White spot Syndrome

Overview

White Spot Syndrome (WSS) is a highly contagious viral disease that primarily affects shrimp and other crustaceans, causing significant mortality and economic losses in aquaculture. It is caused by the White Spot Syndrome Virus (WSSV), a large double-stranded DNA virus from the family Nimaviridae. First identified in the early 1990s in East Asia, WSS has since spread globally, becoming one of the most devastating pathogens in shrimp farming industries.

The name "white spot syndrome" comes from the characteristic white spots that appear on the exoskeleton of infected shrimp. The disease can cause mortality rates approaching 100% within a few days of onset, making early detection and biosecurity crucial to control outbreaks.

Causes

White Spot Syndrome is caused by infection with the White Spot Syndrome Virus (WSSV). The virus is highly virulent and can infect a wide range of crustaceans, including:

• Penaeid shrimp (e.g., Penaeus monodon, Litopenaeus vannamei)

• Crabs (e.g., Scylla serrata, Portunus pelagicus)

• Crawfish and lobsters (less commonly)

WSSV can be introduced into shrimp farming systems through several routes:

• Contaminated water: Use of untreated or virus-contaminated water sources.

• Infected broodstock or larvae: Vertical transmission from parent to offspring or horizontal spread in hatcheries.

• Vectors: Wild crustaceans, insects, birds, or equipment carrying the virus into farming environments.

• Human activity: Movement of infected animals, inadequate biosecurity, or reuse of contaminated equipment.

Symptoms

The symptoms of White Spot Syndrome appear rapidly and can lead to mass mortality within 3 to 10 days. Clinical signs are often obvious and include:

External Signs

• White spots: 0.5–2.0 mm in diameter, often on the inner carapace, especially the cephalothorax and abdominal segments.

• Reddish discoloration: Due to expansion of chromatophores or hemorrhage beneath the cuticle.

• Loose shell: The exoskeleton may become soft or fragile.

• Lethargy: Infected shrimp show reduced activity and feeding behavior.

• Erratic swimming or surface aggregation: Due to neurological impairment.

Mortality

• Sudden and widespread mortality often begins within 1–2 days of symptom onset.

• Death rates can reach nearly 100% within 3–7 days if left unmanaged.

Diagnosis

Diagnosis of WSS can be made based on clinical signs, but confirmatory testing is essential for accurate identification and differentiation from other diseases.

Diagnostic Methods

• Gross examination: Presence of characteristic white spots and rapid mortality in affected populations.

• Histopathology: Shows intranuclear inclusion bodies in infected cells of the gills, cuticle epithelium, and connective tissue.

• Polymerase Chain Reaction (PCR): The gold standard for detecting WSSV DNA in tissue or environmental samples.

• Loop-mediated isothermal amplification (LAMP): A rapid molecular test useful in field conditions.

• Electron microscopy: Visualization of virus particles, primarily for research or confirmation in reference labs.

Treatment

There is currently no effective antiviral treatment or cure for White Spot Syndrome once infection occurs. Management is focused on prevention, containment, and biosecurity.

Preventive Measures

• Biosecurity protocols: Strict control of water sources, disinfection of equipment, and quarantine of new stock.

• Screening and certification: Use of WSSV-free broodstock and post-larvae in hatcheries.

• Water treatment: Chlorination, UV treatment, or filtration of incoming water to remove pathogens.

• Limiting vector exposure: Covering ponds, using bird deterrents, and controlling wild crustaceans.

• Proper pond preparation: Drying, liming, and sanitizing ponds between production cycles.

Containment During Outbreak

• Immediate culling: Removal and safe disposal of infected and exposed shrimp to prevent virus spread.

• Disinfection: Cleaning of pond surfaces, equipment, and effluent water with approved disinfectants.

• Temporary suspension: Halting farming activities in affected areas until containment is achieved.

Research and Experimental Approaches

• Immunostimulants and probiotics: Being explored to boost shrimp immunity.

• Genetically resistant strains: Breeding programs aiming to develop WSSV-resistant shrimp lines.

Prognosis

The prognosis for a shrimp farm affected by White Spot Syndrome is poor if the infection is not detected and managed immediately. Once WSSV enters a farming system, it can spread rapidly, leading to catastrophic economic losses.

Key prognostic considerations include:

• High mortality rates: Up to 100% mortality in infected ponds within a week.

• Long-term impact: Affected farms may require weeks to months for full decontamination and recovery.

• Recurrence risk: Persistent environmental contamination can lead to future outbreaks if strict biosecurity is not maintained.

• Economic consequences: Major financial losses due to lost harvests, stock replacements, and biosecurity upgrades.

With rigorous preventive measures, early detection, and responsible farming practices, the risk of WSSV outbreaks can be minimized, protecting both animal health and the viability of aquaculture operations.