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52
undoubtedly be stated that our knowledge of the subject at the present time is insufficient to justify
the introduction into London of a system of compulsory segregation.
The view that pulmonary phthisis is necessarily the result of inhalation of tubercle bacilli
derived from the sputum of persons suffering from this disease was a natural inference from the
frequency with which the lungs appeared to be primarily infected and from the finding of the bacillus
in the sputum. In children, where the abdomen was the frequent site of the disease, it was recognised
that the method of infection was in all probability by ingestion, but the relative infrequency of
local lesions of the intestine led to the opinion that this method was comparatively rare. There is now
much reason for re-casting the views which previously prevailed on these questions, and for recognising
that the lungs may be infected by ingested bacilli without the occurrence of local lesions of the
intestine.
There is no doubt that the relative frequency with which the disease invades the lungs of adults
has also had its influence in determining the view that pulmonary phthisis is necessarily of human
origin. Indeed, in his address in London in 1901, Professor Koch said: "In far the majority of cases
of tuberculosis the disease has its seat in the lungs and has also begun there. From this fact it is
justly concluded that the germs must have got into the lungs by inhalation."
A further reason which had led to the conclusion that tuberculosis in man was almost entirely
derived from human sources has been the difference observed between the tubercle bacilli of man and of
the lower animals, diSerences which are morphological and cultural, and differences in the ability with
which they can be inoculated into other animals.
It is upon this question, the amount of risk to man from bovine tuberculosis, upon which the
Royal Commission on Tuberculosis throw light in their second interim report of January, 1907.
The Royal Commission studied the nature of the tuberculous material obtained from 60 cases
of the disease in man. They state that their results led them to divide these cases into two groups,
very clearly distinguished from each other by the properties and characters of the bacillus found in each.
Nearly all the cases fall readily into one or the other group, but a few presented exceptional features.
Group 1 contained 14 cases, and the viruses were obtained from sputum, from tuberculous cervical
glands, and from the lesions of cases of primary abdominal tuberculosis. These were found to produce
in bovine animals, rabbits, and other animals, effects which appeared to be absolutely identical with the
effects of the bacillus of bovine tuberculosis, and the Commission state that they have wholly failed
to discover any essential differences between the one and the other; both are equally virulent, i.e., equally
able to set up tuberculosis in bovine and other animals.
Group II was made up of 40 cases and the material was derived from cervical, and mesenteric
glands, from lungs, bronchial glands, tuberculous kidney, testis, joints and bones. The bacillus of this
group produced eSects which presented a marked contrast to those produced by group I. It was much
less virulent, and this lesser virulence manifested itself in a number of ways-, but, "in all its essential
features the disease set up by the bacillus of bovine tuberculosis is identical with that set up by the
bacillus of group II; such diSerences as are met with are differences of amount and phase only."
The essential diSerences between Group I and Group II were that the former manifested more
virulence and grew with greater difficulty in artificial media, and in this way resembled bacilli of
bovine origin; the latter manifested less virulence and less difficulty in growing in artificial media,
and in this way resembled bacilli of human origin.
A third group, however, Group III, diSered from those in the other groups, and interest in the
future will chiefly centre in the bacilli of this group, for they behaved diSerently from those in the other
groups in certain important particulars. In the cases included in this Group, and in cases of feeding
with human sputum, the Commission say they met with—
(a) Bacilli of a character intermediate between Group I and Group II.
(b) Replacement of a eugonic* bacillus of low virulence by a dysgonic* bacillus of high
virulence.
(c) Indications of instability in virulence and cultural characters and want of
accordance between virulence and cultural characters.
The Commission say that the results may be explained on the supposition that there was a mixture
of viruses, of a virus from a human source and a virus from a bovine source, each possessing stable
characters, but they say they have met with no direct evidence of the simultaneous presence of bacilli
of Groups I and II; on the other hand if it be assumed that the characters of the bacillus of tuberculosis
(whether human or bovine) are not always absolutely stable the following alternative view may
be adopted : "We may suppose that in the case of each of the passage experiments, the original
"material contained only slightly virulent human bacilli, but that these were in a special condition
"of instability; so that when subjected to certain influences they become modified in character,
"and transformed into highly virulent dysgonic bovine bacilli, these influences being supplied by the
"tissues of the bovine animals, through the bodies of which the bacilli were passed. Similarly,
"we may suppose that highly virulent bacilli from a bovine source, lodged in the human body,
"may, under certain conditions, manifest instability; may, under the influence of human tissues,
"become modified in character, and so may be transformed into bacilli possessing all the features of
"the bacilli of Group II."
They point out that, "should it be proved that the cases in question were due to an admixture
with the bacilli of human source of a few bacilli of bovine source, the two kinds always remaining
* The Commission found it desirable to adopt the words dysgonic and eugonic to denote respectively that the
bacillus grows with difficulty or with readiness on a medium or on several media.