VIRAL HEPATITIS IN A GROUP OF BOSTON HOSPITALS. II. A PROSPECTICE CONTROLLED EPIDEMIOLOGIC STUDY

Viral depuration by assaying individual shellfish

A study was carried out to further evaluate the practicability of viral depuration by assaying individual shellfish. The Northern quahaug and a strain of the type 1 attenuated poliovirus were used as the working model. Two types of depuration systems were employed: the small experimental tanks and a pilot-size tank with a capacity of approximately 24 bushels (836 liters) of shellfish. Volumes of the individual shellfish samples were found uniform throughout the experiments when a prior selection for the weight of the shellfish was made. There was also no significant difference in volumes of the individual samples during the course of depuration (24 to 96 hr). Under controlled hydrographic conditions, however, the uptake of virus in individual shellfish varied considerably. In general, the individual variability reached 10- to 100-fold. This wide variation would explain the variability of viral contents obtained in pooled samples during depuration as reported previously. During a later phase of depuration, although a great majority of shellfish were free of the virus, a few still harbored minimal amounts of contaminants. The presence of virus in some of the shellfish after various periods of depuration would, theoretically, be obscured by the pooling of the sampled shellfish. Further examination of the negative samples by assaying larger quantities than those routinely used revealed that a few still contained virus. To simulate naturally polluted shellfish as closely as technically possible, shellfish were polluted with minimal amounts of virus. The shellfish were cleansed more rapidly by the depuration process than were those polluted with more virus. Since the naturally polluted shellfish were shown to contain less virus than those studied in the laboratory, it is anticipated that the former type of shellfish may be cleansed more readily by this process within a reasonable period of time. Justification for a field trial of depuration in this country is presented.

Viral depuration of the Northern quahaug

A study was conducted to evaluate critically the feasibility of using the self-cleansing mechanism as a practical means to obtain virus-free shellfish. Two systems supplied with fresh running seawater, three strains of human enterovirus and the Northern quahaug, were used as working models. Preliminary experiments in the experimental system under arbitrarily selected conditions showed that depuration of poliovirus-polluted quahaugs could be achieved by the method used for the Eastern oyster. The factors affecting viral depuration studied so far included: (i) initial concentration of shellfish pollution; (ii) temperature of seawater; and (iii) salinity of seawater. It was shown that purification of the lightly polluted shellfish was achieved sooner than of the heavily polluted ones. The efficiency of viral depuration was roughly a function of the water temperature within the range tested (5 to 20 C). Reduction of salinity to 50 to 60% of the original level stopped this process completely, but 25% reduction in salinity did not affect significantly the rate of depuration. Preliminary study in the pilot system showed that viral depuration in the large tank appeared to be equally as efficient as that in the small experimental tanks under the particular conditions.