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Germany for instance. But with an incidence as high as in Germany, the disease would still be unimportant. The real protec- tion of the public is their own immunity. In these circumstances though it is important that farmers should not relax their efforts to reduce the extent to which their herds are infected with abortion, it is doubtful whether any measures that are practicable would appreciably reduce the present incidence of undulant fever in man; and still more, whether, from the point of view of public health, the effort used in such an attempt could not more profitably be expended in other directions.
(iii) Milk-borne epidemics.
60. The Ministry of Health has supplied us with a list (given in appendix 6) of the milk-borne epidemics which have been reported in the United Kingdom since the beginning of this century. It cannot be regarded as exhaustive, for many epidemics, though recognised as being due to contaminated milk, are not reported, and many slight outbreaks of common diseases, due to milk, are not recognised as such. It will be observed that there have been approximately 100 outbreaks recorded since 1903, ranging in importance from the outbreak in Hove in 1929, in the course of which 1,000 families were infected and 65 deaths occurred, to incon- siderable outbreaks responsible for as few as three cases.
A great variety of diseases is recorded as being conveyed by milk, the most common being scarlet fever, enteric fever, diphtheria, paratyphoid and septic sore throat. In some cases infection has originated from an infected cow; but more frequently infection has been traced to a human source. In the latter case it appears that the milker is responsible for the majority of outbreaks, though there are several instances of infection occurring at a later stage, e.g., when milk is heat-treated. Indeed, the most serious outbreak of milk-borne disease brought to our notice, which resulted in the death of over 500 persons from typhoid in Montreal in 1927, was traced to a typhoid carrier working in a pasteurising plant.
61. The available evidence is not conclusive regarding the importance of milk in the spread of infectious diseases. In spite of occasional serious outbreaks, those reported are neither so numerous nor so severe as to contribute much to the total incidence of the diseases in question. For example, the 28 outbreaks of scarlet fever due to milk in the last twenty years were responsible for between 2,500 and 3,000 cases of infection. But the total number of cases notified has fluctuated between 80,000 and 120,000 annually in England and Wales, and has averaged 17,500 in Scotland. On the other hand, some witnesses expressed the view that unrecorded cases of infection through the milk supply were of considerable importance. It is only where a large number of cases of infection occur that the milk supply is likely to be suspected. Consequently, we cannot rule
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out altogether the possibility that infection through milk is more extensive than appears at first sight. In support of this possibility we may quote the results of inquiry undertaken by the United States public health service. It was found that among 1,875 children who were reared exclusively on heat-treated milk (which is unlikely to convey these infections) the following incidence of certain infectious diseases per thousand children was reported: diphtheria 17 1, scarlet fever 23-0, various intestinal disturbances 227·0. The incidence of the same diseases among 1,762 children reared principally on raw milk was, diphtheria 22-7, scarlet fever 41·4, various intestinal disturbances 278 0.
62. One peculiarity of these milk-borne infections is of great importance in large cities. The organisms responsible for them multiply rapidly in milk, and the bulking of milk consequently adds enormously to the chances of infection. Fortunately, however, most of the bulked milk brought into the large cities undergoes heat treat- ment a process which, if properly performed, destroys the organisms in question.
(c) Milk in human diet.
63. It is generally recognised that milk is the most satisfactory individual food material elaborated by nature, just as
it is beyond all question the nutriment par excellence for infancy. It is essential in babyhood, but with advancing years and the increasing variety of food materials available, although it always remains a most valuable addition to the diet, it can no longer be considered an essential constituent.
64. Milk holds a unique position amongst the foods of animal origin as it contains the three proximate principles of a diet, protein, fat and carbohydrate, in addition to an adequate supply of inorganic constituents in such proportions as to be peculiarly adapted for the period of active growth. Its value as a foodstuff is of course enhanced by the presence in it of the fat soluble vitamins A and D, and the water soluble vitamins B and C. Of these vitamins, the only one which is, so far as is known, materially influenced by heat treatment is C, or the anti-scorbutic vitamin. The only drawback milk would appear to possess to the claim of being the perfect food for infants, is that it is somewhat deficient in substances necessary for the formation of blood.
65. As regards the individual components which render milk such a unique source of material both for growth and energy it is not necessary here to discuss them in detail. The proteins present, but not in equal proportions, in cow's and human milk are of two kinds. The most characteristic protein of both milks is caseinogen, a so-called conjugated protein of peculiar type, which contains phosphorus in its molecule and, more predominant in human milk,
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