While this article is focused on AHPND, it should be noted that many the recommended biosecurity protocol is applicable as well to other shrimp diseases. AHPND or Acute hepatopancreatic necrosis disease was initially referred to as early mortality syndrome (EMS) when scientists were still scrambling to determine its etiology. It was later found to be caused by “unique isolates of Vibrio parahaemolyticus (VPAHPND) that carry a plasmid (pAP1) that contains two genes that produce toxins that are capable of acting together to cause AHPND” (Bondad-Reantaso and Richard Arthur 2018). This disease is characterized by mass mortality during the first 35 days of culture where affected shrimps show empty gut and an atrophied, pale hepatopancreas. Major species affected by this disease is the whiteleg shrimp (Litopenaeus vannamei) but there also reports on black tiger prawn (Penaeus monodon) and fleshy shrimp (Penaues chinensis). This non-viral disease causes severe damage and significant financial losses to the shrimp industry.
Vibrio parahaemolyticus is ubiquitous in marine and brackishwater environment, in both tropical and temperate regions and has been isolated from water, sediments, crustaceans, molluscs, finfish and other animals. Growth and survival of this organism is affected by salinity, dissolved oxygen, temperature, zooplankton and tidal flushing.
AHPND first appeared in China in 2009 and there are also records from other Asian countries such as Vietnam (2011), Malaysia (2011), Thailand (2012) and Philippines (2015).
The risk factors for the international spread of AHPND as identified by the Food and Agriculture Organization of the United Nations (FAO) (2016) are movement of live shrimp from a geographic region where AHPND is prevalent to an unaffected region for aquaculture and the importation of live animals (e.g. polychaetes, clams) as feeds for shrimp broodstock. Other potential vectors of this disease are crabs, crayfish and other crustaceans, predatory birds and mammals, transfer via untreated wastes from infected shrimp in processing plants and via use of infected shrimp fry. Salinity (>5 ppt), pH (>7), high concentration of nutrients in pond water by addition of fertilizers, molasses, etc., low water turnover coupled with low planktonic biodiversity and presence of unconsumed pelleted feed, shrimp carcasses, leading to accumulation of organic-rich sediment are some potential factors that may also promote infection of V. parahaemolyticus in shrimp ponds.
AHPND cannot be excluded from the farm but it can be prevented through proper shrimp farm management. Effective farm-level management procedures include ensuring good farm biosecurity and best management practices (BMPs). Initially, you have to select PLs derived from broodstock verified to be free of AHPND (specific pathogen free or high health broodstock). It also important to avoid overfeeding as uneaten pellets is substrate for AHPND bacteria to grow. Live and treated feeds should be free of infection. Ensure all facilities and equipment is properly disinfected before stocking of PL. To allow better biosecurity, modify farm and pond designs (e.g. use of smaller-sized ponds with plastic liners that can be fully drained, dried and disinfected between culture cycles). To guarantee elimination of fish and other possible disease carriers, increase number of reservoirs and water filtration. It is also an effective measure if water utilized is drawn from deep well and the salinity preferred for growing shrimp is 5 ppt. Following measures should be avoided: heavy chlorination pre-treatment of water, traditional fertilization schedules with commonly used products, especially if these strategies have been used previously and were found to not reduce AHPND losses and stocking ponds during the high-temperature season. It is also advised to apply “designer” pre- or probiotic preparations and “designer” phages that specifically target Vibrio strain causing AHPND (if available) (FAO, 2016)
If the disease is already present in the environment, these are management measures that may use to delay infection: stocking of larger PL; co-culture of shrimp with finfish (e.g. tilapia) or use water from tilapia pond, use of appropriately designed grow-out systems which mitigate the environmental conditions that support growth of V. parahaemolyticus causing AHPND (i.e. central drainage); stocking at appropriate density according to farm capacity; monitoring of shrimp health and removal of infected animals; if diseased shrimp are found, conduct laboratory analyses to aid decision making (FAO,2016).
Bondad-Reantaso, M and J Richard Arthur, 2018. FAO Technical Assistance Efforts to Deal with Acute Hepatopancreatic Necrosis Disease (AHPND) of Cultured Shrimp. Asian Fisheries Science. 31S(2018): 01-14
Food and Agriculture Organization of the United Nations, 2017. Second International Technical Seminar/Workshop on AHPND: There is a Way Forward. 23-25 June 2016. Bangkok, Thailand