed for future investigations and diagnostics.

Sequences were obtained from the brain, gastrointestinal tract, lung, kidney and thyroid. The brain was most frequently infected, with granulomatous encephalitis consistently present. Enteritis, thyroiditis and nephritis was also observed. Two distinct genotypes were identified and were associated with different tissues, indicating clear tropisms. Phylogenetic analyses placed the first genotype closest to the lankesterellid genus Schellackia, while the second was paraphyletic relative to the eimeriids.

Our analyses revealed previously unrecognised diversity among sea turtle coccidia, and also suggested that the classification of these important parasites requires review. This study is the first molecular characterisation of these emerging pathogens, and provides an important basis for parasite life-cycle elucidation, epidemiological investigation and diagnostics.