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51. From the foregoing table it appears that in 1981 in England and Wales 6-0 per cent. of all deaths from tuberculosis arose from infection with the bovine bacillus, the percentage being 0-9 in the case of pulmonary, and 30-0 in the case of other forms of tuber- culosis. If the cases notified are divided in the same proportion, nearly 450 cases of pulmonary, and over 4,800 cases of other forms of tuberculosis occurred in 1931 from bovine infection. In Scotland the percentage of bovine infections is higher in all forms of tuber- culosis than in England and Wales. Applying these percentages in the same way as for England and Wales, we find that there were 129 deaths and 175 notifications of pulmonary, and 336 deaths and 825 notifications of other forms of bovine tuberculosis in Scotland in 1931. Thus the totals for Great Britain were in 1931, 2,612 deaths and about 6,250 notifications. We do not suggest that such calculations lead to very dependable results. They have obvious statistical imperfections. The number of cases investigated is small; they are probably not a random sample of all cases occurring; they occurred over a number of years, and percentages based upon them are not strictly applicable to the year 1981; there may not be a strict correspondence between the forms of tuberculosis falling under each heading in the deaths taken from the registrar-general's reports and in the published results of experiments. But making all allowance for the error so introduced, it is clear that bovine tuber- culosis is responsible in Great Britain for a large number of deaths which is more likely to exceed than to fall short of 2,500 and for a still larger amount of serious illness.
52. Bovine tuberculosis in man is due in practically every case to alimentary infection. Even in the case histories of those suffering from pulmonary tuberculosis (phthisis pulmonalis) due to bovine tubercle bacilli there seldom appears to be any evidence of contact with other human cases of the disease which might account for infection, and post-mortem examinations of such cases usually indicate that the channel of infection has been through the digestive system. Recent observations, however, have suggested that persons who come much into contact with tuberculous cattle, such as milkers, cowmen, &c., may contract bovine tuberculosis through inhalation of infective dust from the hides of cattle or in the shippons or byres. Of milk products, cheese is not likely to harbour infection if kept in store for the time usual in commercial cheese manufacturing, as the chemical changes which take place have the effect of destroying the tubercle bacillus; dried milk is rendered safe by the heat used in the process of manufacture; but butter, cream and cream cheese and ice-cream, if made from raw milk, are capable of conveying infection. It is not possible to say to what extent these products are in fact responsible for the infection which takes place. Cases of alimentary tuberculosis due to bovine bacilli and the consumption of milk are at their highest in early childhood, and show a parallel diminution as age advances. Other foods besides milk may
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be contaminated with bovine bacilli. We have already referred to the large quantity of meat condemned on that account. It is not to be expected that the precaution of inspection is infallible, and a certain amount of infected meat must reach the consumer. meat is generally well cooked, and as the tubercle bacillus is destroyed by exposure to comparatively low temperatures, this is unlikely to be a serious source of infection. It is to infection from milk, the food of childhood, that most of the bovine tuberculosis in human beings is attributable.
53. The extent to which milk as ordinarily sold is infected with tuberculosis is not easily determined. The infected milk of a single cow may be mixed with that from many whose milk is free from infection. The degree to which milk samples prove to contain tubercle bacilli will consequently depend in part on the percentage of cows giving infected milk, and in part on the extent to which milk is mixed before samples are taken. It has sometimes been estimated from the published results of veterinary inspections that between two and three cows in a thousand yield tuberculous milk. This estimate seems to us too low. The evidence that we have received indicates that at least five cows in a thousand yield tuberculous milk. As a result of the mixing of the milk of cows yielding infected milk with that of other cows, a much larger percentage of the samples of milk from individual herds prove to be tuberculous. English local authorities have reported a percentage of infection varying between 5 and 10, and showing an average of under 7. A careful investigation of farm milk coming into the four principal cities of Scotland showed an infection of 10 per cent. The percentage of infection in samples of the bulked milk from a number of farms is naturally very much higher. The extent to which milk, when retailed, is infected with tubercle bacilli depends not only on the original quality of the milk, but also on whether it has been heat treated. In London, for example, the London County Council found that 3.2 per cent. of 282 samples (of which, however, only one-fifth were from milk sold by the large distributors) were infected. In Scotland it has been estimated that in the four principal cities 5.26 per cent. of the milk sold at retail is infected.
54. It is sometimes suggested that the mixing of tuberculous milk with tubercle-free milk is advantageous for two reasons. In the first place it is argued, the dilution of the tuberculous milk reduces to negligible proportions the risk of anyone contracting serious tuber- culosis, and, in the second, the ingestion in childhood of a few bovine bacilli in milk raises resistance to later infections.
55. With regard to the first point, there is no doubt that a small dose of tubercle bacilli swallowed with food is less certain to
* In a report by the Department of Health for Scotland on tuberculous infection in milk published by the Medical Research Council.
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infect than a large dose. This fact has been clearly established by experiments on animals. For example, the highly susceptible guinea-pig may escape infection altogether when the bacilli given in food are few in number. This is because the bacilli are not absorbed but pass out of the body with the fæces. But if a non-tuberculous animal or a human being daily consumes slightly infected milk some bacilli will sooner or later invade the tissues. The outcome of this first infection is determined more by the state of bodily resistance at the time than by the numbers of the invading bacilli. Human beings vary widely in their susceptibility to tuberculosis and are in general less resistant in childhood than in adult age, and probably at all times when suffering from another illness, notably measles. resistant individual a slight infection with bovine (or with human) tubercle bacilli has not generally any serious consequence; the lesion remains localised and eventually heals completely.
The great frequency with which healed tuberculous lesions are found at autopsies on persons who have died from causes other than tuberculosis is evidence that many persons become infected and overcome the infection. On the other hand, in persons whose powers of resistance are naturally low, as generally in infants, or have been lowered by illness a few bacilli entering the tissues multiply and soon spread about the body, causing either acute general tuberculosis or the more chronic forms of the disease, such as bone and joint tuberculosis, kidney and even lung tuberculosis. There is abundant clinical and pathological evidence that much serious disease is caused in young children through the ingestion of small numbers of bovine tubercle bacilli. For example, the majority of cases of bone and joint tuberculosis due to these bacilli must have been caused by quite slight infections since they arose without any clinical evidence of previous tuberculosis of the digestive tract. The bulking of milk therefore, though doubtless making the milk less dangerous to those who are endowed with some degree of resistance, increases by spreading and keeping up the risk-the probability that tuberculosis will be contracted by those who are naturally, or become temporarily, susceptible to infection with the tubercle bacillus.
56. The second argument that a few tubercle bacilli of bovine type in milk are permissible for their inmunising value is based on a misconception that the bovine tubercle bacillus is less virulent for man than the human tubercle bacillus and produces generally a mild non-progressive disease. This belief, originating with Robert Koch, has been definitely disproved by observations in this country. The evidence which has been accumulated goes to show that the bovine bacillus is at least as virulent for man as the human bacillus. Although the bovine bacillus is limited practically to one channel of entry, the alimentary canal, which is a more uncertain route of infection than the respiratory tract, it is able to produce every variety of human tuberculosis and is a frequent cause of death. Numerous instances of phthisis (consumption) due to bovine bacilli in
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