Abstract
The annual production of European flat oysters (Ostrea edulis) in Croatia is about 50 to 65 tons, and it has a long tradition. All Croatian oyster farms are subjected to the national surveillance program aiming to detect the presence of Bonamia ostreae and Marteilia refringens according to the Council Directive 2006/88/EC. Within the surveillance program, the first findings of the parasite Bonamia spp. occurred in 2016 in two production areas in the north and south of the Eastern Adriatic coast. The repeated findings of the parasite were noted up to 2020 but also on two additional sites in the north. The parasite was detected by cytological analysis of stained heart smears, histological examination, and PCR. PCR positive samples were sequenced for SSU rDNA gene, and BLAST analysis confirmed infection with Bonamia exitiosa. Attempts to prove the Pacific oyster as a putative vector of the parasite failed. The infection prevalence from 2016 until 2020 ranged from 3.3 to 20% in different sites. No mortalities were reported from the infected sites, and it seemed that infection of flat oysters with B. exitiosa did not affect their health. The study has not shown the source and way of infection spread, which imposes the need for more comprehensive molecular and epidemiological studies.
Highlights
The production of European flat oyster (Ostrea edulis) has a long tradition and economic significance in many countries of the Mediterranean basin and the East Atlantic coast
This paper presents the findings, distribution, and prevalence of the Bonamia exitiosa in the samples of flat oyster analyzed within the national surveillance program for the infection with Bonamia parasites
There were no reported mortalities, and there were no symptoms of the disease on the oysters from the production areas included in the surveillance program
Summary
The production of European flat oyster (Ostrea edulis) has a long tradition and economic significance in many countries of the Mediterranean basin and the East Atlantic coast. Diseases have caused large losses and dramatically affected production [2,3,4]. One of these pathogens is the protozoan parasite Bonamia ostreae, belonging to the clade “microcell” within Haplosporidia [5]. Identifying parasite species due to morphological similarity is time-consuming and requires experience and expertise. Both these methods have low sensitivity and specificity [6,7,8,9], similar to TEM, which seems to be insufficient in the identification of species [10]. Diagnostic sensitivity has significantly improved with the implementation of molecular methods, such as PCR [11,12] or in situ hybridization
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