MILK AS A ROUTE OF INFECTION i. Milk as a Medium for Bacteria.—In this connection it is necessary to call attention to a phenomenon observed in milk which is commonly referred to as the "germicidal" property. In the first few hours after milk is withdrawn from the cow there is a transitory reduction in the number of colonies which develope on plates poured from unit volumes of milk, as com pared with the number originally present. In a few hours the numbers show no further reduction, commence to in crease and soon greatly exceed those originally present. This phenomenon is not apparent in milk which has been heated to 5o degrees C. . Several explanations have been advanced.
(a) Some consider this phenomenon as a manifestation of a true germicidal property in milk and that the temporary diminu tion is due to a partial destruction of the bacteria originally introduced.
(b) Or it may be due to an inhibition of those species which find milk an unfavorable environment for development.
(c) Others consider the phenomenon is due to the action of agglutination, i.e., the reduction in colonies is due to the clump ing of bacteria and that their numbers are not actually reduced. This view is strengthened by the influence of heating as already noted.
2. Sources of Pathogenic Organisms which may be En countered in let us consider organisms derived from the producing animals: (a) Cows.
r. The mammary gland.
Mycobacterium tuberculosis.
Virus of foot and mouth disease.
Bacillus anthracis.
Virus of cow pox.
Streptococci.
2. From fecal contamination.
Mycobacterium tuberculosis. Bacterium enteritidis (b) Goats.
I. The mammary gland.
Bacterium melitensis. (Malta fever).
Equally important are infective agents derived from human sources, thus: • T. Fecal and urinary discharges.
Bacterium typhosus.
Various non-specific organisms producing diarrhea. 2. Nasopharyngeal secretions.
Mycobacterium diphtherice.
Virus of scarlet fever.
Streptococcus hcemolyticus. (Septic sore throat).
3. Development of Pathogenic Bacteria in Milk.—All of the rapidly growing pathogenic bacteria find milk a very favorable medium, consequently their numbers will increase.
Thus individuals consuming contaminated milk will ingest large numbers of the organisms, in other words, the dosage of infection will be great. The extent to which multiplication takes place is ,modified by a number of factors among which may be mentioned the following: Germicidal property, tempera ture of storage, age of the milk and the time at which the in fective agents gain introduction. Of these temperature is one of the most important. The nearer the temperature of storage approaches body heat the greater will be the development of pathogenic organisms.
The incidence of milk-borne disease in a given population is greatest among those who use milk as a beverage. A lighter incidence will be found among those who only employ milk on cereals, or as a diluent in tea, coffee or cocoa. Since women and children are usually the heaviest milk drinkers, the heaviest evidence of milk-borne disease is found in these age and sex groups. As we have seen, bacterial multiplication in milk is directly proportionate to the external temperature, conse quently the dosage of infection in a given sample of infected milk will be greater in warm weather. As a rule milk-borne epidemics occur during the warmer seasons of the year, or follow ing periods of warm weather in winter. The large dosage of infective agents received reduces the incubation period to its lower limits, but peculiarly, in the case of typhoid at least, the disease when milk borne is commonly milder than usual. In the care of typhoid and diphtheria transmission by milk it will be found that the contamination of the milk with these organ isms has been intermittent, frequently only a single lot being infected. investigation will reveal the dairy or distributing agency responsible and the distribution of the cases will be found to coincide with the distribution of that supply. The possibilities in the contamination of milk and the range of its distribution are indicated in Fig. 64.