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Phan Thi Van, Le Van Khoa, Dang Thi Lua, Kim Van Van and Nguyen Thi Ha

Research Institute for Aquaculture No.1
Dinh Bang, Tien Son
Bac Ninh, Vietnam

Phan Thi Van, Le Van Khoa, Dang Thi Lua, Kim Van Van and Nguyen Thi Ha. 2002. The impacts of red spot disease on small-scale aquaculture in northern Vietnam. p. 165-176. In: J.R. Arthur, M.J. Phillips, R.P. Subasinghe, M.B. Reantaso and I.H. MacRae. (eds.) Primary Aquatic Animal Health Care in Rural, Small-scale, Aquaculture Development. FAO Fish. Tech. Pap. No. 406.


Small-scale aquaculture plays an important role in livelihoods in rural areas of Vietnam. The main cultured species are Chinese carps (Ctenopharyngodon idellus, Hypophthalmichthys molitrix and Aristichthys nobilis), Indian major carps (Labeo rohita and Cirrhinus cirrhosus), common carp (Cyprinus carpio) and tilapia (Tilapia spp.). A disease, the causative agent of which has not yet been identified, but known in Vietnam as "red spot disease" (RDS) because of the presenting signs, which are similar to those of epizootic ulcerative syndrome, is causing significant economic loss. This disease is the major constraint to improving output from freshwater aquaculture in Vietnam.

The objective of this study was to provide an overview of RDS in Vietnam and the socio-economic impacts on small-scale fish farming in the north of Vietnam. A total of 145 farmers in Thai Nguyen and Bac Ninh provinces, representing highland and lowland aquaculture systems, were interviewed using a questionnaire provided by the Network of Aquaculture Centres in Asia Pacific (NACA), and the Epi-Info software program was used to analyse the data. Findings from the study confirmed that grass carp is the most susceptible species to RSD and, out of 81.4% farmers who had disease problems during the growing cycle, 83.1% had this disease problem. The disease has a severe negative socio-economic impact in both lowland and highland small-scale aquaculture systems in the north of Vietnam. It appears to have a seasonal pattern, occurring mainly in March-April and October-November. The findings also showed that government extension officers have been working very closely with fish farmers to assist in controlling this disease. However, farmers have very limited knowledge of disease prevention and control In order to improve this situation, there is a pressing need for more training in basic disease recognition and fish health management for both farmers and extension officers. Research to identify the aetiology of RDS, investigate its epidemiology and find methods for prevention and control is also required. If successful, the experience would serve as a useful model on which to build improved aquatic animal health management in small-scale aquaculture systems in Vietnam.


Freshwater aquaculture production in Vietnam has increased from 111,157 mt in 1988 to 369,000 mt in 1997 (FAO 1999), an increase in tonnage of more than 330% in a decade. Freshwater aquaculture is dominated by small-scale systems, such as household integrated farming systems (VAC system), rice-fish, cage culture in rivers and reservoirs and culture-based enhancement reservoir fisheries. The main freshwater cultured species are Chinese and Indian major carps and tilapia. Production from freshwater aquaculture is mainly used for local, domestic consumption.

A disease, the causative agent of which has not yet been identified, known in Vietnam as "red spot disease" (RSD), is the major constraint to improving the output from freshwater aquaculture. It causes significant economic loss and presents signs similar to those of epizootic ulcerative syndrome (EUS).

In this paper, the history of RSD in Vietnam is described and the findings obtained from a questionnaire survey to evaluate the impact of the disease on small-scale aquaculture systems in two northern provinces are presented.

1EpiInfo is a specialised program for epidemiological analysis and is distributed free by the Centres for Disease Control, Atlanta, Georgia, USA. A copy can be obtained from their website at


RSD was first reported in grass carp (Ctenopharyngodon idellus), common carp (Cyprinus carpio) and (Hypothalmichthys molithrix) in 1962 in Ninh Binh Province (Ha 1995). Since then, it has spread to almost all northern provinces of Vietnam. The disease is characterised by the presence of one or more red, circular lesions on the body of affected fish. In 1973 and 1976, RSD outbreaks were also reported in catfish in the Cuu Long River Delta (Bui et al. 1992), in 1981 in Nghe An Province, and in 1982 in Da Nang and Hoang Lien Son Province (Ha 1992). From 1983 to 1984, RSD was reported to have infected grass carp, bighead carp (Aristichthys noblis), common carp and snakehead fish in the Cuu Long River Delta. In 1986-1994, RSD was widespread in cage-cultured grass carp. In 1994-1998, RSD was reported to be prevalent in pond culture.


Little information is available on the impact of RSD between 1962 and 1982. During the outbreak of 1983 in the Cuu Long River Delta, mortalities amounted to 20-30% of the total fish production from cage culture. In Tuyen Quang Province, between 1986-1987, an RSD outbreak forced the farmers to stop growing grass carp in all of the 339 cages. In early 1993, due to an overabundance of labour, farmers again started cage culture of grass carp. An outbreak of RSD occurred again and affected 357 of 390 cages. This time, mortalities were low and farmers maintained fish production, although levels were not as high as expected. In 1994, the disease re-occurred, causing 70-80% mortality and reducing the number of cages in use to 100. During 1993-1994, the disease appeared to have a seasonal pattern, occurring mainly during the wet season from May to August (Ha 1995). In 1992, RSD affected 42 of 43 grass carp cages in Hoa Binh Province, causing a 100% loss. In the Da River in 1988, 80 of 100 cages were affected by RSD, also causing severe losses (Bui et al. 1992).

In the Hanoi region, cage culture has developed substantially since early 1993, with 619 grass carp cages in the Red, Duong, Nhue and North Hung Hai rivers. From October 1993 to 1994, 105 cages in the Red River were affected by RSD, causing high mortalities. Thirty cages in the Duong River were also affected, and there were several infected cages in the North Hung Hai and Nhue rivers. Due to this disease, the number of cages in the Hanoi region was reduced to 238 by the end of 1994, and in 1996, the number of cages in use was insignificant (Truong et al. 1996). The disease affected not only cage culture, but also pond culture. In Coloa, Dong Anh District, the disease caused a significant loss in broodstock farms. Around Gialam Airport, the disease resulted in losses of nearly 100% in seed production in 1995 (Bui et al. 1998).

In what was then known as Ha Bac Province, now divided into Bac Ninh and Bac Giang provinces, the first outbreak occurred in 1988-1989 in broodstock, causing a drop of 30-40% in seed production. Since then, the disease has occurred annually. In Mao Dien District, known as the "cradle" of seed production in the lowland region in the north of Vietnam, almost all fish farmers have changed to other activities due to severe losses caused by RSD (Bui Quang Te, pers. comm.).

In the centre of Vietnam, including Nghe An and Thanh Hoa provinces, RSD has been reported to cause losses of 30-40%.

RSD has caused heavy mortality in grass carp in Thai Nguyen Province, with the highest mortality (90% to 100%) being in Phu Binh District. In general, during the period of 1995-1997, in the north of Vietnam, 80% of the 5,000 or so cages and 80-90% of ponds were affected by RSD, resulting in severe losses estimated at more than US$500,000 (RIA 1 1998).


Survey Area and Samples

The survey took place in two provinces during August 1999: Thai Nguyen and Bac Ninh, representing both highland and lowland aquaculture systems. These provinces were once part of Bac Thai and Ha Bac provinces, respectively, but have since been separated.

Bac Ninh is a lowland province with a population of 920,460 people, 797,854 of whom are engaged in agriculture (including aquaculture). Bac Ninh has a long experience in aquaculture, such as seed production, nursery, grow-out and fry trading (Nguyen 1998). Bac Ninh has five districts and one town, all of which have aquaculture activities. Two of the districts and the town are considered to have a higher standard of living than the rest. Gia Luong and Que Vo districts were randomly selected as representing districts of higher and lower standards of living, respectively (ranking by Agricultural Extension Officer).

Thai Nguyen is a highland province where 390,000 of a population of 1,019,299 are engaged in agricultural activities. Five of nine districts/towns are considered to have a lower standard of living than the other four. Thai Nguyen town and Dai Tu District were randomly selected as representing higher and lower standards of living, respectively.

A total of 145 farms in seven villages located in DaoVien, Phuong Mao (Que Vo District) Trung Chinh, Phu Luong (Gia Luong District) Tan Thai, Tan Lap (Dai Tu District) and Thinh Dan (Thai Nguyen town) were visited. Data were collected from farmers using a questionnaire provided by the Network of Aquaculture Centres in Asia-Pacific (NACA). The questionnaire was divided into two sections. The first section contained six parts (General, Description of Operation, Revenue, Cost of Production, Household Information and Problems Occurring), while the second section focused on specific disease problems.

Data Analysis

"EpiInfo," a data management and analysis programme, was used to analyse the data.

Other sources of data, taken from reports of the local extension centre and extension staff and from talks with farmers, have also been used in compiling the results of the survey.

Case Study Approach

The survey covered only pond aquaculture systems. Farmers are involved in growing or nursing fish, but some also practice a combination of these two activities.

Poverty ranking, based on family income, was carried out by extension officers. There was only a very slight difference between groups with low and high standard of living; this is because most freshwater, small-scale farmers in northern Vietnam are poor.

Survey Limitations

Not all the questions were completed, as some farmers were not able to provide reliable answers, especially for those questions where numerical answers were required.

In addition, some interviews were attempted at a time when the national football team was playing, and this was broadcast on television. Consequently, some farmers refused to be interviewed, or did not concentrate on the interview. In some cases, this problem was overcome, either by waiting until the football match was finished or by arranging to visit at another time. This pointed out the importance arranging interviews at times that suited the farmers.


Description of Small-scale Aquaculture Systems

All respondents had pond culture systems, and the main culture species were grass carp, common carp, silver carp, mrigal and rohu (Labio rohita). Grass carp is the most popular cultured species because of the low investment cost and also, because family labour can be used. Cash income was considered the main reason for growing fish by 76.6% of farmers. Food consumption by the family was considered the main reason for growing fish by 22.8% of farmers surveyed, while only 0.6% of farmers were growing fish because of status. None of the farmers was growing fish as a hobby (Fig. 1).

In general, growing rice and livestock were the main activities supporting the farmers; however, other activities such as growing vegetables and fruit also provided income. The farmers in Dai Tu District of Thai Nguyen Province were able, due to geographic differences, to pursue another activity, tea growing, which was a significant source of income for the family. Most of the farmers did grow-out of fish, while some of them also nursed fish for fry production. Nursing requires a lot of labour to take care of the fish, and therefore, larger families carried out this activity.


The most important issues affecting farmers were disease (65.8% of the farmers surveyed mentioned disease as the most important problem), insufficient water (15%), theft (8.6%), and flooding (5.7%). Other responses (predation, water temperature too hot or too cold, unidentifed) made up the remaining 4.9%. It has to be noted that most of the farmers do not have a water inlet and outlet canal, they, therefore, depend on rain or ground water supply. More than 8% of farmers considered theft the most important constraint; these farmers will have to invest either money or time in prevention (e.g., by building a fence).

Disease affected 81.4% of farms surveyed. Of these, 83.1% reported that their fish had been affected by RSD, 11.9% by unknown diseases, 4.2% by parasitic diseases and 0.8% by fungal diseases. Insecticides may have caused the unknown diseases that were reported, since they were reported to occur just after rain. This is especially likely in Thai Nguyen Province, a mountain province with fishponds located in the valley and where tea is grown on the sides of the mountain. Farmers very often apply insecticides to the tea bushes and, when it rains, the insecticides are washed into the ponds, causing the death of the fish. These unknown diseases could also be caused by parasites.

The signs of RSD disease are haemorrhage, red spots on the body, scale loss, swollen vent and darkened skin. Grass carp were the species most frequently reported to be affected by RSD, although in some farms mrigal and rohu were also infected. In those two species, no red spots were seen on the body of the fish, but they tended to have dark skin. The size of fish affected by RSD varied.

From the survey, RSD appeared to have a seasonal pattern, occurring mainly in March-April and October-November (Fig. 2).


Of the farmers interviewed, 60.4% said that they could choose the fry they purchased. Of the farmers interviewed, 92.7% said that they had access to healthy fry and only 7.3% said that they received unhealthy fry. Farmers check fry by visual examination, making a judgement about their health based on personal understanding of the movement and colour of the fry.

Poor water quality was considered by 84.5% of farmers as the cause of RSD. Other suggestions were bad food (5.6%), too little water (2.8%) and cold water (7%). All farmers thought that cold water contributed to the occurrence of the disease, probably because their fish were affected by RSD during October-November. Due to shortage of water and capital, (water exchange is expensive, using rented water pumps with additional cost for gasoline), very few farmers cleaned their ponds after every crop, even when the previous batch of fish had a disease problem. Only in Tan Thai Village (Dai Tu District) could farmers readily change water between crops, as it is located near a reservoir with a canal that provides easy access.


Of the farmers interviewed, 88.3% considered that their crop did not perform as well as expected, 3.5% considered it did as well as expected and 7.7% did not know. Only one farmer considered that the farm performed better than expected. The reason he gave was that he was able to sell by-catch fish caught when harvesting. Normally by-catch fish can be harvested from every pond, however, there are not usually saleable.

In farms where RSD occurred, it caused an average loss of 45.2% of production (range of 5-100%, SD=24.2). The total in 1998 for the 58 farmers who answered the question was 71,170,000 VND (US$5,120). Average percentage contribution from aquaculture to the total household income was 38.4% (SD=18.7), which excluded fish consumed by the family (on average each family consisted of five persons).

For those farmers facing disease problems, there was a decrease in the market price of fish leading to a reduction in household income. However, 95.4% of farmers considered that disease did not cause an increased household debt, even though they were poor, as they had not borrowed any money from the government.

It was very clear that RSD caused losses that affected the household income. Despite this, farmers still tended to continue with aquaculture activities because they still considered that aquaculture was profitable (see Table 1). In this situation, "profitable" means some income from selling fish, some fish for food or additional income from family labour. Very few farmers changed to growing rice in fish ponds, since farmers perceived this as a short-term alternative crop, with aquaculture remaining as their main goal for ponds. Farmers considered that rotation between rice and aquaculture was an innovative idea, and by changing crops, they hoped that pathogens remaining in the pond soil would be destroyed. Insecticide or herbicide residues were not considered as a possible cause of problems in aquaculture. There are few other choices for farmers, and they are not able to leave their ponds empty. A number of farmers reported that, if they did not have a good yield in the present year, they would change to rice growing the following year. This was because they saw that people who grew rice in the ponds were sure of good rice production. However, they again stressed that ponds were better for fish growing, and they would continue to grow fish in their ponds, with rice as a crop in alternate years.

Table 1. Changing attitude to aquaculture caused by disease.

Changing fish species was not popular among small-scale farmers because, as discussed above, grass carp do not require a big investment and require minimum labour, such as that provided by children after school. While taking care of the buffalo, grass can be cut for the fish; therefore, grass carp is still the main species for their pond. Some farmers tend to change the ratio of fish species grown; for example, some farmers have increased the proportion of tilapia in the ponds. However, at the moment, sex-reversed tilapia culture is not popular among small-scale farmers in the north of Vietnam.

A general picture of aquaculture in rural northern Vietnam is shown in Table 2.

Another issue for farmers was the use of antibiotics to treat RSD. Among the farmers using antibiotics, 85.7% eat sick fish. There is, therefore, a risk in these communities of consumption of fish containing antibiotic residues that may lead to development of antibiotic resistance in human bacterial pathogens.

Table 2. General picture of aquaculture in rural northern Vietnam.

At the beginning of the survey, according to government extension information, farmers were classified into two standards of living. However, while carrying out the survey, the authors saw that there was no significant difference in terms of standard of living between the two groups; essentially all the farmers covered by the study were poor.


Most farmers used some kind of treatment for RSD, some of which are shown in Table 3.

Antibiotics and lime were the most popular treatments applied when fish were affected by RSD. However, effectiveness was not high, as most farmers reported that treatment was rarely or never successful. Of the farmers surveyed, 4.9% applied more fertiliser when their pond was affected by RSD; such advice (though never successful) came from other farmers. This indicates that farmers need more training in culture techniques and pond management. Apart from using Neem and banana leaves that can be obtained from local gardens, the average cost of treatment was 132,414 VND (US$101).

The findings also show that government extension officers work closely with fish farmers to assist in controlling this disease, as 63.2% of farmers asked for help from government extension staff, while 33.7% sought help from other farmers. Farmers often establish aquaculture groups with a nominated leader, so that they can help each other with problems, as well as share experience.

Table 3. Percentage of farmers using different treatments.


There are four institutes under the Ministry of Fisheries. Each has a disease unit responsible for a particular region in Vietnam. All the institutes have the capability to carry out routine microbiology, parasitology and histopathology for research and diagnosis. Farmers can contact the institutes directly and submit samples for diagnosis, or ask for a field visit from institute staff. Disease diagnostic units only operate at the national level, while provincial and district-level operations focus on extension activities. The disease reporting system in Vietnam is shown in the diagram below.

Extension staff, especially those at the district level, require more training in diagnosis, treatment and prevention of specific fish diseases in order to improve their skills in fish health management. At the moment, there are not many fish extension staff working at the provincial and district levels. Most are from Agriculture Extension, as most inland provinces do not have Department of Fisheries offices. Therefore, with the limited human resources available, farmers who lead aquaculture groups should also be involved in extension activities and will, therefore, require training. As they are located near other farmers, exchange of information will be easier than through government extension staff.


RSD is the main disease causing severe losses in small-scale fish farming systems in northern Vietnam and is affecting cultured grass carp. This disease has a major impact on the livelihoods of small-scale farmers.

Farmers were keen to learn and to exchange information with other people. However, they have very limited knowledge of culture techniques, pond management and disease prevention and control. Although very few farmers measured water quality parameters, most of them considered the cause of RSD to be poor water quality.

Improving water quality management is recommended in order to reduce the impact of disease outbreaks. In some circumstances, changing the ratio of the species cultured (i.e., increasing the numbers for species less susceptible or resistant to RSD) may reduce the impact of RSD on fish production. More care should be taken during those seasons when disease outbreaks are more likely to occur (March-April and October-November). At the moment, no treatment is available for RSD, although emergency harvest is recommended in certain cases.

There is a pressing need for more training in basic disease recognition and fish health management for farmers and extension officers. Research is also required to identify the pathogen(s) that cause RSD, investigate its epidemiology and find methods for prevention and control. If successful, the experience would serve as a useful model on which to build improved aquatic animal health management in small-scale aquaculture systems in Vietnam.

Present institutional capacity and human resources in aquatic animal health need to be strengthened, not only in research, but also in extension activities, as disease prevention is the most important issue. Each time government officers visit farmers or collect samples, they should service the needs of farmers by giving sound extension advice. During the present survey, most farmers took the opportunity to ask questions, not only relating to fish diseases, but also about culture techniques and management.

A national workshop on RSD would be of great benefit. Such a workshop would provide the opportunity for all fish disease staff in Vietnam to participate and develop a uniform approach to dealing with the problem. Such a workshop should include invited fish disease specialists from other countries, in order that a greater range of ideas on the possible causal agent can be considered. In this workshop, fish disease staff would be able to exchange information, present hypotheses on RSD and develop future research plans. A case definition for RSD should also be developed, as other diseases are sometimes mistakenly called RSD by farmers and extension staff.


The authors would like to express their thanks to FAO and NACA for initiating this project. Thanks are given to Drs. Rohana Subasinghe, Michael Phillips and Melba Reantaso, and to Ian MacRae and other staff at NACA for their help and encouragement during the study. Dr Le Thanh Luu, RIA 1, is also gratefully acknowledged for his encouragement throughout the study, as well as the staff of RIA 1. Very special thanks are given to the farmers and field staff who participated in the survey, and to Drs. Chris Baldock and Angus Cameron, Austvet Animal Health Services, Australia, who assisted with database development and analysis.


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