mosquito plays a part in the infection. The negroes of the Usambara
Mountains, who acquire the disease when they descend to the plains, even
use the same word to denote the disease and the mosquito. In Assam, in
Italy, and in Southern Tyrol, the belief in the mosquito origin of
malaria obtains. Experienced travellers, like Livingstone, Emin Pasha,
and General Gordon, insisted on the importance of mosquito-nets,
thinking that the netting ‘acted as a filter against the malarial
poison,’ and knowing by experience that its presence diminished the
tendency to the disease. The whole epidemiological evidence was put
together in a masterly essay on the mosquito theory, read before the
Philosophical Society of Washington in 1883, by Professor A. F. A. King.
There was thus a considerable body of opinion in favour of the
mosquito-malaria theory, when, in 1894, Manson explained his views to
Major Ross, at that time a surgeon in the Indian Medical Service.
Manson’s own epoch-making researches on _Filaria_--another human
parasite whose intermediate host is the mosquito--no doubt strengthened
his faith and helped to encourage Major Ross, who in 1895 began in
Secunderabad a series of investigations, which, after much weary work,
were crowned with brilliant success. The difficulties of the work were
very great. Hardly anything was known about the great number of gnats
and mosquitoes which are found all over India, and it was often
impossible to have them accurately determined. Then no one could predict
the appearance of the parasite within the body of the mosquito--if it
were there--or in what part of the body it should be looked for. The
mosquito had to be searched cell by cell. The difficulty of dissecting a
mosquito is great even in temperate climes, and when we recollect that
hundreds of all the available species were dissected in the most
malarious districts in India, we must recognize that it was only a faith
akin to that which moves mountains which sustained the courage and
stimulated the perseverance of the tireless worker. For nearly two years
and a half Major Ross searched in vain. No matter what species of
mosquito he worked at, the results were negative. A less determined man
would long ago have abandoned the research; Major Ross only tried new
methods. At Sigur Ghat, near Ootacamund, a peculiarly malarious
district, he noticed for the first time a mosquito with spotted wings
which laid boat-shaped eggs. Shortly afterwards he was able to feed
eight specimens of this mosquito on a patient whose blood contained the
parasites in the gametocyte stage--and it should have been mentioned
above that all mosquitoes dissected were first fed upon the blood of
malarious patients. Six of these insects were searched through and
through, organ by organ, but without result. The seventh showed certain
unusual cells in the outer surface of the stomach, which contained a few
granules of the characteristic black pigment or melanin of malarial
fever. The eighth and last specimen showed the same characteristic
cells with the same characteristic pigment; but the peculiar cells,
quite unlike anything hitherto met with in the mosquito’s body, were
larger and further developed. ‘These fortunate results practically
solved the malaria problem.’
Without following in detail the various stages of the further
investigations carried on by Major Ross, we must endeavour to give an
account of the final results obtained by him and later investigators.
Being unable to obtain material for the study of malaria in man owing to
the scare caused by the outbreak of plague amongst the natives, Ross
worked out the life-history of an allied organism which causes malaria
in birds. It is to the brilliant researches of the Italian
school--prominent among whom are Grassi, Bastianelli, and Bignami--that
we owe the first complete accounts of the life-history of the human
parasite. It has already been explained that some of the parasites do
not form spores, but persist in a more or less unchanged condition
whilst in the blood of man as gametocytes. We have also seen that when
removed from the human body some of these gametocytes throw off actively
mobile filiform bodies. In 1897 MacCallum of Baltimore showed what these
filiform bodies really are. Certain of the gametocytes do not produce
them, but lie passively still on the microscope-slide, or in the blood
within the mosquito’s stomach. These are destined to form the female
cell; the filamentous bodies which break off from the first-named
gametocyte were seen by MacCallum to fuse with them, and, in fact, to
play the part of the male cell or spermatozoön. This, in fact, happens
when a mosquito feeds on a malarious patient. The gametocytes, unchanged
in the blood of man, as soon as they reach the stomach of the insect,
swell and burst from their red corpuscle. The male gametocyte throws off
the filiform bodies, which actively swim about seeking a female
gametocyte (Fig. 2, 1). When found they fuse with it, and thus produce a
fertilized cell or zygote (Fig. 2, 3). This zygote is produced on the
microscope-slide, and in the alimentary canal of certain mosquitoes, but
so far as is known at present it undergoes further development only in
the stomach of the various species of the mosquito genus _Anopheles_. In
all other cases it dies or is digested. In _Anopheles_, however, the
zygote travels to the walls of the stomach, pierces the inner coats and
comes to rest underneath the muscular tunic which ensheaths that organ
(Fig. 2, 4 and 5).
At first the zygote is very small, about the size of a red
blood-corpuscle; but it grows, and in the course of about a week it has,
roughly speaking, increased to five hundred times its original bulk
(Fig. 3, 1 and 3). Its contents have not only increased, but have
divided into some eight or twelve cells, called meres; and each of these
meres has given off round its periphery a number of filiform cells,
called blasts (Fig. 3, 2). The structure of the mere, with its coating
of blasts, may be easily understood by a zoologist when it is mentioned
that it very closely resembles that stage in the formation of the
spermatozoa of the earth-worm just before the spermatozoa separate
themselves from the blastophor; the lay mind may gain a better idea of
its appearance by recalling the head of a mop. As the zygote, still
resting on the outside of the mosquito’s stomach, matures, the cells
which are giving rise to the blasts diminish in size and disappear,
leaving the capsule packed with thousands of minute filiform slightly
spindle-shaped blasts (Fig. 3, 3). Then the capsule bursts and the
blasts make their way into the body-cavity, or space between the stomach
and the wall of the mosquito’s body. It is not known whether they have
any movement of their
[Illustration:
FIG. 3.--FORMATION OF THE BLASTS OF HÆMOMENAS PRÆCOX (ROSS) WITHIN
THE BODY OF THE MOSQUITO ANOPHELES. MAGNIFIED 2,000 TIMES. AFTER
ROSS AND FIELDING-OULD.
No. 1, The full-grown zygote dividing up into meres; No. 2, an
isolated mere which has developed its filiform bodies or blasts;
No. 3, the zygote crammed with blasts is bursting; No. 4, the
blasts are making their way into the salivary gland of the mosquito
_a_, through it into the œsophagus _b_, and finally into the
proboscis _c_.
_To face page 144._]
own, but in some way or another they make their way into the salivary
glands of the insect and accumulate in the cells which secrete the
saliva. Thence the blasts pass into the salivary duct and down the
grooved proboscis of the insect (Fig. 3, No. 4). The next time the
mosquito has a meal off a man, some of these blasts will be washed into
the man’s blood by the saliva which causes the irritation set up by a
mosquito’s bite. It is known that when an infected insect bites a
healthy man malaria ensues; and though the blasts have not hitherto been
seen to enter the blood-corpuscles, they certainly give rise to the
disease, and it can hardly be doubted that they force their way into the
red corpuscles and form the young amœbulæ which we described at the
beginning of this article.
The appended scheme will perhaps make clear the very diverse phases of
the somewhat polymorphic organisms. Those stages which occur in the
blood of man are printed in ordinary type, but those which occur in the
mosquito are in italics:
AMŒBULÆ.
|
+----------------------+----------------------+
| | |
Sporocyte. Female Gametocyte. Male Gametocyte.
| | |
Spores | |
(in liquid of blood). | |
| _Female Gametocyte._ _Filamentous bodies_ or
Amœbulæ. | _Spermatozoa_.
| | |
Sporocyte. +----------------------+
| |
| _Zygote._
Spores |
and _da capo_. _Meres._
|
_Blasts._
|
Amœbulæ.
|
+-----------------------+----------+-----------+
| | |
Sporocyte. Female Gametocyte. Male Gametocyte.
The foregoing account of this varied and romantic life-history is no
hypothetical one. With the exception that, so far as we know, no one has
yet seen the blasts enter the corpuscles and become amœbulæ, every
stage in the story has been verified over and over again by competent
observers, and their observations are now accepted by all whose opinion
in such matters has weight. Further, the facts here recorded are not
peculiar to parasites in man. Allied forms of Protozoa attack other
vertebrates, and, in fact, the first hæmatozoön whose life-history was
thoroughly worked out by Ross was the _Hæmamœba (Proteosoma)
relicta_, which causes a malaria-like disease in birds, and is conveyed
from one bird to another by means of the common gnat, _Culex pipiens_.
Again, the parasite which causes so much loss to stock-owners, the Texas
fever organism, _Pyrosoma bigeminum_, is, thanks to the researches of
Smith and Kilborne, now known to be conveyed from one ox to another by
the cattle-tick, _Boöphilus bovis_. Thus, however strange the
life-history of the malarial parasite may seem to the unscientific, it
is very much what might have been expected by zoologists who have worked
on allied organisms, and it is vouched for in its main features by the
most expert workers in England, France, America, Italy, and Germany. The
whole literature of the subject of transmission of disease by insects
has been ably sifted and brought together by Dr. Nuttall in a monograph
whose title is mentioned in the Bibliography.
For two years and a half Major Ross dissected mosquitoes, looking for
traces of the malaria organism and finding none, but at last found what
he sought in a species of mosquito that had hitherto escaped his
attention. This means that, like most other parasites, the Hæmamœbidæ
will develop in one kind of animal and in one kind only. If taken up by
another kind they are simply digested. The mosquito with the
[Illustration: _ANOPHELES MACULIPENNIS._
MALE, IN CHARACTERISTIC ATTITUDE.]
[Illustration: _ANOPHELES MACULIPENNIS._
FEMALE.
_To face page 146._]
spotted wings and boat-shaped eggs undoubtedly belonged to the genus
appropriately named _Anopheles_; and only the species of this genus, so
far as we know, are capable of conveying the infection from man to man.
In their bodies only will the gametocytes develop. If swallowed by other
biting insects or by leeches, etc., they disintegrate, and are no more.
The word mosquito has no scientific import; derived from the Spanish or
Portuguese, it simply means ‘little fly’; it is used popularly to denote
a gnat which bites, and most gnats bite when they have a chance. The
word is sometimes extended to include certain midges. The Dipterous
family, Culicidæ, to which the gnat belongs, contains, according to
Major Giles, some 242 species, divided amongst 8 genera. The great
majority of species, some 160, however, belong to the genus _Culex_;
_Anopheles_ includes 30; whilst the remainder are divided amongst the
other 6 genera, none of which are large. The collections which have been
made at the British Museum, and which were worked out by Mr. Theobald,
contain many species of _Anopheles_ new to science; so that we have now
some half hundred species of the genus ‘which has been hopelessly
convicted of being the medium by which the malaria parasite is
transmitted from person to person.’ According to the last-named
authority, we have in England 17 species of _Culex_, and 2 of
_Anopheles_, _A. bifurcatus_ and _A. maculipennis (claviger)_, though
some authorities are inclined to add a third, _A. nigripes_. Five other
species, belonging to the smaller genera of Culicidæ, make a total of
some 24 species of gnat or mosquito found in England. _Culex pipiens_,
probably the commonest gnat the wide world over, conveys the parasite
_Proteosoma_, or, as Ross now calls it, _Hæmamœba relicta_, of the
avian malaria from bird to bird; but it will not carry the parasite of
human malaria. Indeed, 14 different species of _Culex_ have been tried
in this respect, and in each case with negative results. The same nice
adjustment of parasite to host is found in _Anopheles_. It will not
convey the bird malaria, that is to say, the gametocytes are destroyed
in its body, but it is readily infected by the human parasite, and at
the present date a considerable number of species have been successfully
tried, and this not only in Europe, but in Africa, India, and the United
States.
_Anopheles_ is obviously worth studying. It has now been found very
commonly distributed in England, _A. maculipennis_ abounding in the
eastern counties. Its boat-shaped eggs, laid, not as are those of the
genus _Culex_, in little rifts, but singly, give rise to a charming
little larvæ, whose diet of minute algæ gives a greenish tint to the
centre of the body, which elsewhere is of a brownish hue. When at rest,
these small larvæ float on the water parallel with the surface, and not
hanging down into the water as does the larval _Culex_. They have a most
beautiful arrangement of minute hairs, arranged like the ribs of an
umbrella turned inside out, along the upper surface of their backs, and
by the action of these hairs they hang on to the surface-film. Their
breathing organs open near the tail, but are not produced into the long
respiratory tube by which the _Culex_ larva can be so easily recognized.
They possess the most marvellous arrangements on the head for setting up
currents conveying food to the mouth, and, in fact, they afford one of
the most charming objects of ‘animated nature’ that one could desire to
watch. After some days, varying in number according to the temperature,
the larva turns into one of those curious active Dipterous pupæ which
are well known in the case of other gnats. Like the larva, the pupa
floats at the surface of the water. When mature its integument splits
along the back; then the perfect insect steps out, rests a moment to
dry its wings, and sails away into the air.
It is very doubtful if the male _Anopheles_, which can easily be
distinguished from the female by its bushy feathered tentacles, quite
visible to the naked eye, ever sucks blood. The habit in the female is
possibly prompted by a desire to obtain material for the growth of the
ova. Out of the numerous genus _Culex_ only four species are known in
which the male bites; and it is probable that malaria is always conveyed
from man to man by the activity of the female. It is difficult to say
how long mosquitoes live in the imago state--certainly, if fed, for many
weeks. The earlier collectors, not knowing how to feed them, used to
cork them up in glass tubes, and then, noticing in a day or two that the
poor insect had died, retired to their studies and wrote moral essays on
the brevity of life, or learned treatises on the duration of life in
relation to the methods of ovipositing. Now we feed the imagos--as a
rule, on bananas--and they live well in confinement. The fertilized
female survives the winter, hibernating in some dusky corner, and it is
probable that some of the eggs also carry the species over the cold
months from autumn to the following spring.
It should, perhaps, be mentioned that the infected mosquito does not
transmit the parasites to its offspring. This was an important point to
ascertain, because it is known that the tick which causes Texas fever
does transmit its parasite to the young ticks, and they in turn
communicate the disease to the oxen. A somewhat similar case of the
transference from parent to offspring of an organism causing disease is
that of the Pébrine, caused by a parasite which attacks silkworms, and
which is conveyed by the infected ova from one generation to another.
The above short résumé of the life-history and habits of _Anopheles_ has
been given as a prelude to the important question: What can be done to
diminish malaria? A few years ago, before we understood the cause of the
disease, much had been done to lessen it. While aiming at other objects,
we drove malaria out of England by draining. Now that we know the secret
of the disease we can direct our efforts more intelligently. There are
two points exposed to attack. The first is the sporulating organism in
the blood of man, the second is the insect. If we could eliminate the
organism from man, the mosquito would be saved much suffering, and would
be powerless to infect man; or, if we could prevent the mosquito from
access to man, either by guarding him against its bites or by killing
off the insect, the hæmotozoön would, in the course of time, gradually
die out.
Both methods should be tried. Malarious patients should, so far as
possible, be treated with quinine, and no effort should be spared to
free their system from the parasite. Special precautions, such as
hanging up mosquito curtains, etc., should be taken to prevent the
access of the mosquito to the patient; otherwise he acts as a centre of
infection. It is almost equally important to protect the healthy man
living in a malarious place. The mosquito net must be carefully made,
and let down over the bed well before sunset; its free edges should be
tucked under the mattress, and the greatest care should be taken to
prevent the ingress of a mosquito, especially when slipping within the
curtains. Punkahs should be employed as much as possible; they certainly
tend to keep the _Anopheles_ at a distance. In the summer of 1899 an
experiment was initiated by Sir Patrick Manson which must convince even
those least open to conviction that malaria is preventable if proper
precautions be taken. That the bite of an infected mosquito can convey
malaria may be taken as proved by the voluntary submission of Mr. T. P.
Manson to the experiment, as recounted in the _Times_.[5] This
gentleman allowed himself to be bitten, in this country, by insects
previously fed on malarious patients; and in due course the
disease--tertian ague--showed itself in him. To prove the other side of
the case required even more courage and endurance. During the spring of
1899, Dr. Low and Dr. Sambon, of the London School of Tropical Medicine,
with Signor Terzi, an Italian artist, and two servants, have been living
in a mosquito-proof hut, near Ostia, in the Roman Campagna, and remained
in perfect health. The spot selected for this experiment is so malarious
that the Romans regard spending a single night there as equivalent to
contracting a virulent type of malaria. Yet, when Professor Grassi and
several other experts visited the mosquito-proof hut on September 12,
1900, they found the inhabitants in perfect health--a fact which they
telegraphed, with their salutations, to Sir Patrick Manson, ‘who first
formulated the mosquito-malarial theory.’ The conditions under which Dr.
Low and Dr. Sambon and their Italian companions lived were all directed
to the avoidance of being bitten by mosquitoes. During the daytime they
were allowed out of their hut, because the chance of being bitten in
broad daylight is so small that it may be neglected; but they were
‘gated’ an hour before sunset, and were not allowed out until an hour
after sunrise. The mosquitoes were kept out of the hut by the use of
wire-gauze doors and windows. By these precautions contact between
mosquito and man has been avoided, and man has now lived for months in
one of the most malarious spots in Europe without acquiring a trace of
malaria. It is most satisfactory to record that a similar success has
attended the efforts of the Italian authorities to improve the state of
things in the great plain of Salerno. Visitors to Paestum and
Battipaglia cannot fail to have noticed how malaria has marked that
district as its own. By taking such precautions as are indicated above,
the peasants and railway signalmen have, during the last few years, for
the first time, escaped the disease; whilst for the first time newcomers
to the district have failed to contract it. The intelligent activity of
the Italian Government, and the well-known interest taken in the
question by the King and Queen of Italy, cannot fail to have a
profoundly beneficial effect upon the lives of some of the poorest and
most hard-working of European peasantry.
The problem in Africa is more complex, owing to the fact that the native
population is thoroughly permeated with the parasite. Mr. Christophers
and Dr. Stephens, in their ‘Further Reports to the Malaria Committee,’
have shown that the children of natives are in the great majority of
cases infected with malaria. In one village where the _Anopheles_ was
found in ‘considerable numbers,’ 90 per cent. of the babies suffered, 57
per cent. of the children up to eight years, 28 per cent. of the
children up to twelve years, after which age the children were ‘very
rarely infected.’ This is but one example out of many, all tending to
show that after a time a certain immunity to the disease is acquired,
and, further, that travellers should as far as possible avoid the
neighbourhood of native villages, and, above all, decline to sleep in
native huts.
The destruction of the mosquito, at any rate in neighbourhoods inhabited
by man, is a matter of difficulty, but is worth attempting. To expect to
destroy the mature insect seems a vain thing, but the larva can be more
easily dealt with. _Anopheles_--unlike the common gnat, which breeds
close to houses, in cisterns, garden fountains, old tubs, drains,
etc.--prefers rain-water puddles, natural hollows by the roadside, small
ponds, and rice-fields. We have occasionally found the larvæ of
_Anopheles_ and _Culex_ in the same water in England, but this is
probably exceptional. In England, so far as our experience goes, the
_Anopheles_ larvæ are usually met with in shallow water easily heated by
the sun’s rays; and we have always found them in association with the
common green water-weed _Spirogyra_, though they are not known to eat
this.
Attention to the standing water round houses or near towns will do much
to diminish the scourge of mosquitoes. All pots and pans containing
water should be regularly turned out once a week, and puddles should be
brushed out. The larva takes some seven days to develop, so that once a
week suffices to destroy each brood. All useless water should be drained
away and stagnant ponds filled up. The introduction of fish has markedly
diminished the number of mosquitoes around the late Mr. Hanbury’s
celebrated garden at La Mortela on the Riviera. They eagerly devour the
larvæ, and should be made use of in all large areas of water. For
smaller areas some ‘culicide’ should be tried, and more experiments in
this direction are urgently needed. One of the simplest remedies known
is kerosene oil. A piece of rag tied to a stick should be dipped into
the oil, and then applied to the surface of the water. The oil diffuses
in a fine film over the surface and clogs the breathing tubes of the
larval insect; it possibly interferes with the action of the surface
tension--at any rate, the larvæ die. Fresh tar has the same effect. This
‘painting’ of the water must be renewed once a week. Wells and cisterns
should be kept closed. A more careful selection of the site for houses,
and a more liberal use of wire-netting mosquito shutters, will do much
to minimize the risk to Europeans in malarious districts.