Investigation on the infectious nature of Running Mortality Syndrome (RMS) of farmed Pacific white leg shrimp, Penaeus vannamei in shrimp farms of India

Shrimp aquaculture in India has made significant strides especially after the introduction of Pacific white shrimp in the year 2009. However, intensification of culture practice has exacerbated several disease issues. Apart from infectious diseases, the Indian shrimp aquaculture has been witnessing certain syndromes, affecting productivity. One such syndrome associated with significant morbidity and mortality is popularly termed as running mortality syndrome (RMS) by shrimp farmers. Since 2011, RMS has been widely prevalent in the shrimp farms in Andhra Pradesh (AP) and Tamil Nadu (TN). The affected shrimp show patches of whitish musculature in the abdominal segments as a clinical sign with continuous low-level mortalities, especially after about 35-40 days of shrimp culture. We tried to investigate if this kind of mortality is infectious in nature. Investigations conducted in 34 farms comprising 25 RMS affected and 9 healthy farms were tested negative for all the major OIE listed and other pathogens such as white spot syndrome virus (WSSV), infectious hypodermal and hematopoietic necrosis virus (IHHNV), monodon baculovirus (MBV), hepatopancreatic parvo virus (HPV), infectious myonecrosis (IMNV), Taura syndrome virus (TSV), yellow head virus (YHV), and Penaeus vannamei noda virus (PvNV). Bacteriological examination of haemolymph and hepatopancreas of RMS affected shrimp showed the predominance of Vibrio spp., such as Vibrio parahaemolyticus and Vibrio azureus. Histopathological examination of the hepatopancreas was found to be largely normal, except for haemocytic infiltration in the abdominal segments. Hemolymph of affected shrimp was observed to have less concentration of major and trace minerals than healthy ones. Bioassay through feeding RMS affected shrimp tissue to healthy shrimp and co-habitation experiment of healthy shrimp with the affected animals failed to induce RMS. When maintained in water with optimal physico-chemical parameters, affected shrimps showed recovery within 6-7 days. On the other hand, environmental parameters of pond waters such as total ammonia nitrogen, nitrite nitrogen and turbidity were relatively higher than the optimal values in RMS affected farms. Multiple correspondence statistical analysis of critical factors indicated running mortality to be associated with high stocking densities, high nitrite-N, and high turbidity. Though the study could not identify any specific known aetiological agent associated with RMS affected shrimp, failure to reproduce the syndrome by bioassay, the recovery of affected shrimp under the optimal environmental conditions and the positive correlation with critical environmental parameters and the stocking densities to mortality rates clearly suggest RMS to be a pond ecosystem or pond management associated syndrome rather than infectious in nature and thus can be overcome through best management practices.