in
the blood were living organisms and the cause of the disease, this did
not explain the mode of infection; how the organisms contained in the
blood passed to another animal, why the disease occurred on certain
farms and the adjoining farms, particularly if they lay higher, were
free. Koch showed that in the cultures the organisms grew out into
long interlacing threads, and that in these threads spores which were
very difficult to destroy developed at intervals; that the organisms
grew easily in bouillon, in milk, in blood, and even in an infusion of
hay made by soaking this in water. This explained, what had been an
enigma before, how the fields became sources of infection. The
infection did not spread from animal to animal by contact, but
infection took place from eating grass or hay which contained either
the bacilli or their spores. When a dead animal was skinned on the
field, the bacilli contained in the blood escaped and became mingled
with the various fluids which flowed from the body and in which they
grew and developed spores. It was shown by Pasteur that even when a
carcass was buried the earthworms brought spores developed in the body
to the surface and deposited them in their casts, and in this way also
the fields became infected. From such a spot of infected earth the
spores could be washed by the rains over greater areas and would find
opportunity to develop further and form new spores in puddles of water
left on the fields, which became a culture medium by the soaking of
the dead grass. The contamination of the fields was also brought about
by spreading over them the accumulations of stable manure which
contained the discharges of the sick cattle. The tendency of the
disease to extend to lower-lying adjacent fields was due to the spores
being washed from the upper fields to the lower by the spring
freshets. Meanwhile Pasteur had discovered that by growing the
organisms at higher temperatures than the animal body, it was possible
to attenuate the virulence of the bacilli so that inoculations with
these produced a mild form of the disease which rendered the
inoculated animals immune to the fatal disease. The description of
Pasteur's work on the disease as given in the account of his life by
his son-in-law is fascinating.
Hides and wool taken from dead animals invariably contained the spores
which could pass unharmed through some of the curing processes, and
were responsible for some of the cases in man.
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