NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
45
as follows :— We have already seen that guinea pigs may live in
contact with plague-infected animals, may run about on floors
contaminated with the urine and fæces of these animals and may
eat food similarly contaminated, and still not contract the disease,
if fleas are excluded. In the above experiments, therefore, we
are led to conclude that the guinea-pigs which became infected
contracted plague, not from contact with plague-infected animals
nor from contaminated floors nor from contaminated food, but pos-
sibly from fleas infected by plague rats. This conclusion receives
support from the fact that rat-fleas, as we shall see, were caught
in considerable numbers on the animals. It receives further sup-
port from the fact, which we have already mentioned, that disin-
fection with perchloride of mercury kills all plague germs
present in the walls and floors, even when the latter are covered
with cowdung, but does not affect the fleas.
Group II.—In this group of observations, which also is made
up of two series, fleas taken in plague houses were fed on rats or
guinea-pigs in flea proof cages in the laboratory. The two series
only differ from one another in the manner in which the fleas were
cbtained.
Series 3.—In this series the fleas were taken on rats, which were
proved at the laboratory to be plague-infected. They were transfer-
red in each instance to healthy guinea-pigs. The results were as
follow :—
(a) Bombay, 1906 : 3 experiments ; 3 successes.
(b) Bombay, 1907 : 3 experiments ; 3 successes.
(c) Punjab villages, 1906 : 6 experiments ; 3 successes.
A total of 12 experiments with 9 successes (75 per cent.).
Series 4.—In this series the fleas were taken on guinea-pigs, which
had been left for some time in plague houses. They were brought
to the laboratory in test-tubes and were then transferred to fresh
animals, guinea-pigs or rats, in flea proof cages.
The results of these observations were as follows :—
(a) Bombay, 1906 : 40 experiments ; 8 animals died of plague
(20 per cent.).
(b) Bombay, 1907 : 31 experiments ; 11 animals died of plague
(35 per cent.).
(c) Punjab villages, 1906 : 25 experiments ; 7 animals died of
plague (28 per cent.).
46
A total of 96 experiments with 26 successful transmissions,
namely, 27 per cent.
It is important to note that the great majority of the fleas caught
on the guinea-pigs placed in the houses were rat-fleas (P. cheopis).
Group III.—In this group of experiments, which is also made
up of two series, the conditions were so arranged that, while all
the animals were protected from soil infection and from infection
by contact with any plague-infected rats which might be in the room
and were equally subject to aerial infection, half of them were pro-
tected from fleas and the other half were not so protected. For
each experiment a pair of animals, one protected and the other
not protected, was used.
Series 5.—In this series the protection from fleas was obtained
by means of a curtain of fine metallic gauze, the mesh of which was
slightly larger than of that used for filtering petrol. In the other
cage a layer of ordinary wire netting took the place of the gauze.
The cages were left in the room for 48 hours : they were then re-
moved and the fleas taken off the animals which were segregated.
The following were the results :—
(a) Bombay, 1906 : 42 experiments.
(1) Protected animals—all remained healthy.
(2) Unprotected animals—4 developed plague.
(b) Punjab, 1906 : 21 experiments.
(1) Protected animals—all remained healthy.
(2) Unprotected animals—4 developed plague.
(c) Total; 63 experiments.
(1) Protected animals—all remained healthy.
(2) Unprotected animals—8 developed plague (13 per cent.)
Series 6.—The experiments of this series differ from those of the
last series in that one of the animals instead of being protected
from fleas by means of a metallic gauze curtain was surrounded by an
area spread with " tanglefoot,'' a sticky, resinous preparation used
for catching flies. The minimum width of this area was six inches,
a distance which no flea can jump. In the case of the control ani-
mal the '' tanglefoot " was replaced by a layer of sand.
The following were the results of these experiments
:—
47
i(a) Bombay, 1906 : 29 experiments.
(1)." Tanglefoot '' animals—all remained healthy.
(2) " Sand" animals—7 developed plague.
Out of 247 fleas caught on the " tanglefoot'' 60 per cent. were
human-fleas, 34 per cent. were rat-fleas, and 6 per cent. were cat-fleas.
Plague bacilli were demonstrated in the stomach contents of
1 out of 85 of the human-fleas dissected and of 23 out of 77 rat-
ileas (30 per cent.).
(b) Punjab villages, 1907 : 30 experiments.
(1) " Tanglefoot " animals—all remained healthy.
(2) " Sand" animals—2 developed plague.
The species of 156 of the fleas caught on the '' tanglefoot'' was
determined ; 138 were rat-fleas, and only 18 were cat-fleas. There
were no human-fleas. Fifty-five rat-fleas were dissected, and the
-stomach contents of three were found full of plague bacilli.
The small number of successes in the experiments of Group
III in comparison with those of Group I, in which the guinea-pigs
were allowed to run free, is due to the fact that fleas did not reach
the caged animals in anything like the same number as they did the
free animals. Thus, in Series 1 the average number of fleas per
house was 20 and in Series 2, 40 ; while in Series 5 an average of
-only three fleas per house were taken on the unprotected animals.
Before passing on to the last group of observations we have
-to draw attention to the fact that by far the most common site of
the primary bubo in the animals which became plague-infected in
all the above experiments was the cervical region. We have
already pointed out the significance of this distribution and have
shown that it points strongly to an infection by means of fleas.
Group IV.—This group consists of a series of observations,
having as its object a comparison of the number of rat-fleas caught
plague houses with the number taken in non-plague houses.
In the course of the observations of Group I with free guinea-
ipigs carried out in 1906 in Bombay and in the Punjab a careful
census was made of the fleas caught in the houses. At the end of
the observations the houses were divided into two classes : (A), a
class in which the houses were definitely shown to be plague
infected, either because plague-infected rats had been found in
the house or because one or both of the guinea-pigs developed the
disease ; and (B), a class made up of the. remainder, in which the
houses were only presumably plague-infected, inasmuch as one or
more plague cases had occurred in them. The data concerning-
the flea census in these two classes of houses are summarised in
the accompanying table :—
No. of houses
examined
Total No. of
fleas caught
Average No. of
fleas per house
Class A.
Houses definitely proved
plague- infected
Bombay.
Houses not
disinfected.
13
445
34
Houses
disinfected.
10
731
73
Punjab.
18
467
26
Class B.
Houses only presumably
plague-infected.
Bombay.
Houses not
disinfected.
29
388
13
Houses dis-
infected.
21
488
23
Punjab.
31
246
8
From this table it is seen that the average number of fleas in.
the houses which were definitely proved to be plague-infected was
in each instance about three times as great as in the houses which
were only presumably plague-infected.
During the plague epidemic in Bombay of 1007 further obser-
vations of a similar nature to those mentioned above were made.
In this instance, however, a third class of houses was added, namely,
houses which were definitely known not to be plague-infected. The
three classes of houses, therefore, in which the fleas were enumerated
were as follows :—
A. Houses deemed plague-infected for one or more of the
following reasons : (1) a dead plague-infected rat or
49
rats had been found in the house ; (2) the guinea-pig—
in this series only one animal was put into each house—
which was allowed to run free in the house developed
plague and died ; (3) one or more of the fleas taken in
the house were found on dissection to be plague infected ;
(4) fleas taken in the house gave plague to a healthy
guinea-pig, on which they had been placed.
B. Houses presumably plague infected, either, (1) because a
plague case or cases had occurred in the house , or, (2)
because dead rats, which had not been sent for exa-
mination, were found in the house.
C. Control houses of the same type and situated in the same
neighbourhood as above, but which were definitely
known not to be plague infected.
The results of the flea census made in these three classes of
houses are set forth in the accompanying table, from which it is
seen that houses which were definitely proved to be plague
infected contained on an average nearly three times the number
of rat-fleas-contained in houses which were only presumably
plague infected and 12 times the number contained in houses
which were free from suspicion.
Total No. of
houses
Total No. of
fleas taken
Average No. of
fleas per house
Class A.
27
784
29
Class B.
73
755
10 3
Class C.
68
169
2 5
A further point of interest which came out of these observations
is, that out of 130 fleas which were dissected and examined the
stomach contents of 41 (32 per cent.) contained abundant plague
bacilli.
19 S
C
E
50
From a consideration of all these observations, both in Bombay
and in the Punjab, the only conclusion which can be come to is that
in a plague-infected house the infection is due to infected rat-fleas
and not to any infection of the soil or of the air.
5. The rat-flea the agent of transmission of infection between
the rat and man.
We may now state the evidence which can be adduced in favour
of the theory that the rat-flea is the agent and the only agent of
transmission of the infection from the rat to man.
In the first place, we have to consider the question of the path
by which the plague bacillus effects an entrance into the human
organism.
In the case of primary pneumonic plague there is no question
that the infection is contracted by inhalation. Pneumonic cases
are, however, rare (2 5 per cent.) and play little or no part in
the spread of epidemic plague. While this is true of pneumonic
plague, almost all observers are agreed that, as far as the common
forms of the disease are concerned, man becomes infected through
the skin, the primary bubo developing in the glands in connection
with the lymphatics which drain the area of skin through which the
plague bacilli gain entrance. A large series of experiments in guinea-
pigs, in which infected fleas were fed on definite areas of skin,
supported this opinion, inasmuch as they showed that in this case
at any rate the bubo always developed in the glands in connection
with that area of skin on which the fleas were fed. This being
admitted, at one time it was thought that Indians, going about as
they do bare-footed, usually became infected by bacilli in the soil
getting rubbed into abrasions on the feet. This hypothesis was
founded chiefly on the observation that a large proportion of plague
cases have the bubo in the groin. The Indian Plague Commission,
however (Vol. V, p. 70), pointed out that in proportion to the area
of skin which drained into them the glands of the groin were not
more commonly affected that the glands of the axilla or of the
neck. Thus, they pointed out that the areas of skin surface,
which drain respectively into the glands of the neck, the axilla
and the groin, stand to each other approximately as the figures
51
1 : 1 8 : 5, while the numbers of cervical, axillary and inguinal
buboes stand to each other severally in the relations expressed
by the figures 1: 1 3: 5 8. Further, it has also been shown
that in communities wearing shoes the percentage of groin
buboes is no less than in those who go about bare-footed.
It would appear, then, from these considerations that in the
case of man there is no seat of election, as it were, of the skin area
through which the plague bacillus
enters.
In considering other possible modes of entrance of the plague
bacillus, the Indian Plague Commission came to the conclusion that
man never or only in the very rarest instances became infected
through the digestive tract. Within recent years, however, certain
workers in Hong-Kong, as a result of clinical experience, post-mortem
examination and feeding experiments, have favoured this method
of infection. The evidence brought forward by them is by no
means convincing, and the results of the post-mortem examinations
and of the animal feeding experiments have not been confirmed by
other observers.
In conclusion, we may take it, therefore, that all the evidence
which is available points to man becoming infected through the
skin and is in favour of the agent of infection being of such a nature
as the flea.
In the second place, we have already seen that both in Bombay
and in the Punjab there is a definite time relationship between the
rat epizootic and the epidemic.
In the case of Bombay City the average interval, as deduced
from the curves showing the relation in time of the rattus epizootic
and the epidemic, was 10—14 days, while in the case of the village
of Kasel in the Punjab the same interval was about a week. The
Bombay epidemic curve, however, was calculated on plague deaths,
while the Kasel curve was on plague attacks. The interval in the
latter case would, therefore, be shorter by a few days than the
Bombay interval. Further, in the village of Kasel an opportunity
was obtained of estimating the average interval between the finding
of dead plague rats in the houses and the occurrence of the first
plague case in them. In five houses, in which plague rats were
found on only one occasion prior to the human case, the average
E 2
52
interval between the two was ten days. In 14 houses, in which
plague rats were found on more than one occasion prior to the
human attack, the average interval between the finding of the first
plague rat and the first case was 12 days, while the average interval
between the last plague rat and the first case was seven days.
These intervals correspond closely with those deduced from, the
curves representing the rattus epizootic and the epidemic both in
Bombay and in Kasel itself. This interval is best explained on the
supposition that there is an intermediate agent between rat and
man. Assuming the truth of the flea theory it is divisible into
three periods as follows :—
(a) A period elapsing before the rat-flea, coming from the dead
rat, bites man, namely, three days.
This period of three days is deduced from a large number of ob-
servations made with rat-fleas in the laboratory. In these experi-
ments it was found that rat-fleas deprived of food for from
72 to 96 hours attack and feed on man most readily.
In this connection it is also to be noted, as will be shown
later, that the percentage of infected fleas taken from plague,
rats is greatest for the first four days and after that rapidly
diminishes.
(6) Incubation period of human plague, namely, average of
three days,
(c) Duration of illness of fatal human plague cases, namely,
average of 5½ days.
The periods (a), (6), and (c) make up the interval between the
rattus epizootic and the epidemic in Bombay, and the periods
(a) and (b) the interval in the village of Kasel.
In the third place, we have already seen that in experiments
in plague-infected houses the only possible explanation of the re-
sults obtained with animals, i.e., guinea-pigs, was that the rat-flea
and the rat-flea alone was the agent by means of which the animals
became infected, and that the infection in the houses was solely
due to the presence therein of infected rat-fleas. Further, in several
of these experiments and in some experiments in the godowns,
which have been referred to already, monkeys were infected under
the same conditions, that is to say, under conditions which excluded
all other means of infection but infected rat fleas. It has, of course,
53
been impossible to make direct experiments with man of a similar
nature to those described and one is always reluctant inferen-
tially to transfer the results of experiments made on animals to
man. In the present instance this reluctance, it appears to us,
would be completely overcome, if it could be shown that rat-fleas
will at all readily use man as a host. With this end in view a large
number of observations have been made by the Commission which
confirm those already made by Liston in Bombay. These observa-
tions are summarized as follows—
(a) Observations made in the laboratory.
In the course of the large number of flea experiments made in
the laboratory it was often noticed that, when a man's hand is put
into a cage containing rat-fleas, the fleas will jump on the hand and
if given time will feed on it. When the fleas are starved they will
more readily bite man and most readily when the starvation process
has lasted for from 72 to 96 hours. While this is so, it was also
noticed that unless the fleas were very numerous they would not
attack man in the presence of their natural host, the rat.
Further, it was found that rat-fleas may be attracted, to mar,
jump on him but take some time to feed on him. This last ob-
servation is of importance in connection with the question
of importation of infected fleas from place to place. Fleas might
be carried by man from one place to another without infecting
him, but would, when brought near a rat, attack and infect it in
preference to man.
Finally, it was ascertained that rat-fleas could be kept alive
for some considerable time, namely, 3—4 weeks, by feeding them
on human blood alone. Rat-fleas when starved never survive
longer than a week.
(b) Observations in the course of the godown experiments.
In the course of the godown experiments, to which reference
has been already made, fleas in considerable numbers have often
been taken on the legs of men who entered the godowns only for a
short time. As many as 44 fleas have been captured on the legs
of a man who went inside four times in quick succession and
remained in the godown each time no longer than was required to
pick up and remove a cage
54
(c) Observations made in houses in Bombay.
In the course of the observations in Bombay the members of the
Commission had very often occasion to enter plague-infected houses.
On many instances they caught rat-fleas on their own persons as
well as on those of the attendants. In one house in particular, which
the inhabitants had evacuated after a very severe rat mortality,
many rat-fleas were caught on successive days on a man who
went inside only for a short time ; thus, on April 18th, 51; on
April 19th, 34, and on April 20th, 60 rat-fleas were taken in this
way.
From these various observations it is evident that the rat-flea,
P. cheopis, under certain circumstances, especially in the absence
of its true host, the rat, is attracted by man and will readily bite
and feed on him.
Additional evidence of a more indirect nature might be brought
forward in support of our present thesis, but it would appear to
be unnecessary.
We can, therefore, now safely conclude that the rat-flea and the
rat-flea alone is the agent of transmission of the plague bacillus from
rat to man.
(6) Further observations on the, transmission of plague by fleas.
We have now reached a stage when we may profitably pause and
consider some subsidiary problems which naturally follow on the
demonstration of the part played by the rat-flea in the transmission
of plague from animal to animal in nature. These problems are —
(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.
(6) Whether a single rat-flea can transmit the infection.
(c) Whether both male and female rat-fleas can transmit the in-
fection.
(d) Whether other species of fleas, as well as P. cheopis, can
transmit the infection.
55
(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.
A study of the anatomy of the mouth parts and of the alimentary
canal of the rat-flea shows that the blood is sucked up from the
wound made by the pricker and is then passed down the gullet by
successive waves of contraction of the muscles from before back-
wards into the stomach, which is guarded anteriorly by a valvular
arrangement.
The stomach is a pear-shaped organ occupying a considerable
part of the abdomen of the insect. That the stomach is capable
of containing a considerable amount of blood is apparent from the
observation that after a flea has fed nearly the whole of the
abdomen is seen to be filled with a bright red mass.
In order to arrive at some conception of the chance of a flea
taking plague bacilli into its stomach and of the number likely to
be imbibed from the blood of a rat dying of plague, the Commission
made both approximate measurements of the average capacity
of the flea's stomach and accurate estimations of the average
number of live plague bacilli contained in the blood of rats dying or
recently dead of plague. The technique of these operations is
too complicated to describe here ; it is sufficient to state the follow-
ing conclusions :—
The average capacity of a rat-flea's stomach has been approxi-
mately estimated to be 0 5 c. mm. The blood of a plague-infected
rat before death may contain as many as 100,000,000 bacilli
per c. c. If, therefore, a rat-flea imbibed the blood of such a rat
it would receive into its stomach 5,000 germs. Further, it is
evident that a flea that imbibed the blood of a rat containing
10,000 or more germs per c.c. would take some bacilli into its
stomach. Now, it was found that the blood of about two-thirds
of plague-infected rats contains more than this number of germs,
so that rab-fleas in sucking the blood of plague rats in nature must
have ample opportunity of taking bacilli into their stomachs.
Evidence was next obtained on the question of the multiplica-
tion of plague bacilli in the stomach of the flea. Fleas were fed
on plague-infected rats until the death of the latter animals. They
were afterwards fed on healthy animals, a fresh animal being sup-
plied each day, so that it was impossible for them to imbibe any