CHAPTER V.
RELATION OF DISEASE-BACTERIA TO MILK.
Practical experience with epidemic disease has abundantly demonstrated the fact that milk not infrequently serves as a vehicle for the dissemination of contagion. Attention has been prominently called to this relation by Ernest Hart,[78] who in 1880 compiled statistical evidence showing the numerous outbreaks of various contagious diseases that had been associated with milk infection up to that time. Since then, further compilations have been made by Freeman,[79] and also by Busey and Kober,[80] who have collected the data with reference to outbreaks from 1880 to 1899.
These statistics indicate the relative importance of milk as a factor in the dissemination of disease.
The danger from this source is much intensified for the reason that milk, generally speaking, is consumed in a raw state; and also because a considerable number of disease-producing bacteria are able, not merely to exist, but actually thrive and grow in milk, even though the normal milk bacteria are also present. Moreover the recognition of the presence of such pathogenic forms is complicated by the fact that often they do not alter the appearance of the milk sufficiently so that their presence can be detected by a physical examination. These facts which have been experimentally determined, coupled with the numerous clinical cases on record, make a strong case against milk serving as an agent in the dissemination of disease.
Origin of pathogenic bacteria in milk. Disease-producing bacteria may be grouped with reference to their relation toward milk into two classes, depending upon the manner in which infection occurs:
Class I. Disease-producing bacteria capable of being transmitted directly from a diseased animal to man through the medium of infected milk.
Class II. Bacteria pathogenic for man but not for cattle which are capable of thriving in milk after it is drawn from the animal.