Seeing spots: Decapod crustaceans and their susceptibility to WSD

White Spot Disease (WSD), caused by White Spot Syndrome Virus (WSSV), is largely responsible for the loss of productivity in penaeid shrimp farms. WD results in high mortalities which affects the entirety (100%) of exposed stock. Turnbull et al. (2005) have described the practice of ‘emergency harvesting’ as a way of preventing complete productivity loss following the discovery of infected individuals within the stock. Even though WD was discovered nearly two decades ago it is still the most significant pathogen in regards to the impact it has upon the continued growth and sustainability of penaeid shrimp farming. This has important implications for humans as emergency harvesting may not be able to produce the crop that would otherwise be produced by a full harvest. This inevitably costs the farmers money in terms of profit gained and may, ultimately, lead to a loss of livelihood.

Guidelines which are currently in place for the methodological approaches to diagnosing WSD are primarily based around detecting viral nucleic acids within infected individuals. This study assessed the susceptibility of non-model species to WSSV infection using the criteria set by the European Food Safety Authority (EFSA). The criteria are; replication of the organism during infection, viability of the organism, any changes to/ presence of the pathogen and the location of the pathogen within the host. The use of non-model species within this study is key in helping to further our knowledge of just how strongly the definition of susceptibility can be applied to non-model species.

The authors found that there is a universal susceptibility to WSSV infection within a range of decapod crustaceans native to European waters when exposed to WSSV via feeding. It was also found that WSSV was able to replicate and retain its ability to cause disease in the ambient water temperatures found in European waters. It was also found that the virus was still viable and able to be passed onto other susceptible hosts, spreading the infection. Even though the decapod species tested within this study were found to be universally susceptible to WSSV, the way in which the disease itself manifested after infection by WSSV varied between species. This is to be expected as different species will usually have different tolerance levels to infections.

The results from this study helped to define three categories of susceptibility for the non-model decapods species used, high, low and moderate susceptibility. These three categories were defined using a variety of different factors. The categories also have application for measuring susceptibility at both the species and individual level. The authors conclude by suggesting that, in order to assess the true risk that WSSV poses to wild decapod populations, experiments investigating transmission from asymptomatic carrier hosts to other hosts of either the same species or disease susceptibility are required.  Overall WSSV and, consequently, WSD are not understood enough for us to be able to start bringing preventative measures to bear to prevent crop loss in farmed penaeid shrimp or other farmed decapods species and further study is needed.  

Bateman KS., Tew I., French C., Hicks RJ., Martin P., Munro J., Stentiford GD., 2012, Susceptibility to infection and pathogenicity of White Spot Disease (WSD) in non-model crustacean host taxa from temperate regions, Journal of invertebrate pathology, 110, 340-351

Turnbull JF., Corsin F., Mohan CV, Padiyar PC., Thakur M., Madhusudan NV., Hao NV., Morgan KL., 2005, Optimising emergency harvest strategy for White Spot Disease in a semi-intensive Penaeus monodon culture system in Karnataka, India, Walker P., Lester R., Bondad-Reantaso MG (Eds), Diseases in Asian aquaculture V, Fish health section, Asian fisheries society, Manila, 405-414