Salinity-dependent vulnerability of whiteleg shrimp (Litopenaeus vannamei) to Vibrio parahaemolyticus: growth performance and antioxidant response

Abstract

Whiteleg shrimp (Litopenaeus vannamei) became one of the most cultured crustacea in marine farms in North Egypt. The bacteriological examination of two shrimp farms (water salinity 4.28 and 28.5 g/L) that exhibited high mortality rates revealed the causative agents, which were three Vibrioparahaemolyticus strains VHLA1-3. They were identified by detecting three genes: integral membrane protein (toxR) and haemolysin genes (tdh and trh). The strains were multidrug-resistant and were sensitive to ciprofloxacin and florfenicol; also, their median lethal doses (LD₅₀) were 3.78 × 10⁵, 1.99 × 10⁵, and 1.95 × 10⁵ CFU/mL, respectively. In the experiment, 180 healthy shrimp (5.3 ± 0.03 g, body weight) were reared in three different water salinities 7, 35, and 50 g/L for 8 weeks and then were challenged with LD50 of the isolates VHLA1-3. At salinity of 50 g/L, shrimp experimentally infected with VHLA1, VHLA2, and VHLA3 showed a higher MR of 60%, 80%, and 80% compared to those of 7 and 35 g/L; florfenicol treatment resulted in mortality rate ranged between 10 and 20% regardless water salinity. Bacterial re-isolation (RI%) was 100% in challenged-untreated shrimp, whereas RI% was decreased after florfenicol treatment by 20%, 30 to 40%, and 0% at salinity of 7, 35, and 50 g/L, respectively. Experimental shrimp reared in high salinity (50 g/L) showed significantly higher final weight (15.6 g), specific growth rate (1.54%/fish/day), and low FCR compared to those of low salinity water (35 and 7 g/L). At salinity of 50 g/L, the expression of catalase CAT and superoxide dismutase SOD genes was significantly higher compared to those of low salinity, while the heat shock protein Hsp70 expression was significantly raised in 50 g/L, followed by 7 g/L then 35 g/L in the hepatopancreas of shrimp. It was obvious that V. parahaemolyticus was the causative agent in shrimp mortality, with different patterns of virulent genes. Water salinity was the control factor in antioxidant status and V. parahaemolyticus infection. Shrimp reared at salinity extremities, 7 and 50 g/L, exhibited high mortality and bacterial re-isolation. So, florfenicol treatment is recommended in V. parahaemolyticus infection, and salinity extremities (7 and 50 g/L) should be avoided during rearing whiteleg shrimp.