Deadly shrimp virus has farmers in China fearing the worst

A virus that has plagued shrimp farmers in China since 2014 may be rebounding with a vengeance, this time in Guangdong Province, a crucial hub for aquaculture production in the country.

The South China Morning Post reported on 12 April that Decapod iridescent virus 1 (DIV1) had been detected once more in a number of shrimp farms in the southern province of Guangdong, along the Pearl River Delta, as of February 2020. According to the newspaper, about a quarter of shrimp farming operations in the province have been infected by the current outbreak, which previously struck stocks in China at the start of 2019 before summer temperatures prevailed.  

First identified in 2014 among adult crayfish and shrimp in China, DIV1 has a demonstrated history of causing large-scale stock losses for the country’s aquaculture sector, Global Aquaculture Alliance President George Chamberlain explained in an update about the virus, shared via LinkedIn. The latest reported outbreak of the disease promises more of the same, Chamberlain noted, and at a potentially accelerated rate.

Chamberlain cited recent testing data from a “major Chinese company,” which discovered a 17 percent DIV1 incidence rate in 209 samples taken from its farms in February 2020; comparatively, in its 2018 surveillance report, the Chinese government calculated a 12 percent incidence rate of the virus across 1,255 samples taken from 871 farms. Although 2020 testing remains limited, with not enough data yet from which to draw definitive conclusions, the available preliminary figures suggest “the incidence of DIV1 in the south of China may have increased from around 10 percent in 2018 to 17 percent in 2020,” Chamberlain said.

Originally named CQIV (Cherax quadricarinatus iridovirus) or SHIV (Shrimp Hemocyte Iridescent Virus), DIV1 can decimate shrimp and prawn stocks in a matter of days, Wu Jinhong, a shrimp producer from Da’ao township in Jiangmen city, told the South China Morning Post.

“The infection rate and lethalness of the virus are terrifying,” Jinhong said. “It only takes two or three days from detecting the first infection for all shrimp in the pond to be killed.”

Director General of the Network of Aquaculture Centres in Asia-Pacific (NACA) Huang Jie said the virus can cause high mortality - above 80 percent - in whiteleg shrimp (P. vannamei) and giant river prawns (Macrobrachium rosenbergii) in particular. Symptoms of DIV1 infection in whiteleg shrimp (L. vannamei) include empty stomach and guts. A slight loss of color on the surface and section of the hepatopancreas, as well as a soft shell, is usually observed in partially infected shrimp, Chamberlain said, with one-third of infected individuals also showing slightly reddish bodies.

Local farmers in China told the South China Morning Post that the first sign of a probable case of DIV1 comes when shrimp begin turning a reddish hue – from there, their shells soften and they sink to the bottom of the ponds. The virus appears to be highly contagious, shrimp farmer Zhong Qiang told the newspaper.

“Once one pond is infected with the virus, there is little our farmers can do, given the very high risk of nearby ponds getting infected a few days later,” he said.

The virus has serious implications for shrimp producers throughout Southeast Asia, Chamberlain said, especially considering new reports that it has been detected in wild monodon shrimp in the Indian Ocean, which is a major a source of broodstock for many regional operations.

“Researchers believe that it seems unlikely that it could have spread to the Indian Ocean and reached a significant presence in the wild P. monodon population there in three years simply by movement of wild, infected shrimp,” Chamberlain said.

In light of this, Huang and his research team recommend “that wild, captured P. monodon from the Indian Ocean intended for use as broodstock be subjected to PCR testing before use in a hatchery.”

An increase in the prevalence of DIV1 has also been reported in Vietnam, according to Shrimp Vet Lab’s Loc Tran, who said the virus has been detected in imported blood worms from China, which are used to feed broodstock.

“We have seen more and more prevalence of DIV1 causing mortalities in Vietnam. It seems that the pathogen has become well-established in the environment now.  Typically, 1 to 2 percent of the shrimp population die everyday, leading to a quite low survival rate at harvest,” Tran said, as referenced by Chamberlain.

Similar to other shrimp viruses, there is no veterinary medical treatment for DIV1, Chamberlain said, which is why proper biosecurity protocols at the management level are imperative. Meanwhile, at the national level, “surveillance programs for major shrimp, prawn, and crayfish producing countries are urgently needed for implementation of an early response strategy,” a Disease Advisory for DIV1 from NACA stated.

“All imported broodstock should come from approved sources with certified specific pathogen-free facilities and certificates showing the stocks have tested negative for DIV1,” Chamberlain said. “At the breeding level, some observers report that there is a greater use of local broodstock in Chinese hatcheries this year, due to logistics challenges of broodstock importation during the COVID-19 crisis. Pond-reared broodstock present a higher risk of carrying DIV1, and should be avoided.  At the hatchery level, vertical transmission has not been demonstrated yet, but it is highly suspected.”

“All hatchery feeds, especially live polychaete worms, should be tested or decontaminated to assure they are free of DIV1,” Chamberlain added. “At the farm level, postlarvae should be tested before stocking ponds to assure that they are free of DIV1. There is no research identifying possible carriers of DIV1 in the environment, and the length of time that the virus remains viable outside the host is also unknown. Operational protocols for farms should include proper dry-out and treatment of ponds between cycles, disinfection of incoming water, controlled movement of personnel and equipment, and routine disease surveillance.”

While fatal for some shrimp species, DIV1 is not known to harm humans.

Photo courtesy of Chuchai/Shutterstock