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Report of the Medical Officer of Health.
119
The lethal effect of different samples of milk.—A study was made of the records dealing with the
number of animals dying on each of the first seven days after inoculation, and in each of the subsequent
weeks, expressed as percentages of the total deaths. This study did not reveal any data of especial
significance. Similarly there was no constancy in the period lived, even by those animals from which
the same bacillus was recovered.
The contribution of naturally occurring diseases amongst the guinea-pigs to the observed mortality.—
A considerable proportion of the guinea-pigs upon the market are already infected with the germs of
one or other of the diseases to which they are subject. In experimental work it is always advisable
to submit the animals to a prolonged period of quarantine, but the serious shortage in the supply last
year occasioned by the greatly increased demand for the work of public health authorities practically
rendered this precaution impossible. Further, even when this segregation of animals destined for
experiments has been carried out, some epizootic has broken out among them, due to the accidental
introduction of an animal already suffering fom some disease in a latent form. An unfortunate instance
of this sort occurred in November and December, 1909, when the segregated stock was smitten and
nearly decimated by a disease associated (v) with the presence in the tissues of the animals affected of
B. aertryck (=B. suipestifer). This organism is not uncommonly found in the intestine of the healthy
pig and has been held responsible for outbreaks of food poisoning.
Some of the animals used for the testing of milk had been taken from the stock just before the
epizootic was discovered and the disease broke out amongst them, and spread to their companions.
B. aertryck was never discovered in the milks injected, so that the inoculations were clearly not
responsible for the high mortality at this period. B. aertryck vel suipestifer was also obtained 21 times
post-mortem from the organs of guinea-pigs inoculated with milk between March and August, 1910,
that is, during a period when no epizootic was present, and when the general non-tubercular
mortality was low. In these 21 instances, though the two animals inoculated from each milk sample
lived together in one pail, there were only two cases where the other animal died before the 28th day.
The avilable evidence goes to show that this bacillus is an occasional inhabitant of healthy
guinea-pigs' intestines, and is under normal circumstances not pathogenic. Should the animal suffer
from any condition which lowers its resistance, e.g., the filter-passer referred to in the footnote below (v),
or the injection of milk sediment (see p. 118), the bacillus migrates from the intestine into the organs
and produces in some cases a septicaemia. The occurence of a septicaemia probably explains the finding
of the bacillus in the local lesion in two cases.
Another common disease of guinea-pigs, often of a chronic character, is due to B. pseudotuberculosis
rodentium. During the first three months of 1910 the B. pseudo-tuberculosis redentium
was frequently obtained from the dead guinea-pigs ; it was also encountered occasionally up to June.
This bacillus gives rise to a chronic disease in guinea-pigs, but occasional outbreaks of a more acute
form are not uncommon. It is evident that the guinea pigs infected with the strains obtained during
this enquiry did not readily communicate the infection to contacts, since in the 30 instances in which
the bacillus was present, the companion animal in the cage was not affected by the bacillus, or at least
not fatally, for in all cases it survived until the 28th day.
Pathogenicity.—The strains isolated when injected subcutaneously or intraperitoneally were
moderately lethal, 3 c.c. of a 24-hour broth culture killing a guinea-pig in 4 to 10 days. Only two
feeding experiments were carried out, each of two guinea-pigs being fed on three successive days with
an emulsified agar slope culture. One animal died in 10 days with no pathological appearances postmortem;
the other died a month later with "psuedo-tuberculosis" in the organs, and from the
mesenteric glands the bacillus of this disease was recovered.
This bacillus was not found in the 111 samples of milk examined culturally during this investigation,
nor has it, as far as I can find, been obtained by direct cultivation from milk by other
observers.
Consideration of charts showing mortality, temperature, &c.—A statistical survey of the facts bearing
on the mortality was made and the data charted. A detailed chart (not reproduced) was prepared, in
order to see if any correlation existed between temperature or season of the year, and the "non-tubercular"
daily mortality, or the mortality amongst each batch of animals inoculated. The mortality
curve did not show any close relation to the seasonal curve of temperature, even after making allowance
for the epizootic, which occurred in November and December, 1909, and was responsible for the unusually
high death-rate from causes other than tuberculosis.
In Chart I. the percentages of different batches of samples, one hundredth of a cubic centimetre
or less of which fermented glucose bile salt broth, are contrasted with the mortality resulting from
the inoculation of those consignments of samples.
The "fermentation percentages" curve follows the temperature curve fairly closely, the bacterial
(v) For a time it seemed probable that this organism, which was highly pathogenic for guinea-pigs by subcutaneous
injection, was actually responsible for the epizootic amongst the stock animals and also for the deaths amongst
the inoculated "milk guinea-pigs." However, experiments by Petrie and O'Brien (Journal of Hygiene, 1910, p. 287),
made to determine the mode of transmission of the bacillus, showed that it was of low pathogenicity when given with
food and that the disease did not spread from one animal to another, when one of them was fed upon the bacillus,
although kept together in close contact. This result was in so striking contradiction to what occurred in the natural
epizootic, where 90 per cent. of the animals died, that it suggested an analogy with swine fever, in which disease the same
bacillus is often found, but which disease has been proved to be due not to this bacillus but to an invisible virus
which passes through a porcelain filter. The authors found that sterile filtrates of organs of guinea-pigs dying amongst
the infected stock, when administered by different methods, frequently killed healthy experimental animals. They
therefore conclude that B. aertryck was not the essential infecting agent in the epizootic, but that the disease was caused
by a virus possessing similar characteristics to those of swine fever.