were unfavourable with such substances. Fermi and Lumbau
[1900] tested many substances without marked success. Celli
[1901] states that the old custom of hanging bags containing
garlic or camphor round the neck originated in their protective
effect against mosquitos.

We should observe that experiments on the point must



264 PREVENTION [Sect.

be very exact to be convincing ; that the effect of volatile
substances is sure to be transient ; and that the use, even
of successful protectives, can only be exceptional, as when
mosquitos are very numerous, because few people will ever
be persuaded to smear themselves constantly with them, and
because they would probably affect the skin after a time.
Such protectives do not come at present within the domain
of practical sanitation.

(8). Constant movement. — As a general rule (but not always),
mosquitos bite only when their victim remains quiet. Hence
I have formed the habit, when in danger of being bitten, of
maintaining constant small movements of the hands, feet and
head. I find this, together with the use of a hand fan, far
preferable to the employment of hot gloves, thick clothing or
inunctions.

(9). Fans and punkas. — A palm leaf can be purchased almost
anywhere in the tropics for a penny, and serves not only to
drive away mosquitos, but also to keep the body cool.

Fans rotated by electricity or small hot air engines are now
frequently used in the tropics by those who can afford them.

Punkas consist of a board or canvas stretched upon a
wooden framework, hung by one edge from the roof, and
swung backwards and forwards by means of a rope or pulley
worked by hand labour or (in barracks and hospitals) by
machines. The constant movement and the current of air
prevent mosquitos biting even a sleeping person. Probably
the alternating movement has a better effect for this purpose
than the uniform movement of a rotatory fan. The punka
should not consist merely of a pole with a fringe of cloth
hung from it, because the yielding fringe gives no current
of air.

(10). So7ue results. — Perhaps the first example of the utility
of ordinary bed-nets was cited by myself [22nd July 1899]. In
October 1898 a company of the "Boys' Brigade" of Calcutta
went to live for one week in a house in Barrackpur Park, near



35] RESULTS OF PROTECTION 265

the River Hoogly. The three officers and a friend slept in
bed-nets, but thirteen boys did not. The officers and their
friend remained well ; but all the boys, together with a sister
of one of them, were attacked a few days later. The first boy
attacked, died. Mosquitos were very numerous. Three native
servants (without nets) also suffered.

In 1899 also many experiments were made on the Italian
railways by screening stations and houses of employees. The
results were summed up as follows by Celli [1902]. Out of
4,363 completely protected persons, r9^ became infected and
2i'i% had relapses. Out of 5,165 incompletely infected persons
3*3^ became infected and 20^ had relapses. Out of 802 persons
protected only during sleep iO'9^ became infected. The local
fever index was 40-60^, or even more.

Laveran [1907] quotes an experiment among Japanese
troops in Formosa, of whom, out of 646 unprotected men,
there were 285 cases of malaria, and out of 115 protected
men there were none ; and also cites several similar small
experiments in Corsica, Algeria, Holland and Senegal. In
nearly all of these cases, however, quinine as well as protective
screening was used. The experiment of Sambon and Low
(section 7) served a useful purpose at the time in advertising
the mosquito theory, but there was no very strong scientific
evidence that the subjects would have been infected during the
experimental period, even without the protection.

Further experiments should be performed in order to ascer-
tain the effect of mechanical protective measures alone.

35. Mosquito-reduetion. — This can be effected {a) by direct
destruction of mosquitos or their larvae, and {b) by rendering
houses or localities less suitable for them or for the breeding
of them.

(i). Destruction of adults by hand. — Large numbers of
mosquitos can be destroyed in houses by a very simple device
— a small hand-net. This is made for a shilling or two by



366 PREVENTION [Sect.

tying a piece of flexible cane or other wood or wire in the
form of a loop, to which a bag of white netting about i8 inches
deep (4"5 decimetres) is attached. The net must be white and
not green, in order to enable us to see the captured insect
through the mesh. Any one can easily secure his persecutors
by means of it, and thus obtain respite for some time.

Sir William MacGregor employed a boy to kill mosquitos in
this manner in his house in Lagos in 1901. In lofty rooms a
long-handled net may be required to catch the insects under
the roof.

Colonel Gorgas and Mr Le Prince advocate (sections 42
and 43) the employment of men for this purpose and give
examples. I think that every householder should possess such
a net and should instruct his servants to use it regularly. The
same thing should be done in hospitals, barracks, military camps
and jails.

Much more could be accomplished by this simple method
than has yet been attempted. Boys become very expert
at catching mosquitos. The offer of rewards of a penny
for fifty Anophelines may quite possibly lead to a large
slaughter by village children. Probably, for unit of cost
and trouble, more mosquitos can be killed by hand than by
fumigation.

(2). Fumigation. — This method has long been employed,
especially in Italy and the United States. The discovery of
the carriage of yellow fever by Stegomyia calopus gave great
stimulus to it for sanitary purposes. In yellow fever it is
absolutely necessary that every Stegomyia which has bitten
the patient should be killed for fear lest it may infect other
persons ; and this is rendered easier because the yellow fever
patient remains infective only for three days, and because
vS. calopus is a domestic insect. But destruction of adults is
not so useful for the prevention of malaria, because patients
continue to be infective at irregular periods for a long time,
and because the carriers are often wild insects (section 29 (4))



35] DESTRUCTION OF MOSQUITOS 267

which may remain only an hour or two in houses. Hence
the expensive and troublesome process of fumigation is not
much employed in malaria prophylaxis.

Celli and Casagrandi made experiments with many sub-
stances [Celli, 1901]. Thick tobacco smoke and chloroform
appear to be the best, and have been known for a long time.
Sulphur dioxyde, hydrogen sulphide, coal gas, formaldehyde, kill
in one to two minutes (but, curiously enough, acetylene gas seems
to be quite innocuous). Fumes of pyrethrum powder causes
apparent death in five minutes and actual death in eight hours.
Numerous agents for the destruction of insects in greenhouses
are on sale and are equally effective against mosquitos.

Boyce [1909] describes very thoroughly, and with good
illustrations, the methods of fumigation practised against
yellow fever in America. His recommendations have already
been given (section 12).

In India fumigation is seldom used : and it would be
difficult to apply in the thatched mud huts or leaf shelters
of the poor. But I think that destruction by hand could be
employed more than it is.

Mosquito-traps were also described in sections 12 and 60.
Wells, cellars, privies, stables and dark rooms are natural
traps in which the insects often congregate in large numbers
and can easily be destroyed.

(3). Introduction of 7iatural enemies. — Bats, birds, lizards,
dragon flies and other flies are known to catch mosquitos ;
but whether their presence makes any marked effect on the
number of the insects is another question, which, so far as
I know, has not been studied.

It has been known for a long time that small fish devour
the larvae ; and indeed the larvae were used for feeding the
fry of British trout introduced into India, in my memory,
before 1890. Fish, however, vary greatly in their liking for
the larvae. Thus in Mauritius the small gold fish which abound
in ornamental waters eat them, but not voraciously. On the



268 PREVENTION [Sect.

other hand, I saw in India in 1890- 1899 minnows which would
devour a dozen or more in a few seconds and would visibly
swell after the meal. The waters of Panama contain a
minute fish, less than an inch long, which is equally fond of
them, and which thrives in quickly running streamlets. The
same is true of the common small fish at Ismailia. On the
other hand, large fish often disdain such small prey. I saw
a ditch in Calcutta full of small fish and also absolutely thick
with mosquito larvae ; and have observed fish and larvae
living together in rice fields.

Boyce [1909] well describes the larvicidal propensities of
Girardinus poeciloides, popularly called " millions," a minute
fish probably allied to the one which abounds at Panama.
Mr C. Kendrick Gibbons in 1905 attributed to it the immunity
of Barbados from malaria. The people keep these fish even
in water receptacles, of which Boyce examined sixty without
finding larvae. The Imperial Department of Agriculture in
Barbados has introduced millions into Jamaica, St Kitts and
Antigua. Further references to larvae-eating fish will be found
in Chapter VII I.i

Tadpoles, Notonecta (the water boatman), and the larvae
of several insects eat mosquito larvae ; and indeed the latter
are often cannibals.

This part of the subject is of great importance and is
now receiving considerable attention. Obviously, the success
of the measure must depend largely upon the habits of the
local malaria carriers — whether they breed mostly in perma-
nent pools where fish and their other enemies also can live, or
whether they abound elsewhere, as in evanescent rain-water
pools. The question depends entirely upon the local con-
ditions and requires local study.

No cheaper and better anti-malaria measure than the

^ J. Chaytor-White suggests the importation of "millions" into India [1910].
He reproaches me for not recommending the measure for Mauritius. This was due
to oversight, and the fish are being introduced.



35] ENEMIES OF MOSQUITOS 269

extermination of Anophelines by natural enemies could be
conceived or devised — were such a thing possible. It may be
possible in some places ; while for other places we may not
be able to find the suitable enemy. Unfortunately, experience
proves that animals and plants often survive, even in large
numbers, the introduction of their most deadly enemies ; they
succeed in finding some refuge or other, and a state of com-
mensalism is finally arrived at. For example, Anophelines
abound in the Panama Canal Zone in spite of the fish referred
to. It is only the academical hygienist who believes that
any measure which suggests itself is sure to be practically
useful. It may often happen, too, that the expense of main-
taining the " enemy " will be greater than that of removing
the breeding water entirely. Nevertheless, we should seek
what allies we can find.^

It has often been observed that parts of India where
M. rossii, which seem not to carry plasmodia, abounds are
comparatively free from malaria. Quite possibly it may
tend to crowd out other species, thus reducing them below
the malaria-bearing limit, although it does not exterminate
them entirely. Experiments on this point are much required,
and I have suggested them tentatively for Mauritius. The
comparative freedom of large tracts where the conditions
appear to be favourable must be due to some cause. If we
can ascertain the factor we may be able to introduce it else-
where (section 30 (10) and (u)).

H. Marcus Fernando, in a suggestive paper just to hand
[1910], points out that though Colombo, Ceylon, is surrounded
by low-lying grass fields, subject to periodic inundations, it
is practically immune from malaria. This he attributes to
swarms of minnows and other enemies of mosquitos. He
states also that very hot dry seasons are often followed by
much malaria, simply because they tend to destroy the
aquatic enemies of the larvae. These views accord exactly

^ See also section 54.



270 PREVENTION [Sect.

with those already expressed in section 30 (i), (6) and (10).
So also rice fields are often not malarious — as in Madras town.

(4). Destruction of larvae — The methods employed depend
on where the larvae are found.

For small movable receptacles of water, such as broken
bottles and crockery, empty tins, flower - pots, gourds, old
buckets and tubs, fire-buckets, fallen palm leaves, etc., emptying
out suffices for the moment. But such articles ought to be
collected in suitable dust-bins, and the laws regarding the
proper deposition of rubbish ought to be strictly enforced.

Oiling. — This method, long known,^ was first elaborated as
a public health measure in my reports on Sierra Leone [1899].
It is employed wherever the previous methods are inapplicable,
or where the water is required for use, or where more permanent
measures cannot be adopted at the time — that is, for water
butts, metal cisterns of drinking water, wells, garden cisterns,
washing waters, cesspits and so on. The method has now
become familiar to all. The oil is poured on the water and
broken up so that it quickly forms a thin film all over the
surface. The larvae and pupae are no longer able to keep
the surface by surface-tension and quickly drown — Laveran
thinks that the oil chokes the breathing tube [1907].

The oil may be poured on the water out of a tin or kettle,
or by means of a special syringe. For small pools, I advocated
that each man engaged on the work should be provided with a
tin of oil, and a bunch of rag tied to the end of a stick. The
rag is dipped in the oil, which is now " painted," on the surface
of the pool — but such details are unimportant.

It is best to use the cheapest oils in the local market.
Refined petroleum spreads rapidly and well, but evaporates
quickly. Crude petroleum is cheaper and makes a much
more lasting film. For Ismailia I recommended an equal
mixture of the two. Celli [1901] said that 0"i c.cm. was the
minimum amount required per square metre of surface, but Fermi

^ Howard [1893] and Nuttall[ 1 899].



35] POOLS AND HOLES 271

and Lumbau [1900] require 5*0 c.cm., and others various amounts.
Experiments in vitro are of little use ; and the amount required
depends on the quality of the oil, the heat of the sun, the amount
of vegetation, dSris, etc., and should be determined locally.
Logan Taylor and I found crude creosote to be very useful in
Sierra Leone ; it is more deadly, but also much more expensive
than crude petroleum. For killing larvae in drinking waters,
Strachan recommended me to use eucalyptus or juniper oil.

For pools it is advisable, and indeed sometimes necessary,
to remove vegetation before applying the oil.

My brother, E. H. Ross, described an automatic oiler for
cess-pits at Port Said, and Gorgas and Le Prince mention
" cotton waste " soaked in oil.

Celli and Casagrandi [Celli 1901] and others describe
experiments with many substances. Aniline dyes are active
destroyers. Gallol and Green Malachite kill to a certainty in
solutions of 0'Oi25/iooo and O'25/iooo, and a mixture called
" larvicide " appears to be very effective. They diffuse them-
selves continuously in water, are not volatile, and do not poison
men, cattle, or crops. Celli says that larvicide costs from 12 to
56 lire for destroying larvae in 10,000 cubic metres of water.
It is said to have little action on the pupae.

Oiling should also be used for the tins of water placed under
the feet of tables and meat-safes in order to exclude ants ; and
for saucers under water bottles, flower-pots and filters.

Small pools of water on the ground, especially those which
form after heavy showers, or in rocks along the margin of
streams, or in gutters, open drains, and stone water-channels,
can often be quickly brushed out, water, larvae, and all, by
means of a broom.

Holes m rocks and trees are best dealt with simply by
filling up with earth or gravel as a temporary measure, or
with concrete as a permanent one.

Plants which breed mosquitos should be removed if possible.
In Mauritius, Bilbergia splendida^ a wild pine-apple with a



272 PREVENTION [Sect.

gorgeous flower, was planted in most of the gardens and kept
the houses supplied with innumerable Scutomyia notoscripta
Skuse, until the wiser inhabitants rooted them up during and
after my visit. Cut bamboo stems should be cut again flush
with the next knot, or with the ground. Unfortunately,
mosquitos often breed in palm trees and sugar cane — in
which case I can recommend nothing but grease being applied
at the off-shoot of the leaves. The householder has often to
determine whether he will sacrifice his plants to his comfort.
It is mostly the Qilicines which breed in such situations.

Rain-water gutters along the eaves of houses are best dealt
with by boring a small hole in the most dependent part of the
gutter where the water stagnates. Heavy old metal cisterns,
etc, not worth removal, can often be treated in the same manner,
(5). The screening of breeding waters, — We now proceed to
consider methods for rendering localities or houses less suitable
for mosquitos or for their breeding. We will deal first with
what I call " minor works."

Mosquitos, more especially the Culicines, breed very much
in collections of water, such as water - butts, cisterns, wells,
cess - pits, etc., which cannot be dispensed with, and which
should be so protected that the insects cannot lay their eggs
in them. At the same time it is often necessary to ventilate
the water. For this purpose we should apply covers which
are so scrupulously well fitted that no insect can enter, and
at the same time protect the ventilating orifice by wire-gauze.
In many countries tubs are used outside houses to collect
rain-water from the roof, A wooden top should be closely
fitted and a hole of about 6 inches or more in diameter
should be cut in it. Nail down a piece of wire-gauze on the
outside of the orifice. The pipe which brings the water from
the roof discharges over the orifice, so that the water flows
directly into the barrel, being drawn out by a tap below.
Similar devices can be employed for cisterns, but if these are
made of metal and have badly fitting covers, considerable



35] WELLS AND CESS-PITS 273

difficulty is often experienced in making the proper arrange-
ments, which must be left to the reader.

The subject of wells is of great importance in the tropics,
as they are often the only source of drinking water. For the
usual hygienic reasons the people should never be allowed to
lower buckets and ropes into wells, as this frequently leads
to serious defilement of the water. In my opinion every well
should be furnished with a closely fitting cover, the water
being drawn by means of a pump of some kind. But in
order to ventilate the water, a hole protected by strong wire-
gauze should be cut in the cover. I have seen excellent wells
of this kind in several localities, notably at Bathurst, Gambia.

Cess-pits are frequently used, especially in French possessions,
and are ventilated by long pipes which discharge at the roofs
of the houses. We observed at Ismailia that Culicines were
breeding in large numbers in these pits, though they were
entirely closed except for the ventilating shafts, down which
adults found entrance and exit. To prevent this it is necessary
only to protect the ventilation with strong wire -gauze. In
badly-made cess-pits there are other orifices by which the
insects can enter. These should be done away with as much
as possible. If they cannot be completely filled up, the pits
must be regularly oiled by some method (section 53).

(6). Dealing with breeding-holes. — I have already mentioned
that holes in trees and rocks should be filled with concrete,
that is, with a mixture of Portland cement and sand or gravel.
The best mixture for the locality should be determined by
local engineers. Two men must work together, one carrying
concrete and the other laying it into the holes. Mosquito
breeding-holes in trees are sometimes very difficult to find,
men or boys having to climb the trees for the purpose. Hence,
when they are found their position should be marked on the
tree by a conspicuous patch of paint for future identification.
Inexperienced persons are apt to think that such work is
altogether too much for human weakness to accomplish, but

s



274 PREVENTION [Sect.

as a couple of men can generally fill fifty holes a day, it is
not long before most of the breeding - places round houses
are disposed of. In many cases, moreover, the water in such
holes can easily be liberated by cutting. If concrete is not
available, earth, gravel, or stones often suffice perfectly for a
considerable period. A little trouble guided by common sense
is all that is required.

Garden cisterns often occasion much difficulty. Water is
required for irrigating plants, and is often allowed to stagnate
for weeks in the cisterns. The best way to deal with them
is to direct that not too many of these cisterns be allowed to
remain, so that those which are permitted to do so shall be
in constant use. The frequent agitation of the water by the
gardener generally suffices to check mosquito-breeding in it.
The gardener should be instructed to deal with his irrigation
water in such a manner as not to cause breeding of mosquitos.
Here, again, details can be better left to the reader.

Irrigation pits are often employed for cultivation, and are
found to contain Anophelines, which live especially in the
presence of grass and vegetation occurring at the edge of
the water. These are often difficult to deal with, as the
people occasionally require the water. If possible, they should
be filled in, but if this is not possible, we can often render them
unsuitable for larvae by dragging out the weeds and " backing
up" the margin of the pit by filling with large stones, the
owners being instructed to prevent the new growth of vegetation,
A gang of men will deal in this manner with a number of
such pits in a day.

Borrow pits, that is, pits from which earth has been taken
for building purposes, are found everywhere, and are not
always easy to deal with. They may often be filled up
with ordinary town rubbish, but this should be ultimately
packed into them closely and covered with a layer of earth.
In some places they can be filled up at greater expense with
stones brought from a distance, if such are available. Where



35] BORROW-PITS 275

they cannot be filled up, the best plan to deal with them is
by the method just suggested for irrigation pits. Where no
stones can be obtained, and where the water cannot be drained
away, our only resource is to deepen the bed. Make the banks
" steep-to " all round, and clear away all the vegetation, leaving
the water as before. Such places are particularly suitable for
the introduction of natural enemies of larvae ; but they must
be examined from time to time to see whether they are really
free from mosquitos.

A number of small questions of this nature are apt to
rise in connection with cultivation, and must be left to the
ingenuity of the director of the works. Railway companies
often cause much trouble by their borrow pits along the course
of railways, and they continue to do so in spite of the protests
of medical men. This is a point which should always be taken
note of by the Government of the colony, who should insist
upon the borrow pits being so made as to allow the water to
run off. I know that in absolutely flat country this is difficult,
if not impossible ; but in such cases much can be done by
making regular pits without marshy borders, and by insisting
that these shall be kept free of vegetation, and shall be stocked
with fish, etc.

Built conduits and drains. — By this I mean water-courses
and drains which run through masonry. In the course of time
the masonry is apt to wear away, thus leaving breeding-holes.
These holes can be dealt with by rebuilding, or by filling with
concrete.

(7). Training the banks of streams, rivers and lakes. — The
word " training " is one used by engineers to signify rendering
banks more regular or solid. Untrained streams and lakes
are apt to spread over the ground beyond their margins, and
to cause innumerable small breeding-pools, and the object of
the training is to check this tendency. The director of the
work must be guided by circumstances how this can best be