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to have information with regard to the prevalence of tuberculous
disease of the udder. Primary udder tuberculosis is extremely
rare, and in one half of the cows found to be tuberculous on slaughter
the disease is generalised in the body. Tuberculous mastitis is much
commoner in older cows than in quite young animals, and it is
possible that the disease is encouraged by the artificial stimulation
of the mammary gland and the unnatural demands made upon it
by milking. The Ministry of Agriculture conclude, from the information
at their disposal, that probably from 1 to 1.5% of cows in
herds in which the disease has been discovered under the operations
of the Tuberculosis Order and the Milk and Dairies Acts, yield
tuberculous milk. But this does not represent the proportion of
tuberculous milk in any area; this will depend very largely upon
the extent to which the milk is mixed or bulked before distribution.
It is calculated that between 6 and 7% of the raw milk sold in the
kingdom contains living tubercle bacilli; it should be noted that
this refers to raw milk and not milk as delivered to the consumer.
The latter has been treated and this will be referred to in a subsequent
paragraph. Of 69,901 unselected samples of raw milk
examined, 4,690 or 6.7% were found to contain living tubercle
bacilli.
Certain estimates have been made of the extent of tubercle
in man due to infection from milk. There are four types of tubercle
bacilli—the cold-blooded, avian, bovine and human which are
distinguishable from each other by certain tests.
Tuberculosis, as it affects man, is due to infection with the
human and bovine types.
The infection of man with the human type occurs mainly
by the respiratory tract.
Infection of man with the bovine type, on the other hand,
occurs almost exclusively by the alimentary canal, and the chief
vehicle is cow's milk.
Since however the type of the infecting organisms can be
determined only by bacteriological methods, it follows that our
knowledge of the extent of bovine infection in man rests solely
on the data accumulated by the examination of specimens of tuberculous
material collected either during life or after death. The
process of 'typing' is arduous, time-consuming and demands considerable
experience. For this reason the type of the infecting
organisms is not usually determined as a routine, and the number
of the determinations that have been made is not very large. It
is necessary to bear this in mind when considering the estimates
that have been based upon them.