30 SEPTEMBER 2013 • WORLD AQUACULTURE • WWW.WAS.ORG Francisella Species, Subspecies Fish Hosts and Geographic Range The fish pathogenic Francisella are divided into two subspecies that correlates with their occurrence in coldwater or warmwater fish species. The coldwater pathogen, Francisella noatunensis subsp. noatunensis (Fnn), causes disease in Atlantic salmon Salmo salar (Birkbeck et al. 2007) and Atlantic cod Gadus morhua (Olsen et al. 2006). The warmwater pathogen, Francisella noatunensis subsp. orientalis (Fno) (syn. Francisella asiatica), causes disease in tilapia Oreochromis spp. (Chen et al. 1994, Mauel et al. 2003, Soto et al. 2009a), hybrid striped bass Morone saxatilis x M. chrysops (Ostland et al. 2006), and threeline grunt Parapristipoma trilineatum (Kamaishi et al. 2005). Francisella noatunensis subsp. orientalis has been detected by PCR in populations of ornamental cichlids (Hsieh et al. 2007) and was found to cause infections in Indo-Pacific reef fish (fairy wrasses Cirrhilabrus spp. and blue-green damselfish Chromis viridis) imported for the aquarium trade in the U.S. (Camus et al. 2013). These cases highlight the need for disease surveillance among shipments of ornamental fish and strict quarantine practices in public aquaria. Franciella noatunensis subsp. noatunensis has been reported in diseased Atlantic salmon in Chile and Atlantic cod in Norway and the North Sea. Francisella noatunensis subsp. orientalis has been detected or isolated from tilapia and ornamental cichlids in Taiwan, Central and South America, Jamaica, United Kingdom, Japan, Canada, and the United States, including Hawaii. Both are considered important diseases affecting aquaculture species. Clinical Signs and Pathology of Diseased Fish Francisellosis in coldwater and warmwater fish species present clinically in a similar fashion. They are dominated by severe granulomatous or pyogranulomatous inflammatory disease internally but few external clinical signs (Gjessing et al. 2011). Cod present with extensive gross internal lesions that appear as white, cream colored, partially raised nodules of varying size in the spleen, heart, kidney and liver. Fish are often anorexic and emaciated and have an accumulation of ascites in the visceral cavity. Occasionally hemorrhagic nodules are seen in the skin. Histopathological examination shows extensive chronic multifocal granulomatous lesions present in many organs in the cod. In advanced, naturally infected fish, well-formed granulomas are found in multiple tissues characterized by necrotic centers surrounded by a layer of epithelioid cells (macrophages) and an outer layer of fibroblasts, vacuolated cells, lymphocytes and small blood vessels (Birkbeck et al. 2011). Recently, tissue and cell dynamics in the spleen were examine from laboratory infected cod. The authors found at least three different cell types. Besides the classically reported macrophage-like cells, granulocyte-like cells staining for peroxidase and lysozyme were also found within infectious foci in the spleen. Thus, the authors suggest that, at least in cod, francisellosis causes a pyogranulomatous inflammatory response (Gjessing et al. 2011). Mortality rates in farmed cod can range from 5-40 percent and incidence in wild Norwegian cod can approach 13 percent (Ottem et al. 2008). The water temperature recorded during outbreaks on Norwegian cod farms was 10-15° C (Olsen 2006, Nylund et al. 2006) and 8-10° C on Chilean salmon farms (Birkbeck et al. 2007). Tilapia and other warmwater species present with similar lesions. Grossly fish have relatively few external clinical signs except for occasional hemorrhages or raised nodules in the skin or in the muscle just below the skin. Internally there is marked swelling of the anterior kidney and spleen, on average 10 times the normal size (Fig. 2). Numerous pale tan or cream colored foci (granulomas) are found scattered throughout the head kidney and spleen, with lesser numbers in the trunk kidney (Fig. 3). The tiny 0.5 × 1.0 µm bacteria may also be visible by Geimsa stain extracellularly in the gill and other tissues, especially in acute infections (Fig. 4). Microscopically the head kidney has up to 90 percent parenchymal replacement by coalescing granulomatous inflammation. Granulomas often have necrotic cores that contain numerous 1- µm wide coccoid bacteria. Splenic granulomatous inflammation is less discrete than in the head kidney, with abundant bacteria in macrophages throughout. Spleens are enlarged and similarly effaced by granulomatous inflammation. In chronically infected fish, well-formed granulomas containing numerous melanomacrophages are visible in wet mounts and squashes of spleen and head kidney tissue (Fig. 5). Livers are FIGURE 2. Tilapia with greatly enlarged spleen with granulomas. (Photo: Juan Morales). FIGURE 3. Numerous cream-colored foci (granulomas) throughout the head kidney and spleen. (Photo: Esteban Soto).
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