NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
34
infective when rubbed into abrasions of the skin. It is surely
justifiable to suppose that these excreta would be still less infective
when diluted with food and brought in contact with a healthy
mucous membrane. This supposition is borne out by a series of
experiments which was made on rats. A large number (194)
of rats were given the urine of human plague cases, in
many of which bacilli were present in the blood at the time the urine
was drawn off, but not one of these animals contracted plague.
2. We have already drawn attention to the contact experiments
which were carried out in godowns 5 and 6 in the absence of fleas.
In these experiments the food of the healthy animals was being
continually contaminated with the urine and fæces of their plague-
infected comrades and still not one of the former developed the
disease.
3. We have also mentioned the observations made with
young guinea-pigs, which were suckled by their plague-infected
mothers until the death of the latter. Not one of these sucklings
contracted plague, although it is reasonable to suppose that the
milk of some at least of the mothers contained plague bacilli.
4. Many observers have shown that it is possible to infect rats
in the laboratory by feeding them with grossly contaminated
material, such as the organs or carcases of animals dead of plague.
On these grounds and on these grounds only it has been held
that in a state of nature rats generally become infected in this way.
the bacilli gaining access through the mucous membrane of the
mouth, throat or intestine. On account of those observa-
tions the Commission devoted particular attention to this question
with the following result—
It was found possible to infect rats with plague by feeding them
with grossly contaminated material of a soft nature, 21 4 per cent. of
wild Bombay rats being susceptible to this method of infection. In
a series of similar experiments made with M. rattus in the Punjab
67 8 per cent. of the rats were susceptible, but in this series a
much larger dose of infected material was given. Further, a large
number of wild Bombay rats were infected by feeding them on
the carcases of their plague-infected comrades. No difference as
regards the post-mortem appearances or the distribution of the
primary bubo was found between rats infected in this latter way and
rats infected by feeding on soft contaminated material. But, while
35
the general pathological lesions found in all rats infected by feeding
were in the main the same as those found in rats naturally in-
fected, there were observed two most important differences.
First, in naturally infected rats, while the primary bubo was as a
rule in the neck (74 per cent.), the axillary and inguinal glands
being also often affected (25 per cent.), in not one single instance,
out of 6,000 post-mortem examinations, was a bubo found in con-
nection with the mesenteric glands in the abdomen. On the other
hand, in the rats infected by feeding, the neck glands were in-
volved in only 29 per cent. of the cases, the inguinal and axillary
glands were practically never affected, while the mesenteric glands
were the site of the primary bubo in no less than 72 per cent.
of the infected animals. Secondly, in the case of naturally infected
Tats the stomach and intestines showed, as a rule no marked patholo-
gical change, while in the case of rats infected by feeding, well-
marked lesions were found in the intestines. It would appear jus-
tifiable to conclude, then, that in nature, infection of rats by feeding
rarely or never takes place and that rats do not become infected
by eating the carcases of their comrades. At least this method of
infection can play no part in the epizootic spread of plague.
As regards man becoming infected through eating plague-
infected food all competent observers are agreed that the evidence
is against such a method of infection. The first Indian Plague
Commission were not able to discover any fact which would
support the suggestion that plague may effect an entrance
into the human organism through the stomach or intestinal
canal. They concluded that, if infection takes place at all
through this channel, it can be only in the very rarest
-instances. There is on record no post-mortem examination which
points to the bacilli having gained entrance through the intestine
and there is no description of primary mesenteric buboes having
been observed.
Further,
it is evident that man could never receive
with his food anything like the number of bacilli which were in-
gested by the experimental rats mentioned above. In this con-
nection it is also to be remembered that in the experiments in
the flea-free godowns none of the guinea-pigs that ate food
contaminated with the urine and fæces of their infected comrades
D2
36
contracted the disease, and none of the rats which were fed on
food contaminated with the urine of human plague cases developed
the disease.
We are, therefore, now in the position to conclude that
the transmission of infection by food plays no part in the
spread of plague in nature either from rat to rat or from rat to
man.
E.—TRANSMISSION of INFECTION BY MEANS OF THE RAT-FLEA.
We have now to consider the possibility of the plague bacillus-
being conveyed from rat to rat and from rat to man by means of
some biting insect, such as the flea. A number of observers had
already suggested and had brought forward a certain amount of
evidence in support of their suggestion, that the rat-flea was the
insect responsible. With the exception of two Frenchmen, Gau-
thier and Raybaud, no worker had, however, succeeded in an un-
equivocal manner in conveying the disease from animal to
animal in the laboratory by means of fleas.
It was soon apparent to the Commission that, if any biting
insect was the agent of the transmission of the bacillus from rat
to rat and from rat to man, fleas, and those species of flea which
infested rats, were practically the only insects which need
be considered.
Investigation then showed that practically the only species of flea
to be found on Bombay rats was Pulex cheopis, a flea first described
by Rothschild, who has expressed the opinion that this species,
except in Northern and Central Europe, is the commonest rat flea—
in some localities almost the only species, found upon rats.
All the experiments which are now to be summarised were made
with this flea and when we refer to the rat-flea it is this species
which is meant.
1. Experiments in the laboratory.
After several failures a command of the conditions necessary
was attained and it soon became a simple matter to transmit
plague from animal to animal by means of rat-fleas.
37
In the first series of successful experiments a glass case contain-
ing two wire cages side by side, each standing in a tin tray which
collected the urine, was used. Both trays were filled with sand,
in order to provide dryness and shelter for the fleas. Each
cage was furnished with a lid through which the rats were
introduced and food and water given to them. The whole appar-
atus was covered in with fine muslin to prevent the escape of the
fleas. From this description it is seen that a rat placed in one of
these wire cages could not come in contact with a rat in the other
cage or with its urine and fæces.
The method of experiment was as follows. A plague-infected
rat and a number of rat-fleas were placed in one of the wire
cages. After the rat died, a fresh healthy rat was put into the other
cage, the corpse of the infected rat being left in for 24 hours longer.
Sixty-six experiments with English white rats and with
Bombay wild rats were done in this way, with the result that 30
healthy rats contracted plague ; that is to say, there were 45 per
cent. of successful transmissions. In all cases a fairly abundant
supply of fleas was present; these could pass freely between the
cages and, except for the air, formed the only apparent means of
transmission of the bacilli from rat to rat.
In order to exclude aerial infection a S2cond series of experi-
ments was carried out, in which fleas were taken from a rat which
had died of plague and were placed on a fresh rat in a clean flea
proof cage of similar construction to that already described, but
containing only one wire cage. Twenty-one (55 per cent.) successful
transmissions out of 38 experiments were obtained in this way,
both English white rats and Bombay wild rats being again used.
The possibility of the rat-flea carrying plague from one rat to
another was therefore demonstrated directly.
Following on these successes a comprehensive scheme of flea
transmission experiments was at once started with the object of
investigating several important questions which suggested them-
selves. The teaching of these experiments will be referred to
later in their proper place. It is sufficient to state here, that many
hundred successful flea transmission experiments were carried out
with guinea-pigs, an animal which is an excellent host for P. cheopis.
Finally, a considerable number of successful transmissions were
obtained by confining fleas, taken from plague-infected rats, in test
38
tubes and allowing them to feed on definite areas of skin of healthy
animals through a layer of muslin covering the mouth of the tube.
It is thus demonstrated, that, when the proper conditions
are obtained, it is an easy matter to infect susceptible animals
by means of rat-fleas taken from plague animals, all other means-
of infection being rigorously excluded.
2. Experiments in godowns or cabins.
We have already made passing reference to the experiments
which were carried out in godowns or cabins especially constructed
for the purpose. Attention has been drawn to the fact that these
godowns differed from one another only in the structure of their
roofs, which were so constructed, that in the interior of godowns
Nos. 1 and 2 the supply of rat-fleas was abundant and regular,
in Nos. 3and 4 it was more or less scanty, and in Nos. 5 and 6
there we e no fleas. We have already seen that if fleas are
rigorously excluded, as they were in godowns 5 and 6, close
contact of plague-infected animals with healthy animals does
not give rise to an epizootic among the latter. We have now
to see what happens under similar circumstances if fleas are
present. With this object in vi w a large number of observa-
tions were carried out in godowns 1 and 2 at all seasons
of the year. When the experiments were carried out during
the plague epidemic season, the disease at once started amongst
the healthy animals and rapidly spread from animal to animal
until not one was left alive. When, on the other hand, the experi-
ments were made during the non-epidemic season, e.g.. in June-
or July, either none of the healthy animals became infected or only
two or three contracted the disease and died. These experiments-
have been many times repeated and always with the same results.
It will p rhaps render the subject more easily understood if one-
experiment of each type is detailed as an example.
(a) Godown No 2, January, 1907.—This experiment was made
at the same time as the one already described as having been car-
ried out in godown 5, that is to say, at the beginning of the plague-
epizootic season in Bombay.
On 8th January, 1907, there were placed in the godown five
guinea-pigs which had been inoculated with a virulent culture of
39
the plague bacillus. By 11th January, 1907, these five animals
had succumbed to acute plague.
On 11th January, 1907, after the last inoculated guinea-pig had
died and been r moved from the godown, there were put in 25
healthy guinea-pigs, which were allowed to run free. An epizootic
of plague of the most rapid description at once broke out amongst
these heathy animals, the first dying on 15th January, 1907, and in
a fortnigh from heir being placed in the godown the last one
had succumbed to the d sease.
Fleas were present in great numbers in the godown through-
out this experiment.
(6) Godown No. 2, June, 1905.—On June 13th, 1905, that is dur-
ing the non-epidemic season, 50 healthy and 10 inoculated guinea-
pigs were placed in godown No. 2. The inoculated animals had
all succumbed to plague by June 29th. Of the uninoculated
guinea-pigs four died of plague, the first on he 2nd July and the
last on 12th July All the other animals remained healthy.
Experiments done in June and July, 1906, gave similar results,
although in some instances not a single uninoculated guinea-pig
became infected.
Again, by making contemporaneous experiments during the epi-
demic season in a godown with many fleas and in another
godown with few fleas it was shown that the rate of pro-
gress of the epizootic is in direct proportion to the number of
fleas present. Thus in an experiment carried out in godowns 2 and
4 during the rise of the plague epizootic of 1907 the following re-
sult was obtained. In godown 2, in which fleas were very aban-
dant, the ep zootic spread through 25 guinea-pigs in nine days,
while in godown 4, in which few fleas were present, it lasted 68
days, the same number of guinea pigs being present.
If further proof were necessary to prove that the fleas present
in the godowns were the agents of transmission of infection, this
proof is afforded by the following observations :—
(1) Fleas taken from infected godowns, placed in test-tubes
and fed through muslin on healthy guinea-pigs, infected the
latter.
(2) We have already referred in passing to some experiments
which were carried out in godowns 1 and 2, in which three-
40
groups of guinea-pigs were exposed to infection in different ways,
one group being allowed to run about on the floor, the second
group being placed in wire cages two inches from the floor and
the third being suspended two feet from the ground, also in
open wire cages. It was already known that a flea could not
jump more than from four to six inches. This experiment has been
repeated on several occasions and a ways with the same result.
The following may be taken as a typical example :—
On 28th January, 1907, into godown 2, in which a plague epi-
zootic had just come to an end by killing all the animals, three
groups of four guinea-pigs each were distributed as follows :—
(a) four guinea-pigs running free on the floor ;
(b) four guinea-pigs in wire cages two inches above the ground ;
(c) four guinea-pigs in wire cages suspended two feet above
the ground.
On 30th January, 1907, these guinea-pigs were removed from the
godown, freed from fleas under chloroform and segregated.
One hundred and twenty-five fleas were taken on the guinea-pigs
of Group A, 27 on those of Group B and none on those of Group C.
The eight animals of Groups A and B were observed to be ill and
to have buboes in the neck. The four guinea-pigs of Group C
appeared to be quite well.
The fate of the animals of these three groups was as follows :—
Group A.—One died under the chloroform ; it was proved bac-
teriologically to be plague-infected. The other three all died of
plague.
Group B.—All four animals died of plague.
Group C.—All four animals remained healthy.
In short, the animals which were within the limit of a flea's
jump all died of plague, while none of those which were outside
this limit contracted the disease.
(3) On several occasions monkeys were placed in pairs in cages
in an infected godown. Both animals were protected from soil
infection and were equally exposed to aerial infection. One
animal was, however, protected from fleas, either by a broad (six in-
ches) strip of sticky material, namely, " tanglefoot," or by a fine
metallic gauzo curtain, while the other was not so protected. They
were left in the godowns for 48 hours.
41
The result of these experiments was that not one of 11 ani-
gnals protected from fleas contracted plague, while six out of 13
unprotected developed the disease.
It will be seen that these godown experiments were of the
most exhaustive nature and yielded most convincing proof of the
part played by the rat-flea in plague epizootics. As the
references made to these observations have of necessity been
somewhat scattered, it will be well to summarize the con-
clusions which may be drawn from them. These conclusions
are—
(a) Close and continuous contact of plague-infected animals
with healthy animals, if fleas are excluded, does not give rise
-to an epizootic among the healthy animals. As the godowns
were never cleaned out, close contact includes contact with fæces
and urine, and eating of food contaminated with fæces and urine
of infected animals. Close contact, even the suckling of young
by plague-infected mothers, does not give rise to the disease.
(b) When fleas are present, the epizootic, if it does start,
varies in severity and rate of progress according to the season of
-the year and the number of fleas present. The season, in which
epizootics are readily produced experimentally and spread rapidly,
corresponds with that of the plague epidemic.
(c) An epizootic of plague can occur in a godown containing
infected fleas without direct contact of healthy animals and infected
animals.
(d) In an infected godown the infection is effective in pro-
portion as the test animals are accessible to fleas.
(e) Infection can take place without any contact with conta-
minated soil.
(/) Aerial infection is excluded.
(g) The experiments lead to the conclusion that fleas and
fleas alone were the transmitting agents of infection.
3. The rat-flea the agent of transmission of infection from rat to
rat in nature.
We have already seen from experiments in the laboratory and in
the godowns, that the only means of transmitting plague from
42.
animal to animal at all comparable to what would occur in nature
are—
(a) by feeding on grossly contaminated material, and
(b) by rat-fleas.
Further, we have excluded the possibility of the first of these
methods ever happening in nature.
We propose now to bring forward more evidence in sup-
port of the second of these methods, namely, flea transmission.
This evidence is obtained from a comparison of the distribution,
of the primary bubo in animals, rats and guinea-pigs, infected,
in nature with its distribution in the same animals infected in the
laboratory by means of fleas. Let us first consider the case of rats.
In Bombay, during both the plague season and the non-plague
season, it was found that the great majority of rats that died of
plague in nature had the primary bubo in the neck, the next
most common sites being the armpit and the groin. This state-
ment holds good both for M. decumanus and for M. rattus.
The same distribution of the primary bubo was found to be
present in plague-infected rats in the Punjab villages.
When we compare this distribution with the distribution in rats
infected in the laboratory by means of fleas we find a remarkable
similarity, the neck glands being again most commonly involved,
those in the axilla and groin next. The following table shows at
a glance this comparative distribution :—
Rats examined
....
Bats without buboes
Cervical bubo
.....
Axillary bubo
....
Inguinal bubo
....
Mesenteric bubo
....
Naturally infected.
0,188
per cent.
16
72
23
18
0
Infected by fleas.
117
per cent.
18
61
22
35
0
Material evidence is here adduced that the rats in nature are
infected by means of
fleas.
43
Among guinea-pigs the distribution of the primary bubo in
three groups was as follows
:—
(a) A. group of 87 animals, naturally infected in plague houses.
(6) A group of 253 animals, infected in the course of the
godown experiments already referred to. (It will be
remembered that the conclusion was come to that these
animals were infected by rat-fleas, all other means of
infection being excluded.)
(c) A group of 108 animals, experimentally infected in the
laboratory by means of fleas.
The following table summarizes very shortly the distribution
of the primary bubo in these three groups of guinea-pigs.
Guinea-pigs examined
Without buboes .
Cervical bubo
Axillary bubo
Inguinal bubo
Mesenteric bubo .
Naturally
infected.
87
per cent.
1
92
1
14
0
Infected in
gcdowns.
253
per cent.
1
92
3
13
0
Infected by
fleas.
108
per cent.
1
91
6
22
0
The close correspondence between the distribution of the buboes
in these three groups of animals points to the conclusion that
the mode of infection was the same in all cases. It would follow
that both in the experimental godowns and in plague-infected
houses the infection was conveyed to the guinea-pigs by fleas.
4. Infection in plague houses due to infected rat-fleas.
We have now to consider
certain observations which
prove both indirectly and directly that in a plague-infected
house the infection is due to the presence therein of rat-fleas
which are capable of transmitting the disease to animals. These
44
observations, which were carried out in Bombay and in the
Punjab, fall naturally into four groups.
Group I.—In this group, which is divided into two series, guinea-
pigs were allowed to run free for about 18 to 40 hours in houses
in which one or more plague cases had occurred or in which dead
rats had been found. They were then removed, cleared of fleas and
segregated in the laboratory. In the first series the houses had
not been disinfected, while in the second series disinfection had
been carried out.
Series 1.—In this series we have to record observations made
in Bombay during the plague season of 1906 and again during that
of 1907, and also observations made in the Punjab villages of Dhund
and Kasel in 1906. The results were as follow:—
(a) Bombay 1906 : 42 houses tested ; 12 houses infected the
guinea-pigs, namely, 29 per cent.
(6) Bombay 1907 : 100 houses tested ; 19 houses infected
the guinea-pigs, namely, 19 per cent.
(c) Punjab villages 1906 : 49 houses tested; 9 houses infected
the guinea-pigs, namely, 184 per cent.
Of a total of 189 houses tested in this way 40, or 21 per cent.,
proved " infective " to guinea-pigs.
Series 2.—The observations in this series were made in Bombay
in 1906 in houses which had been disinfected before the guinea-
pigs were put in. In most cases the process of disinfection car-
ried out by the Health Department of the Municipality of Bombay,
consisted in washing thoroughly the walls and floors with an acid
solution of perchloride of mercury of a strength of 1 in 750, a solution
which, as has been abundantly proved, readily kills the plague
bacillus even in organic mixtures. In a few instances gaseous
disinfection with the fumes of burning sulphur was used, while in
still a few other cases a combination of these two methods was
carried out. The results were as follow :—
Thirty-one houses were tested and in nine instances (29 per
cent.) the animals became infected. It is noteworthy that in
many instances the number of fleas trapped by the guinea-pigs
was very large, as many as 263 being taken from one house,
while the average of the 31 observations was 40 fleas per house.
The significance of this group of experiments may be stated