White Spot Syndrome Virus (WSSV) in Farmed Shrimp: A Guide
White spot syndrome virus (genus Whispovirus) causes near-total mortality in farmed shrimp within days. No treatment exists; control depends on biosecurity.
Overview
White spot disease is one of the most economically devastating diseases in global shrimp aquaculture. It is caused by white spot syndrome virus (WSSV), a large, rod-shaped, enveloped double-stranded DNA virus of the genus Whispovirus, family Nimaviridae. First reported as an epidemic in Taiwan in 1992, the virus spread rapidly across major shrimp-farming regions and is now a notifiable disease listed by the World Organisation for Animal Health (WOAH). Reviews estimate cumulative global losses on the order of tens of billions of US dollars since its emergence.
Host range
WSSV has an exceptionally broad host range among decapod crustaceans. Major cultured penaeid shrimp hosts include Penaeus monodon, Litopenaeus vannamei (also written Penaeus vannamei), Marsupenaeus japonicus, and Fenneropenaeus indicus. The virus also infects many other crustaceans, including crabs (Scylla spp., Portunus spp.), crayfish (Procambarus clarkii, Cherax spp.), freshwater prawns (Macrobrachium spp.), and lobsters, several of which can act as carriers or vectors.
Clinical signs
- White spots roughly 0.5 to 2.0 mm in diameter on the inner surface of the carapace, appendages, and cuticle
- Sudden reduction in feeding and lethargy
- Loose cuticle and frequent reddish discoloration of the body
- Erratic swimming and gathering at pond edges
- Rapid onset of mass mortality
Transmission
WSSV spreads both horizontally and vertically. Horizontal transmission occurs through waterborne contact, ingestion of infected tissue, and cannibalism of dead or moribund animals, and can be amplified by infected vectors. Vertical transmission passes from infected broodstock to post-larvae in hatcheries. The virus is highly persistent in pond sediments and can be detected in water shortly after an outbreak begins, which makes contaminated ponds a lasting reservoir.
Diagnosis
Diagnosis is confirmed by molecular testing, principally nested or quantitative PCR, supported by histopathology and, where needed, in situ hybridization or electron microscopy. Laboratory testing distinguishes WSSV from other shrimp viruses such as yellowhead virus and infectious hypodermal and hematopoietic necrosis virus, which can present with overlapping signs.
Treatment, control and prevention
No effective treatment for white spot disease is available. Management is built entirely on prevention, biosecurity, and pond management.
- Stock specific-pathogen-free (SPF) and PCR-screened post-larvae from certified hatcheries
- Disinfect ponds, equipment, and incoming water; remove and properly dispose of dead animals to break cannibalism cycles
- Exclude and control crustacean carriers and vectors around production sites
- Maintain stable water quality; higher and stable water temperatures (around or above 29 degrees C) have been associated with increased resistance
- Apply compartmentalization, screening of broodstock, and selective breeding for tolerance