Effect of salinity on transmission of necrotizing hepatopancreatitis bacterium (NHPB) to Kona stock Litopenaeus vannamei

Elevated salinity and temperature have been observed prior to devastating necrotizing hepatopancreatitis (NHP) outbreaks in several geographically isolated shrimp ponds. These observations have led to the hypothesis that the NHP-bacterium (NHPB) is hindered by reduced salinity, even though the mechanism is not understood. The objective of this research was to examine the effect of salinity on transmission of NHPB. The transmission rate of NHPB was estimated through laboratory experiments whereby individuals of Kona stock Litopenaeus vannamei were orally exposed to a dead NHPB-infected shrimp. For each replicate, 12 susceptible shrimp were placed with a dead NHPB-infected shrimp in a 1 m2 bottom area cylindrical tank maintained at 30 degrees C for a period of 24 h. Four salinities of 10, 20, 30, and 40 per thousand were replicated 2 times in 2 trials, giving a total of 192 shrimp exposed per os to infective material. In each trial, a negative control group was included at each salinity, giving a total of 96 shrimp exposed orally to uninfected material. After the 24 h exposure period, susceptible shrimp were individually isolated at the same physical conditions for up to 60 d to determine NHPB transmission. The NHPB was transmissible regardless of salinity: nearly a quarter of susceptible shrimp exposed to NHPB at the lowest (10 per thousand) and highest (40 per thousand) salinity examined acquired NHPB. Transmission rates were highest at the intermediate salinities of 20 and 30 per thousand, suggesting that those salinities are optimal for NHPB transmission. The observed association between high salinity and NHP outbreak in a shrimp pond is not explained by these results because reduced transmission occurred at very low and very high salinities.