Food Transmission.—Food contaminated by dejecta from cholera patients or carriers is dangerous in proportion to its condition of moisture. Drying and the development of inimical organisms are the two chief factors in destroying the cholera vibrio. Temperature and sunshine are operative in assisting the drying process.
Lettuce and celery are particularly dangerous because of the favorable condition of moisture in their folds and imbrications. Furthermore these vegetables are eaten uncooked and may have been fertilized with night soil (human excrement) which material, if containing cholera dejecta, would infect the plants. Milk is a splendid culture medium for cholera vibrios but, upon becoming acid, sterilizes itself of these vibrios. In sterilized milk, however, they live for extended periods, as long as sixty days and, even when such milk is contaminated by faecal material containing other organisms besides the cholera vibrio, the vibrios live much longer than they do in raw milk.
Milk is liable to be contaminated by flies which have been in contact with cholera stools. Water that has been boiled and food that has been cooked should subsequently be scrupulously protected from flies or other contaminating agents. Uncooked shell fish are peculiarly dangerous in cholera outbreaks.
In India, sun-dried fish, which are frequently covered with flies during the curing process, are a factor in the spread of cholera.
Transmission by Carriers.—This is now universally recognized as the most important factor in the spread of cholera. Dunbar was the first to draw attention to the presence of virulent cholera spirilla in the faeces of apparently healthy persons during the Hamburg epidemic of 1892.
Since that time these observations have been generally confirmed. In some instances as many as 20% of those who have been in immediate contact with a cholera patient have become carriers, some showing symptoms of cholera but a larger proportion excreting cholera spirilla while continuing in health.
While cholera prevailed in Manila, McLaughlin found from 6 to 7% of carriers among healthy persons living in the infected districts.
Pottevin has recently reported that of 13,000 pilgrims examined 1.7 per thousand carried cholera vibrios. The carriers were especially common among the dysenteric patients. During the Naples epidemic of 1911 it was found that on the average 10% of healthy people in contact with cholera cases became carriers. It was estimated that 90% of the cases in this epidemic were infected by sick or healthy carriers.
Sergeant has recently reported the case of a healthy carrier who continued to excrete cholera vibrios for two months and during this time was in contact with 8 persons, 7 of whom became infected and 4 died. In Manila it was found that many of the children reported as dying of meningitis or infantile beriberi were cholera cases.
The vibrios are rarely excreted in the faeces of the cholera patients longer than seven to ten days. Frequently they disappear in three or four days.