A systematic surveillance program conducted since 1980 along the coasts of Brittany has followed the evolution and epidemiological state of the natural O. edulis oyster beds and seed areas in the intertidal zones and deep water.
Presently, Marteilia refringens exists endemically in Brittany estuaries; the rate and degree of infestation varies from year to year depending on the hydrological conditions and the geographical region. In no case, however, is the return of flat oyster culture envisioned.
Bonamia ostreae is present not only in the estuaries but also in the deep-water rearing areas (Baie de Quiberon, Cancale, Baie de St-Brieuc). The infestation increases with age, but also depends on the quantity of the stock and the culture practises. In particular, the transfer of oysters older than 18 months is associated with general infestation and major mortalities 6-8 months later.
The natural oyster beds situated outside the oyster rearing areas remain untouched by the diseases. This emphasizes the importance of developing prophylactic measures to limit the spread of the diseases when an epizootic occurs.