Effect of temperature shifts on shrimp lightly infected with white spot syndrome virus (WSSV)

Abstract

Healthy juvenile kuruma shrimp, Marsupenaeus japonicus, and giant black tiger shrimp, Penaeus monodon, were immersed in a filtrate prepared from the epidermis of either white spot syndrome virus (WSSV)-infected M. japonicus or healthy P. monodon (control). Ten days after immersion, infection of the experimental shrimp were confirmed by two-step polymerase chain reaction (PCR) diagnosis. The sublethally infected and the healthy control shrimp were both subjected to different temperature conditions. For both shrimp species, the experimental groups began to die after 1 d of high temperature (29 ℃) treatment, while only a few shrimp died in the control groups. At low temperatures (15 ℃), the mortalities were lower in the first 24 h than when the shrimp were maintained at a water temperature of 24 ℃. With an initial exposure to low temperatures for 24 h, after the water temperature was increased to 24 or 29 ℃, P. monodon mortality reached 90% 100% after a further 36 h at either temperature, while at 29 ℃, M japonicus mortality reached 100% within a further 48 h, but only reached 40% at 24 ℃. When WSSV DNA specific primer sets were used for one-step and two-step diagnostic PCR, the moribund experimentally infected shrimp under temperature shifts all yielded the specific 1447-bp (WSSV DNA) PCR product in the first step. No 1447-bp PCR product was found in the control groups, whether experiencing a temperature shift or not. These results demonstrate that temperature shifts alone can induce an outbreak of WSS disease in populations in which viral infection is only two-step WSSV positive.