NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
56
more bacilli. Each day a number of the fleas was dissected and
the stomach contents examined as to the presence or absence
of plague bacilli. In a certain proportion abundant germs were
found up to the 12th day and in one instance on the 20th day. We
have good evidence in this observation that multiplication of plague
bacilli may take place in the flea's stomach, for, if there had been
no multiplication, the bacilli-laden blood imbibed originally would
soon have been diluted to a marked degree and the bacilli washed out
by the blood taken in at each feed on the healthy animals.
The next point to determine was the approximate proportion
of fleas in the stomachs of which multiplication of plague bacilli
takes place. This proportion was found to vary with the season
of the year, being six times greater in the epidemic season than in
the non-epidemic season. In the epidemic season the percentage
of infected fleas was greatest for the first four days (43 per cent.)
after removal from the plague rat, but a certain number were found
with bacilli in their stomachs up to the 12th day ; thus, on the 6th
day there were 15 per cent., on the 8th day 16 per cent, and on
the 12th day 9 per cent. On one occasion the stomach contents
of a 20th day flea were found full of plague bacilli. In the non-
epidemic season no flea was found with plague bacilli in its
stomach after the 7th day and of those examined during the first
six days only 5 2 per cent. were infected.
It was important now to determine the duration of infectivity
of rat-fleas taken from plague rats when tested on susceptible ani-
mals. This was done by feeding fleas, which had imbibed blood
containing plague bacilli, on healthy guinea-pigs, a fresh animal
being supplied each day, until all the fleas had died. After being
in contact with the fleas for 24 hours each guinea-pig was carefully
cleared of fleas and segregated. In this way several series of experi-
ments were done with the following results :—One series done
during the epidemic plague season and in which a limited number
of fleas were used showed that, in these circumstances, fleas
could remain infective for at least 10 days. A second series, also
carried out during the epidemic season, but in the presence of a
very large number of fleas, gave the time that fleas might remain
infective as 15 days. In a third series of experiments, conducted
under the same conditions as the second series but during the non-
plague season, the fleas remained infective for only seven days and,
57
further, far fewer (one-third instead of two-thirds) animals than in
the second series contracted the disease. It is to be remembered
that in these experiments the fleas were fed throughout. Starved
fleas, even when placed under the most favourable conditions, never
survive longer than a week.
The next questions which suggested themselves were, (1) are
the bacilli to be found anywhere in the body of the flea outside the
stomach ? and (2), how do the bacilli leave the flea's body ? The
blood after the end of the digestive process in the stomach passes
into the rectum of the flea as a thick, slimy, dark-red mass and ap-
pears at the anus as minute, dark-red or black, tarry droplets. It is
easy to demonstrate by dissection and examination that the
rectal contents are often crowded with plague bacilli, and
it has been repeatedly shown, both by microscopical examination
and by animal tests, that the fæces may contain abundant and
virulent plague bacilli.
On the other hand, a very large number of fleas, which had
come off plague rats at intervals varying from a few hours to several
days, have been dissected and the various parts of the body
examined for the presence of bacilli. On rare occasions a few
bacilli have been seen in the gullet but only in these cases in which
the flea has been killed immediately after feeding. As regards the
other regions of the body, in not a single instance have any
plague bacilli been observed outside of the organs already men-
tioned. No infection of the body cavity has been seen and,
although particular attention was paid to the salivary glands,
nothing at all resembling a plague bacillus has ever been detected
in them. It would appear, then, that nowhere outside the stomach
and the alimentary canal posterior to the stomach are plague
bacilli to be found in any numbers and that they leave the body
chiefly, if not entirely, along with the fæces.
These observations have a distinct and important bearing on
the next question which we have to consider, namely, the question
of how the flea transmits its infection to the healthy animal. It
is evident that several methods of transmission are possible, such
as (a) the animal eating the infected fleas ; (b) the mechanical con-
veyance of the bacilli by the pricker; (c) the injection of the bacilli
along with the saliva as the result of an infection of the salivary
glands ; (d) the regurgitation of the stomach contents or the retention
58
of the infected blood about the mouth parts or in the pharynx,
the bacilli being then injected with the saliva or carried down on the
pricker or rubbed into the wounds made by the pricker; (e) the depo
sition of the infected fæces on the skin, the bacilli being then in
jected by the pricker or rubbed into the wounds made by the pricker.
After careful consideration of all the available evidence bear-
ing on the subject the Commission were of opinion that
it pointed to the conclusion that the bacilli after multiplying
in the stomach leave the body of the flea with the fæces and then
become injected into the healthy animal either by being carried in
by the pricker or by being rubbed into the pricker wounds. In
addition to the evidence already mentioned pointing to this con-
clusion we may mention the fact, that the flea while sucking has a
habit of squirting blood per anum. Further, experiments have
shown that the wounds made by the pricker afford a sufficient
avenue for the entrance of the bacilli, even when no rubbing is re-
sorted to. While all the evidence points to the inoculation of the
fleas' fæces being the method of infection, the possibility of infection
by contaminated mouth parts or by regurgitation from the stomach
cannot be altogether excluded, although no evidence could be
obtained in favour of either of these methods. Infection by the
eating of infected fleas and by infected saliva can be almost
certainly excluded.
(b) Whether a single rat-flea can transmit the infection.
Rat-fleas were taken off plague-infected rats after the death of
the latter. They were then transferred to healthy guinea-pigs in
clean flea-proof cages, one flea being added to each cage.
Sixty-seven experiments were made in this manner, but only in
one instance was there a successful transference of the disease.
As we can estimate that about half of these 67 fleas contained
plague bacilli in their stomachs, it would appear that the chances
of an animal contracting plague after being bitten by a single
infected flea are remote.
(c) Whether both male and female rat-fleas can transmit the
infection.
Observations were made to ascertain if both the male and
female P. cheopis could transmit plague from animal to animal.
59
Six experiments were carried out with each sex, with result that
one animal in each series be came plague infected.
(d) Whether any other species of flea, as well as P. cheopis,
can transmit the infection.
Three other species of flea were available for these observations,
namely, Ceratophyllus fasciatus, Pulex felis and Pulex irritans.
1. EXPERIMENTS WITH Ceratophyllus fasciatus.
This is the common rat-flea of Northern Europe. It is not found
at all in Bombay, but in the cold weather was taken on rats in the
Punjab to the extent of 2 per cent. On account of its scarcity
and because the experiments were made towards the end of the
observations at Dhand and Kasel, only two experiments were com-
pleted with this flea. Both these experiments were successful,
showing definitely that Ceratophyllus fasciatus can convey plague
from infected to healthy animals.
2. EXPERIMENTS WITH Pulex felis.
This is the common flea found on cats and dogs. It was found
not to live well on guinea-pigs and rats. Transmission experiments
with it were carried out by feeding the fleas first on Bombay rats
infected with plague and on the death of the latter transferring
them to fresh guinea-pigs in clean flea-proof cages. Although 27
experiments were made in this way, not a single successful transmis-
sion was obtained. It should be added that these experiments were
made during the height of the plague epidemic in Bombay.
3. EXPERIMENTS WITH Pulex irritans.
Experiments with this flea, the common species found on man,
were carried out in the same manner as those with cat fleas. Out
of 38 experiments three of the guinea-pigs became plague infected.
This small number of successes may be explained by the fact that
this flea does not live well either on rats or on guinea pigs. It was
found that, if kept on rats, only 1 2 per cent. of the number originally
put on could be recovered after 24 hours and that after 72 hours
this was further reduced to 1 per cent.
61
PART IV.—RELATION OF THE HUMAN PLAGUE CASE TO
THE SPREAD OP THE EPIDEMIC.
A.—Introduction.
B.—Experience in hospitals.
C.—Influence of imported cases on the spread of the epidemic.
D.—Previous history of plague cases as regards contact with
other plague cases.
E.—Relative frequency of the occurrence of single and multiple
cases of plague in houses and buildings.
A.—INTRODUCTION.
Considering that we have already arrived at the conclusion
that the plague epidemic is solely attributable to the rat epizootic
it may seem superfluous further to examine the question as to
the part played by a human plague case in the spread of the
epidemic. This question is, however, of so much importance
to the sanitarian, who is called upon to devise prophylactic
measures, that we propose now to gather together all the
evidence on the subject which is available, in order that such a
definite conclusion may be arrived at as to enable the administrator
to act upon it with confidence.
In the following remarks it is to be understood that pneumonic
plague cases are excluded. The contagiousness of this class of case
and the mode of transmission of the infection have never been
disputed ; but on account of their rarity pneumonic cases can play
only a minor part in the spread of the epidemic.
A little consideration will make it evident that an ordinary plague
case can only be infectious, in so far as the excreta are in-
fectious or in so far as the infection is carried by means of a suctorial
insect such as the human flea. We have already brought forward
experimental evidence to show that the excreta of plague cases, as
well as of plague rats, are not infective when the methods of
inoculation practised are such as would occur in nature. As
regards the human flea being an agent in the conveyance of the
disease, it cannot be denied that this flea can transmit the infection
62
from animal to animal in the laboratory. Other experimental data,
however, such as, the comparatively small number of plague bacilli
present in the blood of plague cases, are against this hypothesis,
so that to conclude off-hand from the transmission experiments
done in the laboratory that such a method of transmission from
case to case takes place in nature would be extremely dangerous
and, as we shall see later, would lead us into grievous error. We
do not now intend to enter again into the experimental evidence,
valuable as it is, which is opposed to the belief that a plague case
is infectious. We propose, however, to take up the question
from the epidemiological aspect, drawing our evidence from
various sources and only referring to experimental data when
necessary.
B.—.EXPERIENCE IN HOSPITALS.
It has long been observed in India that the attendants in a
plague hospital remain singularly free from infection, although
they are brought frequently into intimate contact with patients in
the acute stage of the disease. So well is this fact recognised that
it has become a saying " that the safest place in a plague epidemic
is a hospital." The Commission's experience in the plague hospitals
of Bombay, as well as in the homes where cases were treated,
forced the members to the same conclusion.
In order, however, further to investigate the point, experiments
of three different kinds were made—
(1) Two guinea.-pigs were allowed to run free in the acute
plague ward of the Maratha hospital, Bombay. Although kept in
the hospital for one week these animals remained healthy.
(2) Bedding recently soiled by the excreta of acute cases just
before death was obtained daily from the hospital and put into a
confined space, namely, godown No. 5, along with 15 guinea-pigs.
Each lot of bedding was kept in the godown for 24 hours, but
the same guinea-pigs were exposed throughout. Although the
experiment was continued for several weeks and in the intimate
contact of a confined space, a contact, which was closer and
more prolonged than any that could possibly happen in the case
of man, none of the guinea-pigs contracted the disease.
63
(3) We have already stated that neither the urine nor
fæces of acute plague cases infect guinea-pigs when rubbed into
abrasions on the skin.
C.—INFLUENCE OF IMPORTED CASES ON THE SPREAD OF
THE EPIDEMIC.
When we come to deal with the subject of importation of plague
into localities hitherto uninfected, we shall see that imported plague
cases are in many instances seemingly the starting point of the
epidemic. Now, we have already arrived at the conclusion that
the epidemic is wholly dependent on the rat epizootic, so that
it follows that there must be a connection between the imported
case and the start of the epizootic. What this connection is
we shall see later on. At present, however, we wish to bring
forward some observations of the Commission in regard to
the fate of the contacts of plague cases imported into the villages
in Bombay and in the Punjab, when there was no evidence of a rat
epizootic resulting.
(1) Out of 28 cases which occurred in Parel village 17 were
undoubtedly imported, having acquired their infection in Bombay.
In the houses in which these cases were treated a large number
of individuals were living in the same rooms as the sick, but in
no instance did any of the healthy become infected.
(2) In the case of Wadhala village early in the year two separate
cases were imported from Bombay City. None of the inhabitants
of the houses in which these cases lived contracted the disease
and there was no extension of the infection in the village.
(3) During the year that Worli village was under observation
three cases of plague occurred. There is no doubt that all these
cases were infected outside the village. Again the contacts remained
healthy and there was no extension of the disease.
Without entering into details we may say that similar observa-
tions were made in the Punjab.
Before leaving this question we would like to make it clearly
understood that while these observations appear to show that
an imported plague case is of no danger per se to those with
whom it comes in contact, it by no means follows that a plague
case may not import with it something, e.g., infected rat-fleas-
64
which, may infect the indigenous rats and thus be the starting point
of the rat epizootic and, in consequence, indirectly, of the
epidemic.
D.—PREVIOUS HISTORY OF PLAGUE OASES AS REGARDS CONTACT
WITH OTHER PLAGUE CASES.
In Bombay City it was absolutely impossible to obtain any
trustworthy information on this point. In the Punjab villages,
however, in the great majority of cases accurate information
was obtained. In Dhund there were 32 cases: 25 of these
(80 per cent.), it was definitely ascertained, did not prior to
attack come in contact with other plague cases. The remaining
7 cases had come in contact with plague cases, so the
possibility of their having been directly infected from the latter
cannot be excluded. In Kasel out of a total of 75 cases it was
ascertained that 53 (70 per cent.) did not prior to attack come in
contact, with other plague cases; that 11 cases (15 per cent.) had
been in contact with cases prior to becoming ill and that with
regard to the remaining 11 cases (15 per cent.) it could not be
determined whether or not such contact had taken place.
In these two villages, therefore, the great majority of cases
did not receive their infection from other plague cases.
Finally, it will be remembered that in the village of Sion Koli-
wada on the evacuation of the inhabitants a guinea-pig population
was put into the empty houses. A severe epizootic spread through
these animals, although there was no chance of contact in any
instance with a sick animal.
E.—RELATIVE FREQUENCY OF THE OCCURRENCE OF SINGLE AND OF
MULTIPLE PLAGUE CASES IN HOUSES AND BUILDINGS.
If plague were an infectious disease, it would certainly happen
that multiple cases in a house would be the rule and not the excep-
tion. The observations of the Commission throw much light
on this point.
(1) In Dhund the 32 cases inhabited 26 houses : of these houses
one furnished three cases, four furnished two cases each and the
remaining 21 houses a single case each. That is to say, 70 per
65
cent. of the cases were single cases. All the cases were treated in
their own houses throughout their illness. In the 21 houses
which furnished single cases the number of contacts was 86,
all of whom came into intimate relation with the patients.
None of these contacts contracted the disease.
(2) In Kasel the 75 cases inhabited 67 houses. One house
furnished three cases, six houses furnished two cases each and 60
houses a single case each. That is to say, 80 per cent. of the
cases were single cases. In the houses which furnished single
cases the number of contacts was 273, of whom 18 had been
inoculated against plague. In other words, out of 255 presum-
ably susceptible persons who came into contact with plague cases
not one was attacked.
Further, both in Dhund and Kasel it was observed that when
multiple cases did occur in a house they were attacked practically
simultaneously, as if from a common source of infection.
(3) In Parel village there were 28 cases, 17 of which were
imported. Of these cases 20 (71 per cent.) were single cases, the
remaining eight being divided amongst four houses, two in each.
In all the houses where cases occurred a number of inhabitants
were living in the same room as the sick. In no instance was
there any evidence to show that the sick communicated the
disease to their healthy attendants or friends.
(4) In Bombay City the experience of the Commission showed
that it is comparatively rare to find two or more cases in a
house. Attention was specially directed to this point, because
throughout the epidemics of 1906 and 1907 a continual endeavour
was made to find instances of this kind with the purpose of using
such houses for guinea-pig experiments. The number of houses
suitable for this purpose was very small when compared with
the total number of plague houses.
A large mass of data relating to this question was ob-
tained from the municipal records for four epidemics, 1903-1906.
The records relating to the incidence of plague for these years
in all the buildings in 10 typical sections of the city were
analysed and statistics under the following headings compiled —
(a) Mean number of inhabitants per building.
(b) Mean number of tenements per building.
19 S C
66
(c) Mean number of plague cases per building in each
epidemic.
(d) Percentage of buildings which yielded one case, two
cases, three cases, etc., in each epidemic.
The results of a study of these statistics are very striking.
Thus, it was found that although the mean number of
inhabitants per building is as high as from 27 to 61 in the
different sections and the mean number of tenements per
building is between 4·5 and 9·1, that is to say, the buildings are
of considerable size, the mean number of plague cases per build-
ing in the epidemic season is very low, never, indeed, rising to 3
per building, and in the different sections varying between 1·5 and
2·9. In the non-plague season the mean number of cases per build-
ing was never in any section higher than 2 and was generally
between 1 and 1·5.
This low mean number of cases per building, when considered
in the light of the fact that the buildings on the whole have a large
population distributed as separate families in houses or tenements
within the building, is without doubt evidence that the mean
number of cases per house must be a very low figure.
This conclusion is supported by the figures which relate to the
percentage of infected buildings which yield single and multiple
cases. Thus, for example, in the epidemic of 1906 in Dongri section
49·7 per cent. and in Kumbharwada section 71 per cent. of the
buildings infected had single cases. The figures for the other
sections come in between these. In order to make the statistics
clear to our readers, the figures for the year 1906 for Dongri section
are given below.
Dongri section, 1906.
Number of
occupied
buildings.
535
Popula-
tion.
Mean num-
ber of inhabit-
ants per
building.
82,663
61
Mean num-
ber of tene-
ments per
building.
9·1
Total num-
ber of plague
cases.
Jan.
to
June.
435
July
to
Dec.
32
Total num-
ber of
buildings
which yield-
ed plague
cases.
Jan.
to
June.
197
July
to
Dec.
24
Mean num-
ber of plague
cases per
infected
building.
Jan.
to
June.
2·2
July
to
Dec.
1·3
Jan.—June = Plague season.
July—Dec. =Non-plague_season.
67
Number and percentage of buildings which yielded cases.
Num-
ber
Per-
cent-
age of
totalin-
fected
build
ings.
1 case.
Jan.
to
June
98
49 7
July
to
Dec.
20
83 3.
2 cases.
Jan. i
to
June.
31
July
to
Dec.
2
1
15 7
3 cases.
Jan.
to
June.
35
17 8
July
to
Dec.
1
4 2
4 cases.
Jan.
to
June.
14
7 1
July
to
Dec.
..
5—10 cases.
Jan.
to
June.
18
8 1
Juy
to
Dec.
4 2
11 cases
and
over.
Jan.
to
June.
3
1 5
July
to!
Dec.
..
"
All the evidence, then, points to the conclusion that in the great
majority of instances a house during a plague epidemic yields
only a single case. Moreover, it was also found that, when two
or more cases do occur in a house, they are attacked practically
simultaneously as if from a common source of infection, and that
in houses with multiple cases the evidence regarding the rat mor-
tality is much stronger than in those instances in which only a
single case occurred in a house.
This phase in the natural history of plague is adequately ex-
plained when we consider certain facts in connection with the
transmission of the infection from rat to man by rat-fleas, which
have been already mentioned. Thus, we have seen that the
percentage of infected fleas rapidly diminishes from the 3rd or
4th day after the flea has imbibed the blood of the plague rat. In
this connection it is to be noted that only 21 per cent. of plague
houses were infective for guinea-pigs, although in these experi-
ments the houses were chosen as being apparently badly infected
and the tests were made as early as possible. We have also seen
that the chances of infection being given by a single flea are remote,
an important fact when taken in conjunction with the observation
that rat-fleas will not attack man unless forced to do so.
It would appear from all these considerations that the chances
of man being infected in the course of the disease amongst the rats
are not very great, that, in other words, he only participates, as
it were, by an off chance in the rat epizootic.
It is unnecessary here to draw attention to the bearing of
these facts on the prophylaxis of the disease. Their teaching
with reference to such measures as disinfection, segregation of plague
cases, etc., is too obvious to require comment.
F 2