NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
THE DESTRUCTION OF FLEAS BY EXPOSURE
TO THE SUN.
INTRODUCTORY.
T
HERE can be no doubt that plague is frequently carried from one
locality to another by means of clothing and baggage which contain
infected fleas, and it follows that any means by which such clothing can be
rendered rapidly and effectually flea-free will be of considerable value in
preventing the spread of the disease.
There is unfortunately considerable difficulty in obtaining an insecticide
thoroughly suited to this purpose.
Plague is now so universal in India that the disinfection of the clothing of
new arrivals should be carried out as a routine measure in every small town and
village. This, however, would entail an amount of work which would be al-
together impracticable unless it were carried out by the people themselves.
A means of disinfection is, therefore, required which is of a simple and
harmless nature, and can be used in all parts of the country without causing
much expenditure. It is safe to say that no efficient disinfectant has yet been
discovered which combines all these necessary qualifications.
In India, however, where the seasons are regular and where there is con-
tinuous sunshine for the greater part of the year, the sun has been employed
as an agent in this matter.
Exact experiments to test the value of the sun as an agent by means of
which " soiled clothing " could be freed from fleas were first initiated by
Lieut.-Colonel Bannerman, I.M.S., and in the report of the Bombay Bacterio
-logical Laboratory for the year 1907 the results of the experiments carried out
under his dirction were detailed. These experiments showed that an exposure
of three hours is sufficient to remove all the fleas from infested rags.
As these first experiments were undertaken to see if rags intended for
export could in this way be rendered safe the ultimate fate of the fleas was not
at that time investigated, and it was thought that these might survive by
sheltering under grass and leaves.
The Government of India having subsequently (Home Department Noti-
fication No. 1822, dated 10th August 1908) removed the restriction against the
export of rags, etc., from infected localities, further experiments were not
made. The subject was again taken up when, as a result of the work of the
Plague Commission, it became clear that there was real danger of importation
of plague by fleas in travellers' clothes or baggage. It then became important
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to find out some cheap and simple method of ridding these articles of fleas
and, at the suggestion of Colonel Bamber,I.M.S., then acting SanitaryCom-
missioner with the Government of India, various reputed powders were tried.
Of these, the only one at all successful was Naphthaline powder, but this acted
very slowly requiring 6½ hours to kill fleas in a closed space and 9 hours in the
open air (vide Annual Report of the Bombay Bacteriological Laboratory for
1909). It was, therefore, out of the question as a pulicide for use by villagers,
who would require something cheap and of speedy action.
It was then that, on the suggestion of Colonel Bannerman, I.M.S., the
1907 experiments were repeated and extended, as detailed in the following
report.
Before passing on to a description of these experiments, however, it will be
necessary to enquire into the various factors which influence the power of the
sun, because any changes of this kind which occur must necessarily alter its
value as an insecticide. Sunlight and heat are generally believed to be the
chief agents in the natural disinfection which is continually being carried on,
and we may assume for the present that these two factors are responsible for
the sun's action on flea-infested clothing.
Now the amount of light and heat which reach the earth are not always
the same, but are liable to changes, which are due to differences of geographi-
cal position, the season of the year and also certain atmospheric conditions.
As a rule the light is not affected to the same extent as the temperature,
in fact there may be actually no change in the former, while the difference in
the latter is very considerable.
For instance, if we examine the changes which are due to geographical
position, we find that the amount of light in different parts of India is more or
less constant, while the temperature varies very considerably according to the
latitude,
The changes which are due to the season of the year and time of day
require no explanation. Everyone knows that the sun is more powerful in the
hot than the cold weather and that the hours about noon are as a rule hotter
and brighter than any other part of the day. These changes are fairly constant
from year to year and day to day.
Lastly, the changes which are due to atmospheric conditions remain to be
considered. The most common cause of these in India is the wind, which is
capable of altering the temperature of the ground according to changes in its.
direction and velocity. At certain times of the year too, small clouds which
pass over the sun without actually producing rain diminish to a certain extent,
the amount of light and heat which reach the earth.
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These changes are not very great throughout the greater part of the year
but they are extremely irregular because the atmospheric conditions which
produce them are hardly ever the same for any length of time. It is clear,
therefore, that the power of the sun is constantly varying within certain limits,
and it is necessary that the conditions which produce these changes should be
considered and carefully recorded in any experiments which are carried out to
test the value of the sun as a disinfectant. The experiments which were per-
formed with this object consisted in exposing flea-infested clothing to the sun
under different conditions. They were carried out in the grounds of the
Bombay Bacteriological Laboratory between the months of December and
June. This period of the year in Bombay corresponds to the latter half of the
cold and the whole of the hot weather. The sun was, therefore, gradually in-
creasing in power during the greater part of the time that the experiments
were proceeding. The total change of power which occurred in this way, how-
ever, was not so great as it would have been in some other parts of the country.
The time of day at which the exposures were made depended upon the
strength of sun required. The experiments which tested its full power were
carried out about midday, but when lower temperatures were required the
exposures were made earlier in the morning. The presence of any wind or
clouds was also noted, but in the majority of experiments these conditions did
not affect the results to any appreciable extent.
The experiments were carried out :
—
(1) In large tin trays, specially designed, so that the movements of the
fleas could be watched throughout the experiment.
(2) On a piece of ground which was prepared after experience gained in
the tray experiments.
The trays were made from sheets of tin rivetted together ; they measured
4 feet broad by 4 feet 6 inches long. Their sides were 4 inches high and stood
at right angles to the bottom. The upper borders of these were turned over on
themselves towards the centre of the tray so as to prevent the escape of any
fleas which succeeded in jumping to the top of the sides. The floor was covered
with linoleum which was coloured so as to imitate the ground as closely as
possible. Any crevices or cracks which existed between the tin and the
linoleum were filled up with putty so as to prevent them being used by the
fleas as a shelter against the sun. The covering of linoleum was necessary
owing to the uncovered tin becoming so hot when exposed to the sun, that the
fleas were scorched and died almost instantly when placed on it.
The description of the piece of ground used for the second series of experi-
ments will be better understood if it is given immediately before the account of
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the experiments themselves. The fleas used for the experiments were rat-fleas,
which had been removed a short time previously from Bombay rats. A large
number of these rats are brought to the laboratory daily throughout the year,
so that the vitality of the fleas was assured.
Dhurries (cotton carpets) were used in the majority of cases to represent
the native clothing. These were selected because they are thicker and more
closely woven than any other kind of material found in native baggage, and for
this reason would be more likely to resist the rays of the sun. While the
experiments were in progress, it was suggested that padded clothing, such as
resais (bed quilts) and quilted coats, might also require disinfection, so that
certain of the experiments were repeated to test the action of the sun on this
type of clothing also.
An indication of the power of the sun was obtained by taking the
temperature on the upper and under surfaces of the clothing. The thermo-
meters were watched carefully during the experiments, and in those cases in
which the temperature throughout the exposure remained within certain limits
the reading which was made at the end of the experiments was the only one
recorded. In some of the later experiments, which were carried out a short
time before the monsoon arrived, the sun was repeatedly overclouded during
each exposure, and a more complete record of the variations in temperature
had therefore to be kept. All the readings have been made according to the
Fahrenheit scale.
The report has, for convenience, been divided into four parts. The first of
these deals with the action of the sun on fleas in clothing, and the fate of these
insects when they have left the clothes. The second describes the experiments
undertaken to devise a method by means of which the sun can be used to the
best advantage as a disinfectant for flea-infested clothing. The third deals
with the experiments which were carried out with the resais. Lastly, an
account of certain experiments which were performed with a view to designing
a simple form of thermometer, has been added as an appendix.
5
PART I.
THE ACTION OF THE SUN ON FLEAS AND THE FATE OF THE FLEAS WHEN
EXPOSED TO IT.
In the experiments published in the Bombay Bacteriological Laboratory
report for 1907, it was shown that rags if spread on the ground in thin layers
were freed from fleas in three hours. In these experiments, however, the
kind of ground upon which the rags were spread was not regarded as of any
consequence. This is an important point, because the fleas on leaving the
clothes must jump on to the ground, and their fate will therefore depend
upon the amount of shelter they can find there.
In the following experiments the question has been treated from this point
of view and observations have been made upon the different ways in which
fleas behave when the clothes are spread out on different kinds of ground.
These experiments were carried out on trays, and in the first of these (Ex. I,
page 17) the clothes were spread on the floor of the tray alone, which had been
made to represent a hard piece of ground with a smooth surface. In the second
series (Ex. II, page 18) the floor was covered in the one case with sand, and in
the other with grass and leaves, to test the behaviour of the fleas when the
clothes were spread on these two kinds of ground.
The procedure was briefly as follows :—
A piece of ground was selected in the most sunny part of the compound,
and a sun shelter roofed with asbestos sheets was erected over it. Both the
control and experimental trays were placed side by side beneath this while the
experiment was being prepared. The dhurries were first spread upon the
floors of the trays, and the same number of fleas were put on each, half the
total number being placed below and half above each dhurrie. The thermo-
meters were then placed in position (in the first experiment, however, the
temperatures were not taken). After a sufficient time had elapsed to allow
the fleas to become accustomed to their new surroundings the experimental
tray was drawn out into the sun, the control tray being left under the shelter.
While the experimental tray was in the sun the behaviour of the fleas in it
was very carefully watched, and when the time of exposure had elapsed the
final reading of the thermometers in both trays was noted and the tray was
pushed back under shelter again. The dhurries were then shaken. up and four
guineapigs were placed in each tray to capture any fleas which were still alive.
At the end of three hours these guineapigs were lightly chloroformed and
searched for fleas, and the total number found on them was recorded. In ex-
periment I (page 17) the experimental tray was kept in the sun for varying
c
6
lengths of time so as to find out the exposure which had the strongest effect on
the fleas. In experiment II (page 18) the exposure which had been previously
found to be most suitable was employed throughout.
As soon as the experimental tray was drawn into the sun the fleas could
be seen moving off the upper surface of the dhurrie. In trying to escape the
glare they turned their heads away from the sun and travelled towards the
side of the tray which faced them. As the floor of the tray grew hotter, the
fleas, which had by this time arrived at the side, began to die, and at the end
of seven minutes none of these remained alive.
A reference to Table I (page 17), however, will show that this exposure
was not nearly long enough to kill the fleas which were on the under surface
of the dhurrie, and it was found that the tray had to be left in the sun for 45
minutes before all the fleas in it were killed.
The majority of the fleas underneath the dhurrie did not try to escape, but
were found dead on the floor of the tray when the dhurrie was raised.
The results which were obtained by spreading the clothes on sand (Table
II, Nos. 1 and 2, page 18) were much more marked.
The surface of the sand appeared to absorb the heat of the sun faster than
the hard surface of the tray, and for this reason any fleas which jumped off the
dhurrie were almost immediately killed. At the end of the experiment, too,
dead fleas could be seen lying on the sand when the dhurrie was lifted up,
and no fleas were to be found on any of the guineapigs. The fleas were care-
fully watched to see if they made any attempt to burrow into the sand to
escape the glare and heat, but they showed no inclination to do so at any time.
In fact as soon as they reached the sand, it appeared as if their chief object
was to jump off it again as soon as possible.
In the experiments in which the dhurries were spread on leaves and
grass (Table II, Nos. 3, 4 and 5, page 18) the results were not so satisfactory.
Those fleas which could be watched certainly left the clothes as soon as they
were exposed, but they immediately dived under the grass and so obtained
ample protection from the sun. This was proved by the fact that at the end
of 45 minutes exposure many of them were recovered on the guineapigs, which
had been put in the tray at the end of the experiment. The fate of the fleas
beneath the clothing appears to depend on the amount of heat which is deve-
loped on the ground underneath the clothes. When the temperatures are high
(Table II, Nos. 1, 2, 3 and 4, page 18) all the fleas are destroyed, but when it
is comparatively low (as in No. 5) a large number of them are able to escape.
This appears to show that the heat of the sun is the factor which causes the
destruction of the fleas. The high temperature which is obtained underneath
the clothes spread on sand is probably due to the fact that the clothes are in.
7
close apposition to the ground, and the sand is therefore able to absorb the
heat without much difficulty. When lying on grass, however, the clothes are
never in close contact with the ground but are separated from it by a space
through which the air can circulate and which therefore prevents the heat
reaching the surface of the ground.*
The conclusions which have been obtained from these experiments may
be briefly summed up as follows :—
(i) The pulicidal power of the sun appears to be chiefly due to the
amount of heat which it gives off.
(ii) The fate of the fleas, which escape from clothes exposed to the sun,
depends on the nature of the ground upon which the clothes are
spread. On hard ground, and more especially on sand, the fleas
are all destroyed ; on grass they will in all probability be able
to find shelter and therefore escape.
(iii) The time of exposure necessary to destroy all the fleas is about 45
minutes when the clothes are spread on hard ground, and is pro
bably somewhat less when sand is employed.
* This has been corroborated by observations made in various parts of the Laboratory compound. The tem-
perature of the ground under the grass is seldom above 95° when the thermometer reading in the sun is 13
0 º
Oro v e r .
c 2 :
9
PART II
EXPERIMENTS CARRIED OUT TO DISCOVER THE MOST PRACTICAL METHOD OF
USING THE SUN AS A. PULICIDAL AGENT FOR FLEA-INFESTED CLOTHING.
The conclusions in the previous section show that clothes can be readily
freed from fleas by spreading them out on a flat surface covered with a layer
of sand which is exposed to the sun.
The practical value of this method of disinfection, however, will entirely
depend, firstly, upon the number of hours per diem the sun will be able to
yield a sufficient amount of heat to destroy the fleas within a reasonable length
of time; and, secondly, upon the area of ground which will have to be covered
with sand to prevent the escape of any of these insects.
It has already been shown that clothes require to be exposed to the sun
-for 45 minutes to kill all the fleas, and it must be acknowledged that this
period is well within the limits of " a reasonable length of time."
If, therefore, the smallest amount of heat which is necessary to destroy
the fleas in 45 minutes is discovered, it will be easy to calculate the probable
number of hours per diem during which this method of disinfection can be
employed.
The experiments which have been performed with this object were carried
out in the trays, and their technique is precisely the same as that of the ex-
periments performed in the previous section. The differences in temperature
were obtained by making the exposures at various times of the day, and
although it was obviously impossible to arrange that each successive degree of
heat should be tested, a good series of temperatures has been obtained,
The results are shown in Table III (page 19). The experiments have been
arranged in order according to the temperature found to be present on top of
the carpet. No attention has been paid to the date or time of day at which
the exposure was made. With a few exceptions, however, it will be seen that
the low temperatures were obtained early in the morning, and the higher ones
later on in the day. The time of exposure in the experiments carried out
earlier in the year, was 45 minutes, in. the later ones this was extended to one
hour to allow for experimental error. A glance at the table (Table III, page
19) will show that the temperatures between 95° and 112° are not strong
enough to destroy all the fleas, although there is a great diminution in their
numbers in the two last experiments which registered 112°. From 116°u
pwards all the fleas were killed. The minimum. temperature, therefore,
lies between 112° and 116°,—probably 116° itself, because in two experiments
which were carried out by gradually heating a test-tube containing fleas by
immersion in warm water, it was found that the exact temperature at which
11
The horizontal piece was 8 inches wide, the larger of the two verticals 6
inches high, and the smaller, one inch. At one end, the larger vertical plate
extended beyond the rest of the side for 8 inches, so that it might overlap the
horizontal part of the one joined to it at right angles. The four sides were
placed upon the ground at right angles to each other, with the horizontal part
flush with the ground and the smaller vertical pushed into the sand so as to
prevent the fleas from crawling underneath
F I G . I I I .
At the angles of the square the horizontal part of one lay in close apposi-
tion to that of the one at right angles to it (Fig. II) and its projecting vertical
part reached across the horizontal part and so joined closely with the vertical
part of the other. The small gap between the two horizontals was covered
over with a narrow strip of paper.
FIG.III.
The whole when spread out on the ground had the appearance seen in
Fig. III and a square was obtained, the area of which could be altered and
12
which was surrounded by a flat tin surface 8 inches wide. This surface was
covered with a mixture of white paint and resin just before any experiment
was performed. The vertical part of the tin sides acted as a shelter which
kept the sand outside the enclosure from blowing on to the paint when there
was any wind.
The experiments were performed by making the square of such a size
that the tin sides were a certain known distance away from the edge of the
carpet. The rest of the technique was the same as that of the tray experi-
ments. The carpet laid in the middle of the square was infested with fleas by
sprinkling an equal number on both sides of it. The exposure was then
made and at the end of an hour the sides of the tray were lifted, taken into
the shade and the white paint searched for any fleas which might have
jumped on to it. The presence of fleas on the paint proved that these insects
were able to travel the distance between the edge of the carpet and the tins,
and in the next experiment the distance between the two was increased. At
the end of the experiment and before the removal of the tin walls four
guineapigs were allowed to run about inside the square, but no fleas were
ever found on them in any of the experiments. All experiments, with the
exception of the first, which was a trial, were made at the time of day most
likely to give a maximum temperature of about 120° Fah.
A control experiment was not performed with this series as it did not
appear to be necessary. The experiments were carried out in two ways, in the
first series (Ex. IV, part I, page 20) the sand on. the square was kept in the
shade until the experiment was prepared and was then exposed to the sun. In.
this way the method was tested under the most adverse circumstances, because
the sand was quite cool and had to absorb the heat after the experiment had
started, thus allowing the fleas to travel over a greater distance than they
otherwise would have been able to do had the sand been exposed to the sun
beforehand.
The second series (Ex. IV, part II, page 21) the square was prepared in
exactly the same way as it would be in practice. The thermometers were
placed on the surface of the sand in the early morning and were watched until
they registered a temperature of 120°. The carpet was then prepared and
placed in the centre of the square, and the rest of the experiment was carried
out in a similar way to that which has already been described. In the first
series of experiments (Table IV, page 20), it will be seen that the fleas were
able to travel across four feet of sand before they were destroyed by the heat.
In the other series (Table V, page 21), however, when the sand was previously
heated, a distance of two feet was sufficient to destroy all the fleas.
These experiments demonstrated very clearly the way in which the fleas
13
always travel away from the sun. As the experiments were carried out in
the early morning the bearing of the sun was always a few degrees south
of east, and the fleas were always found in largest numbers on the western
side of the square, and a few were seen on the northern border, while none
were ever found on the east or south borders. Thus in one of these experi-
ments 36 fleas were seen on the north, 67 on the west, none on the east and
none on the south; and on several occasions the western border was the only
one on which any fleas were discovered.