Oysters can be a health hazard

Street food like oysters are an integral part of life in T&T and more so with Carnival in the air. In this guest column, Daana Kanhai MPhil student of Environmental Biology from UWI warns us of the risks associated with raw oyster consumption.

“Environmental contamination resulting from human activities is an issue which is of prime concern for the scientific community because of the potential impacts on human health and the overall well being of ecosystems. Yet, to certain members of the public, this issue often remains an abstract concept until it becomes glaringly apparent that human health is being threatened. Here, I use the example of the mangrove oyster to illustrate how environmental contaminants in Trinidad can be a serious issue which has the potential to adversely and directly affect human health.

The mangrove oyster has traditionally been harvested on a commercial basis for human consumption in Trinidad. The popularity of the raw oyster cocktail as a delicacy amongst consumers is due in part to its perceived aphrodisiac properties. Yet, few consumers are aware of the risks associated with oyster consumption. In order to understand the severity of the issue, one needs to understand the biology of the oyster.

Mangrove oysters (Crassostrea rhizophorae) are sessile organisms that are usually found attached to the prop roots of the red mangrove (Rhizophora mangle). These organisms are ‘filter feeders’ or ‘suspension feeders’ and usually obtain their food (primarily phytoplankton and zooplankton) by filtering large volumes of water when submerged. This unique feeding pattern is advantageous since it allows them to obtain vital nutrients which are essential to their survival but at the same time they also take up non-essential contaminants which may be present in the aquatic environment. The waters which oysters are exposed to may potentially contain dissolved contaminants as well as contaminants adsorbed unto suspended particles. It is from these two sources, the water and their diet, that oysters are capable of accumulating contaminants into their tissues. This accumulation of substances into their tissues is referred to as bioaccumulation. As time progresses, the levels of contaminants in the oyster tissues exceed levels in the ambient environment. Contaminants in oyster tissues pose a threat to human health when levels exceed those stipulated guidelines or standards for contaminants in food.

Research conducted on oysters harvested at certain sites in Trinidad and the wider Caribbean region has indicated that oysters are capable of accumulating both chemical and biological contaminants in their tissues. Contaminants such as metals and polycyclic aromatic hydrocarbons (PAHs) have been detected in the tissues of oysters in Trinidad. Copper and zinc were the main metals of concern. Copper levels in oysters investigated by Rojas de Astudillo (2002); 5.73-52.97 ppm, and Norville (2007); 39.90-144.5 ppm, exceeded the local maximum permitted levels (MPLs) for copper (20 ppm) in fish and fishery products (GORTT 1998)! Likewise, zinc levels in oysters investigated by Rojas de Astudillo (2002); 138.47-540.12 ppm, and Norville (2007); 918.89-3994.05 ppm, also exceeded the local maximum permitted levels (MPLs) for zinc (50 ppm) in fish and fishery products (GORTT 1998)! Although both metals are considered ‘essential’ trace elements in humans, if concentrations exceed certain levels organs may become damaged. With respect to the PAHs, although several were detected in the oyster tissues, the one of particular concern is benzo[a]pyrene (Banjoo 2006). The International Agency for Cancer Research (IARC) classified benzo[a]pyrene as being carcinogenic to humans (Group 1). In the Caribbean, oysters have also been shown to bioaccumulate a host of chlorinated hydrocarbon compounds (CHCs) which include PCBs (polychlorinated biphenyls), DDTs (dichlorodiphenyltrichloroethanes) and chlordanes. DDT and its residues are potential endocrine disrupters (e.g. they have been associated with preterm pregnancies and reduced fertility) and have been shown to affect both the central and peripheral nervous system in humans. Furthermore, the IARC classified polychlorinated biphenyls (PCBs) as probably carcinogenic to humans (Group 2A) and DDT as possibly carcinogenic to humans (Group 2B). In addition to chemical contaminants, oysters harvested from our local waters have also been found to retain a host of pathogenic (or disease causing) bacteria in their tissues. Among those detected were Enteropathogenic Escherichia coli (EPEC), Enterococcus spp.,

Salmonella spp., Klebsiella oxytoca, Clostridium perfringens and Vibrio mimicus. All of these pathogenic bacteria have been reported to negatively affect human health. For example, EPEC has been implicated in human haemorrhagic gastroenteritis (bloody diarrhoea) and haemolytic uremic syndrome (HUS).

It is therefore possible that mangrove oysters are capable of accumulating a host of biological and chemical contaminants which have the potential to impact human health in their tissues. The ironic twist in the oyster story is that certain chemical (e.g. metals) and biological contaminants (e.g. bacteria) are natural components of our environment and thus are ubiquitous. Yet, our activities contribute to the release of excessive amounts of these substances into our environment. Run-off from animal husbandry practises, dumping of raw sewage into watercourses, leaching of chemical contaminants from dump sites, release of domestic and industrial wastes from point sources, combustion of fossil fuels and the use of agricultural chemicals are just a few of our activities which result in the contamination of our environment. At the end of the day, we need to be cognizant of the fact that environmental contamination is not a purely abstract concept. It is real, it is current and the consequences are literally life-threatening!

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