Prevention and treatment of hemorrhagic disease in pangasius
Causes
The pathogen is the motile Aeromonas spp. group of bacteria, including Aeromonas hydrophila, A. sobria, and A. caviae. Among them, Aeromonas hydrophila is considered the most significant pathogen for freshwater fish. These are Gram-negative, short rod-shaped bacteria with rounded ends, measuring 0.5 x 1 – 1.5 µm, and are motile via a single polar flagellum.
Features
The disease was first recorded in 1979 in the United States on (freshwater) salmon, after which it appeared in several Asian countries. In Vietnam, it was documented before 1993. Currently, the disease is present in nearly all freshwater fish farming areas and causes substantial economic losses to farmers in the Mekong Delta provinces.
Susceptible species: Striped catfish (P. hypophthalmus), basa catfish (Pansianodon bocourti), and several freshwater fish species. In our country, the disease occurs in almost all fish farmed in cages, rafts, and freshwater ponds. The disease strikes fish at all farming stages. It spreads rapidly, with high mortality rates (up to 90%) in severe cases.
Disease occurrence season: In Vietnam, hemorrhagic disease caused by A. hydrophila occurs year-round, primarily at the beginning of the dry season, especially when fish are stressed, such as after rainfall.
Transmission route: The disease spreads from sick fish to healthy fish within the same pond. Pathogens present in the environment can directly infect healthy fish via contaminated handling equipment (basins, hand nets, drag nets, etc.). The pathogens can survive and reproduce in the pond water environment, spreading from one pond to another and from one farming area to another.
Symptoms
In the early stage of infection, fish eat poorly or stop feeding, floating sluggishly near the water surface. The skin often turns dark without a silvery sheen, and the fish loses its slime coat. In the subsequent stage, hemorrhages appear on the body, fin bases, around the mouth, eyes, and anus.
Ulcers appear, penetrating deep into the muscle. The ulcers are often infested with fungi and parasites. Eyes are cloudy and exophthalmic, the periorbital area is swollen, mucus is lost; the anus is inflamed and hemorrhaged; the abdomen is severely distended, and fins are frayed.
Pathological lesions
Intestines may be filled with gas; the liver and kidneys are often necrotic. Abdominal cavity hemorrhages, liver is pale, gallbladder is swollen, kidneys are enlarged and hemorrhagic.
In acute cases, dissecting the fish reveals abundant reddish and bloody fluid in the abdominal cavity, visceral hemorrhaging, and high mortality rates within a short period.
Diagnosis
Clinical diagnosis: Based on typical symptoms.
Laboratory diagnosis: Collect samples following guidelines. Send diseased fish samples to a designated testing laboratory.
Disease prevention
The pond is enclosed with netting, and the pond dikes are firmly embanked without leaks; pond renovation and environmental treatment follow proper technical procedures. Water supplied to the rearing pond must be treated to ensure quality, filtered through a two-layer mesh net with a 40 µm mesh size.
Pangasius seed stock must be healthy, have clear origins, ensure disease safety, and comply with National Technical Regulations on aquatic breeds; aquatic breeds sourced from outside the province or imported must have a Quarantine Certificate and undergo quality inspection according to regulations.
Selecting pangasius fingerlings: must be healthy and of clear origin. Photo: LHV
Before stocking, fish can be bathed in certain antiseptic solutions to eliminate pathogens.
Comply with stocking density guidelines set by aquaculture management authorities.
Administer vaccines in accordance with the instructions of the manufacturer or specialized aquaculture veterinary authorities. Only use drugs, chemicals, and biological products on the List of aquatic veterinary drugs permitted for circulation in Vietnam and in compliance with guidelines from the specialized aquaculture veterinary management agency; observe withdrawal periods before harvest as directed by the manufacturer.
Use commercial feed in powder, flake, or pellet form with sizes suitable for the fish's mouth size; protein content and other essential components must meet the nutritional requirements for each growth stage. Properly manage the amount of feed provided to prevent waste and pond water contamination; compound and farm-made feeds must be stored properly to prevent mold and bacterial infection; live and fresh feed must be treated to ensure it is free from pathogens before feeding.
Supplement vitamins, minerals, etc., to boost fish immunity before and during peak disease periods.
Daily, monitor fish health indicators such as color, activity levels, and feed consumption. Check pond environmental factors: DO (daily); pH and alkalinity (every 2 days); H2S and NH3 (once a week).
Do not share tools among different ponds, cages, and tanks. Tools used in production must be cleaned and disinfected before and after use.
Prevent fish from experiencing shock caused by adverse environmental shifts: temperature, dissolved oxygen levels, and water contamination.
For fish farming in cages and raft enclosures, bags of lime should be hung regularly: once every 2 weeks during the disease season and once a month at other times. Lime disinfects and alkalizes the water. The average dosage is 2 kg of quicklime/10 m3. Hang more bags in larger cages and raft enclosures and fewer in smaller ones, concentrating them around feeding areas and on the upstream side.
Personnel working at the farming facility must perform cleaning and disinfection when entering and exiting the facility.
Sample fish for pathogen testing periodically at least once a month per pond for commercial fish, and twice a month per pond during the nursery stage.
Aquaculture facility owners need to monitor and stay informed about the disease situation in the area/region, weather forecasts, and disease warnings from specialized agencies to take proactive disease prevention measures. When fish show abnormal signs, facility owners must immediately notify veterinary authorities to promptly identify pathogens.
Diseased fish, dead fish, and waste from infected ponds must be collected and treated promptly according to instructions from technical staff and aquatic veterinary authorities.
Pangasius farming facilities must maintain logbooks to monitor fish health and the use of medications, chemicals, biological products, and environmental remediation substances throughout production. Records must be fully preserved to present upon request.
The pond monitoring logbook must be certified by the competent aquatic animal health agency to meet inspection and traceability requirements when requested by importing countries.
Disease treatment
To treat hemorrhagic disease in pangasius, treat the water with a disinfectant and add antibiotics (such as florfenicol, doxycycline) to the feed for 5 – 7 days, following the manufacturer's instructions for dosage and use.
In the case of fingerlings or high stocking density, antibacterial bath products can be used for fish with appropriate dosages and duration.
Along with that, supplement probiotics and vitamins C and E to help the fish recover faster. At the same time, water quality should be controlled, high-quality feed used, and minerals supplemented to boost the fish's resistance.