Deadliest Catch?
Researchers and regulators work to identify and prevent seafood contamination.
Randall C. Willis
Imports from China hit hard times in 2007. First, tainted wheat gluten provided by one Chinese company prompted a widespread recall of potentially toxic pet food. The U.S. Food and Drug Administration (FDA) banned imports of wheat gluten from that company. Later, lead paint found on children's toys made in China launched another round of recalls. Soon after, FDA had more bad news. At a June 28 press conference, the agency announced new controls on several species of seafood imported from China because unauthorized animal drugs and food additives had been detected in those products (1). This was yet another economic blow to the East Asian powerhouse, and once again the "Made in China" label became suspect.
The compounds found in the seafood were the antimicrobial agents malachite green, nitrofurans, gentian violet, and fluoroquinolone, many of which are carcinogenic. Fluoroquinolone is linked to the development of antibiotic resistance in pathogenic bacteria.
What made this announcement startling, however, was that unlike the recalls of tainted toys and toxic pet food, this ban wasn't directed at one or two exporting companies but rather at the seafood industry of an entire nation.
"Despite extensive communications between FDA and appropriate Chinese authorities to correct the problem," said Margaret Glavin, associate commissioner for regulatory affairs with FDA, during the press conference, "we have continued to find residues of certain veterinary drugs or food additives that are not permitted for use in the United States in farmed fish products produced in China and exported to the U.S."
The problem of unsafe seafood isn't restricted solely to China, however, as developed and developing nations alike face the challenge of maintaining a constant vigil for gaps in their environmental, farming, and production practices to keep the global seafood industry safe. America is no exception; according to the U.S. Environmental Protection Agency (EPA), almost 4000 fish and shellfish advisories were issued throughout the country in 2006 alone, up more than 10% from 2005 (Figure 1; 2).
Paul Duchesne, a spokesperson for Health Canada, says that the key challenges to determining the safety of seafood include the availability of analytical methods for detecting substances of potential concern, including natural toxicants and traditional and emerging environmental contaminants, and the availability of the types of data needed to conduct risk assessments on these substances (e.g., toxicological data as well as occurrence and exposure data). These challenges are particularly problematic, he says, with emerging contaminants.
Metals as contaminants
Seafood contaminants generally fall into one of three broad categories—metals, organic pollutants, and biological organisms—each of which presents government agencies and seafood producers with unique challenges.
With the possible exception of Hg, metal-based seafood contamination is not a significant problem within the U.S., according to EPA statistics. It is apt to occur within confined regions for limited periods, often as a result of industrial discharge or release from geological sources through flooding or erosion. At the same time, according to EPA's James Harvey, concern remains about legacy chemicals residing in estuarine or coastal sediments and the impact of these compounds in fish tissues.
"Our research group is interested in establishing estuarine baseline conditions using a probabilistic sampling design that could help us assess water quality, sediment quality, and biological quality," he says. "Many of the contaminants we found are long-lived in the aquatic environment. We did find that there were regional differences in the highest level of specific contaminants, even though the same five or six contaminants consistently appeared in all regions." Similar concerns are expressed in other countries around the world.
Recently, Thierry Guérin and colleagues at the Agence française de sécurité sanitaire des aliments (AFSSA) tried to estimate the amount of organotin compound (OTC) in the seafood diets of French consumers. The researchers examined fresh and frozen samples from around the country and correlated contamination levels with seafood consumption statistics (3).
"Even though some studies have recently estimated OTC exposure from ingestion of especially fish and shellfish, the amount and percent of exposure from food in the French population has not been well characterized," says Guérin, who heads Contaminants inorganiques et minéraux de l'environnement, a unit of AFSSA. "The food sample size is largely higher than in earlier studies and above all more representative" of French people who consume a lot of seafood.
The researchers used mi-crowave digestion to extract the OTC constituents from the fish samples. Then, they identified OTCs by GC coupled with microwave-induced plasma atomic emission spectroscopy (AES). They found that most seafood samples were contaminated with a variety of OTC species but that butyltin compounds were most prevalent, particularly in halibut and swordfish. They also noted that contaminant concentrations varied widely with geographical location, seasonal changes, and spawning period. Perhaps the most striking finding was that the highest OTC levels occurred near harbor areas and especially during periods of intense boating activity, which correlated well with the prevalence of OTCs in antifouling paints on the hulls of boats.
Despite the extreme variability in OTC levels, the AFSSA scientists noted that most levels were well within the European legislation limits. They stressed, however, that efforts must continue to reduce the environmental and dietary spread of these compounds. As Guérin explains, the OTC dietary exposure in children and teenagers has not yet been determined, and consumers are exposed to a variety of sources of OTC, such as pesticides. In addition, there is a paucity of epidemiological studies on chronic low-level oral exposure to OTC, so dietary organotin levels and their associated health effects could be seriously underestimated.
Further complicating matters is the fact that metal contamination rarely occurs in the form of a single, isolated species. Instead, one high-level contaminant often indicates an overall contamination problem. "There are so many environmental contaminants which are generally cumulative and have chronic effects [that] their additive effects on health are still not understood," Guérin says. "Cadmium, mercury, arsenic, lead, and also tin are the main inorganic contaminants of interest for the European legislation, for the moment, but [nanoparticles] contamination could be a major problem in the very near future. Their potential threats remain to be estimated.
"The main problem is that toxicologists have mainly evaluated only the effect of one pure molecule and not a mix of some contaminants with other essential elements," he says. "Most of the agonist or antagonist effects of contaminants are not well known, actually."
In recognition of the potential cumulative impact of chronic diverse contaminant exposure, Jose Domingo and colleagues at the University of Barcelona and Rovira i Virgili University (both in Spain) undertook a similar but wider analysis of human consumption of metal contaminants in seafood sold in major centers around Catalonia (4). Using inductively coupled plasma MS (ICPMS), the researchers monitored the levels of As, Pb, Cd, and Hg in fish and shellfish species.
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As with the French study, the researchers found that although metals are relatively widespread, different metals are present at higher levels depending on the fish and shellfish species. For example, Cd peaked in clams and mussels, whereas Hg was most abundant in swordfish. Similar variability was found when the researchers correlated the seafood contaminant levels with food consumption by different age and gender groups. Nevertheless, each metal was below the European legislation-mandated limits.
Although metal contamination is largely identified and quantified through ICPMS or some form of AES, these methods can accomplish only so much on their own and are being taxed by new challenges, according to Guérin. "Despite advances in instrumentation, methods, standardization, and even legislation, there are still many problems that remain unsolved, and many questions are still not answered," he says. "For example, molecular detection techniques greatly improved the reliability of species identification; however, matrix effects, low concentrations, or the instability of element species are preventing the successful application of these techniques in many cases."
Organic pollutants
Historically, persistent organic pollutants (POPs) such as DDT, polychlorinated biphenyls (PCBs), and dioxins have been a concern in the U.S. and much of Europe, but during the past few decades, these governments have legislated tighter controls on such compounds. Now, the focus is on countries such as China and India, where rampant industrial growth has outpaced environmental concerns. This lack of control may be problematic for Western dinner tables, because the search for less expensive seafood has meant ever-expanding importation from these regions.
For example, as a result of the rapidly growing electronics industry in South China, a secondary industry has expanded just as quickly in the past few years: electronics waste management and recycling. Unfortunately, in the rush to reclaim components, recycling firms are at risk of releasing into the environment organic compounds such as polybrominated diphenyl ethers (PBDEs), which are quickly incorporated into an already overburdened food chain. To quantify the impact of these activities, the Guangzhou Institute of Geochemistry's Eddy Zeng and colleagues recently monitored the uptake of PBDEs in seafood products throughout South China (5).
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"We conducted a comprehensive sampling program in 2005 and 2006 in 11 coastal districts of Guangdong Province, collecting samples of 13 fish species and 21 nonfish seafood products," Zeng says. "These samples have been analyzed for persistent halogenated hydrocarbons—that is, organochlorine pesticides, PCBs, and PBDEs. We also collected riverine runoff samples from the eight major runoff outlets of the Pearl River Delta between March 2005 and February 2006."
Using gel permeation chromatography and GC/MS, the researchers found BDE congeners in 98.5% of the seafood samples, with oysters showing the highest total BDE concentration. However, they found great variation among species in the prevalence of different BDE congeners. The researchers speculated that these variations may represent differences in feeding habits and metabolism.
Despite the widespread contamination, the researchers determined that overall PBDE levels in the region were lower than in many regions around the world, including The Netherlands, Singapore, and Hong Kong. However, they warn that the study was largely qualitative and that a more quantitative analysis would require a better understanding of the local BDE mixtures as well as more solid knowledge about the mechanisms governing environmental transformation of PBDEs.
"One of the challenges is to ensure the quality of sample extraction and purification," Zeng says. "Seafood generally contains high levels of lipid contents, which may interfere with the identification and quantitation of target analytes.
"Another challenge is to prevent or minimize degradation of BDE-209, which can degrade under sunlight or at high temperatures. To do so, we had to use amber vials to store extracts and a 15 m column to analyze BDE-209 separately from other congeners."
Perhaps surprisingly, industry is not necessarily the main contamination source, as noted in the recent study by Umeå University's Peter Haglund and colleagues in Sweden and Japan (6). The researchers monitored the development of a new environmental stressor—polybrominated dibenzo-p-dioxins (PBDDs)—in the Bothnian Sea and the Baltic Proper off the coast of Sweden. They then compared the spatial distribution and bioaccumulation of these compounds with those of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), which are largely the result of industrial effluent.
The researchers examined several species of marine and freshwater fish, shellfish, and mussels by using GC/high-resolution MS to identify the different chemical constituents (Figure 2). They noted that unlike PCDD/Fs, which are almost exclusively linked to freshwater sources, the PBDDs were found only in marine seafood. Furthermore, PBDD levels were consistent along the entire coast, suggesting that the source is diffuse and therefore likely to be natural. On the basis of earlier research, the scientists speculate that the PBDDs are produced by algae or cyanobacteria and then passed up the food chain through filter feeders, such as mussels, that are consumed by fish and eventually humans.
According to the researchers, the introduction of new POPs to the seafood market is particularly problematic, because the present levels of PCDD/Fs in many catches of fatty Baltic fish already exceed European Commission limits. Given that EU legislation only recently recognized brominated organics as a potential problem, the scientists say, the health risks associated with Scandinavian seafood consumption may be underestimated.
According to Health Canada's Duchesne, trend analyses have shown that the levels of older POPs, such as PCBs and dioxins, in seafood and other foods have been in a steady decline during the past few years and thus so has human exposure. Continued vigilance is needed to ensure that newer chemicals—particularly those of a lipophilic nature—do not accumulate in the environment and the food chain.
With particular reference to natural toxicants, Duchesne adds, continued monitoring and surveillance are needed to ensure that regulatory agencies remain aware of potential changes in the occurrence or levels of various toxicants in different regions. "This may become more important in the future due to external influences, such as climate change," he says.
Biological organisms
Of the three major forms of seafood contamination, microbial contamination—bacteria, viruses, parasites—may be the most challenging. This contamination can occur at almost any point in the process chain from wild or cultivated growth to food processing to meal preparation. Furthermore, unlike heavy-metal or POP contamination, which can take years of chronic consumption before becoming dangerous to public health, microbial contamination can become a health threat rapidly: some bacterial or viral infections become deadly within days. Depending on the introduction and importation of foreign food stocks and dietary practices into the country, the incidence of different foodborne microbes has varied significantly in the U.S. over the years (Figure 3; 7).
Crab or krab?
In a small number of cases, the adulterant that regulators find in seafood is not a contaminating chemical or microbe but an entirely different species of seafood.
According to a recent report by the Food and Agriculture Organization of the United Nations, to produce a wider array of seafood-based products cheaply in the face of dwindling supplies of traditional food stocks, many seafood producers are turning to more abundant or more easily farmed species to supplement their supplies (11). And given that so much of the food is processed—so that most distinguishing morphological features such as fins, shells, and scales have been removed—there is little concern about being caught by inspectors.
Thus, the question becomes: how much of that Maryland crab cake is actually Maryland blue crab?
Recent legislation in Europe requires that foodstuffs carry a complete record of their species, geographical, and cultivation history. But because no validated analysis protocols are available to fully authenticate fish or shellfish, importers have had to rely largely on the honesty of the exporter. This reality is changing, however, as agencies are looking at a variety of methods to identify seafood.
Genomics and proteomics methods have provided some assistance, by enabling researchers to identify the key genetic or protein signatures of different species within a mélange of sample. Unfortunately, processing and cooking can have a deleterious effect on DNA and proteins. In this case, immunohistochemistry approaches (e.g., ELISAs) or NMR-based methods may be necessary to identify the seafood sources.
The one challenge with most of these methods, however, is that samples have to be analyzed off-line, often following several time- and labor-intensive steps that can be very damaging to time-sensitive foodstuffs.
Recently, Scott Hale and Javier Gayo of North Carolina State University used visible and NIR spectroscopy to identify the contents of tinned crabmeat obtained from a local supermarket (12). The researchers combined samples of two known crab species—Atlantic blue and blue swimmer—in 10% increments to see how well they could distinguish the relative contributions in the vis–NIR spectra (Figure 4).
They found that by scanning samples of known crab species at 2 nm intervals across 400–1700 nm, they could generate absorption spectra that would allow them to clearly distinguish homogenous crabmeat samples from adulterated samples. Furthermore, unlike the other authentication methods, vis–NIR offers the opportunity of performing analyses quickly on-line.
Historically, food-processing companies and regulators have relied on the same basic culturing techniques used by medical specialists to isolate and identify potential microbial contamination. Unfortunately, these methods can be both labor- and time-intensive, and the results may come too late for some people. During the past decade or so, however, molecular methods have accelerated the identification timelines, and these methods are reaching the point where on-site testing is often feasible.
Because of their binding specificity, antibodies have frequently been used for contaminant testing, whether through ELISAs, dipstick technology, or bead-based tests. The advantage of immunologic methods is that they are often portable and easy to use, and ELISAs are amenable to high-throughput screening. The challenge, however, is that the antibodies generally bind to a relatively common cell-surface target that is capable of highlighting the presence of a microbe without necessarily offering any information about the particular strain(s) involved. Thus, a food shipment can be flagged for possible contamination, but further analysis is necessary to say specifically what the contaminant is.
To address the strain identity issue, researchers are increasingly relying on genomic technologies, such as microarrays and PCR. For example, Jessica Nordstrom and colleagues at FDA's Gulf Coast Seafood Laboratory and the Alaska Department of Environmental Conservation developed a multiplex, real-time PCR assay that allowed them not only to detect the presence of Vibrio spp. bacteria in oysters but also to identify and quantify pathogenic V. parahaemolyticus strains in a background of excess nonpathogenic species (8). (V. parahaemolyticus is related to the microbe that causes cholera and can trigger gastrointestinal problems such as watery diarrhea, abdominal cramping, nausea, and vomiting.)
The researchers used three gene sequences as markers of species identity and pathogenicity to develop oligonucleotide primers for their PCR assay, and they tested the system on oyster samples from several coastal areas around Alaska. They found that by adjusting the relative concentrations of the oligonucleotides, they could identify as few as 10 colony-forming units (CFUs) of pathogenic bacteria per reaction in the presence of high concentrations (103-fold greater) of nonpathogenic strains. Furthermore, they detected single microbe species down to 1 CFU/reaction.
Even with the genetic methods, however, assay preparation time continues to be a problem. Most assays require several rounds of culture incubation in general and selective growth media, both to subdivide the microbial mixture and to generate enough sample to be detectable. Furthermore, microbes evolve rapidly, and researchers must be vigilant that a negative result is due to a lack of pathogen and not a mutation within that strain.
An action plan
With the globalization of the seafood industry and the changing seafood consumption habits of people worldwide, state, federal, and international agencies are making great efforts to develop programs that address the import–export aspects of seafood safety and the root causes of the problems.
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FDA's seafood hazard analysis and critical control point (HACCP) system, which is just over a decade old, was designed to help companies identify problem points within their food-handling businesses and to offer advice on how to rectify potential threats before they become health issues (9). In the case of the seafood industry, HACCP covers everything from getting the fish into the processing plant to transporting the products to consumers, highlighting opportunities to improve processes along the way. Unlike the European programs, however, which apply to the complete production chain, U.S. seafood HACCP regulations apply only to processors (10).
From the seafood importation perspective, the challenge is in convincing international trading partners to implement similar programs in their jurisdictions to reduce the domestic inspection workload. "[Our] inspection effort is directed at foreign processors that have demonstrated a history of poor compliance with Canadian standards," says Mary Ann Green, director of the Fish, Seafood, and Production Division of the Canadian Food Inspection Agency (CFIA). "On the other hand, inspection effort is reduced through the establishment of Memoranda of Understanding or Mutual Recognition Agreements with other countries having reliable inspection systems."
Ultimately, however, international agreements don't eliminate the risks of something slipping through, and inspection agencies can't examine every shipment. So, they play a numbers game. "CFIA uses a risk-based approach to determine the frequency at which to inspect imported product," Green says. "The frequency varies based on food safety risk, the history of compliance of a particular product, the history of compliance of the processor, and the country of origin of the product."
From Zeng's perspective, the challenge to ensure safer seafood from China will be for the government to find ways to balance economic growth and environmental protection. "There have been accidents in which seafood products imported to Europe [from China] contained substantially higher levels of toxic substances than allowable thresholds, due to the lack of attention to quality control," Zeng says. "As a result, seafood products exported by China to Western countries will inevitably be subject to rigorous inspection by Western agencies until better protective measures are in place."
Nevertheless, China is making headway, adds Zeng. The evidence is a trend of diminishing contamination of seafood with organic compounds such as DDT and PCBs. This follows the government's enactment of stricter environmental policies on these compounds in the 1980s and 1990s. "We believe that some of the environmental issues facing China will have to be resolved with stringent law enforcement and international collaboration," he says.
With the continuing assistance of organizations like FDA and the development of new technologies to identify contaminants before they become problems, this dream will, hopefully, become reality.
Randall C Willis is a freelance writer based in Toronto.
References
- U.S. FDA. Transcripts of FDA Press Conference on Seafood Imported from China; July 2007; www.fda.gov/bbs/transcripts/transcript062807.pdf.
- U.S. EPA. 2005/2006 National Listing of Fish Advisories; July 2007; www.epa.gov/waterscience/fish/advisories/2006/tech.html.
- Guérin, T.; Sirot, V.; Volatier, J. L.; Leblanc, J. C. Sci. Total Environ. 2007, 388, 66–77.
- Falcó, G.; Llobet, J. M.; Bocio, A.; Domingo, J. J. Agric. Food Chem. 2006, 54, 6106–6112.
- Guo, J. Y.; et al. J. Agric. Food Chem. 2007, 55, 9152–9158.
- Haglund, P.; et al. Environ. Sci. Technol. 2007, 41, 3069–3074.
- Centers for Disease Control and Prevention Foodborne Diseases Active Surveillance Network. Preliminary FoodNet Data on the Incidence of Infection with Pathogens Transmitted Commonly Through Food—10 States, 2006. MMWR 2007, 56, 336–339; www.cdc.gov/mmwr/preview/mmwrhtml/mm5614a4.htm?s_cid=mm5614a4_e.
- Nordstrom, J. L.; et al. Appl. Environ. Microbiol. 2007, 73, 5840–5847.
- U.S. FDA. Seafood HACCP; www.cfsan.fda.gov/~comm/haccpsea.html.
- Buzby, J. C., Ed. International Trade and Food Safety: Economic Theory and Case Studies; Economic Research Service, U.S. Department of Agriculture, 2003; www.ers.usda.gov/Publications/aer828.
- Martinez, I.; James, D.; Loréal, H. Application of Modern Analytical Techniques To Ensure Seafood Safety and Authenticity; United Nations Food and Agriculture Organization Fisheries Technical Paper 455, 2005; www.fao.org/docrep/008/y5970e/y5970e00.htm.
- Gayo, J.; Hale, S. A. J. Agric. Food Chem. 2007, 55, 585–592.
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