World Aquaculture 39 Developments in selective breeding for resistance to Aeromonas hydrophila in fish S. Das1 and P.K. Sahoo1 Aeromonas hydrophila is a gram-negative, facultatively anaerobic, oxidase-positive bacterium causing a wide range of acute, chronic and covert infections in fish, such as dermal ulceration, fin rot and tail rot, ocular ulcerations, erythrodermatitis, hemorrhagic septicaemia (Fig. 1), redsore disease, red-rot disease, and scale-protrusion disease. Many freshwater fishes (including Nile tilapia, channel catfish, rainbow trout, catla, rohu, mrigal, catfish, eel, goldfish, Puntius sp., Asian catfish, gizzard shad), a few brackish water species, and some marine fishes are affected. Although A. hydrophila is part of the normal intestinal flora of fish, it can cause severe disease under stress. The disease caused by this bacterium is considered a principal microbial disease called Motile Aeromonad Septicaemia (MAS). Once a disease outbreak occurs, the spread of this type of septicaemia is rapid and can cause high mortality in cultured fishes. Aeromoniasis has also been reported in outbreak forms in intensive carp farms (Mohanty et al. 2008). Emphasis has been placed on prevention of this disease. Different types of vaccines have been tried to prevent A. hydrophila infection in fish, including whole cells (WC), outer membrane protein (OMP), extracellular products (ECPs) such as extracellular proteases, lipopolysaccharide (LPS) preparations, and biofilms. Recently a recombinant S-layer protein vaccine of A. hydrophila has been shown to render protection to common carp Cyprinus carpio against six virulent isolates (Poobalane et al. 2010). However, the bacterium is so heterogeneous that available vaccines are not effective against all strains. In addition to vaccines, some polyherbal immunomodulatory formulations enhance growth, survival, and disease resistance against A. hydrophila (Kumari et al. 2007). A large number of immunomodulatory substances have been screened and some are partially effective in rendering protection to aeromoniasis and raising the vaccine titer in fish (Sahoo 2007, Sahoo and Sakai 2010). Disease prevention with antibiotics and chemotherapeutic agents cannot bring a permanent cure and there will always be a risk of accumulation of these substances in the fish body and the environment, with the threat of developing antibiotic-resistant bacterial strains (Sahoo and Mukherjee 1999). The possibility of large-scale mortality in young hatchling stages, when vaccination cannot be considered, is of great concern. Thus, one of the few major alternaFig. 1. Rohu showing distended haemorrhagic ventral surface after experimental infection with Aeromonas hydrophila. tives remaining is the development of aeromoniasis-resistant fish through selection, bringing long-term protection against the disease. Selective Breeding There are two basic types of selection: 1) mass selection for a single trait and 2) family or combined selection for multiple traits such as growth, disease resistance, meat quality, feed conversion efficiency, salinity and low-temperature tolerance, and reduced fat content. Selective breeding for disease resistance can result in improved lines for aquaculture, reduced use of antibiotics and drugs, and ultimately reduced fish losses to disease. Selective breeding programs have been reported for different economically important fishes for resistance against different diseases, including common carp against dropsy (Kirpichnikov et al. 1993); rainbow trout against enteric
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