Dr Simon Davies: Understanding Decapod Iridescent Virus
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As we stay in lockdown and I remain in my home at Plymouth, UK it is possible to reflect on the changes in our world with major disruption in our way of life and the constraints on society. I would certainly have been overseas by now on exciting missions to conferences in aquaculture and taking my first real vacation in many years. However, with flights grounded it will become more difficult than ever to connect with our industry colleagues and conduct business.
This reminds me of the many biosecurity requirements for aquaculture where we have in essence strict quarantine controls on fish and shrimp movement and many authorisation documents to process for export and importation and scrutiny. This applies to the transportation of stock within countries as well as international movement between zones.
Unfortunately, the rural type of subsistence aquaculture and open backyard operations in tropical regions may be more vulnerable to risk due to lack of compliance with regulatory authorities and weaker management practice. We have seen many examples in South East Asia and other parts of the world such as Latin America where major disease outbreaks can occur and even spread in pandemic fashion to other continents. We must wait for more scientific advice from governments and agencies.
Newly emerging diseases have always been an issue in aquaculture and we have been mostly able to deal with serious bacterial outbreaks but, just recently, we are aware of a viral disease outbreak affecting decapods called Decapod Iridescent Virus or DIV-1. In China this is making shock waves due to heavy mortalities in specific provinces. This new emergence affects the Giant Fresh Water Prawn (Macrobrachium rosenbergii) but can be also observed to have serious consequences for the Pacific White leg shrimp (Penaeus vannamei).
A specific case study report by Qui et al 2019 from the Yellow Sea Fisheries Research Institute at Qingdao assesses a detailed examination of the pathology in giant freshwater prawn in the scientific journal 'Viruses'. These scientists have identified a new strain of the virus that is particularly virulent and infectious. It is recognised by the red colouration of the carapace and in particular a whitish lesion or 'spot' appears on the dorsal region of the cuticle of the head and just in front of the rostrum (giving rise to the term 'whitehead').
Internally, we see severe atrophy of the hepatopancreas and a yellowing colour of this major organ and also significant damage to gills and hematopoietic tissues important for hematocyte production in shrimp that are vital cells for defensive physiology. The stomach and intestines become pale and empty, indicating severe appetite suppression.
Shrimp become very lethargic and rest on the bottom of ponds becoming moribund. It takes just three days to lead to major mortality with up to 80 percent loss in worse cases. It affects all size classes of shrimp and prawn without discrimination. By 2018, the virus had been found in 11 Chinese provinces and has given much hardship to farmers. There is much speculation that the disease can spread from adjacent ponds stocked with P.vannamei. Other susceptible species may include red river swamp crayfish and oriental wild river prawn.
Obviously, there is much interest for us in the role of superior feed formulations to enhance the nutritional status of prawns and shrimp as prophylactic measures to boost defence mechanisms prior to pathogenic challenges. We have many previous articles and reports on feed additives and supplements that can benefit shrimp and I have given talks in several conferences on this topic. This is a theme we will be surely revisiting in future editions of International Aquafeed.
The examples we are seeing in China are likely related to changing environmental situations and will, undoubtedly, be associated with the different husbandry methods employed by rural farmers. Pollution and climate changes are other factors and adverse water quality variations will also add to risk. We cannot ask prawns and shrimp to keep two metres apart and implement social isolation, but we will need to review stocking densities and standard operational procedures. We will need to address optimum stocking density and feed management and especially transportation of post larvae, hatchery output quality, and shipments to market where problems may arise.
Polyculture may have its advantages but can be the cause of cross infections and having mixed species stock on farms could be detrimental compared to monoculture and close containment of single species under better control. Given the nature of aquaculture it could be easily envisaged that DIV-1 could spread to other regions and beyond. This would be very serious scenario with huge costs to the global shrimp industry.
For a strong aquafeed industry to be viable, we need robust animals that can make the best of our feed products and ensure a healthy economic return on investment. Possibly extended genetic selection for resistance to these new types of diseases can be considered in future but will be very costly. We must await and see!
I do hope that our readers across the world are keeping safe and facing this global crisis with great resolve. As Britain's Queen Elizabeth II stated in her address to the United Kingdom and the Commonwealth 'We will meet again' and hopefully soon.
Indeed, we at International Aquafeed would be most keen to learn about how you are coping through the coronavirus pandemic and how it has affected your business operations. We are here to listen and learn from our vast readership network and please visit us online for our blogs and enjoy our magazine wherever you are! Keep safe and strong!