The danger from infected milk is probably very great. This has been repeatedly proved experimentally for bovine tuberculosis by experimenting on calves and pigs. A cow may suffer from tuberculosis of its udder, and yet go on freely secreting milk. The milk from such an udder readily produces tuberculosis in calves or pigs drinking it; but if another animal be fed during the same period with boiled milk obtained from the same udder, it remains well.
The presence of tuberculosis in cattle can be determined with almost complete certainty by the tuberculin test. A glycerine extract of pure cultivations of tubercle bacilli (filtered so as to be free from bacilli) was found by Koch to contain substances which, when injected into guinea-pigs suffering from tuberculosis, produced a febrile reaction, and appeared likely to cure the disease. So far as man and larger animals are concerned the hope of cure by this means has not been realised; but as a means of diagnosis, i.e. detection of tuberculosis, injection of a small quantity of tuberculin under the skin, has been found most valuable. If the cow thus injected is suffering from the slightest tuberculosis, it becomes feverish for a few days; if it is healthy no “febrile reaction” occurs. By using this test tuberculous cattle can be detected, they can then be kept in separate sheds, the former sheds cleansed and disinfected; the milk of these cattle kept separate from that of the rest of the herd, and boiled before being drunk, or the infected cattle sent to the butcher. If the disease is strictly localised the carcases can be utilised for food, after careful destruction of all diseased portions. If these means were generally adopted, tuberculosis might gradually be eliminated from the cattle of the entire country, and a serious source of loss to farmers, as well as of danger to children drinking the infected milk, removed. The presence of tubercle bacilli in cow’s milk is detected by microscopic examination and by injection of small quantities of the suspected milk into guinea-pigs. The proportion of infected samples found when examinations of milk supplies have been made in different towns has varied from 10 to over 50 per cent.
It is probable that tuberculosis is conveyed by cow’s milk only when the tuberculous disease affects the udder. But inasmuch as the udder of a tuberculous animal may become tuberculous very rapidly and without being detected for a considerable time, it is evident that no tuberculous animal of any kind should be allowed to remain in any cowshed where milch cows are kept.
Recently (July, 1901) Koch has thrown doubt on the identity of bovine and human tuberculosis, which was previously accepted, because (unlike some other observers) he has been unable to produce in 19 cattle, on which he experimented, tuberculosis by mixing with their food expectoration of consumptive persons, or inoculating under their skins similar material. Even if these negative results should subsequently be confirmed, the converse proposition does not follow, that bovine tuberculosis cannot be communicated to man; and apart from this possibility milk containing the bacilli of bovine tuberculosis cannot be regarded as wholesome.
The boiling of milk destroys tubercle bacilli. So does a temperature considerably below 212° F. In Denmark, where butter and cheese are manufactured on a large scale, and the raw milk is collected in central dairies, a law was passed in 1898, obliging every proprietor of a dairy to heat all skimmed milk, or butter milk, to a temperature of 85° C. (185° F.) before returning it to the farms. Pasteurization, i.e. the heating of milk in a special apparatus to a temperature of 70° C. (158° F.), and keeping it at this temperature for 30 minutes kills tubercle bacilli. If it is rapidly cooled, the nutritive value and taste of the milk are not spoilt. It is safer, however, to go beyond this point, and the use of an apparatus like the Aylmer or Sentinel Sterilizer can be recommended. More recent experiments have made it doubtful whether tubercle bacilli in milk are always killed in milk at a temperature of 70° C., the pellicle formed on milk when it is heated appearing to shield the bacilli from the effect of the heat. Hence it is desirable that “no sterilizer should be looked upon as thoroughly efficient for the purpose in which a temperature of at least 85° C. (185° F.) is not attained.”
The following test may be used to determine whether milk has been efficiently pasteurized:—
Natural milk contains a ferment or enzyme, which is destroyed at a temperature of 176° F. This enzyme splits up hydrogen peroxide (H₂O₂) into water and oxygen, but this effect is not produced in milk heated above 176° F. Take one drop of a dilute aqueous solution of hydrogen peroxide, add it to one teaspoonful of the milk. Next add two drops of a watery solution of paraphenyldiamine. A dark indigo colour is produced with uncooked milk, no change of colour if the milk has been pasteurized. The same test can be used for determining whether butter has been made with pasteurized milk.
Infection is not the sole determining cause of tuberculosis. Certain conditions of environment may determine whether the infection will succeed in “taking root” or not. Of these the following are important:
The nutrition of the individual if defective favours infection. Probably one chief reason why consumption has declined nearly 50 per cent. in the last 50 years is the better, more varied and more abundant food of the population.
Improved housing of the population has greatly helped in the same direction. Tubercular diseases increase with density of population, and are most prevalent in overcrowded tenements. Probably overcrowding chiefly acts by favouring direct infection, but it must also lower the health and power of resistance of the individual against infection.