Photobacterium damselae subsp. piscicida Infection, a Persistent Cause of Mortalities in Old Hatchery-reared Gilthead Seabream (Sparus aurata) Broodstocks: Molecular-pathological Evidences and Control Strategy

Abstract

Almost all Egyptian marine hatcheries collect their young Gilthead seabream brood stock from open water at Deeba Triangle or Damietta. Some of these wild broodstock could possibly harbour hidden mild Photobacterium damselae subsp. piscicida infection. In the current study, a total of 20 mortal/moribund 5-7-years old broodstocks were clinically examined and then subjected to microbiological and histopathological examination. Moribund brood stocks were lethargic and anorexic. Externally, broodstocks showed generalized skin darkening, haemorrhages at the base of fins together with pale gills. Internally, the spleen, liver, and kidney were remarkably infiltrated with several whitish various-sized nodules together with an unpleasant odor originating from the abdominal cavity of necropsied brood stocks. Pathology has revealed multiple granulomas together with lymphocytic and inflammatory cell infiltrations through the anterior kidney, spleen and liver tissues together with severe melano-macrophage center (MMC) activation. Bacteria-laden macrophages were also noticeable through stained tissue sections. Photobacterium damselae subsp piscicida was presumptively isolated from the internal organs of moribund and mortal fishes. Initial identification was based on bipolarity staining, API 20 E profile, and antibiotic susceptibility testing. The final identity was confirmed using partial sequencing of 16S rRNA gene. The control strategy was based initially on oral antibiotic therapy (florfenicol) to combat systemic infection and tank water disinfectant (Hydrogen peroxide) to minimize bacterial load followed by an immune-boosting feed additive program utilizing probiotics, B-glucan, nucleotides, vitamin C, organic zinc and selenium. Brood stock tanks' follow-up for 1 month after the initiation of the control strategy has revealed no mortalities among reared broodstocks.