NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
23
spread of infection amongst the rats living in different parts of a
building. It would seem that these facts confirm the conclusion
that the place infection of man and of M. rattus is the same.
In the case of the Punjab villages direct evidence of the place rela-
tionship of the epizootic and the epidemic was obtained.
In Dhund the 32 cases can be divided into two groups as
follows—Group 1 comprises 22 cases inhabiting 18 houses, in which,
or in the vicinity of which, plague-rats were found before or at the
time of the occurrence of the cases; Group 2 comprises ten cases
from eight houses, in which, or in the vicinity of which, no plague-
rats were found before or at the time the cases were attacked.
Three of these eight houses (four cases) were, however, shown by
guinea-pig experiments to contain infected rat-fleas. Of the other six
cases two were imported, two lived in the neighbourhood of one of the.
houses shown to contain infected rat-fleas and the remaining two were:
children, whose movements before their illness could not be exactly
ascertained. Plague-rats, however, were found in the vicinity of the-
dwellings of these two children a few days after they fell ill.
In Kasel we may summarize the data which have reference to-
our present point as follows:—
(a) Of 75 cases, four were imported.
(b) Of the 71 indigenous cases, 61 occurred in houses, in which,
or in the vicinity of which, plague-infected rats had
been found prior to the attacks.
(c) Plague-rats were not found in or near the residences of the
remaining ten cases prior to their attacks, but six of these
persons had visited houses where plague-rats had been
found.
(d) Of the remaining four cases, one was doubtful, and
plague-rats were found in the vicinity of the houses of the
three other infected persons some days after they fell ill.
It would appear, then, that a more intimate association between
plague cases and plague-rats was traced in Kasel than in Dhund.
There is no doubt that this difference in the results obtained in the
two villages can be in part explained by the fact that information
of rats having died was obtained more readily in Kasel than in
Dhund, as the Kasel people had ceased to look on the Commission,
with suspicion at the time the epizootic began in that village.
24
Finally, in attempting to estimate the closeness of the associa-
tion between rat-plague and human plague in a Punjab village we
must take into account the small size and close aggregation of the
houses and the free communications by means of rat burrows between
neighbouring houses. It will then be readily apparent that the
association which exists between a plague case in a house and
plague-rats found in an adjoining house or in a house near by,
but not immediately adjoining, may have been more intimate than
that which obtains between cases and plague-rats found in different
rooms or even on different floors of large buildings such as
exist in cities. Further, it is also certain that many plague-rats die
in the burrows and do not come to view. That this is so is shown
by. the trapping of infected rat-fleas on guinea-pigs in houses in
which no dead rat had been found.
In conclusion, it is important to point out that plague cases do
not necessarily follow the finding of dead plague-rats in a house.
In Dhund there were eight houses in which plague-rats were
found and none of the inmates of which, 31 in number, contracted
the disease. In Kasel out of 86 occupied houses, in which plague-rats
were found and which were not evacuated, only 15, or 17 per cent.,
furnished cases.
3. Relation of the epizootic to the epidemic in quantity.
Little need be said on the quantitative relationship of the
epizootic and epidemic. In Bombay City during the non-plague
season, when there are few plague-infected rats found, there are also
few plague cases. In the Bombay and Punjab villages during the
non-plague season, if there are no acute plague-rats, there are no
human cases. During the plague season in Bombay the quantita-
tive variations of the epidemic curve followed very closely those of
the rattus epizootic curve. The same relationship was observed in
the curves of the Kasel epizootic and epidemic.
Further, in Dhund there were far fewer plague-infected rats
both actually and in proportion to the size of the village than in
Kasel. There were also far fewer plague cases in Dhund than in
Kasel.
Lastly, it was soon apparent that the risk of infection of the occu-
pants of a house increased with the number of plague-rats found.
25
Thus, in Kasel out of 80 houses in which single plague-rats were
found only three, or four per cent. of the whole, furnished cases,
whereas plague cases among the occupants followed in 16 out of 57
(28 per cent.) of the houses in which more than one plague-rat
was found. It was also the experience of the Commission in Bombay
that if multiple cases of plague occurred in a house more than
one dead rat had been found in it, and the greater the number of
dead rats, the greater the number of cases.
Before finishing this part of the subject brief reference must
be made to an experiment that was made in the village of Sion
Koliwada which shows well the relationship between the rat
epizootic and the epidemic.
Sion is a small isolated village to the north of the Island
of Bombay. The huts are well separated from one another
and there is no crowding together as there is in a Punjab
village.
In January 1906, plague was introduced into the village by
importation from Bombay. As soon as a few dead rats had
been found and one or two human cases had occurred, almost
the entire population, which numbered 375, evacuated the village
and occupied a temporary camp in the adjoining fields.
Taking advantage of the
voluntary evacuation of the
village by its inhabitants the Commission substituted in as
many houses as possible a guinea-pig population. These animals
were confined in wire cages in the houses and were fed daily by the
staff of the Commission, so that there could be no possible communi-
cation between the animals in different houses. Corresponding
to the rat epizootic there was a guinea-pig "epidemic."
This " epidemic," moreover, was roughly co-extensive with the rat
epizootic and coincided with it in point of time. Further, in certain
of the houses the number of plague-rats found corresponded to
the degree of infectivity of the houses, as judged by the success of
experiments carried out in them. We may, although perhaps some-
what prematurely, draw attention to the fact, that all the evidence
in this experiment pointed to the rat-flea being the agent of trans-
mission of the infection to the guinea-pigs and excluded contact as
playing any part in the dissemination of the disease among the
guinea-pigs.
26
In conclusion, before considering the means by which the
infection is conveyed from animal to animal, it would be well
to state the very definite conclusion which was arrived at as
regards the relation of the rat epizootic to the epidemic as follows
The rat epizootic precedes and is alone accountable for the human
epidemic. In Bombay City it is the rattus epizootic which deter-
mines the epidemic.
27
PART III.—METHOD OF TRANSMISSION OF THE INFEC-
TION FROM RAT TO RAT AND FROM RAT TO MAN.
A.—Transmission of infection by direct contact.
B.—Aerial transmission of infection.
C.—Transmission of infection through the soil.
D.—Transmission of infection by means of food.
E.—Transmission of infection by means of the rat-flea :
1. Experiments in the laboratory .
2. Experiments in godowns or cabins.
3. The rat-flea the agent of transmission of infection from
rat to rat in nature.
4. Infection in plague houses due to infected rat-fleas.
5. The rat-flea the agent of transmission of infection between
rat and man.
6. Further observations on the transmission of plague by
fleas.
(a) The fate of the plague bacillus in the body of the rat-flea
and the mechanism by means of which the flea infects
a healthy animal.
(b) Can a single rat-flea transmit the infection?
(c) Can both male and female rat-fleas transmit the
infection?
(d) Can other species of fleas transmit the infection?
We have seen that the plague epidemic is dependent upon
the epizootic amongst the rats. It is obvious, therefore, that
the next problem to be solved is the method of transmission of
the infection, that is to say, of the plague bacillus, from rat to rat.
and from rat to man. Several possibilities will at once suggest
themselves to the reader, as they did to the Commission. These
possibilities have been carefully examined and experiments con-
ducted with the view either of disproving them or of proving
them to be true. It is the result of this work which we would.
now bring forward.
28
A.—TRANSMISSION OF INFECTION BY DIRECT CONTACT.
It is conceivable that a sick or dead plague-infected animal
might infect a healthy animal by merely coming in contact with it.
The work of the Commission, however, definitely proves that this
method of conveyance may be excluded. The evidence which
bears on this question may be briefly summarised as follows:—
1 A very large series of experiments was made with guinea-pigs
in specially constructed godowns or cabins. As we shall have
to refer constantly hereafter to these observations in the godowns,
it will be well to give a brief account of their structure.
The godowns are six in number and with the exception of the
roofs are of exactly the same construction. In the case of godowns
1 and 2 the roofs, being of country tiles, offer good protection and
shelter to the wild rat of Bombay ; the flea supply in the interior
is abundant and regular. In the case of godowns 3 and 4 the
roofs, being of flat Mangalore tiles, offer only poor protection
to rats and in consequence the flea supply is more or less scanty
and intermittent. The roofs of godowns 5 and 6 are made of
reinforced concrete and are quite rat and flea proof. These
godowns, therefore, could be easily kept quite free of fleas.
The experiments to which we would now refer were carried out
in godowns 5 and 6. In these godowns plague-infected animals
were kept in close contact with healthy animals, that is to
say, running about together in a confined space and eating out of
the same dishes. The plague-infected animals were allowed to
die and the corpses were not removed until some time after
death. In some instances the concentration of infection was
very great, in one case 21 infected animals being at one time in
contact with 25 healthy ones. Further, the infection was often
kept up for a long period by the daily addition of freshly infected
animals. It is also to be noted that, as the godowns were never
cleaned out, close contact includes contact with urine and fæces
of infected animals and contact with, and eating of, food con-
taminated with fæces and urine of infected animals.
In all these experiments, done under various conditions, the
result was always the same, namely, if fleas were rigorously ex-
cluded not one of the healthy animals contracted plague. As
these observations are of great importance a typical example
29
will be given, which will help their being understood. On.
8th January, 1907, in godown No. 5 there were placed 25
healthy guinea-pigs and five guinea-pigs which had been inoculated
with a virulent culture of the plague bacillus. By 11th January,
1907, all the inoculated guinea-pigs had died of plague. After
this date more infected guinea-pigs were added daily, 25 in all being
put in between 15th January, 1907, and 24th January, 1907 All
these guinea-pigs died of plague, the last death taking place on
31st January, 1907. It is to be noted that on 18th January, 1907,
there were 21 plague-infected animals in contact with the 25 un-
inoculated guinea-pigs. The latter, however, remained healthy
until the experiment was ended on 21st February, 1907. No fleas
were observed on the guinea-pigs throughout the course of the
experiment.
2. Young guinea-pigs in the absence of fleas may be suckled by
plague-infected mothers up till the death of the latter and still not
contract the disease. This observation has been made on several
occasions.
3. Contact of healthy animals with plague-infected animals is
not necessary to start an epizootic among the former. Thus, in
godowns Nos. 1 and 2, in which abundant rat-fleas were present,
the following experiment has often been made. A few plague-in--
fected animals have been put into the godown. A day or two. after
the last of these guinea-pigs had died and been removed, a.
number of healthy animals have been placed in the godown.
An epizootic at once breaks out amongst these animals. In
these experiments the healthy animals, it is evident, were never
in contact with the plague-infected animals.
4. In plague-infected houses animals, placed in cages so con-
structed as to prevent contact with plague-infected animals which
might be present in the house, have in many instances contracted
the disease. Similar observations have been made with monkeys
in the godowns which contained fleas and in which plague-infected
animals had. been present a short time previously.
We shall deal with the infectivity by contact of human
plague cases later.
30
B.—AERIAL TRANSMISSION OP INFECTION.
The same experiments which have been noted above as evidence
against contact infection may with equal force be brought forward
as evidence against serial infection. In addition to these experi-
ments we can, however, cite the following observations, which
all point to the same conclusion :—
1. In godown No. 1 or 2, in which many fleas were
present and which had recently contained plague-infected animals,
guinea-pigs were suspended in cages two feet above the ground.
Not one of these animals contracted plague, while animals exposed
in the same godown at the same time, but suspended in cages two
inches above the floor or running about on the ground readily
sickened and died of the disease. It is to be noted that a flea
cannot jump more than 4 inches. All these groups of animals
were equally exposed to serial infection.
2. Animals placed in infected godowns or in plague-infected
houses in cages protected by a curtain of fine wire gauze or surrounded
by a strip of " tanglefoot " never contracted plague. Their
neighbours in similar cages, but not protected as described, often
succumbed to the disease. The animals of both these groups were
equally exposed to serial infection.
We shall have occasion to refer more in detail to all these
experiments when we come to speak of the conveyance of
infection by means of fleas. In the meantime we may note
that we may exclude serial infection as playing any part in the
conveyance of the infection from animal to animal in nature.
C.—TRANSMISSION OF INFECTION THROUGH THE SOIL.
The conception which can be formed of this method of infec-
tion is as follows :—Rats or human beings may contaminate the
soil or floors of houses with their infected excreta. Healthy
animals or man may become infected by the rubbing into skin
abrasions of soil or dust contaminated in this way.
This has been in recent years generally accepted as
the most probable way in which man becomes infected. As it
was recognised that the infection was inside the houses it was
only natural to suppose that it was in the soil. This deduction
seemed to receive support from the fact that in the great majority
31
of human cases the position of the primary bubo is in the groin,
indicating that the bacilli had gained entrance through the skin
of the legs or lower part of the body. Such a mode of entrance
seemed to be in harmony with the well-known customs of the
native of India, namely, that he is practically always bare-footed
in the house and as a rule takes his food squatting on the floor
and also sleeps thereon. The theory of soil infection, moreover,
offered an explanation of the problem of recrudescence, inasmuch
as it was supposed that the bacillus had an existence during the
non-plague season in the soil. We shall return to the question of
the position of the primary bubo in man ; but, in the meantime,
we have to draw attention to the fact that up to date no com-
petent worker has ever recovered the plague bacillus from
the soil of infected houses, and now the work of the present Com-
mission definitely proves that this method of infection can be
excluded. The evidence which justifies this conclusion is briefly
summarised as follows :—
1. The urine of both infected rats and human beings may
contain plague bacilli in large numbers, but they are never
present in them in such large numbers as in the blood. The urine
-can in some cases infect guinea-pigs when injected in consider-
able amount under the skin, but in no instance, out of a large
number of experiments, did it infect an animal when tested by
natural methods, e.g., rubbed into abrasions and scarifications.
It is of course evident that in nature injection of infected urine
under the skin could not occur, the only probable way of
infection being through abrasions. A large number of experi-
ments on the infectivity of the fæces of both rats and man
were made. These tests were carried out by rubbing the fæces into
scarified surfaces on the skin of susceptible animals. Only on
one or two occasions out of several hundred experiments did
infection result, even when the blood of the animal, of which
the fæces were used contained abundant plague bacilli.
2. We have already seen that in godowns 5 and 6, from which
fleas were excluded, healthy animals ran about and lived on the
floors which were continually being contaminated with the urine
and fæces of plague-infected animals and still did not contract
the disease. A large number of such experiments were carried
out and always yielded the same result.
32
3. Several series of experiments were made to ascertain the dura-
tion of infectivity of floors which had been grossly contaminated
with virulent cultures of the plague bacillus. In these experiments-
the contamination was made artificially and was extremely gross,
the floors being flooded with thick cultures, of the bacillus.
Such gross contamination could never possibly occur in nature.
When the test of infectivity employed was to allow healthy
susceptible animals to run about on the floors, it was found
that cowdung floors remained infective for 12 but not for
24 hours after contamination, while chunam floors kept their infec-
tivity for 6 but not for 12 hours. In these experiments only a very
small percentage of the animals exposed to infection contracted
the disease and those only when the floors were still wet when the
animals were put on. It is again to be noted that the contamina-
tion in these experiments was very gross, much more so than
could ever possibly occur in nature.
Further, when we come to analyse the data which refer to
the distribution of the primary bubo in the few animals which
did become infected through soil contamination and compare this
distribution with the distribution in the same species of animals,
namely, guinea-pigs, infected in nature, e.g, by running about
in plague houses, we find a marked contrast, which points to a
different method of infection. Thus, in the case of the soil
infected animals the position of the bubo was in the great
majority of instances in the groin, or in the axilla, the neck glands
being involved in only 22 per cent. of the animals, while in the
animals infected in nature the bubo was situated in the neck in
over 90 per cent. of the cases.
4 That animals may become infected with plague, although
they are carefully protected from soil infection, is definitely shown
by experiments which were made both in infected godowns in the
presence of fleas and in plague-infected houses. These animals
were placed in cages of such a description that they were protected
from soil infection, and still, in the instances in which the access of
fleas was permitted, some of the animals contracted the disease.
Again, in the experiments in infected-flea godowns, in which the
animals were suspended two inches from the floor and were never
33
allowed to come in contact with the ground, they readily contracted
the disease. Two inches is an easy jump for a flea.
5. The last argument which we shall bring forward against
soil infection playing any part in the epidemiology of plague is
afforded by two series of experiments made in plague-infected
houses. In one series the houses had not been disinfected, while
in the other series disinfection had been thoroughly carried out
with an acid solution of perchloride of mercury of strength 1 : 750,
a
solution which, as has been abundantly proved, kills the
plague bacillus even in organic mixtures. The same proportion of
disinfected as of non-disinfected houses, namely, 30 per cent. was
found to be infective to guinea-pigs allowed to run free over the
floors. If the infection had resided in the soil it is only reason-
able to suppose that the disinfection would have rendered
the houses non-infective. It
will be observed that the
disinfection had no effect on the fleas in the rooms.
D.—TrANSMISSION Of INFECTION BY MEANS OP POOD.
It has been frequently asserted lately, especially in respect of
rats, that infection is introduced by means of food. Thus, it has
been supposed that rats might become infected by eating the
excreta either of plague rats or of human plague cases or perhaps
by eating food contaminated with such dejecta. It has also been
stated that a common way of infection of rats might be
the eating of the carcases of their dead infected comrades.
In the case of man it is held by several observers that infection
takes place through eating food contaminated either with the
excreta of infected rats or by flies, etc., coming from infected
material.
These hypotheses receive a certain amount of support from
successful feeding experiments carried out by various workers
in the laboratory. But the evidence which has been accumula-
ted by the present Commission goes far to prove that infection
does not thus occur in nature. Let us briefly consider this
evidence ; first as regards rats—
1. We have already seen that the fæces and urine of plague
infected rats and of human plague cases are not at all or only slightly
19 S
C