NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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occasionally quite white. During the cold weather the fur is longer
and thicker than that of the M. rattus found in Bombay. Mus rattus
in the Punjab as in Bombay is essentially a house rat. The houses
are closely packed together, often back to back, and at the best with
only narrow lanes separating them. The walls are built of sun-dried
bricks or of large clods of caked mud. The roofs are flat, made of
beaten earth supported by twigs, matting or cane. The floor con-
sists merely of beaten earth, which may or may not be plastered
with cowdung. On account of this mud construction of the houses
M. rattus adopts burrowing habits. The mud walls, floors and
roofs, present no obstacle, and it is indeed the exception to
find a village house in which rat burrows cannot be found.
These burrows are very extensive. They have been followed
extending the entire length of the wall and into the wall of an
adjoining house. In short, a Punjab village may be looked upon
for our present purpose as being honey-combed with
rat
burrows which ramify in all directions. Further, the nests
of M. rattus are to be found in these burrows and seldom
in the interior of the house as in Bombay. In the houses
themselves there is kept an abundant food supply for the rats.
Each house has its granary, in which is stored the grain supply
for the household. In earthen vessels there are also kept such
articles as flour, sugar, ghee, potatoes, etc. All these are easily
accessible to rats.
From what has been said above it will not surprise the reader
to find that the rat infestation of a Punjab village is very great, and
that the rats live in close relation with the human population, so
much so indeed, that a common complaint of the villagers is that
their sleep is disturbed by rats running over them at night.
As further evidence of the enormous rat infestation it is only neces-
sary to state that during the year in Kasel the total number of M.
rattus trapped was 4,639, an average of 1 2 rats per inhabitant, the
corresponding figures for Dhund being 2,518 and 1 3. Vigorous
trapping was continued during the year in both these villages. At
the beginning of the operations in Kasel 148 rats per 100 traps
set were taken. This proportion gradually diminished until
seven months after the commencement, namely, in July, only 17
rats per 100 traps set could be obtained. From this date until
the end of the operations in November the numbers caught steadily
increased, until during November 69 rats per 100 traps set were taken,
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or about half the number caught at the beginning. In Dhund the
result was similar, so that there is no doubt that in both villages
at one season of the year breeding more than compensated
for the loss sustained by trapping. In this connection it was found
that while breeding goes on all the year round, during the cold
weather months (November-March, inclusive), it takes place to a
less extent than during the hot weather and that the most favour-
able months are April, September and October, when the mean tem-
perature is about 80°F. The average number of young born
at one time was found to be 6 per rat.
The question of migration of the rats from village to village
and from villages into the fields at harvest time was also investi-
gated. Traps were set around Kasel for one and a half months and
in one instance only, and that when the traps were placed close to a
house, were a few M. rattus caught. During the harvesting of the
spring crops in April, May and June, when the rat epizootic in Kasel
was steadily progressing, no rats, except a few specimens of
N. bengalensis, were found in the fields. In short, no evidence of
migration of any kind could be obtained and the more intelligent
villagers, who recognise the distinction between M. rattus and N.
Bengalensis, are emphatic in denying that the former species ever
leaves the villages.
Before passing on to the question of plague amongst the rats, a
few words on the fleas infesting the rats in Bombay and in the
Punjab are necessary.
On the rats in Bombay and in the Bombay villages practically
only one species of flea was found, namely, Pulex cheopis. The degree
of infestation of M. decumanus was found to be considerably greater
than that of M. rattus ; throughout the year more than twice as
many fleas being taken on the former as on the latter.
In the Punjab villages as well as P. cheopis another species of
flea, namely, Ceratophyllus fasciatus, was found on the rats. This
species, however, made up only about 2 per cent. of the total,
and during the months May to November entirely disappeared.
Another point of interest and importance which comes out
of the flea census is that M. rattus in the Punjab harbours con-
siderably more fleas than it does in Bombay. In the Punjab
its degree of flea infestation approximates that of M. decu-
mahus in Bombay. It has also been a common observation that sick
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Tats habour many more fleas than healthy ones. It is not uncommon
to find 100 fleas and even more on a sick, plague-infected rat.
The question of the seasonal prevalence of fleas will be dealt
with later on when discussing the seasonal prevalence of plague.
The Commission examined the physiological anatomy of the
mouth parts and alimentary canal of P. cheopis, investigated the
external anatomy of this flea and its differentiation from some other
common fleas and enquired into its habits and life history. Papers on
these subjects have appeared in the reports,* to which we
refer the reader whose interests lie in these directions. We shall
note those points which are essential for our present purpose
in their proper place.
C.—OBSERVATIONS ON PLAGUE IN THE RAT—THE EPIZOOTIC.
As the accuracy of the data which have reference to the rat
epizootic and to its relation with the epidemic depends upon the
accuracy with which plague-infected rats can be diagnosed, a few
remarks on natural rat plague and its diagnosis will not be out of
place. The following statements on this subject are founded on the
detailed post-mortem examination of over 5,000 plague-infected rats,
both M. decumanus and M. rattus, found dead in Bombay and in
the Punjab.
Plague rats may be divided into two classes, according as to
whether or not a bubo is present. The bubo if present, and it is
present in 85 per cent. of the cases, is the most important sign of
plague. Its most common situation is, as we shall see, the neck, the
next most common being the armpit. Subcutaneous congestion is
common (69 per cent.) and when combined with a purplish red
appearance of the muscles affords strong suspicion of the animal
being plague infected. Of other characteristic appearances those
occurring in the liver are the most important; it has a waxy, mot-
tled or finely granular appearance, in the latter case as if dusted
over with grey pepper. Hæmorrhages in various parts of the body
are commonly found and an abundant pleural effusion constitutes
when present a noteworthy sign of plague in the rat. In putrid
rats at least three of these signs may persist and when recognised
are of the greatest assistance, namely, a bubo, granular liver and
pleural effusion.
* Journal of Hygiene, Vol. 6, page 486, and Vol. 7, page 446.
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If recourse be had to the microscope, the bubo is the best
place to find plague bacilli in large numbers, the spleen next and
then the blood.
As the result of a number of severe tests it was found that the
naked eye is markedly superior to the microscopical method as
an aid in diagnosis, that is to say, in the case of an experienced
observer. Such an expert would be prepared to make a diagnosis,
of plague on the strength of the macroscopical appearances alone,
even though the other tests were negative and the animal showed
marked signs of putrefaction.
In short, it may be said that neither in Bombay nor in the
Punjab did the Commission experience any difficulty in diagnosing
plague from other diseases among rats.
The bacilli found in naturally infected rats, both during the epi-
zootic and during the non-epizootic seasons, were fully virulent, 62
per cent. of the inoculated animals dying of acute plague in five
days or less. Further, the bacilli retained their full virulence
after passage through a large number of rats, no matter whether the
subcutaneous or cutaneous method of inoculation was used.
Finally, it was ascertained that a considerable percentage of wild
Bombay rats were immune to plague infection, this insusceptibility
being also observed, when fleas were used as the agent of trans-
mission. As a point of epidemiological importance we shall have
occasion later on to refer, to the relative infectivity of the blood,
urine and fæces of plague rats.
With these few remarks on the disease as seen in the rat and its
diagnosis, we may proceed to record these facts concerning the
epizootic which have a direct bearing on the epidemiology of the
disease-
In Bombay City, for which a large mass of data has been
accumulated and analysed, M. decumanus and M. rattus are by far
the most important species in relation to the spread of plague.
Both these species were found plague infected, but from the
statistics it could be concluded that M. decumanus was twice as
liable as M. rattus to plague infection. The explanation of this
difference of plague incidence in the two species does not lie in any
difference between them of susceptibility to plague infection, but
in the fact, already mentioned, that the degree of flea infestation of
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M. decumanus is more than double that of M. rattus. Before
going further we would insist that in Bombay City, just as there
are two common species of rats, so there are two epizootics,
namely, a M. decumanus epizootic and a M. rattus epizootic,
which have to be considered separately.
The infection
amongst the rats in Bombay must be characterised as being
exceedingly severe, 4,381 M. rattus and 13,377 M. decumanus
being shown to be plague infected during the year, a total of
nearly 18,000 plague-rats out of a total of 117,000 rats examined.
The year, however, may be roughly divided into two seasons, (a) the
non-epizootic season, June to November inclusive, and (6) the epi-
zootic season, December to May inclusive. During the epizootic
season the largest number of infected M. rattus for one week was
432, the largest number of M. decumanus for a similar period
being 1,334. This was in March. During the non-epizootic season
never less than 20 to 30 plague-infected rats were examined during
any one week. We see, therefore, that even during the off-
plague season acute plague persists amongst the rats, but to a much
less extent than in the plague season. The factors which govern
this seasonal prevalence will be discussed later on.
A study of the section maps, the method of preparing which
has been described, leads to the conclusion that M. decumanus
is chiefly responsible for the diffusion of the infection throughout
the city, as might be expected in consideration of the out-door
life and wandering habits of this species. While this is so, it is to be
carefully noted that the infection does not spread in any definite
manner from one section to another; infection amongst the rats
persists during every month of the year in most of the sections.
Further, in this connection we have already mentioned that no evi-
dence of a general migration of rats from one place to another,
when plague breaks out amongst them, could be found.
As regards the relation of the decumanus epizootic to the rattus
epizootic it would appear from a study of the curves which were pre-
pared from the figures collected during the year, that the former
preceded the latter by a mean interval of about ten days. Further,
the evidence obtained by the Commission led them to the
conclusion that the decumanus epizootic was directly accountable
for the rattus epizootic. By this statement the general relation
between the two epizootics is expressed, but it is not for one moment
19 S
C
o
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to be supposed that in every instance M. rattus received its infection
from M. decumanus. It was also considered that the persistence in
the off-season was due ultimately to plague in M. decumanus.
These conclusions can be adequately explained, if it be granted
that the conditions for epizootic prevalence are more favourable in
the decumanus population than in the rattus population. Now, the
only factor concerned in the severity of epizootic prevalence
which does not affect the two species equally is, as far as can be as-
certained, the degree of flea infestation of each, M. decumanus har-
bouring a much larger number of fleas than M. rattus. In this fact,
therefore, is to be found the key to the elucidation of the relationship
of the two epizootics. It is scarcely necessary to point out the
extreme importance of this relationship in the problem of the
organisation of measures for combating plague in Bombay.
As regards the four villages on the outskirts of Bombay nothing
need be said with respect to the natural disease in the rat. It is
necessary, however, to refer to certain points of epidemiological
importance which came out of the investigations in these villages.
In the first place, it was found that the infection was not main-
tained in the rats (M. rattus) during the off-season, either in the
acute or in the chronic form. It could, therefore, be concluded,
as we shall see later, that the annual epizootics in these villages
must be ascribed to importation of infection, generally, no doubt,
from Bombay. All the evidence pointed to such importation taking
place, and we shall return to this question later on.
In the second place, in one of the villages, Sion Koliwada,
as soon as the mortality amongst the rats began and one or two
human cases had occurred, almost the entire population evacuated
the village. The human population was replaced by the Com-
mission by a guinea-pig population. We shall have occasion
later on to refer more in detail to this experiment, and in the
meantime would only draw attention to a few of the facts concerning
the rat epizootic.
First, in proportion to the severity of the epidemic the number
of plague-rats found was very small, notwithstanding the very
thorough and extensive search made. The experience both in Sion
Koliwada and in the Punjab village of Dhund in this respect points
to the danger of concluding that plague-rats are absent from
an infected locality unless a very thorough search is carried
out.
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Secondly, it was found that the infection persisted among the
Tats in the village for two months and that in the case of two
individual huts it lingered amongst the house rats, in one for at
least ten days and in the other for 13 days.
Thirdly, a study of the map showed that the epizootic spread
from a central focus towards the periphery and the rate of
spread is indicated in the statement that the infection in the rats
took six weeks to travel 300 feet.
We have already seen that in the Punjab villages M. rattus is
the only rodent commonly present. Our remarks, therefore, refer
to plague in this species.
The outstanding fact as regards rat-plague in the two Punjab
villages which were under observation is, that during the non-
epizootic season, when no plague cases were occurring, rats suffering
from a chronic form of the disease were caught alive. These rats
showed abscesses in various parts of the body, but chiefly in the
abdomen and more especially in connection with the spleen. These
abscesses contained plague bacilli, in small numbers certainly, but
still virulent. Otherwise the rats appeared to be quite healthy and
it is remarkable that no rats with this form of the disease were found
dead. The two main conclusions at which the Commission arrived
after a careful study of this condition were, first, that these rats
had been infected by fleas in the same way as plague-rats which
develop the acute disease; and, secondly, that, from the evidence
bearing on the question, no reason could be discovered for believing
that chronic plague, as it occurs in the Punjab villages, possesses
any significance in the seasonal recurrence amongst the rats of the
infection in the acute form. It is to be carefully noted that the
absence of acute rat-plague during the off-season in the two villages
of Kasal and Dhund does not warrant our concluding that it is
absent in all Punjab villages at this time. On the contrary, there is
no question, as we shall see, that in some villages the disease
persists in the rats in the acute form, as indicated by the occurrence
of a certain number of human plague cases throughout the non-
epidemic season.
In the village of Dhund the first acute plague-rat was found on
the 27th January and the last on the 21st April: the epizootic may,
02
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therefore, be said to have lasted about three months. In Kasel it
lasted from 2nd April till 17th July, namely, about three and a
half months.
In Dhund the epizootic was of very moderate severity, only-
34 plague-infected rats being found. In Kasel, on the other hand,
253 rats were proved to be plague-infected and 89 more, too putrid
for examination, were brought to the laboratory. The lightness of
the epizootic in Dhund is probably due to the large number of rats
which had been removed by trapping before it began.
The mode of spread of the epizootic in both villages was charac-
terised by considerable irregularity. It cannot be said to have ex-
tended outwards from the original focus as a wave with a definite
margin leaving the area passed over free from infection. On the
contrary some plague-rats were found-at a considerable distance
from the original focus of infection early in the epizootic and also
infected rats continued to be found in the vicinity of the original
focus at a time when the epizootic had spread considerably
In
spite, however, of these irregularities it may be admitted that the
general direction was centrifugal and that the areas furthest from
the original focus were the last to become infected
D.—OBSERVATIONS ON PLAGUE IN MAN—THE EPIDEMIC.
We may dismiss this part of our subject in very few words, as
the results of the observations differed in no particular from those
already published by numerous observers. Thus, the figures both
in Bombay and in the Punjab which have reference to the incidence
of the disease in the population classified according to sex, age and
caste are in harmony with what is already well known. In Bombay
City during the year there were reported 12,245 cases and
11,010 deaths. Data concerning 10,880 of these cases were entered
in the case cards, already mentioned, and analysed by the Commis-
sion. Deaths from plague were reported from the City during every
week of the year, the smallest number, 5, being in the week
ending 25th November and the largest number, 925, in the
week ending 28th April.
In the village of Dhund the last case of plague before the present
epidemic was attacked on 14th July 1905. During the epidemic
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which was now observed there were 32 cases, the date of the first
attack being 6th February 1906 and of the last 2nd May. There
were 19 deaths, a case-mortality of 60 per cent.
As regards Kasel the last plague case in 1905 occurred on the
28th April. The epidemic of 1906 consisted of 75 cases with 41
deaths, a case-mortality of 55 per cent. The first case occurred
on 5th April and the last on 6th July.
E.—RELATION OF THE EPIZOOTIC TO THE EPIDEMIC.
We now come to the most important part of the epidemiological
observations, that is to say, the relationship between the epizootic
and the epidemic.
In Bombay City we have seen that it was concluded
that the rattus epizootic was dependant upon the decumanus
epizootic. The conclusion which we shall now come to is that the
epidemic is directly attributable to the rattus epizootic. The same
conclusion holds good for the villages, both in Bombay and in the
Punjab, but in these villages the problem is not complicated by the
presence of the decumanus epizootic. This conclusion has been
arrived at after a careful consideration of the relation of the
epizootic to the epidemic in time, in place and in quantity.
1. Relation of epizootic to epidemic in time.
In Bombay City we have seen that the decumanus epizootic pre-
cedes the rattus epizootic by an average interval of ten days. Using
the same methods of observation it was found that the curve of the
rattus epizootic was separated from that of the epidemic by an
average interval of 10 to 14 days. The explanation of this
interval on the basis of the flea theory will be dealt with
later.
In the maps for the sections the same time relations of the
epizootic and the epidemic are well illustrated. It need hardly be
mentioned again that during the non-epizootic season, that is, when
there are only few plague-rats, there are also few plague cases.
This time relationship is also well shown in the case of the two
Punjab villages. In both villages during the season when no acute
plague-rats were found, there were no plague cases. When acute
22
plague appeared amongst the rats, human cases immediately began
to appear.
In Dhund both plague-rats and cases were too few in number
to allow of curves being constructed. It will be remembered,
however, that the first acute plague-rat was caught on 27th January
and the last on 21st April, while the first human case was attacked
on 6th February and the last on 2nd May.
For Kasel curves were constructed in the same manner as for
Bombay. These curves of rat-plague and human plague exihibit
a very close correspondence, the variations of the curve of
human plague following those of the rat-plague curve at an interval
of about a week.
In both these villages, therefore, rat-plague preceded human
plague and human plague ceased shortly after or just before the
cessation of acute plague amongst the rats.
Observations of the same nature were made in the Bombay
villages in which plague occurred.
2. Relation of the epizootic to the epidemic in place.
In the case of Bombay it was practically impossible to obtain
direct evidence bearing upon this point. Still, in a considerable
number of instances plague infected M. rattus were found in
houses in association with plague cases. There are, however, two
valuable pieces of indirect evidence which show that there cannot
be the slightest doubt that the place infection of man is in-
timately related to that of M. rattus. First, we have already in-
sisted upon the fact that M. rattus is essentially a house rat and that
it lives in close association with man. It necessarily follows that
the place infection of M. rattus must correspond closely with that
of man—in other words, that both must be referred to inhabited
buildings. This association is shown in a general way in the
maps of the sections. Secondly, statistics collected and analysed
for the whole of Bombay and for the different sections separately
show that the incidence of plague on persons living on different floors
in buildings is the same. Now, we have already seen that M. rattus is
found on every floor of the buildings. Moreover, the construction
of the buildings is such that there are abundant facilities for the