VACCINE FOR FISH AGAINST HEMORRHAGIC SEPSIS

Bacterial hemorrhagic sepsis (formerly known as hemorrhagic septicemia) is a severe systemic disease that affects numerous fish species, particularly under confinement conditions such as in aquaculture facilities, where stress, high stocking density, and skin injuries facilitate infection. The pathogens responsible for sepsis are able to overcome the immune barriers, spread through the bloodstream, and cause internal and external hemorrhages, multi-organ inflammation, and high mortality. This disease can be caused by multiple bacterial species, including Vibrio vulnificus, V. harveyi, Photobacterium damselae subsp. damselae (Pdsd), V. parahaemolyticus, and Aeromonas salmonicida. Among these, V. vulnificus and Pdsd stand out as zoonotic marine pathogens, capable of infecting both fish and humans. In V. vulnificus, the ability to cause sepsis in fish depends, among other factors, on a "fish-blood survival kit" composed of an outer membrane protein and an outer membrane lipoprotein both encoded on transferable plasmids. Only strains that have acquired these genes are capable of causing sepsis in fish. Both genes have also been identified in strains of Pdsd and V. harveyi, among other species. Currently, most commercial vaccines against fish hemorrhagic sepsis are designed to target a single bacterial species and all of them are conventional formulations. The increase in seawater temperature, associated with climate change, is further promoting the proliferation and expansion of V. vulnificus and Pdsd, while also enhancing their virulence, resulting in a greater zoonotic risk for animals and humans. Consequently, it is urgent to develop effective preventive and control strategies against bacterial hemorrhagic sepsis that not only protect against the disease but also limit the transmission of virulence genes among bacteria in aquaculture facilities, thereby contributing to the integrated control of infection and its genetic dissemination within the aquaculture ecosystem, in the framework of a One Health strategy.