Bacterial distribution and tissue targets following experimental Edwardsiella ictaluri infection in Nile tilapia Oreochromis niloticus.

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.