In addition to data of this sort that is practically experimental in character, there are also strong clinical reasons for considering that infection of human beings may occur through the medium of milk. Naturally such infection should produce intestinal tuberculosis, and it is noteworthy that this phase of the disease is quite common in children especially between the ages of two and five.[84] It is difficult to determine, though, whether primary infection occurred through the intestine, for, usually, other organs also become involved. In a considerable number of cases in which tubercular infection by the most common channel, inhalation, seems to be excluded, the evidence is strong that the disease was contracted through the medium of the milk, but it is always very difficult to exclude the possibility of pulmonary infection. Tuberculosis as a bovine disease has increased rapidly during recent decades throughout many portions of the world. This has been most marked in dairy regions. Its extremely insidious nature does not permit of an early recognition by physical means, and it was not until the introduction of the tuberculin test[85] in 1892, as a diagnostic aid that accurate knowledge of its distribution was possible. The quite general introduction of this test in many regions has revealed an alarmingly large percentage of animals as affected. In Denmark in 1894 over forty per cent were diagnosed as tubercular. In some parts of Germany almost as bad a condition has been revealed. Slaughter-house statistics also show that the disease has increased rapidly since 1890. In this country the disease on the average is much less than in Europe and is also very irregularly distributed. In herds where it gained a foothold some years ago, often the majority of animals are frequently infected; many herds, in fact the great majority, are wholly free from all taint. The disease has undoubtedly been most frequently introduced through the purchase of apparently healthy but incipiently affected animals. Consequently in the older dairy regions where stock has been improved the most by breeding, more of the disease exists than among the western and southern cattle. [Illustration FIG. 21: Front view of a tuberculous udder, showing extent of swelling in single quarter.] ~Infectiousness of milk of reacting animals.~ Where the disease appears in the udder the milk almost invariably contains the tubercle organism. Under such conditions the appearance of the milk is not materially altered at first, but as the disease progresses the percentage of fat generally diminishes, and at times in the more advanced stages where the physical condition of the udder is changed (Fig. 21), the milk may become "watery"; but the percentage of animals showing such udder lesions is not large, usually not more than a few per cent. (4 per cent. according to Ostertag.)