As the Global Aquaculture Alliance (GAA) and Sustainable Fisheries Partnership (SFP) throw their combined weight behind a letter from a group of 25 seafood buying companies urging governments in Asia to act to prevent the outbreak of hitherto little known diseases, experts are warning of the potential consequences for an industry that appears to have learned little from past episodes.

Amid concern about two recently reported shrimp diseases from China, the buyers sent a letter to authorities in China, India, Indonesia, Thailand and Vietnam, the world’s top shrimp producers.

DIV1 (Decapod iridescent virus 1), originally named CQIV (Cherax quadricarinatus iridovirus) then SHIV (Shrimp Hemocyte Iridescent Virus), was first detected in farmed juvenile and adult crayfish and shrimp in China in 2014.

The other virus has been tentatively named “hepatopancreas and digestive tract necrosis virus” (HINV) or "hepatopancreatic translucence virus" (HPTV) by separate teams of scientists.

As with other shrimp viruses, there is no veterinary medical treatment for DIV1 or HINV/HPTV.

"We don’t yet know the potential for epidemic spread of DIV1 and HINV/HPTV to susceptible populations beyond China," Global Aquaculture Alliance (GAA) President George Chamberlain told IntraFish.

"We know from past experience that some pathogens like WSSV and AHPND have spread widely and caused massive losses and shifts in trade. Other pathogens like IMNV only caused major losses in a limited geographic range (Brazil and Indonesia)," he said.

Devastating impacts

While seafood companies have diversified shrimp sourcing strategies to avoid placing all their eggs in one basket, the impact of a new disease or virus can be devastating for affected supplier countries.

Between 2012 and 2014 as EMS swept through Asia, shrimp production in Thailand, then the world's leading producer, was cut in half to 300,000 metric tons, causing massive economic losses.

The gap in the market created by EMS allowed India to take over the mantle as the world's leading shrimp producing nation, as Chinese and Vietnamese industries also suffered.

Andrew Shinn of the Fish Vet Group and his coworkers published a 2018 paper in Asian Fisheries Science estimating the impact of APHND and other shrimp diseases in Thailand during 2010–2017 at $ 7.38 billion (€6.2 billion) in production losses with a further $4.2 billion (€3.5 billion) in export losses.

It doesn't have to be like that, Chamberlain argued. India and Indonesia, for example, fared much better against AHPND than neighboring countries in the region because they established policies to prevent entry of AHPND-infected broodstock by only allowing SPF broodstock from approved suppliers.

Lax regulation

There is a fear that to keep businesses going during the coronavirus downturn, shrimp producers will turn to broodstock producers in China or other locations that have suffered from these new diseases and in the process import disease. There is also concern regulations will be relaxed allowing the potential importation of the diseased broodstock.

"We shouldn't be absolutely paranoid about these diseases. It's the breakdown in the system to prevent diseases from country to country that is the challenge and a reminder to each country to improve what they do on a general basis," Anton Immink, aquaculture director at the Sustainable Fisheries Partnership (SFP) told IntraFish.

"Their systems don't work particularly effectively right now and they could be doing a lot more to reduce the impact of disease on their entire industry."

With the exception of Thailand, which has built a "pretty rigorous" testing system, preventive efforts to contain the threat of new viruses in Asia is being compared with the handling of the coronavirus pandemic by US President Donald Trump and UK Prime Minister Boris Johnson, who have been heavily criticized for their lack of emergency response plans.

"The whole system is just not fit for purpose," Immink said. "So if you see a disease problem starting to emerge, who is going to do what?"

Local media reports in Indonesia also mention concern over the prospect of EMS raising its ugly head for the first time, comparing it with acute hepatopancreatic necrosis disease (AHPND) and the negative impact the two diseases had on reducing shrimp production in China, Thailand, Vietnam, Malaysia, Mexico and the Philippines.

Although Indonesian authorities are putting thousands of dollars into one-off testing regimes to test for its presence, their actions are considered insufficient to deal with a widespread outbreak of the disease.

How we got to here

Chamberlain provided an update on what researchers have learned so far about the diseases in a highly detailed email to IntraFish.

Symptoms in shrimp of DIV1 include empty stomach and gut, slight loss of color on the surface and section of hepatopancreas, and soft shell.

The crustacean may develop a slightly reddish body. Moribund shrimp can also lose their swimming ability and sink to the bottom of water.

DIV1 can cause high mortality (above 80 percent) in L. vannamei and Macrobrachium rosenbergii, according to Huang Jie, a former senior researcher at the Yellow Seas Fisheries Research Institute (YSFRI) in Qingdao, China, and now director general of the network of Aquaculture Centres in Asia-Pacific (NACA) in Bangkok, Thailand.

However, the time required to reach 50 percent mortality after infection by mouth of L. vannamei with DIV1 is around eight days, double the time required for 50 percent mortality with an infection of white spot virus.

DIV1 infections have been detected at a rate of 12-18 percent in L. vannamei mainly during the winter in Guangdong province in the Pearl River Delta.

Huang warns that DIV1 has also been detected in wild P. monodon harvested in the Indian Ocean, recommending that if intended for use as broodstock they be subjected to PCR testing before use at hatcheries.

Weak and low incidence of DIV1 positives have been detected in frozen krill and polychaetes, raising biosecurity concerns because feeding of infected worms to maturing shrimp broodstock could lead to DIV1 infection of postlarvae used to stock ponds.

Chinese shrimp hatcheries have also reported a new disease, commonly known as “glass post-larvae disease," which causes post-larvae to appear discolored and transparent.

In separate tests, Huang’s research group at YSFRI and Jianguo He's group at China’s National Shrimp and Crab Industry Technology Station found a new RNA virus associated with this disease.

He’s group has tentatively named the virus “hepatopancreas and digestive tract necrosis virus” (HINV), while the YSFRI team led by Dr. Xuan Dong has tentatively called the unclassified picorna-like virus "hepatopancreatic translucence virus" (HPTV).

It’s too early to confirm if the genetic sequence of these two findings are the same. Dr. Huang suspects that some EMS-like cases reported in different countries may be caused by this virus.

Recommended management is to implement proper biosecurity protocols to minimize disease introduction and transmission.

"The prudent approach for all shrimp farming regions is to be proactive with heightened diagnostic testing and biosecurity both at the government and producer level," Chamberlain said.

At national level, he said surveillance programs for major shrimp, prawn, and crayfish producing countries are urgently needed for implementation of an early response strategy.

As part of this it, is recommended that imported broodstock should come from approved sources with certified specific pathogen-free facilities and certificates showing the stocks have tested negative for DIV1 and HINV/HPTV.

Among breeders, some observers report that there is a greater use of local broodstock in Chinese hatcheries this year because of logistics challenges of broodstock importation during the COVID-19 crisis.

Pond-reared broodstock pose a higher risk of carrying DIV1 and HINV/HPTV, leading to warnings that it should be avoided.

At the hatchery level, vertical transmission has not been demonstrated yet, but is strongly suspected.

Huang warns that DIV1 can be easily transmitted by mouth, meaning all hatchery feeds, especially live polychaete worms, should be tested or decontaminated to ensure they are free of disease.

On farms, post-larvae should be tested before stocking ponds to assure that they are clean. There is no research identifying possible carriers of DIV1 and HINV/HPTV in the environment, and the length of time that these viruses remain viable outside the host is also unknown.

Operational protocols for farms should include proper dryout and treatment of ponds between cycles, disinfection of incoming water, controlled movement of personnel and equipment, and routine disease surveillance.

While Asian countries have been sharply criticized for failing to heed the lessons of past outbreaks, Ecuador by contrast has won praise for its good selective breeding approaches and lower stocking densities, which are considered a luxury in Asia.

Because there is no spread of shrimp diseases to humans, however, governments put the issue well down their list of priorities.

"There are loads of lessons from beef and chicken production to put an emergency response plan in place but in shrimp we just don't do it, and I think it's because shrimp disease doesn't kill anyone," Immink said.

Shrimp

Spate of new viruses raises alarm bells in shrimp farming sector