Edwardsiella ictaluri, a Gram-negative enteric bacterium, is the known etiological agent of enteric septicemia of catfish. In the last few years, different strains have been implicated as the causative agent of mortality events in cultured fish, including Nile tilapia Oreochromis niloticus L. Due to the emergent nature of edwardsiellosis in non-ictalurid fish, little is known about the dynamics of E. ictaluri infection in tilapia. The purpose of this study was to gain a better understanding of the pathogenesis of edwardsiellosis in tilapia by determining the median lethal and infective doses, tissue targets of infection, rate of bacterial dissemination, and the specific tissue response to E. ictaluri following an immersion challenge with bacterial strains recovered from outbreak events in tilapia. In addition to histopathology assessment, the bacterial burdens in several tissues of infected fish were determined over a 2 wk course of infection using quantitative real-time PCR (qPCR). The collected data suggest the cutaneous and oral routes as the main ports of entry for the organism, which later spreads hematogenously throughout the body. Even though histopathological assessment of infected fish revealed involvement of a wide range of tissues, the severity of the necrotizing and granulomatous lesions in the spleen and head kidney, with concomitant high levels of bacterial DNA in these organs determined by qPCR, identifies them as the main targets of infection.
KEY WORDS: Edwardsiella ictaluri · Histopathology · Pathogenesis · Real-time PCR · Tilapia
Resale or republication not permitted without written consent of the publisherDis Aquat Org 104: [105][106][107][108][109][110][111][112] 2013 Hanson 2011). In Edwardsiella ictaluri-infected tilapia, the most consistently reported findings are multifocal nodulations in the spleen, head kidney, and hepato megaly with a reduced fat reserve in the liver and peritoneal cavity. Moreover, microscopically, a severe chronic inflammatory infiltrate is observed in the spleen, head kidney, and liver, with multifocal areas of necrosis and granuloma formation (Soto et al. 2012).The use of experimental animal models remains critical to elucidating disease pathogenesis, especially when studying emergent diseases. In the present study, we used quantitative real-time PCR (qPCR) and histological analysis to evaluate the dissemination of Edwardsiella ictaluri in Nile tilapia under controlled conditions.
MATERIALS AND METHODS
BacteriaEdwardsiella ictaluri strain RUSVM-CO-2-5 was originally a clinical isolate obtained from a recently reported outbreak of edwardsiellosis in cultured tilapia (Soto et al. 2012). The bacterium was grown on tryptic soy agar (TSA) supplemented with 5% sheep blood at 28°C for 48 h. Three colonies were suspended in 100 ml of brain-heart infusion broth (Remel-Thermo Fisher Scientific), and grown overnight at 150 rpm in a shaking incubator maintained at 27°C. The next morning, bacteria were pelleted, washed twice in 1× phosphate-buffered saline (PBS), ...
