5.2 Chemical hazards

There are no toxic chemicals, but there are toxic concentrations of all chemicals. Very few chemicals are present in high enough concentrations to pose a threat to human health. Mass toxication has occurred in connection with accidental exposure to high concentrations but in reality the risk of acute chemical intoxication is very low. However, long-term low level exposure to some chemical contaminants may be associated with serious diseases such as neurological damage, birth defects and cancer.

The chemical contaminants with some potential for toxicity are (Ahmed, 1991):

• Inorganic chemicals: arsenic, cadmium, lead, mercury, selenium, sulphites (used in shrimp processing)

• Organic compounds: polychlorinated biphenyls, dioxins, insecticides (chlorinated hydrocarbons). This is a very diverse group with a wide range of industrial users. Unfortunately the chemical stability allow them to accumulate and persist in the environment

• Processing related compounds: nitrosamines and contaminants related to aquaculture (antibiotics, hormones).

5.2.1 Industrial and environmental contaminants (Hans Henrik Huss)

A modest concentration of contaminants is ubiquitous in the clean aquatic environment. A few metals such as copper, selenium, iron and zinc are essential nutrients for fish and shellfish. Contamination occurs when there is a statistically significant increase in the mean levels in the comparable organisms.

Problems related to chemical contamination of the environment are nearly all man-made. The ocean dumping of hundreds of millions tons of material from industrial processing, sludge from sewage treatment plants, draining into the sea of chemicals used in agriculture and raw untreated sewage from large urban populations all participate in contaminating the coastal marine environments or freshwater environments. From here the chemicals find their way into fish and other aquatic organisms. Increasing amounts of chemicals may be found in predatory species as a result of biomagnification, which is the concentration of the chemicals in the higher levels of the food chain. Or they may be there as a result of bioaccumulation, when increasing concentrations of chemicals in the body tissues accumulated over the life span of the individual. In this case, a large (i.e. an older) fish will have a higher content of the chemical concerned than a small (younger) fish of the same species. The presence of chemical contaminants in seafood is therefore highly dependent on geographic location, species and fish size, feeding patterns, solubility of chemicals and their persistence in the environment.

In a review on chemical residue concerns in seafood, Price (1992) concluded that risk from chemical contaminants in commercially harvested fish and shellfish is low and not a problem. Risk from chemical residues (mercury, selenium, dioxins, PCPs, kepone, chlordane, dieldrine and DDT) are primarily a concern with recreational fish and shellfish, caught in coastal waters and (possibly) in highly polluted waters.

In a more recent review, Smith and Gangolli (2002) similarly concluded that organochlorine levels in fish intended for human consumption are low and probably below levels likely to adversely affect human health. However, they are of potential concern for two groups: populations for whom seafoods form a major part of the diet and infants and young children who consume substantial quantities of oily fish.

A large section of a committee report concerned with Seafood Safety in U.S. (Ahmed, 1991) has been devoted to occurrence of chemical contamination and related health risks. Some of the general conclusions and recommendations from this report are cited below:

• From both natural and human sources, a small proportion of seafood is contaminated with appreciable concentrations of potentially hazardous organic and inorganic chemicals. Some of the risks that may be significant include reproductive effects from PCBs and methylmercury, and carcinogenesis from selected PCB congeners, dioxins, and some chlorinated hydrocarbon pesticides

• Consumption of some types of contaminated seafood poses enough risk that efforts toward evaluation, education and control of that risk must be improved

• Present quantitative risk assessment procedures used by government agencies can and should be improved and extended to non-cancer effects

• Current monitoring and surveillance programs provide an inadequate representation of the presence of contaminants in edible portions of domestic and imported seafood, resulting in serious difficulties in assessing both risks and specific opportunities for control

• Because of the unevenness of contamination among species and geographic areas, it is feasible to narrowly target control efforts and still achieve meaningful reductions in exposures

• The data base for evaluating the safety of certain chemicals that find their way into seafood via aquaculture and processing is too weak to support a conclusion that these products are being effectively controlled.

The principal recommendations of the committee are as follows:

• Existing regulations to minimize chemical and biological contamination of the aquatic environment should be strengthened and enforced

• Existing FDA and state regulations should be strengthened and enforced to reduce the human consumption of aquatic organisms with relatively high contaminant levels (e.g. certain species from the Great Lakes with high levels of PCBs, swordfish and other species with high methylmercury levels)

• Federal agencies should actively support further research to determine the actual risks from the consumption of contaminants associated with seafood and to develop specific approaches for decreasing these risks

• Increased environmental monitoring should be initiated at the state level, as part of an overall federal exposure management system

• States should continue to be responsible for site closures, and for issuing health and contamination advisories tailored to be specific consumption habits, reproductive or other special risks, and information sources of specific groups of consumers

• There should be an expanded program of public education on specific chemical contaminant hazards via governmental agencies and the health professions.

The conclusions from the Committee report (Ahmed, 1991) are still valid (2002), although some of the recommendations have been put into practice. Environmental monitoring at state level is done by many countries, and government agencies are responsible for closures of harvesting areas and management of risks related to chemical contaminants. Most countries have laws and regulations defining the conditions for use of agrochemicals. Usually, a holding period is required between the use of such chemicals and harvest or slaughter (aquaculture). Maximum levels have been established for a number of compounds. Examples are shown in Table 5.28. Results from a large number of surveys have shown, that residues of chemical contamination normally are lower than the limits shown in Table 5.28, and do not give rise to any concerns regarding health of the consumer.

A class of compounds made up of the polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF), collectively known as dioxins has recently received widespread attention.

Table 5.28 Environmental chemical contaminants. Tolerances and critical limits in fish and fish products (EC, 2001a; FDA, 1998).

Dioxins are commonly formed when organic substances smoulder or burn in the presence of chlorine. This may happen in industrial operations within metallurgy, paper mills, chemical industries (the Seveso case) and others. Due to the high persistence of dioxins these compounds are relatively stable once released into the environment. Due to the chemical nature of dioxins, the compounds will accumulate in the fat deposits of fish and animals and amounts will increase in higher levels of the food chain. The WHO has recently re-evaluated the toxicity of dioxins and is recommending a Tolerable Daily Intake (TDI) of max. 1-4 picogram TEQ (toxic equivalent)/kg body weight. Examples of dioxin amounts in food are shown in Table 5.29.

Table 5.29 Dioxin amounts in common foods (Compilation of EU dioxin exposure and health data, October 1999)

5.2.2 Veterinary drugs (Allan Reilly)

As aquaculture has developed, a range of fish and shellfish diseases have been encountered that have led to major economic losses and the failure of the industry in some parts of the world. This has led to the increased use of veterinary drugs and vaccines in intensive production systems to combat diseases in farmed fish. Antibiotics are commonly used in aquaculture worldwide to treat infections caused by a variety of bacterial pathogens of fish including Aeromonas hydrophila, Aeromonas salmonicida, Edwardsiella tarda, Pasteurella piscidida, Vibrio anguillarum, Vibrio salmonicida and Yersinia ruckeri. They are commonly used as in-feed medications or surface coated onto feed pellets and dispersed in water. There is a wide range of antimicrobial agents used in aquaculture (Alderman and Hastings, 1998; GESAMP, 1997; ACMSF, 1999). Where antibiotics are approved for use, specific doses and withdrawal periods are specified by manufactures. Since fish are poikilotherms, their metabolic rate is determined by environmental temperatures. Withdrawal periods are specified as degree days, for example, 10 days at 5°C equals 50 degree-days. Some of the antibiotics in common use are shown in Table 5.30.

The use of antibiotics in fish farming is associated with new hazards in fishery products that are not encountered in wild captured species. The main hazards are antibiotic residues and the development of antimicrobial resistance in bacteria that may be transferred to consumers of farmed fish.

5.2.2.1 Antibiotic residues

With the increased use of veterinary drugs in food production, there is global concern about the consumption of low levels of antimicrobial residues in aquatic foods and the effects of these residues on human health. This concern is not limited to only aquaculture products but to all foods of animal origin where the use of antibiotics has become an integral part of intensive animal husbandry.

The potential hazards associated with the presence of antimicrobial drug residues in edible tissues of products from aquaculture include allergies, toxic effects, changes in the colonisation patterns of human-gut flora and acquisition of drug resistance in pathogens in the human body (WHO, 1999).

Table 5.30 Examples of antibiotics used in aquaculture.

1. Use permitted in Canada (http://www.syndel.com/msds/canada_approved.htm)
2. Licensed for use in the UK (Alderman and Hastings 1998)
3. Use permitted in Norway (Alderman and Hastings 1998)
4. Use permitted in Japan (Okamoto 1992)
5. Tan (1999).

Antibiotics are used in aquaculture as prophylactics, as growth promoters and in the treatment of diseases. Prophylactic use of antibiotics is defined as the administration of antibiotics in advance of disease occurrence and this is a common practice in shrimp hatcheries in Asia to reduce the incidence of diseases (GESAMP, 1997). A recent review (Graslund and Bengtsson, 2001) report the widespread prophylactic use of antibiotics in both shrimp hatcheries and in shrimp ponds in Southeast Asia. Antibiotics are usually administered in aquatic feeds and most commercial shrimp feeds contain antibiotics (Flaherty et al., 2000). In contrast, antibiotics are not used either as prophylactic agents or as growth promoters in temperate water aquaculture production in Europe and North America (Alderman and Hastings, 1998). In recent years the use of antibiotics has fallen dramatically in the farmed salmon industry in Norway form about 50 tonnes to less than one tonne annually (Figure 5.17). This is largely as a result of the successful development and use of vaccines against the principle fish pathogens (Alderman and Hastings, 1998).

While vaccines have been developed for finfish, the same success story has not be true for farmed shrimp. Vaccines are of little use in shrimp culture because of the nature of the shrimp host defence system is such that no long term specific immune memory has been demonstrated to exist. Control over the sale and use of antibiotics in some shrimp producing countries is limited which has led to problems in overseas markets. The occurrence of antibiotic residues in cultured shrimp from Asia has led to the rejection of products in export markets (Saitanu et al., 1994) and more recently, the European Union has introduced new legislation requiring the testing of all shipments of farmed shrimp from China, Vietnam and Indonesia for residues of chloramphenicol (EC, 2001b,c).

5.2.2.2 Antimicrobial resistance

There are a number of ways in which bacteria become resistant to antibiotics. When a population of sensitive bacteria is exposed to an effective antibiotic, the majority will be killed or their growth will be inhibited. However, within a population there may be a few relatively resistant organisms that are capable of survival and growth. These have a selective advantage over the sensitive organisms and are able to survive and grow. Bacteria develop resistance through random mutations in bacterial genes or they can acquire resistance from another bacterium. There are three ways in which genes can be transferred between cells:

• by transformation where naked DNA released by a bacterium is taken up and incorporated into the chromosome of the host cell

• by transduction, which involves a bacteriophage transferring DNA into the hosts' chromosome

• by conjugation, which involves direct transfer between cells and where genes are carried on plasmids or conjugative transposons.

Conjugation is thought to be the principal way in which transfer of antibiotic resistant genes occurs between bacteria. Large plasmids that encode resistance to several different antibiotics have been found in human pathogens such as Salmonella Typhimurium DT 104.

The emergence of antimicrobial resistance following the use of antimicrobial agents in aquaculture has been identified in fish pathogens. (WHO, 1999; Midvedt and Lingass, 1992). For instance, plasmid-mediated resistance to antimicrobials have been identified in a number of bacterial fish pathogens including Aeromonas salmonicida, A. hydrophila, Vibrio anguillarum, Pseudomonas fluorescens, Pasteurella piscicida, Edwardsiella tarda (Aoki, 1988) and Yersinia ruckeri (DeGrandis and Stevenson, 1985). Transferable R-plasmids have been found in A. salmonicida encoding resistance to chloramphenicol, sulphonamide and streptomycin in Japan, and to combinations of sulphonamide, streptomycin, spectinomycin, trimethoprim and/or tetracycline in Ireland (Aoki, 1997). In Scotland, transferable R-plasmids were found in 11 out of 40 oxytetracycline-resistant A. salmonicida isolates (Inglis et al., 1993). Transferable resistance was detected to combinations of oxytetracycline, streptomycin, sulphamethoxine and/or trimethoprim. These are all examples of the emergence of antimicrobial resistance in fish pathogens following the use of antimicrobial agents in fish farming.

Use of antimicrobial agents in aquaculture also selects for resistance in bacteria in fish in the local environment and in sediments close to fish farms. Medicated feeds that are not eaten by fish fall to the bottom of ponds or through the bottom of cages. Additionally, some of the antimicrobials in medicated feed that is consumed by fish will be excreted in faeces into the local environment. Uneaten feeds may be consumed by other fish in the vicinity of a fish farm. Samuelson et al. (1992) reported residues of oxolinic acid in wild fish, crabs and mussels in the vicinity of a Norwegian fish farm up to 13 days post-treatment. A number of authors have reported oxytetracycline in sediments in the vicinity of salmon farms (Coyne et al., 1994; Bjorklund et al., 1991; Samuelson et al.,1992a).

Potential risks to consumer health exist in that antimicrobial resistance arising from the use of antibiotics in aquaculture can be transferred to human pathogens, such as Vibrio parahaemolyticus (Hayashi et al., 1982) and Vibrio cholerae (Nakjima et al., 1983). The strain of Vibrio cholerae O1 that caused the epidemic of cholera in South America in 1991 was multidrug-resistant and the epidemic in Ecuador began among persons working on shrimp farms (Weber et al., 1994). Similar multi-drug resistance was found in non-cholera Vibrio that were pathogenic to shrimp which may have been transferred to the V. cholera O1 (Weber et al., 1994). Bacteria on farmed fish and shrimp can be transmitted to humans when these are eaten or when such bacteria are transferred to food that are subsequently eaten. Vibrio parahaemolyticus is a common cause of food-borne illness in Japan and Salmonella species have been isolated from farmed fish and shrimp (Reilly and Twiddy, 1992). Other bacteria that are human pathogens, such as Streptococcus iniae and Vibrio vulnificus, have been associated with wound infections in fish handlers and can cause serious illness (Weinstein et al., 1997; Bisharat and Raz, 1996). Another route for the transfer of antimicrobial resistant bacteria to man is by ornamental fish. Multi-drug resistant strains of Mycobacterium marinum have been isolated from ornamental fish and are the cause of "fish tank granuloma" in man. There is also a potential for resistance development in bacteria in integrated fish/poultry/animal production systems in parts of Asia where the waste from animals is used to fertilize fish ponds. Antibiotics administered to poultry and animals are inadvertently dose the fish ponds in faeces and urine with subsequent selective pressure for resistance development.

5.2.2.3 Control Strategies

There can be little doubt that the use of antibiotics in aquaculture selects for antimicrobial resistance among bacteria in farmed fish and in the environment surrounding fish farms. It is well established that antibiotics given to animals have resulted in the emergence of some resistant germs that can infect humans via the food chain. Additionally, illegal residues have been reported in aquaculture products in export markets.

Antibiotics should never be used as an easy alternative to good fish farming practices. National governments need to put in place control programmes for residues of antimicrobials in aquaculture production. Such control programmes should control the approval or licensing of antimicrobials and should control their sale and use in fish farming. What is required at national level is up-to-date legislation and standards that are based on sound science, a monitoring programme and adequate resources for enforcement of the legislation.

Consumers can protect themselves against antibiotic resistant bacteria as these are just as susceptible to heat and hygiene as their non-resistant counterparts. Thorough cooking, frequent hand washing, prevention of cross-contamination by separating raw seafoods from other foods and proper chilled storage will minimise the incidence of seafood poisoning.