While recognizing that in the absence of the tubercle bacillus there can
be no tuberculosis yet we must not ignore the fact that many conditions
of the animal system and its environment contribute largely to the
propagation of the disease on the one hand, or to hinder its progress on
the other. None of these conditions can call the germ into being _de
novo_, but in its presence, they greatly favor its diffusion or even its
malignancy. Like any other seed, this bacillus requires a suitable soil
and favorable climate, to bring out its most destructive development. In
striking the balance, we have to guard against the error of so many, who
would attribute to the germ alone the deadly results and who assume that
these should be the same under all conditions, and the opposite error no
less prevalent, that ascribes the evil to the conditions, and holds that
without these the germ would be harmless.
_Hereditary Predisposition. Racial Vulnerability._ Before the discovery
of the specific bacillus, when as yet tuberculosis was held to result
from a constitutional weakness, or cachexia in which deterioration of
cells was held to be the main factor, the disease was held to be mainly
hereditary, and its every day transmission in the line of descent, and
the increasing mortality to extinction of given families were
confidently appealed to in support of the doctrine. Now, however, we
recognize that congenital tuberculosis in man or beast is very
exceptional, and that the morbid process almost invariably takes its
start from the germ implanted after birth. In Saxony when the
tuberculous cows were 16.5 per cent. tuberculous calves were but .2 per
cent., though the latter had been fattened on the milk of the former. In
Munich but two calves were found tuberculous in 400,000 killed, and in
Lyons but five in a similar number. Up to the present the number of
calves recorded as tuberculous at birth does not exceed seventy.
That the young almost always contracted the disease, only after birth,
virtually disposes of the alleged heredity of the tuberculosis but it by
no means antagonizes the heredity of the racial vulnerability. As man,
cattle, swine and Guinea pigs show a much greater vulnerability than the
carnivora in general, so certain families in each of these genera show a
more decided susceptibility to tuberculosis under similar conditions
than do certain other families. This goes far to explain the appearance
of tuberculosis, in certain lines of blood, and its advance to the
extinction of the family, while under no better environment, other
families can count on a practical immunity. In the Burden herd of
Jerseys in 1877, I condemned eleven animals, verifying my diagnosis by
necropsies, and found to my surprise that I had taken every
representative, even the grades, of a given family, and left all the
pure bred members of a second family untouched. Both families had
mingled freely in the pastures and yards, yet the second family
furnished no cases of tuberculosis then nor for many years afterward.
The case is all the more striking that the non-tuberculous family gave
the largest yield of milk and might have been expected to run down
rather than the other on account of this drain.
_Close Buildings. Lack of Ventilation._ Air rendered impure by being
breathed again and again, predisposes strongly to tuberculosis, and has
been even looked upon as the sole cause (Macormac). Everywhere city
dairy cows, kept in confined, close buildings, suffer severely (6 to 20
per cent. and upward); in Berlin 75 per cent. (Ostertag); in Denmark 61
per cent. (Bang), while in the same districts country cows are
comparatively immune (often 1 to 2 per cent.), and steers raised in the
open air still less (0.02 per cent. for our Plains cattle). For the
slighter cases of tuberculosis in man and beast, life in a pure open
air, day and night, in a genial climate, gives the best hope of
improvement or recovery. In the Burden Jersey herd above referred to,
animals condemned in spring as tuberculous, were turned out to pasture
during the summer where they maintained an appearance of robust health,
yet when returned to the barns in fall they fell off rapidly, so that
some had to be helped to rise in the stall. “The stabled cow, the tame
rabbit, the monkey, the caged lion, tiger or elephant are almost
invariably cut off by scrofulous affection” (Aitken). It has long been
noticed that sailors sleeping in close spaces (Bryson, Parkes), soldiers
confined to close stone barracks even in the tropics (Parkes), suffer
much more than the officers in more spacious rooms (Clark). Monks and
nuns (two-thirds of the deaths, Leudet) occupying confined cells, and
the inmates of prisons (four times the average outside, Baer) have shown
an extraordinary prevalence of tuberculosis and attendant mortality.
While this can be attributed mainly to the preservation and
concentration of the bacillus in such places, a considerable allowance
must be made for the impure and rebreathed air.
_Dark, Foul, Damp Stables._ Dark stables are usually close, dirty and
damp as well, and all these conditions alike conduce to tuberculosis.
Darkness hinders the development of organic coloring matter in living
bodies, whether chlorophyll in plants or hæmoglobin in the blood of
animals. Hæmoglobin is the main oxygen carrier in the blood, and in case
of its deficiency the tissues are not properly ærated. The result is as
if the inhaled air contained little oxygen, so that darkness further
intensifies the evil of rebreathed, deoxygenated air. The extraordinary
mortality from tubercle among prisoners, monks, nuns and miners serves
to further accentuate this conclusion.
Trudeau’s experiment with rabbits is instructive in this connection. Of
a number inoculated with the same number of tubercle bacilli, one half
were kept in the open air and the rest in a dark, damp, underground
place deprived of sunlight. When killed on the same day, it was found
that the open air rabbits showed only slight lesions or none, while the
underground lot had extensive tuberculosis.
The impure stable air lowers the vital tone of the animal, especially if
the impurity has come from animal exhalations.
The same is true of damp air, “a damp ship is an unhealthy one,” and
Bowditch and Buchanan pointed out the especial prevalence of tubercle in
cold, damp, undrained, foggy localities. This does not hold for all damp
lands, yet damp, fog, and cold are especially hurtful in presence of the
germ.