ABSTRACT

Other factors may also contribute to the predominance of viruses in recent United States shellfish-associated disease outbreaks. Once discharged into marine waters, human enteric viruses can survive for extended periods of time (La Belle and Gerba, 1982; Feachem et al., 1981). Some viruses are extremely infectious; for example, Norwalk virus has a theoretical infectious dose of from one to ten particles. Hard clams (Mercenaria mercenaria) have been shown to bioconcentrate hepatitis A virus 900 times over the concentration in water (Goswami et al., 1993); in addition, they show a marked seasonal increase in the concentration of bacteriophages (viruses that attack bacteria) in the spring, which may indicate a propensity to do so for other viruses (Burkhardt et al., 1992). Viruses can be retained within actively pumping shellfish much longer than bacteria (Enriquez et al., 1992; De Mesquita et al., 1991; Power and Collins, 1990a & b; Lewis et al., 1986). Viruses can be absorbed intracellularly into shellfish, as opposed to merely being held in the gut, and as such are much more difficult for the shellfish to eliminate (Hay and Scotti, 1987). Once harvested and refrigerated, Coxsackie B3 and polioviruses have been shown to survive basically unreduced for 28 days at 5'C (Tierney et al., 1982; Metcalf and Stiles, 1965). Other work has shown gradual inactivation up to approximately 90 percent for poliovirus over 30 days when refrigerated, and over 12 weeks when frozen (DiGirolamo et al., 1970). Many enteric human viruses are fairly heat-resistant (hepatitis A and Norwalk); even if partially cooked, the viruses may survive (Millard et al., 1987; Truman et al., 1987; Morse et al., 1986; Peterson et al., 1978).