Protecting the health of farmed aquatic animals: Prevention first, then comes timely treatment

Several diseases severely affect aquaculture production

Vietnam’s aquaculture is currently suffering from many dangerous aquatic animal diseases, particularly in key farmed species such as black tiger shrimp, white-leg shrimp, pangasius, and other high-value aquatic species, resulting in substantial economic losses for farmers. Some notable shrimp diseases include:

Monodon baculovirus disease (MBV): Found in species of the Penaeidae family. Severely infected shrimp are typically weak and swim sluggishly. Their body color changes to bluish or greenish-black. They grow slowly and molt unevenly. Cumulative mortality can reach 90% if environmental conditions are unstable. Harvested shrimp are often undersized and regarded as “needle shrimp” after 3 - 4 months of farming.

White Spot Disease (WSD): One of the most dangerous shrimp diseases, especially in black tiger and white-leg shrimp. Caused by white spot syndrome virus (WSSV), it spreads extremely rapidly and can lead to mass mortality within just 3- 10 days of onset.

Enterocytozoon hepatopenaei (EHP): The leading cause of 50 - 70% growth reduction, size variation, soft shells, and poor nutrient absorption. EHP outbreaks are most severe in the rainy season, with detection rates nine times higher than in the dry season.

White Feces Disease (WFD): Common in ponds with high algal concentrations, overfeeding, and microbial imbalance. Caused primarily by Vibrio, often in coinfection with EHP.

Acute Hepatopancreatic Necrosis Disease (AHPND): Caused by Vibrio parahaemolyticus, particularly affecting black tiger and white-leg shrimp, with the most susceptible stage being 20 - 45 days old. Mortality can reach 100%.

Infectious Myonecrosis (IMN): Caused by the infectious myonecrosis virus (IMNV). This disease results in the highest mortality in white-leg shrimp, 40 - 70% of the stock. IMNV can strike at any growth stage, is chronic, and can cause up to 70% mortality by the end of the grow-out cycle. Affected shrimp display extensive white necrotic areas in striated muscle, particularly in the abdomen and tail, and turn reddish after death, resembling cooked shrimp.

Taura Syndrome Virus (TSV): Particularly damaging to postlarvae and juvenile white-leg shrimp aged 14- 40 days. It can cause mortality rates of 40- 95%. In the acute stage, shrimp appear pale red, with reddish tails and swimmerets, soft shells, empty guts, and typically die at the end of a molting cycle.

Fish farmers are also suffering heavy losses due to the appearance of several fish diseases, including:

Viral Nervous Necrosis (VNN): Affects around 50 marine fish species, such as Asian seabass and grouper. The disease impacts larvae and postlarvae stages, but high mortality is also recorded in market-size and adult fish. Clinical signs include abnormal swimming (circling, upside down, disoriented), loss of appetite, darkened skin, bloated swim bladders, and death occurring within 3 - 5 days.

Bacillary necrosis of pangasius (BNP): Caused by Edwardsiella ictaluri. This occurs at all stages, but mainly in fish under 400 g. Clinical signs include lethargy, reduced or zero appetite, hemorrhages on skin, fin bases, and anus, swollen eyes, and enlarged heads. Liver, kidney, and spleen show white spots.

Streptococcus agalactiae infection: Affects many fish species, especially tilapia. Occurs year-round, mainly during high temperatures, with mortality rates up to 90 - 100% within 2 - 3 weeks. Signs include abnormal swimming patterns (spiralling or spinning), darkened body color, cloudy and protruding eyes, hemorrhages at fin bases and operculum, abdominal distension, and inflamed anus.

Disease information must be recorded, managed, and reported promptly and accurately

At a recent workshop, Nguyen Van Huu, Acting Head of the Aquatic Animal Disease Management Division, Directorate of Fisheries and Fisheries Surveillance (Ministry of Agriculture and Environment), mentioned the suffering of some localities from aquatic animal diseases due to multiple contributing factors.

Unstable farming environments are the number one factor. Diseases often break out when water temperature fluctuates, organic load is high, or sudden changes in salinity occur. A phenomenon known as “microbial collapse” can occur after rain, as rainwater contains hydrogen peroxide (H₂O₂), disrupting beneficial microbiota, weakening shrimp, and causing mass mortalities. Discharging untreated wastewater and waste into the environment not only violates regulations but also spreads pathogens.

Another major issue is the lack of full compliance with disease prevention and biosecurity measures. Farm owners are responsible for regular cleaning, disinfection, and waste treatment in farming areas and equipment, but in many places, these practices are not strictly followed. The use of raw or home-prepared feed without biosecurity safeguards also poses significant risks.

Disease management and surveillance in some localities remain limited. Despite directives from higher authorities, the development and implementation of aquatic animal disease control plans are sometimes inconsistent. Inadequate updating or delayed reporting of disease data makes forecasting and response efforts difficult.

Nguyen Van Huu, Acting Head of the Aquatic Animal Disease Management Division, Directorate of Fisheries and Fisheries Surveillance, said, “The most important principle in aquatic animal disease prevention and control is prevention first, then comes timely treatment and urgent outbreak response. However, given the complexity of these diseases, close monitoring and swift action are required from specialized management agencies as well as from farmers themselves.”