• 19 Aug 2015

Investigations on EMS/AHPNS and their findings

Pathology of EMS / AHPNS was explicitly described by Dr. DV Lightner of University of Arizona, in both P. monodon and P. vannamei and he informed that the pathology appeared to be limited to the hepatopancreas (HP). He described EMS / AHPNS as idiopathic since no specific disease causing agent (infectious or toxic) was so far found to be associated. The clinical signs such as significant atrophy of the HP, which may be often pale to white due to pigment loss, sometimes with visible black spots or streaks, which does not squash easily between the thumb and forefinger could be used for presumptive diagnosis in cases of shrimp mortality starting as early as 10 days post-stocking. Progressive degeneration and dysfunction of the HP tubule epithelial cells progressing from proximal to distal ends of HP tubules and its degenerative pathology suggested of a toxic etiology. However, he also mentioned that the anecdotal information suggested that disease spread patterns may be consistent with an infectious agent.

According to Dr. Tim Flegel of CENTEX shrimp, in the EMS affected shrimp, in addition to various well studied pathogens such as WSSV, YHV and vibrios that are commonly linked to EMS, they also found certain groups of bacteria using metagenomics tools. While the role of certain crustaceacides such as cypermethrin was ruled out, he mentioned that the disease transmission trials were inconclusive.

According to Dr Peter Walker of CSIRO, the potential pathogens are integral components of all ecosystems and disease emergence and subsequent spread often resulted from some disturbance in the ecology, which can upset the natural balance resulting in a normally innocuous organism emerging as a new disease agent. He mentioned that the current aquaculture practices, which are artificial and un-natural high density culture activities, promote emergence of pathogens. Understanding biological and ecological drivers of pathogen emergence can help in reducing or limiting the impact of emerging infectious diseases. Considering the need for identifying the etiology of EMS / AHPNS of shrimp, Dr Jeff Cowley of CSIRO in his presentation on the molecular tools for discovering unknown pathogens mentioned that a number of new methods such as sequence-assisted and sequence-independent virus discovery that could be applied to help discover viruses or other pathogens if these are the cause of AHPNS.

The epidemiology and risk factors involved in EMS / AHPNS required systematic studies. Dr Flavio Corsin and Dr Matthew Briggs opined that until epidemiological approaches are applied systematically to include hatchery, transport, pond, farm and location specific data, it will be very difficult to pinpoint and prioritize risk factors for AHPNS. The potential risk factors included most of the generic factors such as high stocking densities, older farms closer to the sea using higher salinity water, farms not employing reservoirs, farms overusing chemicals, inadequate aeration, and presence of toxic levels of H2S etc.