Aquaculture for all

Facts on White Spot Syndrome Virus (WSSV)


By LSU AgCenter. White Spot Syndrome Virus (WSSV) has been confirmed for the first time in Louisiana in crawfish from a pond in St. Martin Parish.

This crawfish disease seems to affect mostly the medium to large crawfish. Symptoms include sluggish crawfish that don’t move much once they are dumped from the trap. They do not pinch hard and can’t walk. There are no color differences or obvious signs other than weakness and slowness. Some dead crawfish are noticeable in the traps while others are noticeable in the shallow water along the edge of the pond.

This viral disease affects only crustaceans like crawfish and shrimp. Humans are not susceptible to the virus, and consumption of infected crawfish does not endanger the health of humans. The virus can apparently cause significant losses of crawfish in a pond.

Following are some facts about the disease:

Where did the disease come from?

The disease was first reported in shrimp in China in 1992-93. In 1995, it was reported in several shrimp farms located in south Texas. The disease was probably transported from its origins in China by infected shrimp that were used as a bait or food for the shrimp in Texas. This is the first report of a natural infection of crawfish in the United States. It is possible that shrimp escaped from those farms and migrated into the Gulf, which may be the source or our present problem in farmed crawfish.

What causes the disease?

It is caused by a specific virus called a baculovirus.

Can the infected pond be treated to eliminate the infection?

Because the disease is caused by a virus, there is no anti-viral treatment or cure that can be effective in a pond or rice field environment.

What animals are affected in Louisiana?

In Louisiana the only species in which the disease has been confirmed is in farmed freshwater crawfish.

Do we know if the free-ranging crawfish in marshland and bayou areas are free of the disease?

No. The Louisiana Department of Wildlife and Fisheries (and their federal counterparts) will work on testing crawfish harvested from wild venues for the disease. The marine shrimp in coastal estuaries will also be tested for WSSV.

How widespread is the problem among the crawfish farmers?

Thus far, it has been disclosed in only one pond (in St. Martin Parish), but several crawfish producers in Vermilion and St. Landry parishes have reported an unusually high death loss. We will not know how widespread the problem is until USDA-APHIS or LDAF have completed surveillance assessments and testing of the crawfish-producing areas in the state.

What are the susceptible species to WSSV?

Only certain crustaceans (decapods) are susceptible to the WSSV. The disease has caused severe problems in marine (sea-water raised) shrimp in Texas and Asia, but it also can affect crabs, lobsters and crawfish.

Can it infect humans?

Humans are not susceptible to the disease.

Are infected crawfish edible?

The consumption of infected crawfish does not endanger the health of humans.

What are the signs and symptoms of the disease in crawfish?

There are no visually conspicuous signs of the disease in an affected crawfish. In shrimp, WSSV causes the formation of white spots on the shell (carapace), but that has not been observed in crawfish. The virus can invade many organ systems in the crawfish, causing a high death loss (mortality). An affected crawfish may be lethargic or weak, but there are no other signs of the disease that are apparent.

What should a crawfish farmer do if he/she thinks that he/she may have a WSSV disease problem?

Report the problem to the local county agent. An increase in death loss may be due to a wide variety of problems, such as a low dissolved oxygen (DO) content of the water, pesticide toxicity, etc. The county agent will contact the AgCenter aquaculture specialist for assistance if it is indicated. If it is located in a parish where WSSV has been confirmed, whole crawfish can be submitted for WSSV testing.

How is the disease diagnosed in crawfish?

Whole crawfish are submitted to the laboratory for the diagnostic procedure. The virus is confirmed in the crawfish by a special test (called PCR, or polymerase chain reaction), in which the genes (DNA) of the specific virus causing the disease are identified. This test requires 24 hours to complete. There is no blood test, or virus isolation test, that can be used to diagnose the disease as with many of the traditional tests in livestock.

How can the disease be spread to a pond or rice field?

There are several ways in which the WSS virus can be spread:

  1. Feeding infected shrimp or using them as bait in crawfish traps.
  2. Introducing infected crawfish (trapped from an infected pond or wild marshland) as stock into the pond.
  3. Shorebirds carrying infected crawfish from one place or the other or introducing the virus into a new location through their digestive tract.
  4. Infected crawfish migrating from one pond to another.
  5. Moving contaminated equipment (traps, boats, etc.) from an infected location to a clean pond.
  6. People caring the virus from one place to another on their boots or personal equipment.

What happens after the infection has been disclosed in a pond or rice field?

Those procedures are being developed.

How is the infection in a pond cleaned up?

Those procedures are being developed.

How can the crawfish farmer be sure that the infection has been cleared from his ponds or fields?

Submit a representative sample of the restocked crawfish to the laboratory for diagnostic purposes after the pond has been re-established for at least one month.

How can an individual crawfish farmer best prevent the infection?

Acquire crawfish for stocking the pond/field from sources that have been shown to be free of WSSV. If there is a problem with shorebirds at the location, consult with the local Louisiana Department of Wildlife and Fisheries representative.

Is WSSV related to “Ich,” or White Spot Disease, in finned fish?

No. Ich is cause by a protozoan parasite that infests the outer surface of the skin of fish. WSSV is a virus that invades the organ systems of crustaceans, causing a “white spot” on the external surface of shrimp and an exceedingly high mortality.

May 2007

Filed as: Health