NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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which again enter other corpuscles and go through the same cycle
This is the asexual method of multiplication [schizogony] by simple
division of a full grown parasite into a number of small ones, and
it is clear that even if originally only one corpuscle contained a
parasite, a very large number of corpuscles will, by this method,
soon become infected.
After this method of multiplication has gone on for a
certain number of days, some of the parasites in the red blood cells
instead of going on to their full size and dividing up into a number
of segments, proceed to the formation of sexual forms [gametes].
These sexual forms are, as a rule, readily distinguished from the full
grown asexual forms. Two kinds are present, viz., male forms
[microgametocytes] and female forms [macrogametocytes]. Having
attained to their full size they appear as coarsely pigmented round
or crescent shaped bodies enclosed within the thin shell or envelope of
the red blood corpuscle (Pl. I, 8, 9). In the blood of man they
undergo no further development. If, however, a mosquito bites a
man with these sexual forms in his blood, some of them are carried
into the mosquito's stomach with the blood which the insect extracts.
If the mosquito is of a particular kind (viz., one belonging
to the anopheles genus), further development of the parasites in
its stomach occurs. As a result of the mixture of the blood with
the fluids in the mosquito's stomach, aided perhaps in part by the
digestive action of the stomach juices, the thin envelope of red blood
corpuscle which protected the parasite while it was in the blood of
man, becomes disintegrated, and the parasite escapes and is free in
the fluid of the mosquito's stomach.* Immediately following its
escape from the corpuscle great changes are seen to take place
in the character of the parasite. The female sexual form becomes a
granular spherical body (Pl. I. 13). The pigment of the male sexual
form is first seen to be violently agitated, and then, from the periphery
* It will be seen by those who have studied the subject that this description
differs in detail from that usually given. It is usual to say that
the gametocytes escape from the red blood corpuscles and become free in the
blood plasma while they are actually in the blood of man, although
it is said that, for some unknown reason, the male gamete does not flagellate
while it is in the man's blood. That flagellation really does not take place
in the blood of man is proved by the fact that flagellated bodies can never
be found in freshly drawn blood. It appears to me probable that the reason
of this is that the sexual parasite has not, of itself, the power of escaping
from the corpuscle, but that some external influence such for example as
the effect of osmosis in a moist-chamber, or in the mosquito's stomach,
is necessary to break down the enclosing envelope of the red cell. As soon
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of the parasite, three or four long filaments (flagella) [microgametes]
are suddenly protruded (Pl. I, 12). After lashing about for a few
moments the flagella break off from the main body of the parasite
and swim about in the fluid in the mosquito's stomach. They are
the true male element in the sexual process. When one of them
meets a female sexual form it enters it and fertilizes the true female
element [macrogamete] contained in it (PL I, 14). After this act
of fertilization has occurred, the female parasite acquires new
characters. Changing from the shape of a sphere to that of an
ovoid with a pointed end [ookinet] (Pl. I, 15), it begins to move
about, and making for the inner wall of the stomach of the mosquito
it passes through the internal coats and comes to rest between the
epithelial and muscular layers. Here it forms a capsule round itself
and begins to grow, until in a few days it has attained a compara-
tively large size (Pl. I, 16). At this stage it is usually called a zygote,
[oocyst]. By a process of division an enormous number of embryo
parasites called sporozoites, elongated in shape and provided with a
nucleus, are formed in the full grown zygote (PL I, 17, 18). After
a time the capsule of the zygote bursts and the sporozoites are
liberated into the lymph sinuses which surround the outer surface of
the stomach. Thence they are conveyed by the circulation to the
salivary glands, and penetrating the outer wall of the gland cells
they come to rest in them and in the salivary duct, which has its
external opening at the point of one of the piercing stylets of the
proboscis which enter the skin in the act of biting. When the
mosquito nest bites a man it injects some of these sporozoites into
his blood with the salivary or poison fluid.* Having got into the
blood of man again the sporozoites after a certain time are found
in the red blood corpuscles as young parasites identical in all respects
with those from which the cycle was commenced.
These two cycles of the malaria parasite are represented
as the male parasite has escaped from the red corpuscle it is able to throw
out its flagella.
This explanation would also account for the fact that phagocytic leucocytes
do not attack sexual forms while they are in the blood of man, although as
soon as they have escaped from the protecting envelope of the red corpuscles
and changed into flagellated bodies, they do so readily, in the same way
as such leucocytes are able to attack the free spores of the asexual parasites as
soon as they have escaped from the red cells.
* It should be particularly noted that the tube down which the salivary
fluid and sporozoites pass when injected into man, is quite separate from that
through which blood is sucked up by the mosquito.
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diagramatically in Plate I, and photographs of the parasite in its
more important stages in man and in the mosquito are shown in
Plates VII and VIII.
The method by which malarial fevers are contracted,
From what has been said above it will readily be seen how
malarial fevers are contracted. A mosquito of the anopheles genus-
bites a man who has been suffering for some days from malarial
fever, and who has sexual forms of the parasite in his blood.
These sexual forms are taken into the mosquito's stomach with
the blood which it sucks in, and there they undergo the develop-
ment that I have just described, resulting after some days in
the formation of numerous sporozoites which lodge themselves in
the salivary or poison gland of the mosquito. When this mosquito
next bites a healthy man, it injects some of these sporozoites
into his blood with the salivary fluid, and so produces a
malarial infection in him similar to that of the man from whom
it originally sucked blood.
Fortunately every kind of mosquito cannot act as a suitable host
for the human malaria parasite, and in all probability only mos-
quitoes of one genus, viz., the anopheles genus, can do so. Mala-
rial fevers can therefore only be contracted by the bite of an
anopheles mosquito. It must be remembered also that malarial
fever cannot be contracted even by the bite of an anopheles mos-
quito, unless that mosquito has sporozoites in its salivary glands.
This means that it must have bitten a man with malarial fever
a week or more previously, and that the temperature and other
factors necesssary for the development of the parasites in the
mosquito's body, were favourable. Again it is not sufficient
for an anopheles mosquito to have bitten a man with malarial
fever unless sexual forms of the parasite (which alone can develop
in the mosquito) were present in the man's blood at the time
he was bitten, and in addition the sexual forms must have been
fully developed, for as a general rule, sexual forms cannot proceed to
further development even in an anopheles unless they have reached
a certain stage of maturity. Thus in order that an anopheles
mosquito may be able to carry infection it must have previously
bitten a malarial fever patient at a particular period of his case.
In malarious places, however, cases of malarial fever are so
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frequent, and the number of anopheles mosquitoes is so great,
that many opportunities occur for all the - required conditions
to be fulfilled, so that it is always possible in malarious places
to find a number of anopheles mosquitoes with sporozoites in
their salivary glands.
It has been proved almost beyond any possible doubt that there
is no other method by which malarial fevers can be contracted
than by the bite of a properly infected anopheles mosquito.
We cannot contract malarial fevers from drinking impure water, or
from the soil, or by breathing impure air, and water and air are only
indirectly connected with malarial infection in the sense that they
are the homes of the malaria-carrying mosquitoes.
What we have learnt so far may be summed up as follows :—
Malarial fevers are caused by the presence in the blood of the
patients of a small parasitic animal belonging to the lowest grade
of the animal kingdom, the protozoa. This parasite passes its life
in two hosts, viz., man and anopheles mosquitoes. In man it multi-
plies itself by a process of simple division for some days, and then
produces sexual forms, which for their further development, require
to be taken into the body of a mosquito of the anopheles genus.
This is effected when such a mosquito bites a man with these forms
in his blood. In the mosquito's body the female sexual form is
fertilized by the male, the cell which results being called a zygote.
By the growth and division of the zygote an enormous number of
new forms of the parasite called sporozoites are produced, which
lodge themselves in the salivary or poison gland and duct of the
mosquito, so that when this mosquito next bites another man, some
of the sporozoites are injected into him with the salivary fluid.
In his body they develop into the young parasitic animals in the
red blood cells exactly identical with the original ones.
As the malaria parasite does not, in all probability, pass any
part of its life in any other environment than men and mosquitoes,
it follows that men and mosquitoes are the only sources of infection
of malarial fevers. The life history of the parasite which has been
described fulfils every requirement for its nourishment and con-
tinued life, and it is therefore extremely improbable that it has yet
another phase existing perhaps in water, or in the air, or in some
other medium, because such an additional phase is unnecessary
for the continued existence of the parasite. For this reason, as
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well as on account of many experiments that have been made in
this connection, we may say with some certainty that the only
way by which malarial fevers can be contracted is by the bite of
a properly infected mosquito of the anopheles genus.
The conditions on which the prevalence of malarial fevers
in different places depends.
It has always been recognised that some districts are more
malarious than others, and it is necessary to refer briefly to some
of the conditions which determine a greater or less prevalence
of malarial fevers in one place than in another.
Prom what has been already said it is evident that the prevalence
of malarial fevers in any place will depend on the following
factors :—
(1) The prevalence of the carrier of infection, that is the pre-
valence of the particular kind of mosquito in which
the parasites can develop.
(2) The prevalence of the source of infection, that is the
prevalence of suitable cases of malarial fever from
which the mosquitoes can become infected.
(3) The presence or absence of favourable conditions for the
passage of the parasite from man to the mosquito
and for its growth in the mosquito.
(4) The presence or absence of favourable conditions for the
communication of the parasite from the mosquito to
man and for its growth in man.
We will consider these factors in order.
(1) The prevalence of the carrier of infection.—The prevalence
of adult anopheles mosquitoes in any village or town will depend
on the number of suitable breeding places, the distance of these
breeding places from the village, and the presence of favourable
climatic conditions for the life of the mosquitoes. If there are many
suitable breeding places near, and if the weather is hot and the air
moist, numerous anopheles mosquitoes will usually be found in the
houses of a village, and, as a general rule, it may be said that a
direct relation exists between the extent and nearness of breeding
grounds, the abundance of adult anopheletes * and the prevalence
of malarial fevers.
* The plural form of the word anopheles is anopheletes ; that of the word
culex is culices.
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It must not be thought, however, that there are no exceptions to
this rule. Although no place has yet been found where malarial fevers
exist in the absence of anopheletes, it is possible to find many places
in India where they are abundant and yet where malarial fevers
are almost unknown, although every condition, including all the
other factors enumerated above, appear favourable for the develop-
ment of the disease. How can we explain this condition of things ?
Recent observations have shown that the explanation for some places
of this nature lies in the fact that all anopheles mosquitoes are not
equally good, carriers of malaria in, nature. It is therefore possible
to have in one place numerous anopheletes of a species which is a bad
carrier accompanied by little or no prevalence of malarial fevers, and
in another place comparatively few anopheletes of a species which is a
good carrier accompanied by great prevalence of the disease.
This fact is a very important one in connection with the con-
ditions which determine the prevalence of malaria, and very im-
portant in connection with efforts at destruction of mosquitoes.
(2) The prevalence of the source of infection.—We have already
' seen that the source from which the mosquito derives the malaria
parasite is man. It has recently been shown that in malarious parts
of India a large proportion of the young native children have malaria
parasites in their blood and suffer from malarial fever, although,
in spite of this, they are often able to run and play about without
showing the marked symptoms which are usually at once brought
to our notice in an adult. In certain parts of India as many as
70 to 80 per cent. of the native children under ten years of age
harbour the malaria parasite, which runs its course in their blood
unchecked, because the parents of the children (though they often
recognise that the children have " fever ") do not, as a rule, think
it worth while to bring them for treatment. Many of the children
have enlarged spleens reaching as low as the level of the umbilicus
or more.
It will be easily understood that, as these children are untreated,
the parasites are able to develop in great numbers in their blood,
and that sexual forms of the parasites are very commonly met with.
These native children form in fact the chief source from which
anopheles mosquitoes become infected.
(3) Conditions favourable for the passage of the parasite from
man to the mosquito are the presence of abundant sexual forms of the
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parasite in a large number of individuals, the presence of a large
number of suitable mosquitoes, and many opportunities for the
mosquitoes to bite the infected persons. These conditions are met
with in every malarious town or village in India. The chief con-
dition necessary for the growth of the parasite in the mosquito is a
suitable temperature; this condition is also necessary for the con
tinued life and activity of the mosquito.
- (4) Conditions favourable for the communication of the parasite
from the mosquito to man are the presence of a large number of in-
fected anopheletes in a thickly populated district. The indifference of
the native (especially native children) to mosquito bites, and the fact
that they usually sleep with the greater part of their bodies uncovered,
gives them a special liability to infection.
An individual may be bitten by a properly infected mosquito
and yet not develop an attack of malarial fever, that is he may, for
some reason or other, be immune against the infection. Very few
individuals are absolutely immune from malaria, but a relative
immunity may be acquired by repeated infections and reinfections.
Thus in very malarious places, although almost all the young
children may have malaria parasites in their blood and be suffering
from malarial fever, few or none of the adults may be found
infected. The only possible explanation of this fact is that the adults
• have, by repeated infections during childhood, become relatively
immune. This immunity of adult natives in India applies only to
those whose lives have been spent in certain very malarious districts,
and the majority of adult natives enjoy little or no immunity.
To any one who has followed the preceding remarks carefully
it will be apparent that a fairly complete knowledge regarding
mosquitoes, and especially regarding mosquitoes of the anopheles
genus, is necessary not only to explain the facts regarding the
endemiology and epidemiology* of malaria, but also because
* Endemiology means the knowledge resulting from the scientific study and
investigation of endemic disease.
An endemic disease is one to which the inhabitants of a particular country
are peculiarly subject and which for that reason may be supposed to proceed
from local causes.
Epidemic means generally diffused and prevalent. A disease is said to
be epidemic in a place when it appears in a great number of cases at the same
time in that locality, but is not permanently prevalent there. If it is perma-
nently prevalent it is said to be endemic.
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it is essential to be able to recognise the malaria carrying species
of mosquitoes before any measures against them can be undertaken.
A short account of mosquitoes will therefore be given here.
The general characters of mosquitoes.*
The general appearance of a mosquito is probably well known
to all, but it may be necessary to mention that mosquitoes may
be distinguished from other flies (such as " sand-flies," etc.)
which seem to resemble them, by the following characters :—
(1) They have a long sucking proboscis.
(2) The veins of the wings are covered with scales (which can
be plainly seen with a small magnifying glass). The
veins of the wings of other flies have no scales.
The following are the stages through which mosquitoes—like
all other insects—pass before they reach the adult state. An
adult female mosquito lays its eggs on a pool or other collection of
water. In a few hours these eggs hatch into small animals called
larvæ, whose life is spent entirely in water. The larvæ of mosqui-
toes can be seen in almost any old tin or tub that has been left full
of water in a garden for some days. After a variable time, depend-
ing chiefly on the temperature, the larvæ change their shape and
character, and become small comma-shaped creatures called pupæ
which spend their time struggling to the bottom of the water and
floating up to the surface again to breathe. After one or two days
the pupæ are transformed into the adult insect or imago, which
emerges at the surface of the water from the skin or pupa-case
enclosing it, and which it leaves behind in the water. After resting
for a while on the surface of the water, the adult mosquito flies away
to seek its food from the blood of some animal or man.
The appearances of the various kinds of mosquitoes differ in
all these stages of the insect's existence, and it is important to
know these differences in order that we may be able to distinguish
the malaria-bearing from the harmless mosquitoes.
For purposes of classification, mosquitoes are divided into a
number of genera or families, each of which contains many species
or varieties, but for our present purpose it is only necessary to have
* For the different- parts of the mosquito, which are referred to in this
description, see Plates II—IV.
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a knowledge of two genera, viz., the culex genus and the anopheles
genus.*
The differential diagnosis between these two genera is very
important, because, as we have already learnt, only mosquitoes of
the anopheles genus are capable of carrying malarial fevers.
The distinctions between CULEX and ANOPHELES mosquitoes,
The differences between the eggs, larvæ, pupæ, and adult insects
or imagines, may be taken in order.
(1) The eggs of culex and anopheles.—The eggs of the culex genus
of mosquitoes are laid on the water in small black masses consisting
of several hundred eggs joined together. These " egg-rafts " or
" egg-boats" as they are called, look like small lumps of soot
floating on the surface. They can be seen on almost any tin or tub
of stagnant water that has been left lying about out of doors.
The eggs of the anopheles genus of mosquitoes are tiny rod-
shaped bodies which, as each egg is laid separately, can only be
seen plainly with the aid of a lens. Each egg is provided with a
delicate rim or float so that they can be easily distinguished from the
individual eggs of culex mosquitoes which have no float (Plate IV).
(2) The larva of culex and anopheles.—If any one who has not
seen a mosquito larva, will look in almost any tin or tub of water
that has remained unemptied in his garden for some days, he will
find there a number of small, very active creatures about \1/16\th to
¼th of an inch long, which wriggle about in the water, now diving
to the bottom and then coining up to the surface to breathe, which
they do by means of an air-tube attached near the tail. If some of
these little animals are put into a glass bottle and watched from
the side, it will be seen that, when they come to the surface to
breathe, they hang obliquely with their heads downwards and with
only the tips of their air-tubes at the surface of the water.
These are the larvæ of the culex genus of mosquitoes. The
* The words culex and anopheles are here used in their original sense
and must be understood to include the new genera separated by entomologists
on account of differences in scale structure. Thus the word culex as here used
includes the genera Culex,Stegomyia, Panoplites, etc., and the word anopheles
includes the genera Myzomyia, Pyretophorus, Mysorhynchus, Nyssorhyn-
chus, etc., of the new nomenclature. In spite of their new names the species
comprised in these so-called new genera (which formerly all came under
the term anopheles) still remain prominent malaria carriers, and from this point
of view it is unfortunate that they should be separated into different genera.