CO129-562-12 Dysentry epidemic- recommendation to enforce compulsory pasteurization of milk 7-6-1937 - 17-8-1937 — Page 17

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There are, however, efforts are made to eradicate the disease. various methods of immunisation now under examination, which Of these, may in time provide a safeguard against infection. immunisation with Bacillus Calmette-Guérin has been the subject of experimentation for some years, and may soon approach the stage at which it will have practical usefulness. Immunisation, however, is not to-day an important outwork in the farmers' defences against tuberculosis. Nor can we hold out hope that it is likely to provide in any reasonable period of time more than a partial solution of the problem.

34. While the percentage of tuberculous animals in the total dairy stock may be put by common consent at about 40, the incidence in different herds, even within small areas, is known to range from complete freedom to 100 per cent. infection, and the variation in the degree of infection in individual herds is considerably greater than would be expected if tuberculous cows were distributed at random among the herds of the country. The eradication of bovine tuberculosis in cattle would lead to the disappearance of the greater part of tuberculosis in swine, and of practically all the tuberculosis of horses, goats and cats. For though infection of cows from these other animals is rare, infection of the latter from cattle is of the first importance.

(ii) Contagious abortion.

35. Little information is available regarding the degree to which cattle are infected with contagious abortion. The practice of testing herds for the presence of this disease is not as common as in the case of tuberculosis, nor, so far as we are aware, have any records been kept of the extent to which carcases of cows slaughtered are infected. We have seen, however, the results of two surveys* of the extent to which the disease in an active form has been found in a number of herds. These show that abortions occurred in more than 40 per cent. of the herds surveyed, and that nearly 9 per cent. It of the cows in these herds aborted in the course of the year. must be remembered that not all these abortions are necessarily due to infection with contagious abortion; and that many infected cows, though they are carriers of the disease, do not abort. The general view appears to be that cattle are infected with contagious abortion to about the same extent as with tuberculosis, namely, 40 per cent. This conclusion, however, is not grounded upon adequate statistical investigation.

36. Unlike tuberculosis, contagious abortion does not result in the death of the infected animal, nor does it unfit the carcase for human consumption. It is, however, responsible for very serious were undertaken by the Agricultural Economics Research Institute, Oxford, in 1932, and by the Department of Agricultural Economics of the University of Reading in 1929.

* These

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loss to dairy farmers, both directly by causing the loss of calves and the loss of milk, whether due to lessened productivity or to inter- ference with normal lactation periods, and indirectly by causing disease of the reproductive organs and consequent sterility.

37. The disease is contracted mainly by ingestion of infected material, but also by contact with such material, especially at points where the skin is broken. The sources of infection are matter discharged at the time of calving, vaginal discharges for about one month after calving, and milk. The calves of infected animals are infected at birth, and remain infected for some weeks, or, in some cases, months. A blood-agglutination test can show whether living animals are infected. The organism responsible for the disease is the bovine type of Brucella abortus.

38. Contagious abortion is a disease from which herds may achieve a comparatively high degree of immunity. There are, in consequence, two policies open to farmers in respect of it. They may in the first place carefully preserve their herds from infection. If they succeed, they avoid abortions in their herds, but they run the risk of a sudden and serious herd infection. Alternatively, farmers whose herds are already infected, and who have therefore incurred serious initial losses, may prefer to encourage the spread of infection in their herds and thus to avoid the risks of sudden and serious outbreaks, though this course insures chronic but lighter losses. In such herds it is preferable to secure such immunity as is possible by vaccination.

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39. Vaccination has been practised in the case of this disease for many years. Experience has shown that vaccination with dead cultures is usually ineffective. If non-pregnant animals vaccinated with a live culture of moderate virulence, the loss of calves is usually very much smaller than that among unvaccinated animals. Inoculation is most effective among animals that are free from infection. Some experimental work has suggested that pregnant animals may be immunised against any but the most severe infection by an attenuated live vaccine, but this method has not yet been shown to be effective under practical conditions, and is ineffective with animals which are not pregnant. The use of

vaccination as a measure of defence does not release the herd owner from the necessity of taking reasonable hygienic precautions against infection, for massive infection may overcome the immunity acquired by vaccination.

40. Vaccination against abortion has been attacked upon several grounds, and it is perhaps less popular to-day than heretofore. The objections which have been raised on grounds of public health will concern us later, here it is sufficient to notice that vaccinated animals may, as a result, yield infected milk, and that the chances of this are increased if the vaccine used is of a virulent strain. Further, there is some evidence that vaccinated animals may be a source of

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