Impact and distribution of Perkinsus olsenii in cultivated Mediterranean mussel Mytilus galloprovincialis from the southern coast of Campania (Italy)

Following a previous description from the Campania region (southern Italy), a survey for perkinsosis was performed at 12 mussel farms along the Italian coast during the period October 2022 to July 2023. The prevalence of Perkinsus parasites was estimated by PCR using genus-specific primers. Seven farms tested positive and sequencing was performed to identify the Perkinsus species. The parasite Perkinsus olsenii was detected in the farm samples with a BLAST identity of 97-100%. One of the affected farms was also monitored for the period of July 2022-June 2023 using histopathology, Ray's fluid thioglycollate culture method, and quantitative PCR. The sensitivity of each of the techniques was also determined. Trophozoite stages were observed mostly aggregated in the connective tissue of the digestive tract and gonad/mantle, and to a lesser extent in the foot and kidney, producing an encapsulated inflammatory response that occupied most of the tissues in some samples. The condition index of affected individuals was lower than in healthy ones (p < 0.05) and larger animals (5.8-6 cm) were more often infected (p < 0.05). This study provides information regarding P. olsenii pathogen infections in M. galloprovincialis in the Campania region, its seasonal variations, and host-parasite interactions over a 1 year period.