RISK FACTORS AND SOCIO-ECONOMIC IMPACTS ASSOCIATED WITH EPIZOOTIC
ULCERATIVE SYNDROME (EUS) IN BANGLADESH

Masud H. Khan¹and James H. Lilley²

¹Bangladesh Fisheries Research Institute,
Mymensingh-2201, Bangladesh
²Institute of Aquaculture, University of Stirling,
Stirling FK9 4LA, Scotland, UK

Khan, M.H., and J.H. Lilley. 2002. Risk factors and socio-economic impacts associated with epizootic ulcerative syndrome (EUS) in Bangladesh. p. 27-39. 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.

ABSTRACT

An interview-based questionnaire survey of a fish farmer and a fisher randomly selected from each of the 64 districts of Bangladesh was carried out to study risk factors associated with outbreaks of epizootic ulcerative syndrome (EUS). The survey was undertaken during the EUS season, December 1998 to April 1999. Data showed that there is a significantly higher relative risk of EUS occurring in farmed fish when wild fish are present in the pond; EUS occurred in the previous season; pond embankments are not high enough to prevent in-coming flood water; ponds are connected to natural waters; ponds are not dried or limed prior to stocking; ponds are not limed post-stocking; nets are not dried or disinfected; and pond water colour is black, indicating high levels of organic waste. Of the wild caught fish, those sampled from haors had a significantly higher relative risk of getting EUS. Fish from rivers and flood plains were at a lower risk of EUS infection.

Out of 64 districts, fish with lesions were recorded from fish farms in 32 districts (50%), and 30 (47%) were confirmed EUS positive, and from wild fisheries 52 districts (81%) demonstrated lesions and 49 (77%) were confirmed as EUS positive. However, the percentage of infected fish was quite low in some sites. A total of 6,433 wild fish and 6,401 farmed fish were examined for lesions, and average prevalence was 16.0 and 15.5%, respectively. Thirty-one species of fish were confirmed as being EUS positive out of 40 recorded with lesions.

Eighty-eight percent of farmers interviewed had between one and four ponds. These small-scale farmers, in particular, are at risk from suffering serious financial difficulties from sudden disease losses, or from reduced production levels due to disease. Losses in wild fisheries could deprive the poorer sections of the community from access to cheap sources of animal protein.

The present study demonstrated that EUS is still the most damaging disease among freshwater fishes in Bangladesh, and probably has significant effects on fish production, although no direct information on mortalities was obtained. Eighty-six percent of farmers and 89% of fishers interviewed considered EUS to be a major problem. Total fish loss due to EUS for 1998-99 is estimated as 39,797 mt and US$3.97 million using the prevalence data obtained from this study.

¹Depressions in floodplains located between two or more rivers, which function as internal drainage basins.

INTRODUCTION

With an increase in unemployment, small-scale fish farming is becoming very popular among the unemployed as a source of earnings. Some of the small-scale farmers have their own ponds, but most of them rent ponds for fish culture business. Consequently, fish disease, and epizootic ulcerative syndrome (EUS) in particular, has a severe socio-economic impact on public life, and especially on rural life.

EUS was a very new phenomenon at the time of the first outbreaks in Bangladesh, and it caused great concern because of the perceived dangers to both staple food crops and to human life. The widespread fear of disease transmission to consumers, although unfounded, led to a drastic decrease in market demand for food fish, including marine species, which were not affected by the disease. Usually, the only animal protein available to accompany the rural people's rice diet is derived from fish, and therefore, an inadequate intake of fish could result in nutritional deficiency. It has been estimated that 250 million families in the Southeast Asian Region depend on rice as a main crop, and much of the incidental fish harvests from these paddies are an important part of the family's diet (Macintosh 1986). The economic loss due to EUS was estimated at 118.3 million Taka (US$3.4 million; 1 US$=35 Taka) during 1988-89. In the second year the disease occurred with lower severity, and the economic loss was estimated at 88.2 million Taka (US$2.2 million). Fish price dropped to 25-40% of the pre-disease level during the first outbreak (Barua 1994).

Since 1988, EUS has been considered the most serious epidemic disease affecting freshwater fish in Bangladesh. As with most other diseases, there is strong evidence that EUS outbreaks occur only when a number of determinants or causal factors combine. A number of factors are considered to be acting at the same level and ultimately lead to the exposure of dermis. These exposed sites could provide the point of attachment and entry for spores of Aphanomyces invadans, regarded as the essential component in all EUS outbreaks (Lilley et al. 1998). Recent studies suggest that there are a number of other sufficient causes for EUS outbreaks. Although every set of sufficient causes for EUS is different from one another, each combination has the common result of exposing the dermis and allowing entry of A. invadans. Callinan et al. (1996), reported outbreaks of EUS in estuarine fish in Australia associated with acid-sulphate soil areas, and reproduced EUS by exposing susceptible fish to acid water and spores of A. invadans. Kanchanakhan (1996) has shown that EUS can be reproduced when susceptible snakeheads (Channa sp.) are injected with a particular strain of rhabdovirus and bathed in spores of A. invadans. Demonstration of the highly invasive abilities of EUS fungus in tissues like bone, gizzard and spinal cord provides an indication that under certain circumstances, the fungus may be able to invade the healthy skin of fish (Vishwanath et al. 1998).

The epidemiology of EUS is poorly studied in many affected countries, including Bangladesh. However, a number of factors have been hypothesised as either risk factors or determinants for EUS outbreaks in Bangladesh. These factors are based on observations of the mode of disease transmission, the species, habitats and culture systems affected by EUS; human interventions; movements of animals; and seasonality of EUS outbreaks. Identifying true risk factors for EUS allows rational control measures to be developed. EUS research requirements, as recommended by FAO (1986), included the need for a greater understanding of the influence of environmental factors and pollutants on the disease and the identification of causative agent(s). During an EUS survey of Bangladesh, Roberts et al. (1989) stressed the need for an epidemiological study of individual waters to collect information on disease transmission, relative species susceptibility, mortality and recovery rate in different species and ages of fish, fish losses and economic impact. The present cross-sectional survey aimed to quantify the degree of the present EUS problem, and also identify risk factors that affect outbreaks. Fish farmers and fishers were interviewed and the information was used to measure the strength of association between EUS and hypothesised risk factors.

² Floodplain lakes, which may hold water permanently or dry up during the winter season.
³ Oxbow lakes.

MATERIALS AND METHODS

A cross-sectional, interview-based survey was conducted in a thana selected from each of the 64 districts of Bangladesh from December 1998 to April 1999. This period is the recognised "EUS season." Three M.Sc. students from Bangladesh Agricultural University interviewed a fish farmer and a fisher randomly in each thana and examined 100 fish for EUS-type ulcers.

Survey Areas

One thana known to have adequate fisheries resources was randomly selected from each of the 64 administrative districts. In August 1998, a letter was sent to the Thana Fisheries Officers (TFO) requesting a list of categorised fish farms (both registered and unregistered) and wild fisheries areas in their respective thanas. From these lists, one fish farm and one wild fishery were randomly selected.

Development of Questionnaire

The questionnaire development procedure followed the methods described by Thrusfield (1995). Both fish farmer and fisher questionnaires were designed to record information in a standard format with in-built error checks. Closed questions were used, wherever possible, to give data in a yes/no/don't know or categorical format to facilitate ease of coding and analysis. Attempts were made to make wording unambiguous, brief, polite and non-technical. Both questionnaires were prepared in English and Bengali, and the Bengali version was used for interviewing. Before starting the survey, questionnaires were pre-tested two times by interviewing target people to identify ambiguous and irrelevant questions.

⁴ Sub-district.

Interviewing and Sampling

The three interviewers were trained together to minimise differences in technique. Training also included examination of fish for EUS-type ulcers and sampling for histology. Each interviewer covered one third of the total districts. TFOs were requested to aid and co-operate with the interviewers. Each interviewer carried with him the required number of questionnaires, fish sampling sheets, photographs of EUS-affected fish, 10% buffered formalin, vials, marking pen, scalpels, cast net and hapa. After completion of the interview, at least 100 susceptible fish from each farm or fishing site were examined for EUS ulcers, irrespective of species, and information recorded on the sampling sheet. One fish of each species recorded with lesions was sampled for histology. Tissue samples were fixed in 10% buffered formalin. In case a sampling net was unavailable, the interviewer supplied his own net for catching fish. During farm visits, in order to avoid re-counting the same individuals, fishes, once examined for ulcers, were separated into the hapa until 100 individuals had been examined. Nets were disinfected between sites. A fish farm or wild fishery was classified as affected with EUS if the presence in one or more fish of any species of characteristic mycotic granulomas was confirmed histologically.

Database Preparation and Analysis

Two MS Access™ databases (for fish farmer and fisher data), and two MS Excel™ spreadsheets (for fish species data) were used to enter the information. Univariate analyses were undertaken using Epiinfo™ to examine the association between EUS occurrence and putative risk factors using crude relative risk (RR) as the measure. Fish farm and wild fishery data were analysed separately.

Histology

Formalin-fixed blocks of lesions and underlying muscle were processed, embedded in paraffin wax and sectioned at 5 µm. The sections were stained with haematoxylin and eosin (H&E) to visualise granulomas, and Grocott's silver stain was used to confirm the mycotic involvement.

RESULTS AND DISCUSSION

Variables were analysed for their effect on the relative risk (RR) of EUS (Tables 1 and 2). RR > 1 indicates that the variable is a putative causal factor of EUS; RR = 1 indicates no association exists between the factor and EUS; and RR < 1 indicates the variable is a sparing factor for EUS (i.e., that it reduces the chance of EUS occurring). Where the lower confidence limit is above 1, there is 95% confidence that the variable is a risk for EUS; where the upper confidence limit is below 1 there is 95% confidence that the variable is a sparing factor for EUS.

Data from Fish-farmer Interviews

Pond connections

The analyses showed that there was over 10 times more chance of EUS occurring in culture ponds containing wild fish. This was the highest RR measured out of the variables examined. The data also show that there was a significantly lower RR (0.39) of EUS occurring on farmed fish when pond embankments were high enough to prevent incoming waters. Similarly, ponds that had been flooded that year showed a significantly higher RR (2.33). Fish farms directly connected to water bodies that allowed the entry of wild fishes also showed a significantly higher RR (2.63) of EUS. Each type of connecting water body (i.e., rice-field, ditch and beel) provided a similar level of risk. Ponds containing water sourced from underground wells or only from rain were at much lower risk of EUS (RR=0.91, 0.52), compared to ponds with water sourced from ricefields (RR=2.36). These results equate with those of Hossain et al (1992) which, when recalculated for RR, show a significantly lower risk of EUS-type lesions occurring on fish from rainfed ponds (0.65) than from flooded or irrigated ponds.

Floodwater and entry of wild fish are risk factors probably because they are routes of entry for pathogens (Kabata 1985). Roberts et al. (1989) described floodwater as a powerful means for spreading EUS throughout Bangladesh. Changes in water quality and agricultural run-off due to floods may cause stress for the farmed fish, and may be a component cause for EUS. There is an absence of parasites and microbial flora in underground water, and the exclusive use of rainwater and underground water would reduce the risks described above (Munro and Roberts 1989).

Pond preparation

Complete draining of pond water, drying, bottom mud removal and liming during pond preparation were found to result in low relative risks of 0.55, 0.41, 0.17 and 0.42, respectively. Fertilisation during pond preparation also resulted in a low, but non-significant, RR (0.50).

Pond preparation techniques described above will exclude A. invadans, and many other pathogens, from the pond environment. It is interesting that the "removal of bottom mud" resulted in a very low RR. Unlike other oomycete fungi, A. invadans does not appear to show strong negative geotaxis, and may possibly accumulate on the pond bottom, although soil assays have not succeeded in isolating A. invadans (Willoughby 1999). Aphanomyces invadans can feasibly survive the warmer months of summer in the thick bottom mud of older or derelict ponds, which generally possess a temperature below 31° C, and with declining temperature or rainfall disturbance, the fungus might be activated to grow. This theory is supported by Ahmed and Rab's (1995) study, which showed that fish cultured in previously derelict ponds had a significantly increased probability of EUS.

Post-stocking management and hygiene of habitat

Post-stocking liming also gave a significantly low RR of 0.46, and again, fertilisation after stocking did not significantly affect RR. Pond-water colour indicating high levels of phytoplankton or zooplankton had low, but not significant, RRs. However, ponds black with high levels of organic waste showed significantly higher RR (2.21).



Table 1. Variables affecting relative risk (RR) of EUS in fish farming areas, giving 95% confidence limits (lower< RR< upper). RR> 1 indicates the factor is a putative causal factor of EUS; RR=1 indicates no association exists between the factor and EUS; and RR< 1 indicates the factor as a sparing factor for EUS.

Table 2. Variables affecting relative risk (RR) of EUS in 64 fishing areas, giving 95%confidence limits (lower<RR<upper). RR>1 indicates the factor is a putative causal factor of EUS; RR=1 indicates no association exists between the factor and EUS; and RR<1 indicates the factor as a sparing factor for EUS.

Liming increases pH, hardness, alkalinity and the buffering system of pond water and also reduces stress for fish, thereby reducing the risk of EUS. Exposure of fish to low pH might be one of the causes of skin damage, necessary for fungal entry to cause EUS. In aquarium trials, EUS lesions were induced in fish exposed firstly to acidified water, and then to spores of A. invadans, and thus, confirming these two factors in combination as a sufficient cause of EUS (Callinan et al. 1996). It is possible that the increase in calcium and magnesium in the pond will also have a more direct effect by benefiting fish skin and inducing encystment in fungal zoospores, thereby making them fall out of suspension.

Hygiene/other disease

Allowing cattle to wash and drink in the pond after ploughing or grazing in other areas gave a high RR (2.90), possibly due to the transport of pathogens with the cattle. Netting with dried or disinfected nets, and requiring buyers to do the same, contributed much lower RR values (0.59 and 0.14, respectively). The use of equipment that has been transported between farms (e.g., by buyers) is likely to provide a source of infective material, and drying or disinfection is recommended.

A high RR (2.65) was also demonstrated in ponds where the farmer said fish were affected by parasites. A number of parasites have been isolated from EUS-affected fish (Tonguthai 1986) and may either be possible vectors for the pathogen, or a stress-inducing factor in EUS outbreaks. Subasinghe (1993) demonstrated such an association between the level of infection by Trichodina sp. and the susceptibility of Channa striata to EUS infection. The mechanism of attachment of these parasites can cause skin rupture, and might facilitate infection by the EUS fungus.

Climate/seasonality

Farmers that reported EUS in the previous season were shown to be at higher risk of EUS (RR=3.00). Reports by farmers that there was unusually low temperature or heavy rainfall 3-15 days prior to the interview were also correlated with EUS occurrences.

EUS has been associated with low temperature, and has often occurred after periods of heavy rain. Phillips and Keddie (1990) observed from data from 1988-89 that EUS outbreaks occurred during months when the mean daily temperature was below the annual mean temperature in Bangladesh, China, India and Lao-PDR. However EUS outbreaks in the Philippines and Thailand were also recorded in warmer months. Chinabut et al. (1995) challenged striped snakehead (Channa striata) by injecting with zoospores of A. invadans and found a weaker inflammatory response, higher mortality rate and more extensive fungal invasion in fish held at 19° C compared to fish held at 26 and 31° C.

Data from Fisher Interviews

Types of habitat

Among the different types of fish habitat sampled, haors showed the highest RR (1.33) and rivers showed the lowest RR (0.54). A haor is the biggest natural depression between two or more rivers, and is lower than the adjacent floodplains. It functions as a small internal drainage basin and receives upland runoff water (Khan 1997). Chemicals, waste and pathogens may enter the haor through the river systems. At the onset of the dry season, the water level of the haors decrease and the aquatic animals and plants are concentrated, often resulting in stressful conditions for fishes. The presence of a wide range of EUS-susceptible fishes under these circumstances make haors susceptible areas for EUS outbreaks. The active movement of the water in rivers may lessen the chances of the fungal pathogen attaching to fish, thereby resulting in the lower RR recorded for EUS in rivers. There was no significant association between artificial stocking of natural water bodies and occurrence of EUS. Water bodies that fishermen reported had been affected the previous season, were at higher risk of EUS (RR = 2.19).

General Observations

During the period of survey, interviewers recorded farmers' general observations and opinions, which are summarised here. Some of these points have been demonstrated by the present study, whereas others require further work investigate possible associations.

• EUS outbreaks have occurred every year since 1988 and affect most of the freshwater fish. Initially the severity of disease was very high, but this has shown a decreasing trend.

• Wild fisheries are much more affected than farmed fishes. Farmers and fishers from beel areas reported that an outbreak occurs every year in their local beels as the temperature begins to fall. It affects snakehead, Puntius, Mastacembelus, escaped farmed fish and others. Later, fish farms very close to those affected wild fisheries become affected, and then more distant farms are affected.

• EUS often occurs in culture ponds directly connected to ricefields through drainage, but other nearby ponds not linked through drainage systems are usually unaffected.

• Some farmers and fishers opined that aquatic birds, fish-eating birds, reptiles and mammals might transmit the disease from one place to another by preying on easy to catch EUS-affected fish and dropping uneaten portions in unaffected water bodies. They may spread the pathogen by repeatedly preying and washing alternately between affected ponds and unaffected ponds.

• Most farmers and fishers believe that floods play a vital role in spreading EUS throughout the country.

• Some observers recorded that moderately affected snakeheads, walking catfish and climbing perch might transmit the disease by entering an unaffected water body.

• Some farmers commented that their unaffected pond became affected after they spread duckweed from a wild water body.

• A remarkable number of older ponds with very high bottom deposition and shade were reported to be repeatedly affected over several years, although not in anyway connected to wild or affected fisheries. It appears, therefore, that the fungus may survive in isolated water bodies under particular conditions.

• Ponds with a high stocking density were observed to be more affected by EUS.

During a separate case-control study of ponds in Mymensingh District undertaken concurrently with the cross-sectional study, the following observations were made:

• A series of adjacent fish ponds were found affected with EUS in late winter, and it was difficult to find an unaffected "control" pond in that area.

• Some ponds, with a common embankment located near to a particular beel, were all affected. However, newly constructed ponds with very high embankments near to the same beel were unaffected

• On a number of occasions, affected ponds were separated from unaffected ponds with similar culture characteristics by a highway or high embankment.

• Unaffected farms with ponds nearby an EUS-affected beel usually became affected within a week. It was thought this was after cattle and items used in the beel were washed in the pond. Some farmers reported that after fishing in EUS-affected beels, ricefields and other ponds, EUS occurred in their fish farms when they washed nets, wild-caught fishes, and themselves in the ponds.
  

Socio-economics

Out of the 64 districts, fish with lesions were recorded from fish farms in 32 districts (50%) and 30 (47%) were confirmed EUS positive. From wild fisheries, 52 districts (81%) demonstrated lesions and 49 (77%) were confirmed as EUS positive. Thirty-one species of fish were confirmed as being EUS positive out of 40 recorded with lesions. Totals of 6,433 wild fish and 6,401 farmed fish were examined for lesions, and average prevalences were calculated as 16.0 and 15.5%, respectively. Eighty percent of 471 ulcerated fish sampled were confirmed as EUS positive.

EUS commonly affects small wild fishes e.g., Channa spp., Puntius spp., Mastacembelus spp, Colisa spp., Mystus spp., Nandus sp., Anabas sp., Heteropneustes sp., Clarias sp. and Ambassis spp. Rural poor people catch these species of fish as a main source of animal protein, as they usually cannot purchase fish or other animal products. People involved in this activity range from small children to professional fisherfolk. Of the fishers interviewed, 89% considered EUS to be a major problem.

Major carps are the most significantly affected farmed fish. Once an outbreak occurs in a carp pond, EUS can damage the entire crop and, as a result, small-scale poor farmers can fall into serious economic crisis, particularly farmers who rent ponds. Of the farmers interviewed, 86% considered EUS to be a major problem. The majority of farmers said that the price of fish dropped during the EUS season and in EUS-affected localities (Box 1). Fish prices given by the interviewees indicated that prices dropped by more than 50% when fish were slightly ulcerated, or when there was an EUS outbreak in the locality (Table 3). Despite the potential dangers, most of the farmers interviewed considered fish farming to be a profitable business.

Economic loss was estimated by relating the present prevalence of lesions on fish to five-months projected fish production data for 1998-99 obtained from the Fisheries Resources Survey System, Directorate of Fishery, Bangladesh (FRSS 1998). The estimate excludes hilsha, shrimp and production from the Sunderbans area. It is also adjusted to exclude the 20% of fish with lesions that were not EUS-affected; another 20% of EUS-affected fish that would be consumed anyway; and a further 10% to allow for recovered fish. Total fish loss for 1998-99 is estimated at 39,797 mt, at a value of US$3.97m (at Tk 50/kg fish and 1 US$=50 Tk). Farmed fish account for 18,140 mt of this figure, and wild fish account for 21,657 mt. The estimated loss is higher, although the severity of the disease is lower, as compared to 1988 and 89, due to the two-fold increase in fish production over the last 10 years.

Table 3. Average fish prices indicated by interviewees.



CONCLUSIONS AND RECOMMENDATIONS

Prevention of disease is always more economical than cure. On the basis of risk analysis and general observation, the following precautionary measures could be adopted to prevent EUS:

• Repair and raise pond embankments above the flood level and close inlets and holes on the bank to prevent entry of flood water.

• Dry and lime ponds.

• In the case of old and derelict ponds, bottom mud should be removed.

• Ponds should be supplied with rainfed, underground or purified water.

• Suitable resistant species might be substituted for susceptible species in severely affected areas.

• Ponds should be stocked with healthy hatchery-reared fish fry. Wild fry should be avoided.

• Fish fry could be treated with 2.5% NaCl for at least 15 minutes before release in ponds. Fry source water should not be released into ponds.

• All wild fish (e.g., snakeheads, catfish, Puntius spp, Mastacembelus spp and Anabas sp.) should be removed and excluded from ponds.

• Avoid washing of ploughing equipment, cattle and people in fish ponds following work in other water-filled areas (e.g., ditches, paddy fields or beels) in winter.

• Nets and other equipment should be disinfected (e.g., using bleaching powder or iodophore), or sun dried, prior to re-use.

• Ideally, feet/boots of farm workers and visitors should be disinfected at the farm entrance.

• Good plankton bloom should be maintained. The water should not be allowed to go black with high organic waste.

• Periodic liming during stocking should be done (depending on pH and alkalinity).

• Severe parasitic infestations should be treated.

Winter is the most common period for EUS outbreaks, therefore particular measures should be taken at this time. Fish farms should be monitored regularly. Liming prior to winter (at 1kg/decimal) is recommended. Awareness of fish health management should be created among fish farmers. Regulations concerning transportation of fish from affected/suspected affected zones to unaffected zones might be effective, although the present study demonstrates that EUS is endemic to a large proportion of Bangladesh. Regulations against the indiscriminate use of chemicals and antibiotics against EUS are necessary to prevent detrimental effects to the environment, fish and ultimately, the consumer.

ACKNOWLEDGEMENTS

Grateful thanks to B. Majumder, M.G.A. Sarker, M. Alauddin and M.A. Hoque for help with the survey and sample processing; Drs. G.U. Ahmed, M.B.R. Chowdhury and S. Chinabut for advice and use of laboratory facilities; Dr. K. Morgan for assistance with the study design; and Dr. C. Baldock for commenting on a draft of this paper. This work was supported by a grant from the Department for International Development of the United Kingdom (DFID) and the British Council.

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