Summary about Disease
White Spot Disease (WSD), also known as White Spot Syndrome Virus (WSSV), is a highly contagious and lethal viral disease affecting a wide range of crustaceans, particularly shrimp. It is characterized by the appearance of white spots on the carapace, appendages, and cuticle of infected shrimp. WSD can cause rapid mortality, leading to significant economic losses in shrimp farming industries globally.
Symptoms
White spots (0.5-3 mm in diameter) on the carapace, appendages, and cuticle.
Lethargy and reduced feeding.
Reddish discoloration of the body.
Rapid mortality (up to 100% within days of symptom onset).
Erratic swimming behavior.
Loose exoskeleton.
Causes
WSD is caused by the White Spot Syndrome Virus (WSSV), a DNA virus. Transmission occurs through:
Ingestion of infected tissue (cannibalism, feeding on infected feed).
Waterborne transmission (virus shed by infected shrimp).
Vertical transmission (from infected broodstock to offspring - less common).
Contaminated equipment or vectors (birds, crabs etc.).
Medicine Used
There is no effective medicine to treat WSD once shrimp are infected. Management focuses on prevention and control measures to minimize the risk of outbreaks. Some researchers are exploring antiviral compounds and immunostimulants, but none are universally effective.
Is Communicable
Yes, WSD is highly communicable. It spreads rapidly among crustaceans in aquaculture settings, primarily through waterborne transmission and ingestion of infected tissues.
Precautions
Strict biosecurity measures on farms (disinfection, restricted access).
Use of specific pathogen-free (SPF) or specific pathogen-resistant (SPR) shrimp stocks.
Regular water quality monitoring.
Proper pond preparation and disinfection.
Quarantine of new shrimp stocks.
Control of potential vectors (birds, crabs).
Good feed management to minimize waste and prevent cannibalism.
Early detection and removal of infected shrimp.
How long does an outbreak last?
An outbreak of WSD can last from a few days to several weeks, depending on the severity of the infection, the density of the shrimp population, and environmental conditions. Mortality can reach 100% within 3-7 days after the onset of visible symptoms in severely affected ponds.
How is it diagnosed?
Visual Inspection: Observing the characteristic white spots.
Polymerase Chain Reaction (PCR): A highly sensitive and specific molecular test to detect the presence of WSSV DNA.
Histopathology: Microscopic examination of tissue samples to identify viral inclusions.
ELISA (Enzyme-linked immunosorbent assay): An immunoassay to detect viral antigens.
Timeline of Symptoms
1. Infection: Shrimp become infected with WSSV. 2. Incubation Period: (Variable, 3-7 days) No visible symptoms. 3. Early Symptoms: Lethargy, reduced feeding, and slight reddish discoloration. 4. White Spots Appear: White spots become visible on the carapace, appendages, and cuticle. 5. Advanced Stage: Increased number and size of white spots, severe lethargy, erratic swimming. 6. Mortality: High mortality rates occur within a few days of the onset of visible symptoms.
Important Considerations
WSD is a notifiable disease in many countries, requiring reporting to authorities.
Prevention is crucial, as there is no effective treatment.
Economic impacts can be devastating for shrimp farmers.
Proper disposal of infected shrimp is essential to prevent further spread.
Research continues to develop more effective diagnostic tools and control strategies.