DAO 103:133-139 (2013)  -  DOI: https://doi.org/10.3354/dao02572

ABSTRACT: Mass mortality of farmed small abalone Haliotis diversicolor occurred in Fujian, China, from 2009 to 2011. Among isolates obtained from moribund abalones, the dominant species AP37 exhibited the strongest virulence. After immersion challenge with 10⁶ CFU ml^-1 of AP37, abalone mortalities of 0, 53 and 67% were induced at water temperatures of 20°C, 24°C, and 28°C, respectively. Following intramuscular injection, AP37 showed a low LD₅₀ (median lethal concentration) value of 2.9 × 10² CFU g^-1 (colony forming units per gram abalone wet body weight). The LT₅₀ (median lethal time) values were 5.2 h for 1 × 10⁶ CFU abalone^-1, 8.4 h for 1 × 10⁵ CFU abalone^-1, and 21.5 h for 1 × 10⁴ CFU abalone^-1. For further analysis of virulence, AP37 was screened for the production of extracellular factors. The results showed that various factors including presence of flagella and production of extracellular enzymes, such as lipase, phospholipase and haemolysin, could be responsible for pathogenesis. Based on its 16S rRNA gene sequence, strain AP37 showed >98.8% similarity to Vibrio harveyi, V. campbellii, V. parahaemolyticus, V. alginolyticus, V. natriegens and V. rotiferianus, so it could not be identified by this method. However, multi-locus sequence analysis (MLSA) of concatenated sequences, including the rpoD, rctB, gyrB, toxR and pyrH genes, identified strain AP37 as V. harveyi. Phenotypic characters of AP37 were identified by API 20E. In antibiotic susceptibility tests, strain AP37 exhibited susceptibility to 7 antibiotics and resistance to 13. This is the first report of a V. harveyi-related species being linked with the mass mortality of adult abalone H. diversicolor in southern China. 

KEY WORDS: Small abalone · Vibriosis · Characterization · Virulence · Multi-locus sequence analysis · MLSA