lymph nodes. The bacteria are engulfed by macrophages and are then
isolated in nodules called tubercles or granulomas which are mainly
composed of a dense accumulation of activated macrophages and
lymphocytes. For many people this is as far as the infection proceeds,
the development of the tubercle is checked by surrounding it with a
fibrous wall and it then calcifies to a yellow gritty mass. In others,
however, illness ensues when the tubercle liquefies causing local tissue
necrosis and releasing the bacteria to spread infection around the body.
7.10.4 Isolation and Identification
Tuberculous lesions can be identified in meat animals by post-mortem
inspection of carcasses but disease can also be identified in the live animal
(and humans) using the tuberculin test. In this, the animal exhibits
delayed hypersensitivity to injection of tuberculin, a protein preparation
fromMyco. bovis. In clinical specimens mycobacteria can be identified
directly on the basis of their acid-fast reaction in the Ziehl–Neelson stain
when the organisms appear red and the surrounding tissue blue. The
organisms can be cultured on simple media but are very slow growing.
7.10.5 Association with Foods
In 1900 at the London Congress on Tuberculosis, Robert Koch caused
consternation when he concluded that the risk of transmission of bovine
tuberculosis to humans was so slight that he did not deem it advisable to
take any measures against it. The impact of this from the world’s leading
bacteriologist and discoverer of the tubercle bacillus can only be imagined
- especially on John McFadyean who was due to talk on the same subject
two days later! Nonetheless, when his turn came he felt compelled to
‘offer some criticism on the pronouncement of one, the latchet of whose
shoes I am not worthy to unloose’. He pointed out that in post-mortem
examinations of hundreds of children in London and Edinburgh, primary
infection appeared to have occurred through the intestines in approxi-
mately 28% of cases, and that 2% of all cows in Britain had tuberculosis
of the udder and were excreting the bacillus in their milk.
Pooling of milk increased the incidence ofMyco.bovisso that in the
1920s and 1930s the organism could be isolated from 5–12% of milk
samples and the high rates of TB in children due toMyco. boviswere
attributed to the consumption of unpasteurized milk. In North America,
compulsory pasteurization regulations were introduced in a number of
the large cities from about 1910 and had a marked effect reducing the
incidence of bovine TB in children. In the UK there was considerable
resistance to the introduction of milk pasteurization, but the available
evidence suggests that its later and more gradual introduction had a
Chapter 7 233