Epidemiological study of Bonamia sp. in the native flat oyster, Ostrea puelchana from San Matías Gulf (NW Patagonia, Argentina)

In 1996 a mass mortality was reported in Ostrea puelchana culture in San Antonio Bay (San Matías Gulf, North Patagonia, Argentina) and Bonamia sp. was identified as a possible aetiological agent. The presence of this pathogen was the first record for this genus in the SW Atlantic Ocean and is the only know OIE (Office International des Epizooties) Notifiable Disease of aquatic organisms known in Argentina. In this study we surveyed natural beds for five years, in order to describe the spread, as well as size and sex-related variation in the prevalence of the Bonamia sp. infection, into San Matías Gulf. The prevalence of infection by Bonamia sp. in 1996–2001 clearly defined the focus of infection from which the parasite spread. Infection radiated from this focus, in the culture area inside San Antonio Bay in 1996, through the oysters beds located at the SW and NE of the bay. Statistical analysis indicated that there were no differences between the mean height of the parasitized and non-parasitized oysters and infections with Bonamia sp. were observed in all sexual categories. Their prevalence varied between beds. Statistical analysis indicated that the probability of infection by Bonamia sp. was independent of the sex. We propose that Bonamia sp. is an enzootic parasite of San Matías Gulf, because of the demographic isolation of the native oyster populations, as San Matías Gulf is a relative close geographical area. Also, there is not existence of other banks with similar densities in the geographical distribution range of O. puelchana. Furthermore, the presence of natural dense beds close to the oyster culture, the existence of strong tide currents in the area, the hydrodynamic of the marine bottom and environmental modifications are all factors that may contribute to the spread of Bonamia sp. infection among SW and NE natural beds of O. puelchana in San Matías Gulf.