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Report of the Medical Officer of Health.
115
III.—Report by Dr. R. A. O'Brien (of the Lister Institute) on the Results of an Investigation
into the Pathological Significance of an Infectious Quality in Certain Samples of
Milk.
The period covered by this report is a little more than twelve months from September, 1909,
the total number of samples during this period being 2,520; but the figures given in some of the tables
are derived from the whole material available in the records of the routine examination of milks for
the presence of tubercle bacilli from August, 1908, to October, 1910.
Details of the methods used in the examination of samples of milk for the presence of tubercle bacilli.
On two days, or in some cases three days, in each week, the samples of milk in lots of 24 or
30 reach the laboratory during the afternoon. In a centrifuge running at about 4,000 revolutions
per minute for a period of about twenty minutes, a quantity of 160 cubic centimetres from each sample
is spun. After removal of the cream and the supernatant fluid, the deposit adhering to the bottom
of the centrifuge glass is emulsified in eight to ten c.c. of sterile salt solution, and the emulsion pipetted
into a sterile conical test-glass. The emulsion so prepared is injected subcutaneously on the inner
side of the thigh into two guinea-pigs. The two animals corresponding to each sample are kept for
28 days in a metal cage, previously boiled in a lysol solution. During this period a certain number
of the animals die, some few showing signs of tuberculosis. These latter are not dealt with in this
report, which is concerned only with deaths from causes other than tuberculosis—the so-called "nontubercular
mortality."
On the 28th day the surviving animals are killed and examined for the presence of tuberculosis.
Extent of the "non-tubercular mortality" amongst guinea-pigs inoculated with sediment of milk samples.
Table showing Total Mortality from causes other than Tuberculosis.
(i.) Number of cases in which both guinea-pigs inoculated with the deposit from a sample of
milk, died in less than 28 days, from causes other than tuberculosis.

(ii.) Number of cases in which one of the two guinea-pigs inoculated with the deposit from each
sample of milk died before the 28th day=l,597.
(iii.) Total number of guinea-pigs dying, "non-tubercular deaths" =(2 × 736) + 1,597 = 3,069.
Total number inoculated = 9,016.
3,069 represents 33 per cent. of the total number.
Of the above 3,069, 1,472 represents the animals from 736 samples, all of which, owing to death
of both guinea-pigs before the 21st day, had to be collected a second time ("repeat" samples), since
the possibility of tubercle bacilli being present in these samples could not be definitely excluded.
Investigation into the actual causes of death of the inoculated animals.
It is obvious that an enquiry of this nature is complicated by the great number of factors
involved, the inter-relation of which it is not easy to define. The guinea-pig itself is not a constant
factor, being subject to natural diseases of its own. For instance, in November, 1909, a severe
epizootic occurred amongst the stock guinea-pigs at the Institute and affected some of the inoculated
"milk guinea-pigs," greatly increasing the mortality amongst them. Further, the sediment injected
introduces a complexity of factors, since it consists of a mass of foreign proteid and other bodies in
the centrifugalised cells and deposits from the milk, as well as a varying number of various kinds of
organisms.
With the exception of a few instances in which the animal was examined after being killed on
the 28th day, the guinea-pigs dying each day were opened and examined. The maximum period
elapsing between the death and the examination of any given animal was thus usually about 18 to 20
hours.
Post-mortem appearances.—Inflammatory exudates and hæmorrhagic conditions at the site of
inoculation were found in the majority of animals dying during the first week, with occasional con
gestion of the spleen, liver and suprarenal capsules. In those dying after the first week, these conditions
were less often met with, though in a certain number of cases abscesses at the site of inoculation were
found when the survivors were killed on the 28th day. Gas was never found in the subcutaneous tissue.
During the winter of 1909-10—the "epizootic period" (see p. 119)—there were frequently found
in the liver and spleen, hard round whitish-yellow nodules or points varying in size from tiny spots
to nodes as large as a pea. The smaller spots in the liver and spleen were found mostly in cases where
the B. aertryck was recovered in pure culture. The larger nodes in liver and spleen were found most
often in association with enlarged mesenteric glands which on section were partly grumous or purulent.
From these cases the B. pseudo-tuberculosis rodentium was almost always recovered.