HEALTH

NEW HAVEN DEPARTMENT OF HEALTH

Vol. XLIII, No. 9	September, 1916

This article excerpted from p. 3 of the issue.

It is generally known that the biological conditions most favorable for the cultivation of oysters are frequently intimately associated with natural agencies of pollution. Depending largely for their food upon substances washed down by rivers, shellfish grow best in bays and estuaries, and in many cases these waters receive the sewage of cities. While it may often be possible to remove the main sources of pollution, there will still remain many valuable areas where such expense would be unwarranted, or where it would not, for a long time, if ever, be possible to make the beds perfectly safe for the direct removal of oysters for the market. Under these conditions it is believed that the following studies may offer a solution of the problem of the purification of polluted oysters.

Certain facts, discovered in 1914 and 1915 during the progress of experimental studies at the Fisherman's Island laboratory, under the charge of Surg. Hugh S. Cumming, indicated the commercial possibility of artificially purifying polluted oysters. The natural cleansing of oysters when transferred to clean surroundings had been previously demonstrated by several investigators, and confirmed by experience. They variously stated the time requirement to insure thorough purification, agreeing generally upon a week or more. Prof. E. B. Phelps, in experiments at Narragansett Bay, found, however, that even after two days' sojourn in pure water, oysters had become satisfactorily cleansed. He did not study shorter times.

Experiments performed at this station showed that oysters heavily inoculated with B. coli freed themselves very rapidly when suspended in baskets placed directly in the tidal current at the mouth of Chesapeake Bay. Within a few hours during the summer months the greater part of the pollution had disappeared, though the last traces required longer periods for removal. Even during the so-called "hibernating" period, a few days in clean water ordinarily sufficed to wash out most of the sewage organisms. It was therefore certain that the purification of oysters could be accomplished in a period so short as to make the suggestion of artificial purification of oysters economically reasonable.

Experimental studies of the oyster physiology, feeding habits, and digestive mechanism explained the rapid disappearance of polluted matter. At feeding temperatures large volumes of water, from 25 to 50 gallons a day, pass through the oyster's gills. The rate at which food materials progress through its gastro-intestinal system is also surprisingly rapid. Feeding experiments showed that in less than half an hour particles touching the gill had moved to the mouth, and that within five hours they were being ejected with the feces. Unlike most animals, an oyster feeds as it breathes, and under favorable conditions a continuous stream of food material flows through the intestinal tract and is deposited in a solid ribbon beneath the oyster. As the total length of the whole passage is about 6 inches, it is not remarkable that so short a time is, required in transit.

All indications, therefore, pointed to the active elimination of accumulated pollution and strengthened the conviction that the time required would not be the prohibitive factor in the artificial purification of oysters.

Basins of filtered sea water have been utilized in France for this purpose, but with the present market price of oysters in this country the expense of filtration would make such a method impracticable. However, the successful treatment of public drinking water supplies with minute traces of calcium hypochlorite suggested a cheap and efficient means of accomplishing the same purpose.

In many places oysters are now kept for varying periods before marketing in basins or floats which, with little alteration in arrangement and at small cost for hypochlorite of lime, could be turned into suitable purification chambers. It is believed, moreover, that aside from the question of economy, water which had not been filtered would be even more effective, since the food particles would assist and hasten the discharge of polluted substances from the gut. Experiments were therefore undertaken to test out the practicability of artificially purifying oysters in water containing calcium hypochlorite.

Technique.

While the experiments were not made upon a commercial scale, all the conditions could be duplicated in practice, and they were in no sense "laboratory" experiments. One of the tanks at the station, having an inside length of 6 feet and a breadth and depth of 4 feet, was available for the purpose. It was filled with water directly from Chesapeake Bay, and in it from 30 to 40 oysters were suspended about 18 inches below the surface, in rectangular galvanized iron wire baskets, about 3 feet long and 18 inches wide and deep. Either liquid cultures of B. coli ("free coli") or B. coli in finely divided agar suspensions ("attached coli") were used to inoculate the tank. After the oysters had become thoroughly infected samples were collected for examination and the water was disinfected with from 25 to 150 cubic centimeters of a 10 per cent suspension of commercial calcium hypochlorite. In most cases after about six hours a second dose was added to kill such bacteria as might subsequently have been discharged by the oyster, thus escaping the first treatment. This overcame any possibility that the oysters might have remained closed until the full strength of the hypochlorite has become appreciably diminished.

The mean of three separate tests, made for another purpose was taken as indicating the initial condition of the oysters: In the first, the B. coli content per cubic centimeter of shell liquor of each of the five oysters taken for examination was averaged. In the second, the number of B. coli per cubic centimeter of a composite sample of equal volumes of liquor was determined; and, in the third, an analysis was made of the mucus obtained by vigorously shaking the five oyster bodies together. In the quantitative estimation of the number of B. coll, suitable dilutions of the material were inoculated into lactose bouillon according to the regular method. Fermentation of the lactose, with subsequent typical appearance when smeared and grown upon endo medium, was considered a positive test for the organism. In this way a satisfactory index of the number of sewage organisms was obtained, and by using the mean of three independent tests on each sample of five oysters, a better value was obtained than would follow the use of any one of the tests singly.

Results.

The mean results show clearly a remarkable purification within 24 hours and a considerable change when tested after six hours. Whereas most of the initial samples were much more grossly contaminated than oysters usually are when taken from condemned beds, all but one had, after treatment, less than 10 B. coli per cubic centimeter, which is the standard set for the conditional approval of oysters by the Rhode Island Shell Fish Commission. The one exceptional case received but a single dose of disinfectant. It should be noted, furthermore, that the index here calculated is more severe than that based merely upon the examination of the shell liquor. With proper adjustment of conditions there is no reason to doubt that the best of these results could be equalled, and probably improved upon, in practice. Longer exposure to the treatment would yield even better results, if such were desirable.

The addition of disinfectant did not seem to interfere with the normal activity of the oysters themselves. Immediately after dosing the tank they appeared to close for a short time. Whether this was due to the effect of the chemical, or was merely the result of the disturbance, can not be definitely stated. Since the decomposition of hypochlorite is quite rapid, however, it was thought advisable to add the second dose, in order to make sure of the destruction of such organisms as might survive within the oyster and be subsequently discharged.

The method is quite analogous to natural purification in pure sea water. It has, however, two advantages, in that the process may be controlled and assured, and that the water need not contain more salt than does that upon the beds from which the oysters were taken, requirements hard to satisfy under natural conditions.

No change in the condition of the oysters could be detected, nor was it possible to distinguish any difference in flavor between the treated and untreated oysters. Considering the small quantities of hypochlorite used and the general adoption of the hypochlorite method of treating public drinking waters, no undesirable effect would be expected. Sea water normally contains comparatively large amounts of the same chemical salts that result from calcium hypochlorite disintegration.

Conclusion.

The experiments are believed to prove that oysters which have lain in polluted water can be artificially purified to such a degree as to pass a most rigid standard by exposure for a short period in water containing calcium hypochlorite, and they appear also to demonstrate the feasibility of such a process.

Reprint from the Public Health Reports, vol. 31, No. 28, July 14, 1916, pp. 1848-1852.