White spot syndrome virus: three days from first sign to empty pond

What the spots are

The white spots are calcium deposits in the cuticle, 0.5 to 2 mm across, most visible on the inside of the carapace. They are not the virus itself but the shell's response to it. Affected shrimp stop feeding, swim slowly at the surface or pond edge, and often turn pink to red. Not every infected animal shows spots, and spots alone can also follow water-quality stress, which is why a positive PCR is required before a farm acts.

How the virus enters a farm

Three routes account for most outbreaks: infected post-larvae from a hatchery, carrier crustaceans such as crabs and wild shrimp entering with intake water, and contaminated equipment or frozen shrimp used as feed. Broodstock carry the virus without showing signs and pass it to eggs. The virus survives in water for days and in frozen tissue for months.

Screening points

Hatchery: PCR on broodstock before spawning and on post-larvae before sale. Farm: PCR on the post-larvae batch at arrival (pooled sample of 60 to 150 animals depending on the protocol), on carrier crustaceans caught at the intake, and on the stock itself at the first sign of reduced feeding. Many farms also run a mid-crop check every two to four weeks in high-risk seasons, since low-level infection can persist until a temperature drop triggers an outbreak.

Reading the result

Real-time PCR reports a cycle threshold that reflects viral load. A strong positive in post-larvae is a reason to reject the batch; a weak positive in a grow-out pond is a reason to reduce stress, avoid water exchange during cold snaps and prepare an emergency harvest. Duplex kits test WSSV together with a second pathogen such as decapod iridescent virus, so one run covers the two most common causes of sudden mortality.

Written by the Medicare Vet product team from manufacturer data, WOAH guidance and published validation. Not a substitute for veterinary or laboratory advice in your market.