Greg Lutz

Production of farm-raised crawfish in Louisiana continues to grow in acreage and value. In 2019, the farm-gate value was approximately $252 million, with an additional added value of $170 million through processing and marketing. Additional economic multipliers via support industries across Louisiana are also significant. Some 1,850 farm operations scattered over approximately 250,000 acres in 31 parishes provide an estimated 5,500 jobs in production, processing and support industries. Crawfish production is also an important economic option for Louisiana rice producers, often making a difference between profit and loss for many farm operations in years when rice prices are less than favorable.

Originally described in shrimp farms in Thailand in 1992, white spot syndrome virus has been a problem in Louisiana crawfish ponds since it was first confirmed in the state in 2007. Cases have been reported in farmed crawfish every year since, usually in March or early April. Although white spot syndrome is technically a virus, it affects only crustaceans. It cannot infect people or other animals.

Typical signs of a white spot syndrome outbreak are a drastic drop in catch over just a few days. Most of the medium-sized and large crawfish die but small crawfish continue acting normally. Dead crawfish are often seen floating throughout the pond or windblown along the levees. Large crawfish that aren’t dead are usually slow-moving and uncoordinated. Estimates of the total impacted acreage peaked during the 2016–2017 season, ranging as high as 40,000 acres.

When a pond “breaks” with white spot syndrome, the producer faces severe economic loss. Factors that trigger these outbreaks have never been well understood, despite numerous documented cases. But with funding from the Louisiana Crawfish Promotion and Research Board, AgCenter researchers have collaborated with colleagues in the LSU School of Veterinary Medicine to determine several factors influencing crawfish susceptibility to the virus. These findings will result in additional management recommendations for producers.

Size Influence on Mortality

Small (12.9 gram), medium (24.5 gram) and large (35.1 gram) crawfish were dosed with the same concentrations of viral particles to investigate effects of size on infection and survival. Results indicated no difference in mortality patterns among size groups. Consumption of infected tissue is a typical route of white spot syndrome spread in crustaceans, and results support the theory that when a crawfish dies from the virus, large crawfish eat it and become infected, while small crawfish avoid interacting with larger ones and so remain uninfected.

Genetics of Resistance

In laboratory trials, a number of crawfish survived virus concentrations that killed other test animals. These survivors were housed in indoor tanks and, as they matured, males were introduced into female holding tanks for mating. Females were placed in artificial burrows with mud and water and maintained in the dark at 81 degrees Fahrenheit to mimic summertime conditions in natural burrows. Hatchlings from resistant parents were stocked into outdoor tanks with soil, rice plants and virus-free pond water. After five months, offspring of resistant and control animals were brought back to the lab and injected with known concentrations of virus. There were no significant differences in survival between the groups, suggesting that resistance to the virus is not transmitted from a survivor to its offspring. This finding will help producers make decisions in how to manage surviving populations and crop rotations in impacted ponds.

Temperature and Infection

Infection studies at 68 F, 77 F and 89 F indicated that the middle temperature resulted in more rapid mortality. The upper and lower temperatures eventually saw the same cumulative mortality, but crawfish were much slower in dying. These results help explain why white spot syndrome outbreaks are usually observed in March and April. Infected crawfish die more quickly at intermediate temperatures, and as they are eaten the disease spreads much more rapidly through the population. Follow-up research will focus on water depth management to minimize the time crawfish are exposed to this temperature-related window of susceptibility (for example, maintaining deeper depths up to the time when water temperatures reach the mid-70s, followed rapidly by shallower depths to speed up warming trends).

Greg Lutz is a professor in the School of Renewable Natural Resources and an aquaculture specialist with Louisiana Sea Grant.

Acknowledgements

Chris Green, formerly in the School of Renewable Natural Resources; John Hawke in the LSU School of Veterinary Medicine; and Mark Shirley, LSU AgCenter Southwest Region and Louisiana Sea Grant.

(This article appears in the winter 2022 issue of Louisiana Agriculture.)

A female crawfish being introduced in an artificial burrow for research on the white spot syndrome. Photo by Greg Lutz

Offspring of resistant and control crawfish were brought back to the lab and injected with known concentrations of virus. There were no significant differences in survival between the groups, suggesting that resistance to the virus is not transmitted from a survivor to its offspring. Photo by Chris Green