Public health issues surrounding the quality and safety of wild and cultured shellfish have been a source of general concern throughout the industry. Much negative information and publicity, especially concerning water pollution and contaminated shellfish areas, has increased public awareness about the potential dangers of eating shellfish, particularly shellfish that has not been adequately stored or cooked. As discussed earlier, bivalve mollusks are filter feeders, straining particulate food from surrounding water (and absorbing elements through substrate). The particulates are caught on the gill surfaces and are transported to the digestive tract. Because of their ability to filter significant quantities of water in relation to their size and the ability to retain microorganisms in the size range of bacteria and viruses, shellfish may ingest and concentrate undesirable pathogens, as well as toxic pollutants and heavy metals, during the filtering process. Some bivalves are so efficient that they may concentrate and/or exceed 1000 times the ambient contaminant concentration. (Chang, 1991) Many of these pathogens may have little effect upon the shellfish, but could be devastating to a consumer.

History

It has been known for years that shellfish harvested from polluted areas can cause illness, particularly if eaten undercooked or raw. Toxins such as pesticides, hydrocarbons, heavy metals, radioisotopes, and marine biotoxins all may be successfully concentrated by shellfish. The major public health concern historically however, has been with the pathogenic bacteria like Salmonellae and viruses capable of causing hepatitis-a, gastroenteritis, and polio.

Pathogens

Fecal coliform bacteria, which lives in the intestines of warm-blooded animals, has traditionally been used as an indicator of sewage pollution. Evidence of fecal contamination in turn, indicates at least the potential presence of pathogens. Therefore, classification of shellfishing is based upon the concentration of fecal coliform bacteria in overlying waters.

According to the National Shellfish Sanitation Program, shellfish growing areas may be approved if, on the basis of fifteen sets of samples taken under adverse pollution conditions as described by the sanitary survey, they meet the following criteria:

1) the median number of total coliform bacteria in the water does not exceed 70 per 100 ml of water sampled, and not more than 10 percent of the samples have a count in excess of 230 per 100 ml, or

2) the median number of fecal coliform bacteria - bacteria associated with the feces of warm-blooded animals - do not exceed 14 per 100 ml, and not more than 10 percent of the samples have a count in excess of 43 per 100 ml.

The methodology is not by any means foolproof. There are problems associated with the use of total coliforms or fecal coliforms being relied upon as the indicators of the level of sewage contamination. The actual degree of risk of infection posed by human pathogens is also uncertain. Coliform counts may mislead, as follows:

1) sewage treatment plants generally use chlorine to sterilize effluent. Although chlorine does reduce bacterial contamination, it may have little effect upon virus populations. In that circumstance, the lack of coliform bacteria in a sample may not indicate the absence of viruses;

2) Escherichia coli (primary fecal coliform component) has a lower survival rate in seawater than other organisms, including viruses. Therefore, concentrations of E. coli may lead to an underestimate about the risk of other pathogens;

3) E. coli in seawater may assume other, non-detectible forms, resulting in false (low) numbers of indicators reported;

4) there are many pathogens that flourish in water that are not associated with fecal contamination and may not be tested for or detected.

Naturally Occurring Pathogens

Vibrio

Concern has been expressed that fish farms could lead to increased numbers of bacteria that cause disease in humans. Sedimentation and organic-loading of finfish waste products could result in uptake, accumulation, and transmission of human pathogens through vectoring shellfish. These concerns have been directed specifically towards the bacteria genus Vibrio, since the genus is common in marine systems and includes fish, shellfish, and human pathogens.

The genus Vibrio contains approximately 20 species. Five of the species, V. cholorae, V. parahaemolyticus, V. vulnificus, V. alginolyticus, and V. mimicus, are known human pathogens, and the pathogenicity of two other species, V. fluvialis and V. metschnikovii, is unclear. V. anguillarum and V. ordalii infect salmonids, but there is no evidence to suggest that these species are human pathogens.

In humans, Vibrio infections most frequently cause gastroenteritis. The clinical symptoms include diarrhea, abdominal cramps, nausea, and vomiting. The disease is normally mild to moderate in severity, and symptoms typically persist for a few days. Exposure to Vibrio suspended in seawater can also cause earaches and wound infections. Primary sepsis, caused by V. vulnificus, is the most serious of the Vibrio diseases. Infection occurs most frequently in persons with chronic liver disease. The V. vulnificus infection causes fever, chills, and nausea, and results in death in about one half of the cases. (NSSP Operations Manual, 1992 Revision).

Vibrio bacteria, including pathogenic and nonpathogenic species, are common members of the microbial community in estuarine environments throughout the world. They play significant roles in nutrient recycling and are probably the principle bacterial group responsible for the mineralization of refractory organic materials like chitin. (Cabrizzi, 1990)

The fact that potentially pathogenic Vibrio species are widespread but the incidence of infection is relatively low is probably attributable to three factors. First, not all strains of a pathogenic species are virulent. Second, many pathogenic Vibrios require water temperatures above 15 degrees C. Finally, the incidence of Vibrio infection is minimized by cooking seafood and killing the bacteria. Vibrio infections are contracted by eating raw seafood, inadequately cooked seafood, or cooked seafood which is subsequently left in contact with raw seafood.

Concerns that aquaculture could lead to increased incidence of Vibrio infections are based on two facts. First, Vibrio suspensions have been found in greatest abundance in areas characterized by high inputs of organic matter and/or particulate material. Such conditions may exist in the vicinity of fish farms. Second, filter-feeding mollusks concentrate bacteria through normal feeding activities. Thus, there is a potential route for human infection if a fish farm promotes increased Vibrio suspensions in the vicinity of harvestable shellfish. Shellfish in these circumstances could not be harvested for direct human consumption.

There is no current evidence to conclude that fish farms contribute to the proliferation of bacteria species and strains pathogenic to humans.

Naturally Occurring Biotoxins

Shellfish contaminants can take the form of naturally- occurring toxins, such as certain phytoplanktons that tend to bioaccumulate in warm weather, microbiological contamination where the shellfish acts as a vector for disease transport to humans, and bioaccumulation of toxics, such as heavy metals and polycyclic aromatic hydrocarbons (PAHs).

Paralytic Shellfish Poisoning (PSP)

Quahogs, soft shell clams, oysters, and mussels may act as transmission vectors for paralytic shellfish poison (PSP) to humans. PSP results from the ingestion of saxitoxin, an extremely potent toxin produced by the dinoflagellate Alexandrium tamarensis.

Anyone who has lived in a coastal area has experienced the phenomenon known as "red tide." Under certain conditions; high water temperatures, calm seas, low salinity, and high-nutrient content, certain naturally occurring plankton will experience population explosions, resulting in tremendous quantities of planktonic organisms reproducing at a massive rate in a very short period of time. It should be noted that all such population explosions may not be toxic and may not discolor sea water; this is dependent upon the kind of organism experiencing the rapid growth, or "bloom." Toxic red tides on the east coast are primarily caused by the dinoflagellate Alexandrium tamarensis. Shellfish ingest this organism and accumulate toxins in their bodies. This accumulation has no ill effect on the health of the shellfish directly, but if consumed by humans, can result in PSP. This toxin is particularly dangerous because it will not be eliminated by cooking and does not change the appearance of a contaminated mollusk. (Northeastern Marine Advisory Council, 1987)

PSP acts quickly. The onset of symptoms will usually occur within thirty minutes of ingestion, causing tingling in the lips, face, neck, and extremities. This is followed by headache, dizziness, nausea, and occasionally vertigo. In cases of severe poisoning, muscular paralysis and respiratory failure may occur, which can be fatal.

All shellfish-producing states have monitoring programs for PSP. In Massachusetts, the Department of Fisheries, Wildlife and Environmental Law Enforcement maintains a testing and monitoring program. DMF, with laboratory assistance from DEP, conducts a monitoring program in shellfish growing areas while DPH monitors shellfish in the marketplace. In the event that tolerance levels for saxitoxin are exceeded in tested shellfish or shellfish growing areas, the shellfish are seized and destroyed, the growing areas are posted and closed, and local cities and towns are notified.

The National Shellfish Sanitation Program

In 1925, the National Shellfish Sanitation Program ("NSSP") was created, the result of a cooperative effort between the shellfish industry, the U.S. Public Health Service (now the Food and Drug Administration) and cooperating states. The goal of the program was to design uniform standards, applied to all participating states, for the maintenance and oversight of shellfish sanitation in growing areas, in harvesting, and in processing. Uniformity was critical; all participating states embraced the same standards to assure consumers of a uniformly healthful product and to eliminate any competitive advantage a state could obtain by not adhering to the same sanitary requirements. Participation in the program is not mandated by law, but a state who does not participate cannot engage in interstate shipment or distribution of shellfish.

The basic obligations of all participating states are as follows:

1. All states shipping shellfish have to adopt adequate laws and regulations for the sanitary control of the industry; complete and maintain sanitary surveys of growing areas, delineate and patrol restricted areas, inspect shellfish plants and laboratories, and implement measures to insure that all shellfish reaching consumers have been grown, harvested, and processed in a sanitary manner. The state agrees to issue numbered certificates to shellfish dealers complying with the sanitary standards and copies of the certificates are then forwarded to the Food and Drug Administration ("FDA").

2. The FDA makes an annual inspection of each state shellfish control program, including the inspection of a representative number of shellfish processing plants. Based upon this information, the FDA determines the degree of conformity the State program has with the NSSP. Based upon this information, a monthly list is generated of the valid interstate shellfish shipping certificates.

3. The private sector has adopted and augmented this management structure and created its own, additional sanitary standards, including tracking mechanisms to record the shipping and final disposition of the shellfish.

In 1982, the NSSP created an organizational structure utilizing the expertise of shellfish control officers, the FDA, cooperating state representatives, and industry officials; the Interstate Shellfish Sanitation Conference ("ISSC"). The ISSC adopted the NSSP Operations Manual in 1983, as well as a process to revise and amend the provisions of the manual.

In 1983, the ISSC adopted the NSSP Operations Manual as the formal statement of standards for shellfish sanitation. In 1984, the FDA entered into a Memorandum of Understanding with the ISSC as the basis for a continuing, cooperative relationship between the states, private industry, and the federal government.

In 1985, the NSSP Manual was formally codified by the FDA and published as regulations in the Federal Register (50 FR 7797, February 26, 1985).

The NSSP Manual is divided into two parts, reflecting the awareness that the growing and processing of shellfish are two distinct processes. Part I: Sanitation of Shellfish Growing Areas is a guide for preparing state shellfish laws and regulations pertaining to the sanitary control of shellfish growing area classification, laboratory procedures, relaying, patrol operations, and marine biotoxins. Part II: Sanitation of the Harvesting, Processing and Distribution of Shellfish is a guide for operating, inspecting, and certifying shellfish shippers, processors and depuration facilities and for controlling interstate shipments of shellfish.

The Manual also provides specific protocols for Reviewing Classification of Areas Implicated in Shellfish Related Illnesses, Reviewing Classification of Areas Implicated by Pathogens in Shellfish Meat Samples, and Approved National Shellfish Sanitation Program Laboratory Tests.

Part I, Section G of the Manual addresses aquaculture. The section is divided into three categories:

Administrative Procedures - state laws and regulations designed to provide an adequate legal basis for sanitary control of aquaculture;

Open Water Aquaculture - the cultivation of molluscan shellfish in natural shellfish growing areas; and

Shellfish Polyculture and Land-Based Monoculture - the establishment of procedures for issuing permits for shellfish aquaculture, approving culturing sites and boundaries, keeping records, imposing quarantine measures, and developing other control measures as may be necessary, recognizing that the potential for pathogens in land-based monoculture facilities is greater than natural areas and prudent control measures must be used. Further, the use of indicator microorganisms may not be related to their use in natural settings. There are also increased public health concerns related to polyculture, as the potential is greater for contamination of molluscan shellfish with pathogens and animal drugs than in monoculture.

Massachusetts is a participant in the ISSC and subscribes to the standards promulgated by the NSSP. As part of its Coastal Zone Management Program, Massachusetts is developing a protocol for the siting of wastewater treatment plants, using as a condition to be evaluated in the siting process the existence and classification of shellfish growing areas.

The process employed by the NSSP to classify shellfish growing areas assumes the existence of treatment plants in the proximity of shellfish growing areas and assumes the potential for shellfish contamination. This assumption has been a necessary operating premise, as waste water treatment plants have historically been sited near shellfish beds without giving due consideration to the need for clean water.

In Massachusetts, the protection of shellfish resources is one of the factors to be evaluated in determining water quality standards and the consequent uses allowed in that particular body of water. The state Coastal Zone Management Program Policies require the preservation of shellfish beds and encourage the development of aquaculture. A siting process that considers preserving the water quality necessary to maintain and enhance existing shellfish resources prior to running the risk of degrading that water quality preserves water quality and forwards achievement of coastal resource management goals.

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Published: September 1995