A Piscirickettsiosis‐like Syndrome in Cultured Nile Tilapia in Latin America with Francisella spp. as the Pathogenic Agent

Abstract

In 2004, cultured Nile tilapia Oreochromis niloticus in several Latin America farms began to succumb to a disease similar to the piscirickettsiosis-like syndrome previously reported in tilapia in Taiwan and the United States. Mortality increased during 2005; reductions in tilapia biomass ranged from 5% to 80% in individual ponds and averaged 50% overall. All ages of fish have been involved. Clinical signs include lethargy, loss of appetite, petechia, exophthalmia, and abnormal swimming behavior. Gross lesions have included splenomegaly, renomegaly, and numerous white nodules observed in the spleen, kidney, testes, heart, ovaries, and occasionally the liver. A previously unreported black granulomatous lesion was reported in up to 30% of the fillets. Histologically, granulomatous infiltrates were observed in the kidney, spleen, liver, testes, ovary, and choroid gland, and rarely in the brain and heart. A small pleomorphic bacterium was observed in Giemsa-stained blood smears and spleen imprints. The bacterium did not grow on standard microbiological media and has not been isolated in cell culture. We obtained a near-complete 16S ribosomal DNA sequence with high similarity to Francisella spp. sequences previously identified in tilapias Oreochromis spp. (Taiwan), Atlantic cod Gadus morhua (Norway), and three-line grunts Parapristipoma trilineatum (Japan).

Outbreaks and genetic diversity of Francisella noatunensis subsp orientalis isolated from farm-raised Nile tilapia (Oreochromis niloticus) in Brazil

“…The diseased fish presented with anorexia, melanosis, exophthalmia, erratic swimming, skin ulcers, and gill pallor. These clinical signs were similar to previously described outbreaks of FNO-related francisellosis in tilapia in Central America (Mauel et al, 2007;Soto et al, 2009), Taiwan (Chen et al, 1994), Indonesia (Ottem et al, 2009), the UK (Jeffery et al, 2010), and the USA (Soto et al, 2011). Our findings represent the first report of outbreaks of FNO-related francisellosis in South America.…”

Development of a real-time PCR assay for identification and quantification of the fish pathogen Francisella noatunensis subsp. orientalis

“…Upon macroscopic and microscopic examination, internal organs are enlarged and contain widespread multifocal white nodules. Histological examination often reveals the presence of multifocal granulomatous lesions, with the presence of numerous small, pleomorphic, cocco-bacilli (Hsieh et al 2006, Mauel et al 2007, Soto et al 2009a). …”

“…During the past 5 yr, bacteria of the genus Francisella have caused significant mortalities in cultured tilapia Oreochromis spp., Atlantic cod Gadus morhua, Atlantic salmon Salmo salar L., hybrid striped bass Morone chrysops × M. saxatilis, three-line grunt Parapristipoma trilineatum, and ornamental cichlids, both in warm-and cold-water environments (Kamaishi et al 2005, Nylund et al 2006, Olsen et al 2006, Ostland et al 2006, Birkbeck et al 2007, Hsieh et al 2007, Mauel et al 2007, Mikalsen et al 2007, Ottem et al 2007, Soto et al 2009a.…”

“…In the majority of the cases, PCR and sequence comparison of the 16S rRNA have made it possible to place the organisms at 97% similarity to F. tularensis, 98% similarity to F. philomiragia, and 99% to other Francisella spp. strains isolated from fish species (Kamaishi et al 2005, Hsieh et al 2006, Ostland et al 2006, Mauel et al 2007, Mikalsen et al 2007, Ottem et al 2007, Soto et al 2009a. The Francisella strain utilized in this research project at the Louisiana Aquatic Diagnostic Laboratory, LSU School of Veterinary Medicine, LADL 07-285A, isolated from tilapia Oreochromis spp.…”

“…Previous diagnosis of francisellosis in fish species has been made with the aid of histopathology, electron microscopy, conventional culture assays, conventional polymerase chain reaction (PCR) using Francisella sp. specific primers, 16S rRNA sequencing, and in situ hybridization (Kamaishi et al 2005, Hsieh et al 2006, Ostland et al 2006, Mauel et al 2007, Ottem et al 2007, Soto et al 2009a). However, the diagnosis of the pathogen remains a challenge, and some of the current techniques are difficult, time consuming, and expensive; some require specialized personnel and are prone to show false negatives because of low sensitivity, or false positives because of low specificity.…”