Warning of new disease appearing on white leg shrimp

About translucent post-larva disease (TPD)

The Department of Fisheries (MARD) published a document that states that translucent post-larva disease (TPD) is emerging in farmed shrimp. TPD was first discovered for the first time in white leg shrimp production in China in March 2020 (Harkell L, 2020 and Zou Y et al, 2020).

This disease often infects shrimp fingerlings, with an infection rate of over 60%, causing high mortality rates in the shrimp seed stage, especially from PL4 - PL7 (90 - 100% after only one day of detecting abnormal signs). The main symptoms are hepatopancreas and transparent white intestines, pale and shrunken body, and transparent muscles. These symptoms are similar to those of acute hepatopancreatic necrosis.

The cause of the disease is the Vibrio parahaemolyticus strain of bacteria. The bacterial species Vibrio parahaemolyticus (Vp- JS20200428004-2) was identified as the causative agent of the disease. However, the Vibrio parahaemolyticus species differs from the previously published Vibrio parahaemolyticus that causes acute hepatopancreatic necrosis. (Zou Y et al, 2020). The experimental infection results by soaking at a dose of 1.83 x 10 x 6 CFU/mL resulting in a 100% mortality rate after 40 hours of infection. The experimental shrimp had the same disease signs as initially described.

However, this study did not provide DNA markers or PCR procedures to identify Vibrio parahaemolyticus, so there is currently no information to use PCR to detect this disease.

Ailan Xu and his colleagues recently reported on glass post-larvae disease (GPD) on whiteleg shrimp seeds in the Journal of Virus Research (2023). The authors described pathological signs on diseased shrimp similar to the research results Zou Y and colleagues described in 2020.

However, according to these authors, the causative agent of GPD in this study is not caused by bacteria but is a new RNA virus of the Marnaviridae family found in diseased shrimp and temporarily named Baishivirus (GenBank: ON550424). Experimental pathogens have also verified the pathogen, and the Realtime RT-PCR process to detect the disease has also been researched and introduced.

In Vietnam, according to results of the ShrimpVet Research Department at the end of August 2023 and early September 2023 in Vietnam, five strains with characteristics of Vibrio parahaemolyticus bacteria were isolated from suspected sudden death shrimp samples. suspected to be due to TPD in hatcheries in Vietnam (these strains tested negative by PCR for the V. parahaemolyticus strain causing Acute Hepatopancreatic Necrosis (Tran L et al., 2013; Han et al., 2015). The five strains suspected of causing TPD are all more virulent than Vibrio harveyi (does not glow, causes muscle clouding), Vibrio parahaemolyticus (does not cause EMS/AHPND) and Vibrio parahaemolyticus cause AHPND.

According to the ShrimpVet Research Department, the bacterial strains suspected of causing TPD are new strains of V. parahaemolyticus that cause TPD in farmed shrimp. These strains are more virulent than strains that cause AHPND. These strains can be a major source of risk for the shrimp farming industry in Vietnam and other countries. The Department of Fisheries is working with the research team to immediately guide some TPD disease prevention measures for businesses and shrimp farming facilities.

The above-mentioned studies all reported high levels of susceptibility and mortality in post-larvae shrimp, especially in the early stages (PL4 - PL7). The disease descriptions were similar, but the causative agent had two outcomes: Bacteria (Zou Y et al. 2020, ShrimpVet Research Department, 2023) and Viruses (Ailan Xu et al., 2023).

However, there have been no studies showing how the disease is transmitted. Research by Zou Y and colleagues in 2020 did not provide DNA markers or PCR procedures to identify the causative agent.

Disease prevention guidelines

The Department of Fisheries said that, in response to information about the TPD epidemic in farmed white shrimp in China, results of the ShrimpVet Research Department, information from scientists and previous related research documents regarding TPD disease, to avoid the spread of this new disease across borders and ensure the safe production of white-legged shrimp varieties and the development of Vietnam's shrimp industry, the Department of Fisheries proposed and recommended that Minister Le Minh Hoan assign the Department of Fisheries to complete the initial guidelines based on scientists’ recommendations to guide the prevention of TPD disease in shrimp to manage better the quality of shrimp seeds produced for commercial farming.

To avoid the spread of the new disease TPD across borders and ensure the safe production of white-legged shrimp varieties and the development of Vietnam's shrimp industry, the Department of Fisheries proposed and recommended that MARD Minister Le Minh Hoan assign the Department of Fisheries complete the initial instructions. Photo: Thanh Cuong.

Organize communication and guide localities, hatcheries and shrimp farmers on good pond management measures from renovation, selecting quality breeds for stocking and good management of the pond environment to TPD prevention.

Monitor developments in the world’s brackish water shrimp farming situation to advise on solutions for sustainable and safe development of Vietnam’s brackish water shrimp industry.

Assign the Department of Animal Health to conduct an epidemiological investigation of TPD disease in a number of hatchery production facilities and key brackish water shrimp farming areas in Vietnam. Advise on solutions and direct strict control of the import of broodstock, shrimp seeds and fresh food from China.

Assign the Department of Science, Technology and Environment to advise on the appraisal/inheritance of research results of the ShrimpVet Research Department, and develop the task of researching effective diagnostic methods and prevention measures for TPD.

Previously, on September 20, 2023, ShrimpVet Research Department, Minh Phu AquaMeKong Aquaculture Technical Services Company Limited) led by Dr. Tran Huu Loc, Director, a leading expert on shrimp diseases, sent a document to the Department of Fisheries warning of the discovery and risk of new pathogens in the production of white leg shrimp varieties in Vietnam. Accordingly, the ShrimpVet Research Department has also proposed a number of measures to manage and prevent TPD, such as:

- When importing fresh food abroad, note that origin security is a risk factor.

- It is necessary to carefully check the origin, quality and health status of shrimp seeds before entering the farm.

- In hatcheries, technicians and managers must be trained to understand the transmission routes of bacterial pathogens. It is necessary to apply safe hygiene measures, such as cleaning equipment, tools, and ponds; freezing fresh food; and following the proper Nauplii washing process before putting them into the production tank.

Proposed application of microbiology to reduce, limit, or prevent TPD in hatcheries. It is possible to use beneficial microorganisms that inhibit the growth of V. parahaemolyticus. These microorganisms can be added to feed or culture water to improve shrimp health and immunity.