White Spot Syndrome

Name

White Spot Disease or WS, which is more familiar among farmers

Clinical signs

Infected shrimp have white circle-shaped spots on their carapace, which are sometimes followed by redness all over the body. The shrimp’s hepatopancreas will enlarge and turn yellowish white. They usually lose appetite, and after a few days, they appear to be dying, swimming on the surface of the water or along the sides of the pond.

Method of diagnosis

In the farm, clinical symptoms are observed to determine a diagnosis. These indications must be validated in the laboratory by testing shrimp samples using PCR and histopathology methods.

WSSV-infected shrimp

Name

White Spot Syndrome Virus (WSSV) from the Nimaviridae family and the Whispovirus genus

Tipe patogen

Virus

Sinonim

WSSV

Karakter

WSSV can survive in seawater for 30 days and has an optimum temperature of 18-30oC. This virus has double-stranded DNA and targets ectodermal tissue (epidermis cuticle, foregut and hindgut, as well as gills and nervous tissue) and mesodermal tissue (connective tissue of lymphoid organs, antenna glands, and hemopoietic tissue) after transmitting through shrimp’s mouth and gills. WSSV also produces nonstructural proteins, which are likely to be responsible for the infection. The protein is CP11, the most expressed protein that prevents the binding of host DNA to histone proteins, and then disrupts the nucleosome assembly. WSSV also infects a variety of decapods, including crabs, lobsters, freshwater shrimp, insect larvae, and artemia. This virus belongs in the C-1 category of pathogenic viruses, the one that may cause mass mortality and are difficult to treat.

Toxicity

This virus can cause a total mortality of up to 70-100% of the shrimp population in the pond. Acute infection causes death within 2-10 days after clinical signs emerge. Shrimp may die 1-2 days after infection, with mass death occurring on the 3rd-10th day and the total rate reaching 80-100%. When the pond temperature rises over 32°C, the mortality rate of WS-infected shrimp decreases.

Predispositional factors

Stress factors, such as sudden changes in salinity, may trigger this disease. Besides salinity, it can also be affected by temperature fluctuations, low DO, and high ammonia concentrations. In principle, diseases may appear due to a combination of environmental conditions, host conditions (shrimp), and the presence of pathogens (virus). Inharmonious interaction between the three causes stress in the shrimp and reduces its immunity, resulting in the shrimp becoming infected with the disease.

Transmission

The virus transmits horizontally, for example through cannibalism and predation, as well as through water flow and into the gills.

Epidemiology

It was first detected in China in 1992 and later spread to Southeast Asia and Latin America. It first occurred in Indonesia in 1994, on the northern coasts of East Java, Central Java, and West Java, before spreading to South Aceh.

Host or vector

This virus can infect shrimp without requiring a biological vector. However, other aquatic organisms, such as crustaceans, zooplankton, molluscs, phytoplankton, and worms, can carry and transmit this virus. This virus can also be carried by shrimp and transmitted to others.

Infectious dose

The virus dose (LD50) with a 100% mortality rate occurs when the virus population reaches 10⁶ CFU/ml.

Incubation period

The incubation period is 2-10 days after the virus infects the shrimp.

Susceptibility to drugs

No data.

Susceptibility to disinfectants/probiotics

No data.

Physical inactivation

WSSV can be inactivated using formalin (>10% concentration), ethanol (≥30% concentration), hypochlorite (100 ppm for 10 minutes), and isodine (>1.25 ppm) at 25°C. The use of UV light radiation at 9.30 x µWs/cm² is also possible. Furthermore, this virus can be inactivated at a temperature of 50°C for 120 minutes and 60°C for 1 minute using pH 3 for 60 minutes or pH 12 for 10 minutes.

Early warning

Detecting clinical signs and behavioral changes in shrimp.

Prevention

Avoiding shrimp fry distribution during the colder season, using SPF or disease-free shrimp fry, implementing biosecurity measures and shrimp-fish polyculture, avoiding the use of natural feed, and adding beta-glucan, vitamin C, fucoidan, and other immunostimulants to feed that can enhance resistance to this virus.

Treatment

There is no effective vaccination for WSSV treatment.

Eradication

The pond must be immediately treated with 30 ppm chlorine to kill the shrimp and potential carriers. The shrimp and other dead animals must be taken and buried or burned. The water that had been treated with chlorine should be left as it is for 4 days.

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Bir, J., P. Howlader, S. Ray, S. Sultana, S.M.I. Khalil, and G.R. Banu. 2017. A Critical Review on White Spot Syndrome Virus (WSSV): A Potential Threat to Shrimp Farming in Bangladesh and Some Asian Countries. International Jorunal of Microbiology and Mycology. 6 (1): 39-48.

FAO Fisheries Technical Paper 402/2.

Ferasyi, T.R., Zulpikar, Sugito, Z.A. Muchlisin, Razali, Nurliana, and Al Azhar. 2015. A Preliminary Study of White Spot Syndrome Virus (WSSV) Infection on Vannamei Shrimp Intensive Ponds in Bireuen District of Aceh Province, Indonesia. AACL BIOFLUX. 8 (5): 810-816.

Genics Pty Ltd. 2023. Genics Education Series: White Spot Syndrome Virus (WSSV).

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Lotz, J.M. 1997. Special Topic Review: Viruses, Biosecurity and Specific Pathogen-free Stocks in Shrimp Aquaculture. World Journal of Microbiology & Biotechnology. 13: 406- 413.

OIE. 2018. Manual of Diagnostic Tests for Aquatic Animals.

Oseko, N., T.T. Chuah, Y. Maeno, B.C. Kua, and V. Palanisamy. 2006. Examination for Viral Inactivation of WSSV (White Spot Syndrome Virus) Isolated in Malaysia Using Black Tiger Prawn (Penaeus monodon). JARQ. 40 (1): 93-97.