Environmental conditions influence susceptibility of striped catfish Pangasianodon hypophthalmus (Sauvage) to Edwardsiella ictaluri

Over the last 20 years the production farmed Vietnamese striped catfish (Pangasianodon hypophthalmus) has increased significantly and in 2016, over 1.2 million tonnes of catfish were farmed and sold globally. Bacterial disease outbreaks due to Edwardsiella ictaluri continue to be one of the biggest threats to the sector, however, little is known on how the environmental conditions affect the survival of the fish during disease outbreaks. Growth of 14 Edwardsiella ictaluri strains recovered from natural disease outbreaks occurring in 4 provinces in Vietnam between 2002 and 2011 was investigated in vitro under different pH and salt concentrations. The results showed that a pH value of 6.5, NaCl concentration of 0.5% was optimal for the growth of the bacteria in vitro. The effect of varied pH and salt concentrations on the susceptibility of striped catfish to E. ictaluri infection was also studied in vivo following an immersion bacterial challenge (1.1 x 10(7) cfu ml(-1) E. ictaluri for 30 s). The cumulative mortality of striped catfish in water at pH 5.5 and pH 6.5 was significantly higher than that of fish maintained in more alkaline water (p < .05). The cumulative mortality of the striped catfish maintained in 0.5% NaCl was significantly lower than those kept in 0%, 1% and 1.5% NaCl (p < .05). This study identified the effect of pH and salinity changes on the susceptibility of striped catfish to E. ictaluri infections.