White Spot Disease in Crabs
White spot disease is a serious viral infection affecting crabs and many other crustaceans, particularly farmed shrimp. It is caused by a virus called white spot syndrome virus (WSSV). The disease spreads very rapidly through farming ponds...
What is it?
White spot disease is a serious viral infection affecting crabs and many other crustaceans, particularly farmed shrimp. It is caused by a virus called white spot syndrome virus (WSSV). The disease spreads very rapidly through farming ponds and can cause the death of large numbers of animals within a few days. It represents a significant economic threat to aquaculture, but poses no risk to humans.
Key point
Notifiable disease in many countries due to its major economic impact on aquaculture; it can wipe out an entire farm within days.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The most characteristic sign is the appearance of small white or whitish spots, round in shape, under the shell, especially visible on the inner surface. These spots correspond to abnormal chalk deposits linked to infection.
Infected crabs become soft, lethargic and stop feeding. They may move toward the water surface or approach the sides of the pond, an unusual behavior reflecting general distress.
The shell may also change slightly in color, becoming more pink or reddish depending on the species. In farming operations, massive and rapid mortality is often observed, sometimes within just a few days.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by white spot syndrome virus (WSSV), a DNA virus that infects the tissues of many crustaceans, including crabs. This virus particularly attacks tissues of epithelial and connective origin, disrupting the function of several organs.
Farms with high stocking density promote rapid spread of the virus. Stress from poor water quality, sudden temperature or salinity changes weakens the crab's defences and facilitates infection.
Introduction of new animals or contaminated equipment without quarantine, as well as the presence of birds or other crustaceans carrying the virus in the environment, are also important risk factors.
Transmission
The virus is transmitted mainly through horizontal transmission, that is, by direct contact between animals, through contaminated water, or by ingestion of carcasses or infected tissues (cannibalism). It can also be transmitted from the female to her offspring. Birds, waterfowl, and certain insects can help spread the virus between different ponds or farming sites.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on observing clinical signs combined with specialized laboratory testing, such as detection of viral genetic material using molecular biology techniques (PCR). These tests are performed by specialized veterinary or aquaculture laboratories, since white spots alone are not sufficient to confirm the disease.
Treatment
There is currently no specific curative treatment for this viral infection. Management consists mainly of isolating affected animals, improving farming conditions and supporting uninfected populations through sanitary management measures.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies mainly on strict biosecurity measures: quarantine of new animals, disinfection of equipment and tanks, monitoring water quality and limiting stocking density.
No vaccine is currently available against this viral disease. Regular farm monitoring and prompt removal of sick or dead animals help limit spread.
Possible complications
The disease can cause very high and rapid mortality within a farm, with significant economic consequences. Once established in a tank or area, the infection is difficult to eradicate completely.
When to see a veterinarian
White spots appearing under the shell of several animals.
Unusual or rapid death in a farming pond.
Abnormal behaviour: lethargy, loss of appetite, clustering at the surface.
Suspicion of recent introduction of infected animals.
Prognosis
The outlook is generally poor if infection is confirmed in a farm, with a very high risk of death within a few days. Some animals may survive, but they often remain carriers of the virus.
Frequently asked questions
Can white spot disease be transmitted to humans?
No, this virus only infects crustaceans. There is no risk to human health, even when consuming animals that have been in contact with the virus.
Can a crab with white spot disease be treated?
There is no specific curative treatment. Management focuses mainly on limiting spread to other animals and improving breeding conditions.
How to prevent disease from entering a livestock farm?
Quarantining new animals, disinfecting equipment and water, and strictly controlling stocking density are the best preventive measures.
Are all crabs with white spots sick?
Not necessarily. Other causes can produce spots on the shell. Only laboratory analysis can confirm the presence of the virus.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 28 August 2026
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