Meat is a less likely source of infection than milk, chiefly because it is rarely eaten without cooking. Opinion regarding the actual frequency of the transmission of tuberculosis by means of the meat of tuberculous cattle has been widely at variance in the past, and must even now be based on indirect evidence. There is no well-established instance of human infection from the use of the flesh of tuberculous cattle. The significance of this fact, however, is diminished by the observation that tubercle bacilli can pass through the intestinal wall without leaving any trace of their passage and can make their way to the lungs or to other distant organs where they find opportunity for growth. This, together with the long period which usually elapses between the actual occurrence of infection and the discovery of the existence of infection, makes the difficulty of securing valid evidence peculiarly great. Opposed to any very frequent occurrence of meat-borne tuberculosis are the facts that the tubercle bacillus is not commonly or abundantly present in the masses of muscle usually marketed as "meat," that the tubercle germ itself is not a spore-bearer and is killed by ordinary cooking, and that the reported cases of the finding of tubercle bacilli of bovine origin in adults over sixteen years of age are extremely rare. This latter fact is perhaps the strongest evidence indicating that tuberculous meat infection, although theoretically possible, is at least not of common occurrence.
Most of the commissions and official agencies that have considered the precautions to be taken against possible tuberculous meat infection are agreed that the entire carcass of an animal should be condemned when the tuberculous lesions are generalized or when the lesions are extensive in one or both body cavities as well as when the lesions are "multiple, acute, and actively progressive." Any organ showing evidence of tuberculous lesions is obviously not to be passed as food. On the other hand, it is considered that portions of properly inspected animals may be put on the market if the tuberculous lesion is local and limited and the main part of the body is unaffected; in such cases contamination of the meat in dressing must be avoided. It is the general belief that when such precautionary measures are taken the danger of tuberculous infection through properly cooked meat is so slight as to be negligible.
Milk is a much more likely vehicle than meat for the transmission of tuberculosis. Freshly drawn raw milk from tuberculous cattle may contain enormous numbers of tubercle bacilli, especially if the udder is diseased. Contamination of milk by the manure of tuberculous cows can also occur. Observers in England, Germany, France, and the United States have found tubercle bacilli in varying numbers in market milk, and have proved that such milk is infectious for laboratory animals. Although, as pointed out with reference to meat infection, the difficulties of tracing any particular case of tuberculosis to its source are very great, there are a number of instances on record in which the circumstantial evidence strongly indicates that milk was the vehicle of infection. Especially convincing are the observations on the relative frequency of infection with bovine and human tubercle bacilli at different ages as shown in the following tabulation:[59]
| Adults Sixteen Years Old and Over | Children Five to Sixteen Years Old | Children under Five Years | |
| Human tubercle bacilli found | 677 | 99 | 161 |
| Bovine tubercle bacilli found | 9 | 33 | 59 |
The large proportion of bovine tubercle bacillus infections in children stands in all probability in causal relation to the relatively extensive use of raw milk in the child's dietary.
The proper pasteurization of milk affords a safe and reasonably satisfactory means of preventing tuberculous infection from this source. The general introduction of the pasteurizing process in most American cities has ample justification from the standpoint of the prevention of infection.
VARIOUS MILK-BORNE INFECTIONS
The facts related in the foregoing pages indicate that of all foods milk is the most likely to convey disease germs into the human body. This is partly due to the fact that milk is sometimes obtained from diseased animals, and partly to the fact that unless great care is taken it may readily become contaminated during the process of collection and transportation; if milk is once seeded with dangerous bacteria these can multiply in the excellent culture medium it affords. It is also partly because milk is commonly taken into the alimentary tract without being cooked. For these reasons the amount of illness traceable to raw milk far exceeds that ascribable to any other food.
There are several infections that may be communicated by milk, but are rarely if ever due to other foodstuffs. Diphtheria and scarlet fever are perhaps the best known of these. Both diseases have been repeatedly traced to the use of particular milk supplies, although various forms of individual contact also play a large rôle in their dissemination. Milk-borne scarlet fever and diphtheria seem to be generally, if not always, due to the direct contamination of the milk from human sources. It is considered possible, however, by some investigators that the cow may sometimes become infected from human sources with the virus of scarlet fever or diphtheria and may herself occasionally contribute directly to the infection of the milk.
A serious milk-borne disease, which has lately been conspicuous in Boston, Chicago, Baltimore, and other American cities under the name of "septic sore throat" or "streptococcus sore throat," originates apparently in some cases from infection of the udder of the cow by an infected milker; in other cases the milk has seemingly been directly infected by a human "carrier." The specific germ is thought to have been isolated and its connection with the disease demonstrated in the laboratory. This disease, like diphtheria and scarlet fever, is sometimes due to contact. It is not known to be caused by any food except milk.