Outlines of Dairy Bacteriology, 8th edition / A Concise Manual for the Use of Students in Dairying

by H. L. Russell

Notwithstanding this, numerous observers[89] especially in Germany have succeeded in finding the tubercle bacillus in market butter, but this fact is not so surprising when it is remembered that a very large fraction of their cattle show the presence of the disease as indicated by the tuberculin test, a condition that does not obtain in any large section in this country.

The observations on the presence of the tubercle bacillus in butter have been questioned somewhat of late[2] by the determination of the fact that butter may contain an organism that possesses the property of being stained in the same way as the tubercle organism. Differentiation between the two forms is rendered more difficult by the fact that this tubercle-like organism is also capable of producing in animals lesions that stimulate those of tuberculosis, although a careful examination reveals definite differences. Petri[90] has recently determined that both the true tubercle and the acid-resisting butter organism may be readily found in market butter.

In the various milk products it has been experimentally determined that the true tubercle bacillus is able to retain its vitality in butter for a number of months and in cheese for nearly a year.

Treatment of milk from tuberculosis cows. While it has been shown that it is practically impossible to foretell whether the milk of any reacting animal actually contains tubercle bacilli or not, still the interests of public health demand that no milk from such stock be used for human food until it has been rendered safe by some satisfactory treatment.

1. Heating. By far the best treatment that can be given such milk is to heat it. The temperature at which this should be done depends upon the thermal death point of the tubercle bacillus, a question concerning which there has been considerable difference of opinion until very recently. According to the work of some of the earlier investigators, the tubercle bacillus in its vegetative stage is endowed with powers of resistance greater than those possessed by any other pathogenic organism. This work has not been substantiated by the most recent investigations on this subject. In determining the thermal death point of this organism, as of any other, not only must the temperature be considered, but the period of exposure as well, and where that exposure is made in milk, another factor must be considered, viz., the presence of conditions permitting of the formation of a "scalded layer," for as Smith[91] first pointed out, the resistance of the tubercle organism toward heat is greatly increased under these conditions. If tuberculous milk is heated in a closed receptacle where this scalded membrane cannot be produced, the tubercle bacillus is killed at 140° F. in 15 to 20 minutes. These results which were first determined by Smith, under laboratory conditions, and confirmed by Russell and Hastings,[92] where tuberculous milk was heated in commercial pasteurizers, have also been verified by Hesse.[93] A great practical advantage which accrues from the treatment of milk at 140° F. is that the natural creaming is practically unaffected. Of course, where a higher temperature is employed, the period of exposure may be materially lessened. If milk is momentarily heated to 176° F., it is certainly sufficient to destroy the tubercle bacillus. This is the plan practiced in Denmark where all skim milk and whey must be heated to this temperature before it can be taken back to the farm, a plan which is designed to prevent the dissemination of tuberculosis and foot and mouth disease by means of the mixed creamery by-products. This course renders it possible to utilize with perfect safety, for milk supplies, the milk of herds reacting to the tuberculin test, and as butter of the best quality can be made from cream or milk heated to even high temperatures,[94] it thus becomes possible to prevent with slight expense what would otherwise entail a large loss.

2. Dilution. Another method that has been suggested for the treatment of this suspected milk is dilution with a relatively large volume of perfectly healthy milk. It is a well known fact that to produce infection, it requires the simultaneous introduction of a number of organisms, and in the case of tuberculosis, especially that produced by ingestion, this number is thought to be considerable. Gebhardt[95] found that the milk of tuberculous cows, which was virulent when injected by itself into animals, was innocuous when diluted with 40 to 100 times its volume of healthy milk. This fact is hardly to be relied upon in practice, unless the proportion of reacting to healthy cows is positively known.

It has also been claimed in the centrifugal separation of cream from milk[96] that by far the larger number of tubercle bacilli were thrown out with the separator slime. Moore[97] has shown that the tubercle bacilli in an artificially infected milk might be reduced in this way, so as to be no longer microscopically demonstrable, yet the purification was not complete enough to prevent the infection of animals inoculated with the milk.

Another way to exclude all possibility of tubercular infection in milk supplies is to reject all milk from reacting animals. This method is often followed where pasteurization or sterilization is not desired. In dairies where the keeping quality is dependent upon the exclusion of bacteria by stringent conditions as to milking and handling ("sanitary" or "hygienic" milk), the tuberculin test is frequently used as a basis to insure healthy milk.

Foot and mouth disease. The wide-spread extension of this disease throughout Europe in recent years has given abundant opportunity to show that while it is distinctively an animal malady, it is also transmissible to man, although the disease is rarely fatal. The causal organism has not been determined with certainty, but it has been shown that the milk of affected animals possesses infectious properties[98] although appearing unchanged in earlier phases of the disease.

Hertwig showed the direct transmissibility of the disease to man by experiments made on himself and others. By ingesting milk from an affected animal, he was able to produce the symptoms of the disease, the mucous membrane of the mouth being covered with the small vesicles that characterize the malady. It has also been shown that the virus of the disease may be conveyed in butter.[99] This disease is practically unknown in this country, although widely spread in Europe.