Perkinsus olseni in green-lipped mussels Perna canaliculus: diagnostic evaluation, prevalence, and distribution.

Perkinsus olseni (Perkinsidae) is a molluscan parasite notifiable to the World Organisation for Animal Health that is reported in several shellfish hosts in New Zealand, including the native green-lipped mussel Perna canaliculus. Green-lipped mussels comprise over half of New Zealand's aquaculture export value and have historically been considered free of serious diseases based on extensive histology-based surveillance. The discovery of P. olseni in green-lipped mussels has raised questions about future disease threats to green-lipped mussels, particularly under changing ocean climatic conditions. Using mussels collected from farmed (n = 358) and wild (n = 236) populations, we aimed to determine the distribution and prevalence of P. olseni in green-lipped mussels around New Zealand, and assess the performance of diagnostic tests, including real-time PCR, conventional PCR, and culture using Ray's fluid thioglycolate medium (RFTM). Prevalence and diagnostic test performance was evaluated using Bayesian latent class analysis with informative priors. The prevalence of P. olseni was 0-3%, except for 1 wild population from a harbour where prevalence was 22%. Real-time PCR had the highest diagnostic sensitivity (87%) compared to 62 and 21% for conventional PCR and RFTM, respectively. Diagnostic specificity was similar among all methods (96-98%). No mortality was observed during the study. Our results suggest that real-time PCR is the diagnostic test best suited for surveillance of P. olseni in subclinically infected green-lipped mussels under New Zealand conditions.