The Diagnostic And Treating Strategies of EHP - An Example from A Chinese Super-Intensive Farming System  

Super-intensive farming system, also known as "shrimp factory", is getting popular in northern China. The average stocking density of these indoor facilities is 500 to 700 pieces per square meter. Ideally, the icy sea water at winter time plays the role as a natural bio-security system to clear any potential threat from acute hepatopancreatic necrosis disease (AHPND) and Enterocytozoon hepatopenaei (EHP) in the water. However, an EHP outbreak still occurred after an unexpected overflow of untreated sea water at one facility in Oct, 2016.

On-site PCR tests were applied after the slow growth was observed, followed by further real-time PCR quantitative analysis done on the biggest and the smallest shrimps. The data showed that the infectious levels of these two groups were similar. Both were above 1000 copies/ng hepatopancreatic DNA. The real-time PCR analysis checked two EHP genes: the small subunit ribosomal RNA (SSU rRNA) gene and the spore wall protein (SWP) gene regions. From the Ct number of the data, the SSU rRNA gene system was more sensitive than the other.

At the same time, the neighboring ponds were confirmed as EHP positive by PCR without slow growth symptoms. The real-time PCR data showed the infectious level was below 10000 copies/ng hepatopancreatic DNA. Several potential treatment procedures were applied after the diagnosis, and the effects were monitored by the farmers' observation.

To establish an EHP control protocol, a strategy which combined the diagnosis, symptom observation, and treatment has been designed at this super-intensive system.  A periodic sampling of 50 animals was applied. The growth curves of the average, smallest, and the biggest animals were plotted to identify the slow growth symptom. The smallest pieces were screened by PCR. When the EHP positive result was found, the real-time PCR analysis was followed to check the infectious level.  A total of 6 infectious scenario were identified for different treatment procedure. Besides the diagnostic and treating protocols, an intensive disinfection protocol was also applied to clean the infected ponds.