White Spot Syndrome Virus (WSSV) has emerged as one of the most prevalent and lethal viruses globally and infects both shrimps and crabs in the aquatic environment. This study aimed to investigate the occurrence of WSSV in different ghers of Bangladesh and the virulence of the circulating phylotypes. We collected 360 shrimp (Penaeus monodon) and 120 crab (Scylla sp.) samples from the south-east (Cox’s Bazar) and south-west (Satkhira) coastal regions of Bangladesh. The VP28 gene-specific PCR assays and sequencing revealed statistically significant (p < 0.05, Kruskal–Wallis test) differences in the prevalence of WSSV in shrimps and crabs between the study areas (Cox’s Bazar and Satkhira) and over the study periods (2017–2019). The mean Log load of WSSV varied from 8.40 (Cox’s Bazar) to 10.48 (Satkhira) per gram of tissue. The mean values for salinity, dissolved oxygen, temperature and pH were 14.71 ± 0.76 ppt, 3.7 ± 0.1 ppm, 34.11 ± 0.38 °C and 8.23 ± 0.38, respectively, in the WSSV-positive ghers. The VP28 gene-based phylogenetic analysis showed an amino-acid substitution (E→G) at the 167th position in the isolates from Cox’s Bazar (referred to as phylotype BD2) compared to the globally circulating one (BD1). Shrimp PL artificially challenged with BD1 and BD2 phylotypes with filtrates of tissue containing 0.423 × 10⁹ copies of WSSV per mL resulted in a median LT50 value of 73 h and 75 h, respectively. The in vivo trial showed higher mean Log WSSV copies (6.47 ± 2.07 per mg tissue) in BD1-challenged shrimp PL compared to BD2 (4.75 ± 0.35 per mg tissue). Crabs infected with BD1 and BD2 showed 100% mortality within 48 h and 62 h of challenge, respectively, with mean Log WSSV copies of 12.06 ± 0.48 and 9.95 ± 0.37 per gram tissue, respectively. Moreover, shrimp antimicrobial peptides (AMPs), penaeidin and lysozyme expression were lower in the BD1-challenged group compared to BD2 challenged shrimps. These results collectively demonstrated that relative virulence properties of WSSV based on mortality rate, viral load and expression of host immune genes in artificially infected shrimp PL could be affected by single aa substitution in VP28.

The black tiger shrimp, Penaeus monodon, is the most economically important penaeid shrimp in Bangladesh for both culture and capture fishery. The culture of these penaeid shrimps is always risky due to different diseases and environmental stressors. This study aimed to assess the relative expression of the Penaeus monodon Rab7 gene in the post larvae (PL) of the P. monodon cultured under different salinity conditions. We challenged the PL of the P. monodon in aquariums with two different salinity and dissolved oxygen conditions during a 7-day test. The relative expression of Rab7 was upregulated during the experimental period of 36 hours. Average differences in the relative expression of Rab7 were 0.3-folds and 0.5-folds, and 0.4-folds and 0.6-folds after 18 and 36 hours of rearing in 2 and 54 ppt salinity treated shrimp PL, respectively, when β-actin was used as an internal control. On the other hand, when elongation factor 1 alpha (EF1α) was used for normalization, the average relative expression of Rab7’s fold differences were 0.2-folds and 0.5-folds, and 0.3-folds and 0.6-folds after 18 and 36 hours of rearing in 2 and 54 ppt salinity treated shrimp PL, respectively. Therefore, our results suggested that the relative expression of the Rab7 was positively correlated with an increased level of salinity and challenge duration. The significant increase in the expression level of Rab7 in the PL of P. monodon exposed to high salinity stress (54 ppt) conditions indicated its functional role in osmotic stress in shrimps. However, further studies are necessary to elucidate the mechanisms of how Rab7 is upregulated in shrimp cells after applying different levels of salinity using a larger sample group.

Climate change is a very common phenomenon which may affect rural livelihoods. A sustainable rural livelihoods approach was taken for this study to observe household incomes and any impact of climate to the livelihoods. It was found that there was negative correlation (Pearson value -0.449) to the income of males and females in 2014. Assumption of impact of climate change on income was not statistically significant in male farmers while it was statistically significant (Pearson value 0.332) in the year 2013 for female respondents. This current study suggests diversification in income generating activities and introduction of fair trade for betterment of the fishers Asian J. Med. Biol. Res. September 2020, 6(3): 418-424