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mediate types of dairymen. Each is likely to predominate where circumstances render it economical, and each has its particular problems from the point of view of public health. The producer- retailer and the large distributor are in competition on the circum- ference of the largest towns and throughout towns of large but not the largest size. According to figures published by the Milk Marketing Board in England and Wales, the number of producer- retailers is approximately 50,000,* and that of producers selling by wholesale contract, approximately 78,000.
II. CATTLE DISEASES AND THE DISEASES IN MAN ASSOCIATED WITH THEM.
(a) Cattle diseases.
(1) Bovine tuberculosis.
29. The incidence of bovine tuberculosis among cattle is probably as high in Great Britain as anywhere else in the world. A number of witnesses before us have expressed the view that 40 per cent. of the cattle in this country are infected with it in such degree that they will react to the tuberculin test. This estimate is based first upon the percentage infection found in herds tested with tuberculin for the first time, and, secondly, upon the results of the inspection of carcases of cows slaughtered. For example, the results of tests in 144 herds which had not previously been tested for tuberculosis were quoted in various memoranda submitted to us, and showed that of 5,199 cows, 2,233, or 43 per cent., gave a positive reaction. It was also recorded
in a report drawn up by the People's League of Health that 39.5 per cent. of 55,318 cows slaughtered at various centres were found on post-mortem to be infected. In considering this latter percentage it must be remembered that many cows which are infected with tuberculosis do not reach the slaughterhouse, as they die upon the farm, or are sent to kennels or knackers.
It may therefore be unduly low. In the light of such figures as these the generally accepted conclusion that at least 40 per cent. of cows in dairy herds are infected with tuberculosis does not appear exaggerated. This percentage of infection may be compared with a percentage of some- thing over four in the United States, and of approximately twelve in Canada, before steps were taken against the disease. We have no evidence to show whether the disease is increasing or diminishing in Great Britain, but the widespread neglect of adequate precautions against its increase makes the latter improbable.
* Of these, a large number are situated near the towns with a population of over 100,000, in regard to which we make recommendations later in our report. Though no direct information as to their numbers is available, we give certain statistics on this point in appendix 5.
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80. In some infected animals the progress of the disease is arrested before it assumes a generalised form. We have received evidence that of animals slaughtered at Edinburgh and found to be tuberculous, about 10 per cent. have to be condemned entirely, about 20 per cent. have to be condemned in part, and the remainder or 70 per cent. are so slightly infected that the whole carcase may be sold. This 70 per cent., in fact, consists of two groups. In the first are those in which the disease is of such recent origin that it has not had time to spread. The second is that in which the disease is not generalised, having been brought under control by the animal's natural powers of resistance. These cows are not free from the danger of a breakdown of their resistance against the disease in circumstances of exceptional strain.
31. Cattle, as has already been mentioned, may be shown to be infected with tuberculosis during life by their reaction to the introduction of tuberculin into their system. This test, however, generally fails to reveal infection incurred less than three or four weeks before the time of testing and sometimes infection incurred as much as three months previously. Unfortunately, reaction to the test does not show what is the stage of the infection, or whether the disease is progressive or quiescent.
32. Cattle become infected with tuberculosis either by inhaling or by ingesting infected material. The source of infection is normally another cow, though the disease may be contracted by contact with other infected animals, for example, with pigs. Indeed, to-day the disease may be contracted from a man suffering from bovine tuberculosis.* Unfortunately cattle, even at an early stage The
of the disease, excrete tubercle bacilli in great numbers. principal sources of infected material are the lungs. Infected sputum, however, is swallowed, and in this way the fæces become infected. The udder may eventually become infected, and the cow will in that case yield infected milk. But only a small proportion of tuberculous cows give infected milk. Generally speaking, tuber- culosis does not appear in the udder until the disease has reached an advanced stage. Thus infection may derive from matter passed through any of the principal channels of the body. The tubercle bacillus may, in favourable circumstances, remain alive for a con- siderable time after it has been excreted. The danger of infection through the use of premises is therefore very real, and there is also risk from the use of infected pastures. The incidence of tuberculosis is normally low in calves and heifers, but rises rapidly after the second year.
Calves born from infected mothers are nearly always free from the disease and are likely to remain free if segregated at once and kept strictly isolated.
33. No cure for tuberculosis is known, and once a herd is heavily infected it is likely to remain in that condition unless special
Namely, ulcerative, pulmonary, or renal tuberculosis of bovine origin,
*
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