bivalve mollusc Bacteria Pacific Oyster Nocardiosis I. Causative Agent and Disease IV. Transmission Pacific oyster nocardiosis (PON), The bacterium is considered to be an previously known as “focal necrosis” opportunistic pathogen that is widespread and “fatal inflammatory bacteremia” in the marine environment. Transmis- (FIB) is caused by an actinomycete sion is likely horizontal and facilitated by bacterium Nocardia crassostreae that other physical environmental stressors. is Gram-positive or Gram variable, acid fast, PAS positive and catalase positive. V. Diagnosis The bacterium contributes to “summer Histological examination of typical mortality” of 2 year and older oysters in lesions in infected oyster tissues indicate shallow, warm eutrophic embayments Gram-positive or Gram-variable, acid fast during August through November when foci of bacteria within surface vesicles seawater temperature is 20°C or greater. and in granulomas eliciting a marked Infection produces small round yellow, host inflammatory response. Confirma- green or brown vesicles of bacteria and tion is by fluorescent antibody staining granulmoas 2 mm to 1 cm in diameter and by transmission electron microscopy throughout oyster tissues resulting in showing typical actinomycete morpholo- significant mortality. gy. Culture of the organism is possible on several medias for fastidious organisms II. Host Species but is not a routine procedure because the This disease is reported to be appearance of colony growth may take restricted to Pacific oysters, although a from 3 weeks to 3 months. similar disease has been observed in Eu- ropean flat oysters cultivated near areas VI. Prognosis for Host of infected Pacific oysters. PON has been Mortality of infected oysters may be reported in Pacific oysters from Califor- significant but has not been accurately nia, Washington, British Columbia, Japan determined in the field. However, ex- and most recently from the Netherlands. perimental injection of the organism has In Alaska, infection by this organism was produced 47-50% cumulative mortality detected in only two juvenile animals of oysters within 30 days. Management imported from the state of Washington techniques have not been investigated but but had been reared in Alaskan waters. culturing oysters in off-bottom gear and Therefore, it is unclear whether the avoidance of shallow warm embayments disease can spontaneously occur in the in the summer may reduce the prevalence colder seawater temperatures typical of and severity of the disease. Alaskan summers. VII. Human Health Significance III. Clinical Signs Aside from aesthetically displeas- Clinical signs of PON include moder- ing, there are no known zoonotic human ate to high mortality of young adult health concerns with Nocardia infection oysters that are undergoing physiological of Pacific oysters. stress from rapid gonadal development with presenting lesions of small, round, yellow, green or brown vesicles of bacte- ria and granulomas in the tissues. 14 15 bivalve mollusc Bacteria Yellow vesicles (arrow) of nocardiosis Histological section of vesicles containing bac- in the mantle tissues of a Pacific oyster teria (arrow) in infected mantle tissue (Photo: R. Elston, AquaTechnics, WA) Typical subsurface radiating granuloma (arrow) containing central Nocardia organisms sur- ounded by host inflammatory cells, histological section 14 15.