Food such as milk that is not only eaten customarily without cooking, but is also suitable for the growth of typhoid bacilli, needs to be particularly safeguarded. It is noteworthy that the compulsory pasteurization of milk in New York, Chicago, and other large American cities has been accompanied by a great diminution in the prevalence of typhoid fever. Until recent years milk-borne typhoid in the United States has been common and hundreds of typhoid epidemics have been traced to this source.
Fig. 5.—Bacteria left by fly passing over gelatin plate. (By courtesy of Doubleday, Page & Company.)
One food animal, the oyster, frequently eaten raw, has been connected on good evidence with certain typhoid outbreaks.[57] The number of well-established oyster typhoid epidemics is not great, however, and the danger from this source has sometimes been exaggerated. The source of oyster contamination is in sewage pollution either of the shellfish beds or of the brackish water in which the oyster is sometimes placed to "fatten" before it is marketed. State and federal supervision of the oyster industry in the United States in recent years has largely done away with the taking of oysters from infected waters, and although oysters—and clams and mussels as well—must be steadily safeguarded against sewage contamination, the actual occurrence of oyster infection at the present time is believed to be relatively rare.
Probably the most effective method of preventing typhoid food infection is to investigate every case of typhoid fever and trace it, so far as practicable, to its origin. In this way typhoid-carriers may be discovered and other foci of infection brought to light. Carriers, once found, may be given proper advice and warned that they constitute a danger to others; the complete control of typhoid-carriers who are not disposed to act as advised is a difficult problem and one not yet solved by public health authorities.
ASIATIC CHOLERA
With Asiatic cholera, just as with typhoid fever, domestic animals are not susceptible to the disease, all cases of infection having a direct human origin. Drinking-water is the usual vehicle of cholera infection, and even in countries where the disease is endemic, food-borne outbreaks of this disease are far less common than those of typhoid fever. Occasional instances of Asiatic cholera due to milk supply and to contaminated fruits or lettuce are on record, but these are exceptional and cannot be regarded as exemplifying a common mode of spread of this disease. The extent, however, to which dwellers in tropical countries—and indeed in all lands—are at the mercy of their household helpers is illustrated by the following experience of the English bacteriologist, Hankin. "I have seen," he says, "a cook cooling a jelly by standing it in a small irrigation ditch that ran in front of his cookhouse. The water running in this drain came from a well in which I had detected the cholera microbe. He cleaned a spoon by dipping it in the drain and rubbing it with his fingers; then he used it to stir the jelly."[58]
TUBERCULOSIS
Animal experiments have shown that both meat and milk derived from tuberculous cattle are capable of conveying infection. The precise degree of danger to human beings from the use of these foods under modern conditions is still in dispute. Since the tubercle bacillus of bovine origin differs from the tubercle bacillus of human origin in certain well-defined particulars, it is possible by careful study to distinguish the human infections caused by the bovine bacillus from those caused by the so-called human tubercle bacillus. Additional comparative investigations are needed in this field, and these may enable us to estimate eventually more fully than is possible at present the extent of human tuberculous infection derived from bovine sources.