The surveillance and control programme for bonamiosis and marteiliosis in European flat oysters, Ostrea edulis, and blue mussels, Mytilus sp. in Norway in 2019

Abstract

This report describes the results from The surveillance and control programme for bonamiosis and marteiliosis and research activities on Marteilia sp. in 2019. The programme is carried out by the Institute of Marine Research according to a contract with the Norwegian Food Safety Authority. Samples were collected from two oyster farms, one wild oyster population, two mussel farms and four wild beds, two mussel farms and five wild mussel populations. Samples were collected in April/May and in October, in order to be able to detect Bonamia sp. and Marteilia sp. during the periods when the potential prevalence is at the highest. Samples from one of the mussel farms and one wild bed were collected after reports of elevated mortality. Except from these samples, no abnormal mortalities were observed during the surveillance. Bonamia ostreae / B. exitiosa were not detected. There have been several reports on mortality or “disappearance” of mussels along the Norwegian coast. The reason(s) for the mortalities have not been determined. However, the parasite Marteilia sp. was detected for the first time in mussels, Mytilus edulis, collected at Bømlo, western Norway in 2016 and Tysnes in 2019. This has been followed up with an extended survey in several research projects. The results show that Marteilia sp. infecting mussels in Norway, Sweden and England are different from Marteilia refringens infecting flat oysters. The name Marteilia pararefringens has been proposed, and there is strong evidence that Marteilia refringens and Marteilia pararefringens sp. nov. are distinct parasites of bivalves and have different European distributions. The work will be continued in 2020 – 2021, linked to research on the distribution of M. pararefringens in wild mussels. We propose a revision of the surveillance programme combined with the establishment of a new model for health control in mollusk farms, application for disease free status for Norwegian flat oysters and a categorization of zones.