WSSV in shrimp: detect before an outbreak

White spot disease can advance through a shrimp farm before animals show visible lesions. A 2026 study in the Journal of Veterinary Diagnostic Investigation describes an assay combining isothermal amplification, CRISPR-Cas12a and a lateral-flow strip. Its distinguishing feature is the use of two white spot syndrome virus (WSSV) targets: the capsid gene VP28 and WSSV366, which the authors associate with early or latent infection.

The researchers tested 36 Pacific white shrimp (Penaeus vannamei) samples collected from farms in Maharashtra, India, all previously confirmed as WSSV-positive by nested PCR. VP28 produced 28 positive results and WSSV366 produced 30. Combining the targets captured WSSV366-positive samples that were negative for VP28. The finding supports further development of rapid screening near the pond. It does not establish clinical sensitivity in an unknown population or justify replacing a validated laboratory method.

White spots are not a diagnosis

WSSV is an enveloped DNA virus in the family Nimaviridae. Penaeid shrimp are particularly susceptible, but infection may remain subclinical or progress rapidly to high mortality. Reduced feeding, lethargy, abnormal swimming, pinkish discolouration and gathering at pond edges can precede losses.

White inclusions in the carapace are suggestive rather than specific. The World Organisation for Animal Health (WOAH) Aquatic Manual notes that high alkalinity or bacterial disease can also produce white spots, whereas infected moribund shrimp may have few or none. Waiting for this sign delays investigation; seeing it never removes the need for molecular diagnosis.

Apparently healthy animals can carry the virus. Water, infected tissue, cannibalism, some aquatic organisms and contaminated equipment can contribute to transmission. Rapid changes in salinity, temperature or pH and other stressors may favour clinical expression. Health management must therefore combine control of introductions, surveillance of stocks and environmental stability.

Two targets to narrow a blind spot

The experimental workflow begins with recombinase polymerase amplification (RPA), which operates at a moderate temperature without a thermocycler. The amplified material then activates Cas12a when the enzyme recognises its target. Activation causes cleavage of a reporter molecule that can be read by fluorescence or on a lateral-flow strip, visually similar in principle to a rapid test.

The authors selected VP28, a major structural protein commonly targeted during active infection, and WSSV366, described as a latency-associated marker. Under their test conditions, the reported analytical limits of detection were 20 copies per reaction for VP28 and 200 copies for WSSV366. The assay did not cross-react with the other shrimp pathogens or host DNA included in the specificity panel.

This dual readout is intended to reduce the chance of missing a particular biological profile, not to measure viral load. The RPA step is qualitative and non-linear, so band intensity must not be translated into copy number. Detecting viral DNA also does not, by itself, prove that infectious particles are present or that clinical disease is imminent.

What the 36 samples actually show

Test material included tissue from gills, stomach, pereopods and pleopods. After preparation of a crude extract, the CRISPR reaction ran for 20 minutes at 37°C before fluorescence or strip reading. This operational simplicity is attractive for sites far from a diagnostic laboratory.

However, these were not 36 shrimp sampled at random from a surveillance population. Every sample had already been classified as WSSV-positive by nested PCR. The study therefore provides neither a robust estimate of diagnostic specificity among field negatives nor predictive values for a pond with low prevalence. The fractions 28 of 36 and 30 of 36 must not be presented as generally applicable sensitivity estimates.

The lateral-flow format was also less sensitive than fluorescence for signals near the detection limit. Sequences were assessed with Indian samples, and both target conservation and test performance require evaluation in other regions and sample matrices. Ambient temperature, inhibitors, contamination when tubes are opened and reagent quality may further change performance outside the laboratory.

Fit a rapid test into biosecurity

A farm-side tool becomes useful when it answers a written decision rule. Before stocking, a plan can define which postlarval batches are screened, who collects samples, which tissue is used, what controls accompany the run and how animals are selected. WOAH identifies gill, haemolymph and pleopod as suitable tissues for non-lethal PCR sampling, but a change in matrix still needs validation for the chosen method.

A positive screen should trigger isolation of the stock, suspension of movements and confirmation by a competent laboratory in line with applicable requirements. In the European Union, WSSV infection is a listed disease for decapod crustaceans under Regulation 2018/1882 (French summary); the exact measures depend on health status and national implementation. This experimental assay does not create an alternative to official methods or procedures.

A negative result should not automatically clear a shipment. Time since exposure, number of animals, sampling design, tissue quality and analytical limits all determine what that result means. Documented quarantine, repeat testing where risk warrants it, and physical separation of water, animal, staff and equipment flows remain essential.

In grow-out ponds, surveillance should combine mortality, feed intake, behaviour, water parameters and analytical results. When an alert occurs, promptly managing dead or moribund animals under the site protocol, stopping transfers between units and contacting the attending veterinarian or competent authority can reduce opportunities for spread. No validated antiviral treatment replaces biosecurity.

A promising screen, not a standalone verdict

The RPA–CRISPR–strip combination addresses a real constraint: bringing molecular detection closer to a pond when transport time to a laboratory matters. The VP28/WSSV366 pair is scientifically interesting because it explores different infection states and recovered samples that VP28 alone did not flag in this series.

The next step is prospective, multicentre and blinded validation in representative positive and negative populations. It should measure diagnostic sensitivity and specificity, repeatability across operators and reagent lots, contamination risks, storage stability and performance under farm temperature conditions. Practical trials also need to compare the time and total cost per decision, not merely the minutes required for the Cas12a reaction.

Vetofish can help farms assess WSSV risk, design a sampling plan, select a diagnostic laboratory and organise decisions after an alert. Speed is valuable only when each result leads to a proportionate, traceable action and the required level of confirmation.