Epidemiological surveillance of Francisella noatunensis subsp. orientalis infection in tilapia farms from São Paulo state and its application in the elaboration of vaccines

Abstract

Tilapia is the main and biggest growing farmed fish in Brazil, however infectious diseases are major obstacles for Brazilian tilapia culture. Currently, Francisella noatunensis subsp. orientalis (Fno) is one of the main pathogens in Brazilian tilapia farming, inducing high mortality especially during the cold season. However, studies on Fno are scarce worldwide, including Brazil, where its first isolation was not many years ago time (2012), and there are no commercial or experimental vaccines against this pathogen. Therefore, it is imperative to develop more epidemiological and vaccine studies involving Fno. The present proposal aims to establish an epidemiological surveillance scheme for Fno infection in intensive cultured Nile tilapia, Oreochromis niloticus, from São Paulo state (SP), by focusing on the isolation of different bacterial strains, their characterization and subsequent use for the preparation of vaccines using different antigen preparation methodologies. Briefly, animals with signs of infectious process will be sampled from 20 fish farms to detect Fno through bacteriological isolation and direct PCR of tissues. The isolates will be confirmed by biochemical tests, conventional PCR and sequencing, and submitted to genotyping. Virulence will be determined by experimental challenges, with further evaluation of histopathological survival and lesions. Depending on the type of vaccine (live or not) the less or the most virulent strains of each characterized genotype will be used as vaccine antigens. Four methods of immunogen preparation will be tested (total inactivated bacterium, outer membrane vesicles, and attenuated bacteria resistant to rifampicin or not). Vaccine safety and efficacy will be assessed by measurement of IgM antibodies anti-Fno at 28 days post-vaccination (pre-challenge), relative survival percentage (PRS) and survival (pre-challenge and post-challenge), anti-caspase 3 immunohistochemistry (pre-challenge and post challenge) and re-isolation of the bacteria (pre-challenge, during and post-challenge). The project will expand the epidemiological knowledge over this pathogen in the second tilapia producer state of the country and will allow to expand the studies on tilapia vaccination in Brazil. (AU)