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50
example, that many cows yielding tuberculous milk are not discover- able by clinical examination. The results of the two investigations bearing upon this point known to us are not easily reconciled. But the proportion of such cows is not likely to be less than one in four. Again, it is highly improbable that having regard to the intervals between inspections, veterinary inspection of the frequency we contemplate would eliminate much more than three- quarters of those cows which yield tuberculous milk and show recognisable clinical symptoms of tuberculosis.
130. That routine inspection has been to some extent effective in reducing the infection of milk, is supported by the following table (table 6), which gives the results of tests for the presence of tubercle bacilli in samples of mixed milk from individual herds in the West Riding, by the county borough councils of the principal towns* drawing milk from that area :—
TABLE 6,
The number of samples containing tubercle bacilli obtained from the mixed milk of individual herds situated in the West Riding of Yorkshire sold in certain county boroughs in Yorkshire and Lancashire.
Year.
Total number of samples taken.
Number of samples found
Percentage of samples to contain containing tubercle bacilli. tubercle bacilli.
1923
174
13
7.5
1924
439
21
4.8
1925
426
33
7.8
1926
586
52
8.9
1927
573
33
5.8
1928
729
45
6-2
1929
670
38
5.7
1930
697
31
4.4
1931
955
48
5.0
1932
855
44
5-1
1933
847
44
5.2
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the proportion in which each town contributes to the result fluctuates considerably.* The figures for individual towns are given in appendix 11.
131. The efficacy of routine clinical inspection would be materially increased if adequate use were made of the microscopic and the biological testing of herd milk samples. This is of particular importance where cows with clinical symptoms have been removed from a herd. For this affords some ground for expecting that other animals in the herd may have reached the stage at which their milk is infected, though they do not yet show any recognisable clinical symptoms. The full biological test, which alone can be relied on to show that milk is not infected, takes six weeks to complete. But an earlier positive result may be obtained, at any rate in a majority of cases, either by the use of a microscopic test, which can be applied to bulked samples of milk or by killing the first guinea-pig used in a biological test within six weeks if it shows glandular enlargement. 132. It thus appears from both practical and theoretical considerations that routine veterinary inspection of dairy herds is capable of reducing appreciably the extent to which milk is infected with tuberculosis, but cannot be relied on to reduce infection to really small proportions, though it has not yet been sufficiently developed to show how effective it might be in this direction.
in man.
133. A reduction in the infection of milk with bovine tuber- culosis is not accompanied by a proportional reduction in the infection Those who are highly susceptible to the disease run almost the same risk of contracting it, whether the percentage of infected samples is seven or five or two. Among those, on the other hand, who are highly resistant to it, and who only succumb to the small minority of very highly infected samples, there is likely to be a reduction in disease almost proportional to the reduction in the infection of milk. We are not in a position to predict whether the reduction which would be achieved by means of veterinary
* The percentages of infected samples disclosed in this table are higher than those recently found in samples sold in the administrative county of the West Riding, which were as follows:--
Milk supplied to schools.
Other milk.
Grade A. Ungraded.
Grade A.
Ungraded.
A chief veterinary officer was first appointed in the West Riding in the year 1927 and routine clinical inspection had been extended throughout that area by the year 1929. In interpreting this table it must be borne in mind that the percentage of infection shown depends upon the technique employed in the test and on the size of herds, that these vary from year to year and from town to town, and that
* Barnsley, Bradford, Dewsbury, Doncaster,
Leeds, Rotherham, Sheffield, Wakefield and York in Yorkshire, and Manchester Oldham and Salford in Lancashire.
Huddersfield,
1931 Samples
Positive samples Percentage positive
25 I
126
100
401
6
4
14
4.0
4.7
4.0
3.5
1932 Samples
6
294
140
637
Positive samples
Nil
8
3
23
Percentage positive
Nil
2.7
2.1
3.6
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