Linda Benedict, Thune, Ronald L. | 5/20/2009 1:02:05 AM
Ronald L. Thune and John P. Hawke
In much of aquaculture, animals are in high density production systems. This can result in stress from crowding and sub-optimal water quality conditions and provide for easy transmission of disease.
In response to anticipated disease problems, the Louisiana Aquatic Animal Disease Diagnostic Laboratory (LAADDL) was established in the LSU School of Veterinary Medicine in 1982. Initially, the laboratory assisted the few catfish and bait-minnow producers. However, as catfish farming expanded and technologies developed for the culture of additional species, the laboratory expanded its duties. Laboratory staff work cooperatively with several state agencies involved in protecting our aquatic resources. The LAADDL staff work with the Louisiana Department of Wildlife and Fisheries to diagnose disease problems in natural bodies of water and in the state’s fish hatchery system. In addition, the lab helps the department’s enforcement division to develop evidence to support the arrest and conviction of individuals involved in the illegal taking and selling of wild fish. The staff is also a part of the LSU School of Veterinary Medicine Fish Kill Emergency Response Team, which helps the Louisiana Department of Agriculture and Forestry and the Louisiana Department of Environmental Quality investigate fish kills.
Most case submissions to the LAADDL, however, have been from commercial aquaculture, involving catfish, hybrid striped bass, soft-shell crawfish and tilapia. Several of the most significant problems are highlighted below.
A primary problem in catfish farming is a bacterial disease specific for channel catfish called enteric septicemia of catfish, or ESC. Control involves giving antibiotic treated feeds to the fish. Only two antibiotics, Romet and oxytetracycline, have been approved by the Food and Drug Administration (FDA). Limited treatment options led to overuse of the individual antibiotics and the development of ESC bacteria resistant to one or both drugs. Alternate treatment options developed, including cessation of feeding in affected ponds to prevent oral exposure of the fish to the bacterium in the water. It was apparent, however, that the preferred option was prevention of disease by vaccination, so we initiated research to develop an effective vaccine. This proved more difficult than anticipated, but has led to the development of an effective product that is being field tested (see page 19).
In the mid-1980s, aquatic diagnostic labs in Arkansas and Mississippi reported a new disease problem that farmers called “Hamburger Gill Disease” (HGD) because of the massive destruction that occurred in the gill filaments. In April 1985, the first case in Louisiana was diagnosed from a newly constructed pond. More than 90 percent of newly stocked 4-inch to 6-inch catfish died within 48 hours to 72 hours of stocking, and similar scenarios were repeated more than 60 times in the next 36 months as new ponds were constructed all over the state.
At the time, HGD was an emerging disease problem with an unknown cause. Because of the high mortality rates and the lack of an available treatment, LAADDL staff recommended rapid harvest if the fish were of marketable size, followed by a complete disinfection of ponds with quick lime. Six years later a protozoan parasite was confirmed as the causative agent. Soon after that, a small aquatic worm was implicated as an intermediate host for the parasite, releasing a small spore into the water that infected the fish. The catfish was an accidental host for the parasite, however, because the parasite was never able to complete its life cycle and produce a stage that could reinfect the worm. At present, with still no treatment available, efforts continue to find a control for the worm host and to identify the true final host. Elimination of the worm host or the final host are the only options for controlling HGD-associated deaths.
Farm-raised crawfish seldom experience disease outbreaks. While growing rapidly, they constantly molt the exoskeleton and, in the process, shed any external parasites. Also, crawfish production is not nearly as intensive as the other aquaculture systems in the state, so the stress/crowding-associated diseases are uncommon. But, in the late 1980s, two developments in the industry led to intensification.
First, attempts to expand crawfish markets to non-traditional markets in other states found that consumers were reluctant to eat crawfish with the dark vein (actually intestine). This led to the development of high density purging systems in which crawfish were held for two to three days without feed to allow the intestinal tract to become void. The resulting product looked cleaner and was more acceptable.
Second, the softshell crawfish phenomenon swept through Louisiana. Using technology developed at the Louisiana Agricultural Experiment Station, producers held crawfish at high density in tray systems, selected crawfish as they shed their exoskeletons and sold them to the restaurant trade for more than $7 per pound.
Unfortunately, as in other situations, harvest and movement to the intensive, crowded conditions stressed the crawfish. In 1985, unexplained mortalities began occurring in an experimental purging system at the Ben Hur Aquaculture Facility, now the Aquaculture Research Station. A non-01 strain of Vibrio cholerae was the cause. Over the next several years, V. cholerae was isolated from a number of purging and softshell operations that were experiencing significant die-offs. Recommendations to reduce harvesting stress by running traps more frequently and to reduce transport stress by keeping the animals cool and moist helped alleviate the problem somewhat. Eventually the softshell industry succumbed to economic problems associated with overproduction, but V. cholerae is occasionally diagnosed in the remaining facilities.
In August 1992, the Department of Wildlife and Fisheries issued the first permit for a tilapia farm in Louisiana. It was very restrictive and limited to indoor, recirculating, intensive systems. The success of the initial farm attracted additional producers, and Louisiana now has five. As with other species, the appearance of a serious disease was inevitable and, in 1994, we began to receive tilapia specimens from all over the United States. A serious bacterial pathogen, Streptococcus iniae, was isolated from the diseased specimens. Fortunately, the disease was not reported from Louisiana farms. Because of the seriousness of the S. iniae problem in other regions, LAADDL staff recommended that Louisiana producers not import any tilapia, and, if importation was necessary, to apply strict quarantine procedures. This recommendation was followed, and as of this writing S. iniae has not been reported from Louisiana tilapia farms. It remains a significant problem elsewhere.
Hybrid Striped Bass
In the late 1980s, intensive culture of hybrid striped bass increased dramatically in Louisiana and surrounding states. This was reflected in submissions of hybrid striped bass specimens, which increased from 18 cases in 1988 to 220 in 1991. Parasitic and bacterial infections were common in the early development of the industry, all of which could be controlled with treatment.
In 1990, hybrid striped bass culture spread to the coastal marshes where fish were raised in cages and raceways. That December, a new bacterium, known as Photobacterium damselae subsp. piscicida, was isolated on one of the coastal farms. Photobacteriosis proved difficult to control because the fish stop eating, which precludes treatment with antibiotics in feed.
Over the next several years, photobacteriosis caused massive mortalities and was at least partially responsible for closing several fledgling hybrid striped bass farms on the coast, where it remains a major constraint to the development of fish culture. Recent research has led to the development of a live attenuated vaccine for photobacteriosis that has promise for preventing the problem worldwide (see page 15).
Ronald L. Thune, Professor, Departments of Veterinary Science and Veterinary Microbiology and Parasitology, and John P. Hawke, Assistant Professor, Veterinary Microbiology and Parasitology, School of Veterinary Medicine, Baton Rouge, La.
(This article was published in the fall 1999 issue of Louisiana Agriculture.)