Strain and dose infectivity of Vibrio parahaemolyticus: The causative agent of early mortality syndrome in shrimp

Diseases of shrimp have contributed to billions of dollars of economic loss in the aquaculture industry. Newly emerging strains of the bacterium Vibrio parahaemolyticus produce a condition in shrimp called early mortality syndrome or acute hepatopancreatic necrosis disease. Three different V. parahaemolyticus strains were evaluated for their respective pathogenicity on shrimp, Litopenaeus vannamei, when the bacterial strains were grown under various laboratory conditions prior to inoculating shrimp. For each trial, feed was inoculated with a known concentration of bacteria and then fed to the shrimp. The early mortality syndrome strain of V. parahaemolyticus was the most lethal resulting up to 100% mortality within 24 h after being introduced to shrimp via a single feeding. The other two strains of Vibrio, one isolated from the environment and the other from a human clinical case, resulted in 0% and 30% mortality within 96 h respectively. The concentration of the early mortality syndrome strain of V. parahaemolyticus that the shrimp were exposed to directly correlated with mortality rate, which allowed for lethal or sublethal short-term disease challenge assays to be established. Infiltration of haemocytes was also evident in the midgut caeca of shrimp infected with the early mortality syndrome strain of V. parahaemolyticus, which has not been previously reported.