Fish Ponds and Public Health in General

The public health implications of livestock-fish integration are investigated in this chapter. Many of these are as important for specialized, stand-alone livestock and fish production as for integrated systems.

Historical associations between human health and livestock production are compared to modern threats and how a large range of inputs to livestock, fish and wider food production systems may impact directly or indirectly on public health.

Identifying the hazards and assessing the risks of pathogens including bacteria and viruses and parasites, are considered, and the involvement of integrated farming in influenza pandemics.

The importance of both chemical and biological hazards is raised. The current debate about anti-microbial resistance is interpreted from the standpoint of extra risk due to integration. Relative risks from bioaccumulation and toxic algal blooms are discussed.

General Considerations

Public health issues can be considered as those of direct importance to both producers and consumers of fish and include broader issues of food production, processing and delivery systems.

Linkages have been made between fish or livestock production and health in terms of communicable diseases, non-communicable disease, malnutrition and injury.

Clearly, the improved availability of low-cost fish and livestock products for people’s nutrition needs to be placed in perspective with likely risks. Threats to public health from both livestock and aquaculture are diverse.

Recently, livestock and fish have been implicated in the irregular occurrence of influenza pandemics; the global impacts on public health of promoting livestock and fish integration are huge if these claims are substantiated.

Certainly throughout history, infectious diseases have largely entered human populations through animals.

It has been known for some time, that common pathogens of warm-blooded animals do not generally cause disease in fish, but the role of cultured fish in the possible transfer of pathogens between livestock and humans is important, particularly in less developed countries.

Livestock and fish are involved in both passive and active transfer of a range of parasites and diseases to humans, broadening the need for risk assessment.

The role of fish and warm-blooded livestock as intermediate hosts for a range of human parasites and control strategies are well known.

However, the increasing use of a range of technologies, chemicals and feed ingredients in both livestock and fish production poses a relatively new set of risks.

Products such as antimicrobials, pesticides and a range of chemo-therapy are often used with little idea of either indirect or long-term risks.

Prophylactic use of antibiotics and growth promoters in intensive fish feeds rival their use in the livestock industry. The development of genetically modified organisms, either as feeds of livestock and fish, or the animals themselves has been raised as both a moral and public health issue.

A holistic, balanced assessment of risks involved with integrated livestock-fish production needs to consider the alternative and more specialized, separate intensive systems.

An example is the impact of livestock and fish culture on water quality as independent or integrated activities.

Pollution of surface and ground water, with direct negative impacts on health may be avoided if wastes are recycled through integrated aquaculture with little to no impacts.

The pooling of water has often been related to the spread of insect-vector borne diseases but use of water for aquaculture may reduce this risk.

Unmanaged water bodies in rural or semi-urban areas are more likely to harbor suitable micro- habitats for hosts’ pests than ponds stocked and managed for fish culture.

Adoption of livestock wastes for use in fish culture may already have made important contributions to improving health and hygiene risk analysis, in which hazards are identified and their relevance and control methods determined is a logical approach to assessing the implications of integrating livestock and fish production on public health.

Identify the hazards associated with the various practices constituting integrated livestock-fish and the risks associated with them will be discussed.

Pathogens

Pathogens can affect human health through both active and passive contact. There are potential risks from handling livestock and their feeds, their production and slaughter house wastes as occur in stand- alone livestock farming.

In addition there is a need to consider hazards associated with transfer and use of wastes for fish culture, in management, harvest and marketing of fish, and in addition, potential risks involved with preparing and consuming waste-fed fish.

Guidelines exist for the use of wastewater in fish culture but are considered to be too conservative and overly restrictive. What is important is not the presence of pathogens in the farming environment but their ability to actually cause human disease.

Guidelines should be based on epidemiology and not solely on presence of micro-organisms. An understanding of the main risk factors and how to reduce them is therefore essential for developing best management practice.

Moreover, in order to obtain a holistic view of risk, any comparison of public health and aquaculture produce derived from livestock waste-fed systems should be compared to those from other production systems.

On a broader level, the risks associated with disposal of untreated livestock waste in fish ponds should be compared with alternative uses that may present greater risks to public health in developing countries.

Fundamentally a fish pond is a treatment system for pathogens present in organic wastes; large diurnal variations in temperature, pH and dissolved oxygen in shallow tropical fish ponds tend to cause rapid attenuation of pathogens.

Bacteria and Viruses

Livestock fecal wastes used as inputs into fish culture contain varying quantities of bacteria and viruses that depend on the health status of the stock and the method used for collection, storage and use.

Identifying Hazards, Assessing Risks

All livestock fecal wastes must be assumed to contain pathogens. Most disease is believed to be transferred via feces at slaughter.

However, there is variation in the risk to human health based on livestock type, diet and their management. Human disease caused by many pathogens carried by livestock is difficult to diagnose.

Typically, little is known about the transferability of such pathogens to humans via fish. It should be assumed that all water used in aquaculture is potentially contaminated with pathogens, whether or not livestock wastes are used.

Salmonella and bacteriophage (used as an indicator of viruses) were sometimes present beforeinput of wastes, suggesting that surface water is often contaminated.

Whilst the levels of micro-organism in manure or pond water are important in understanding risks to the producer, the level of pathogens contained in the fish at harvest is of key importance in determining risk to those preparing and consuming the fish.

Levels of microorganisms found in the digestive tract of fish are much higher than in the water illustrating a likely route to infection is via contamination of hands and surfaces during preparation and cooking of fish.

In contrast to bacteria, indicators for pathogenic viruses, such as bacteriophage give a measure of fecal contamination rather than the presence of pathogens.

It is thought that enteric viruses are also rapidly attenuated in waste-fed ponds but their low infective dose suggest that serious attention be given to their persistence in fish ponds.

Reducing Risks

Risks of passive transfer of pathogens through handling of live fish during production, harvest and processing can be reduced if physical exposure is minimized through use of appropriate clothing, especially gloves.

Attention to minimize the risk of cross-contamination during processing should be avoided, as the digestive tract is the major source of pathogens.

Depuration, the holding of fish in clean water without feeding, facilitates this task by reducing the amount of digestive tract contents Depuration may not always be an effective method to remove micro-organisms from fish.

Depuration in clean water for a six week period is ineffective because the micro-organisms had already entered the muscle tissue.

The process was more effective with tilapia raised in optimal growth conditions in wastewater-fed ponds as they contained initially lower concentrations of bacteria, with none present in organs or muscle.

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Consumption of raw, certain types of processed, or undercooked fish should be avoided. Removal of visceral and major organs, in addition to the digestive tract, prior to marketing ‘whole fish’ would also reduce risk.

Pre-treatment or processing of livestock waste prior to its use as a fishpond fertilizer or feed ingredient also reduces risks associated with transfer of pathogens.

Streptococcussp. infections of fish are a relatively newly identified threat to humans. Increasingly found in cultured tilapias, S. iniae and other Streptococcithat infect fish may also infect humans.

Infections have been contracted when people market live fish, or consumers are cut or spined during handling or preparation of the fish.

The disease appears most prevalent in intensive tilapia production systems, in which water quality is marginal and/or there is environmental stress or trauma to the fish.

Parasites

A variety of parasites (Trematodes, Nematodes, Cestodes) may be transferred through livestock waste to aquatic plants and animals (fish, amphibian, molluscs or aquatic vegetables) and then back to humans.

An understanding of current systems and the potential reduction or increase in risk through integration is required.

The exposure of livestock to parasites, through foraging on human feces is often a critical part of the life cycle in lesser developed countries lacking adequate sanitation but if animals are penned, risks are minimal.

The risks of promoting integration of pigs and fish among groups in which pigs are allowed to forage on human waste may, in contrast, be important.

Keypoints to reducing public health risks from pathogens in livestock-Fish systems include the following:

Good husbandry of the livestock, an adequate level of nutrition, hygienic accommodation and control of scavenging on human waste.

Storage and/or composting of wastes reduce pathogens and parasites.

Water quality suitable for optimal growth of fish contains bacteria below the critical concentration that lead to infection of fish organs and muscle.

Fish digestive tracts typically contain high levels of bacteria.

Although depuration is not effective when bacteria occur in fish muscle, holding fish for a short time after harvest effectively reduces bacteria in the digestive tract.

Contamination of hands and surfaces during cleaning and evisceration of fish is a common route of pathogen infection through cross-contamination of other food.

Adequate cooking of fish ensures fish is safe for human consumption; fish eaten raw, undercooked, or improperly processed or preserved increases risk.

Reducing Risks

Improved sanitation or human waste disposal is a key element in the control of parasites, as is the control of pond-side vegetation that provides cover for snails which are often intermediate hosts.

Education and the availability of anti-helminthes drugs are also prerequisites for successful improvement of public health at the community level.

Aquaculture may also reduce the health impacts of parasite diseases. Key aspects of this are through habitat modification and host control.

Abandoned or poorly managed fish ponds have been associated with schistosomasis in Africa (McCullough, 1990) but well managed, productive systems in which aquatic weeds and molluscs are removed or managed are probably less of a problem.

Insect-Vector Borne Diseases

Poorly-managed fish ponds often become mosquito-breeding sites (Birley and Lock, 1998).

Removal of surface and emergent vegetation, as a part of intensified aquaculture, reduces shelter for mosquito larvae. Introduction of aquaculture has actually decreased the incidence of disease through reduction of the habitats of the vectors or intermediate hosts such as mosquitoes and snails in some countries.

Influenza Pandemics

A major issue regarding the promotion of integrated livestock-fish production has been the possible connection between such practices and the emergence of influenza pandemics.

The farming of pigs, poultry and fish together on the same farm is predisposing the world, to the emergence of new virulent strains of influenza virus. Human influenza viruses are similar to poultry viruses but require change before they can infect humans, a change that can occur in pigs or between chickens.

Chemical Hazards and Associated Risks

A major issue is how integrating livestock and fish together can reduce the level of chemical hazards and associated risks compared to stand- alone enterprises.

Generally, hazards associated with intensive aquaculture, particularly of carnivorous fish, are likely to be greater than less intensive culture of herbivorous and omnivorous species because of the greater likelihood of bioaccumulation and exposure through the higher levels of water exchange required.

Wild fish in unmanaged aquatic systems may suffer more from the effects of chemicals than cultured stocks as aquatic habitats often serve to drain effluents and run- off from agriculture and complex natural ecologies are more likely to be disturbed than closely monitored culture systems.

Chemicals may accumulate more in slow-growing, carnivorous species than well-fed, short-lived farmed fish. We first explain the range of chemical hazards with a tabulated assessment of their importance to both integrated and non-integrated aquaculture compared to, where appropriate, reference to wild stocks (Table 3.2).

Exposure to chemicals can be accidental or purposeful. Contamination of the surrounding environment, water or feed source for fish or livestock integrated with fish can be either acute or chronic in nature.

Chemicals are also often used as part of disease control, general husbandry or as feed additives. The tendency in integrated systems is for the fish culture component to be semi-intensive i.e. relatively low densities of fish feeding low in the food chain and these are less likely to require treatment for disease, intensive disinfection or specialized feed additives.

Range of Chemical Hazards

Chemical hazards may arise from the use of agrochemicals, chemo- therapy, metals, feed ingredients and organic pollutants.

Agrochemicals i.e. chemical fertilizers, water treatment compounds, pesticides and disinfectants are widely used in both commercial and smallholder food production.

Chemo-therapy include a range of compounds used to control the impact of disease on both livestock and fish i.e. antimicrobials, parasiticides and hormones.

The issue of bacterial resistance induced through prophylactic use of antimicrobials and drug residues that risk human health are of key interest.

Exposure to metals, in addition to that from chronic or acute pollution of aquatic systems, may occur due to their use as anti-foulants and molluscides or through their inclusion as growth promoters in livestock diets.

Other feed ingredients have come under recent scrutiny and, especially as use of manures as fishponds inputs may increase the pathways through which these compounds can enter the human diet, should be considered.

Aquatic systems are particularly sensitive to organic pollution. In this regard exposure to chlorinated hydrocarbons that are persistent and can bioaccumulation.

Integrated management of livestock waste and fish production ideally leads to good practice that reduces the necessity for use of chemicals to control pests and maintain optimal conditions.

The frequent collection and use of manure for fish culture can reduce problems associated with its accumulation and storage such as the spread of flying insect-borne diseases or ammonia-related respiratory problems.

Moreover, many farmers have found that siting of livestock pens over ponds improves the environment for the livestock through evaporative cooling; this is particularly important in the tropics where more expensive methods of controlling heat losses are uneconomic and heat stress is a major factor predisposing livestock to disease.

In summary,key points to reducing public health risk due to parasites and other biological and biochemical agents is through proper and careful handling.

Reduction in risks from parasites in livestock-fish systems needs to consider current feeding and management practice and minimizing contact with intermediate hosts.

Improved sanitation, anti-helminth drugs and biological control are also important. Well-managed productive ponds, in which aquatic weeds and molluscs are controlled, reduce risk of parasite transfer and insect vector-borne disease.

There is no evidence that livestock-fish integration has been linked to the incidence of influenza pandemics. But multi-species livestock production, especially pigs and poultry should be avoided in specialized units or integrated with aquaculture.

Livestock-fish integration probably reduces the overall risk of contamination with chemicals compared to production of intensive, stand-alone fish production or fish captured from the wild.

The risks to human health through microcystins produced by blue-green algae in livestock waste fed ponds are very small.

Eutrophication of any surface water, through use of fertilization, or occurring as a result of feeding fish intensively, need to consider the multipurpose use of water.

The use of human and livestock waste in semi-intensive fish culture, subject to certain safeguards, can generally be considered positive in any holistic assessment of risk.

Semi-intensive systems fertilized within the carrying capacity of the system are healthy environments for fish. The fish production unit can act to ‘treat’ wastes that themselves may contain pathogens, provided certain precautionary steps are observed.

Moreover, non-integrated intensive livestock and fish production carries its own risks to public health, which should be considered in any balanced comparison.

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