MOSQUITOES AND MALARIA
Ever since the beginning of history we have records of certain fevers
that have been called by different names according to the people that
were affected. As we study these names and the various writings
concerning the fevers we find that a great group of the most important
of them are what we to-day know as malarial fevers. Not only are these
ills as old as history but they have been observed over almost the
entire inhabited earth. There are certain regions in all countries where
malaria does not occur, but almost always it will be found that other
regions near by are infected and it very often happens that these
infected regions are the most profitable parts of the land, the places
where water is plentiful and vegetation is luxuriant. Indeed the
coincidence of these two things, low-lying lands with an abundance of
water, particularly standing water, and malaria has always been noted
and gave rise to the earliest theories in regard to the cause of the
disease.
For instance, we find some of the very early writers emphasizing the
point that swampy localities should be avoided for they produce animals
that give rise to disease, or that the air is poisoned by the breath of
the swamp-inhabiting animals.
These views of the origin of the fever prevailed until about the
beginning of the eighteenth century when the recently discovered
microscope began to reveal the various kinds of animalculæ to be found
in decaying material.
In 1718 Lancisi held that the myriads of insects, particularly gnats or
mosquitoes, that arose from such swampy regions might carry some of
these poisonous substances and by means of their proboscis introduce
them into the bodies of the people, and although he had made no
experiments to test the assumption he did not consider it impossible
that such insects might also introduce the smallest animalculæ into the
blood. It took almost two centuries of study and investigation before
this guess was proved to be right.
One reason why the mosquitoes were not earlier associated with these
diseases was that all who investigated the matter at all turned their
attention to the bad condition of the air in these swampy regions.
Malaria means bad air. We all know that we can see the mists arising
from such regions, particularly in the evening or at night, and as
exposure to these mists very often meant an attack of malaria they were
naturally supposed to be the cause of the disease. So for a long time
the whole attention of investigators was turned toward studying and
analyzing these vapors, and various experiments were made which seemed
to show conclusively that the malaria was caused only by these
emanations. The investigations even went so far that the exact germs
that were supposed to cause the fever were separated and experimented
with.
THE PARASITE THAT CAUSES MALARIA
The blood had been studied time and again and the characteristic
appearance of the blood of a malarial patient was well known. In 1880
Laveran, a French army surgeon in Algiers, began to study the blood of
such patients microscopically and soon was able to demonstrate the
parasite that caused the disease. His discoveries were not readily
accepted, but other investigations soon confirmed his observations and
the fact was gradually firmly established. Not until recently, however,
did this distinguished physician receive a full recognition of his work.
A few years ago he was awarded the Nobel prize for medicine, perhaps the
highest honor that can be bestowed on any physician. It is interesting,
too, to note in this connection that it was another French surgeon who
in 1840 discovered that sulphate of quinine is a specific for malaria.
[Illustration: FIG. 96--Horse and cattle tracks in mud filled with
water; good breeding-places for Anopheles.]
[Illustration: FIG. 97--A malarial mosquito (_Anopheles maculipennis_);
male.]
[Illustration: FIG. 98--A malarial mosquito (_A. maculipennis_);
female.]
The next important step was made in 1885 by Golgi, an Italian, who
studied the life-history of the parasite in the blood and distinguished
the three forms which cause the three most familiar kinds of malarial
fevers, the tertian, the quartan and the remittent types. From this time
on this parasite has been studied by physicians of many nationalities
and the whole course of its life-history worked out. In order that we
may understand how it was that mosquitoes were determined to be the
means of disseminating this parasite we will discuss first its
life-history in the human blood.
The parasites that cause the malarial fevers are Sporozoans and belong
to the genus _Plasmodium_. Other names such as _Hæmamoeba_ and
_Laverania_ have been used for them, but the term _Plasmodium_ is the
one now most commonly employed. The three most common species are
_vivax_, _malariæ_ and _falciparum_, causing respectively the tertian,
quartan and remittent fevers.
LIFE-HISTORY OF PARASITE
The life-history of all of these is very similar, the principal
difference being in the length of time it takes them to sporulate. Let
us begin with the parasite after it has been introduced into the blood
and trace its development there. At first it is slender and rod-like in
shape. It has some power of movement in the blood-plasm. Very soon it
attacks one of the red blood-corpuscles and gradually pierces its way
through the wall and into the corpuscle substance (Fig. 99); here it
becomes more amoeboid and continues to move about, feeding all the
time on the corpuscle substance, gradually destroying the whole cell. As
the parasite feeds and grows there is deposited within its body a
blackish or brownish pigment known as melanin.
During the time that the parasite is feeding and growing it is also
giving off waste products, as all living forms do in the process of
metabolism, but as the parasite is completely inclosed in the corpuscle
wall these waste products cannot escape until the wall bursts open.
After about forty hours if the parasite is _vivax_ or about sixty-five
hours if it is _malariæ_ it becomes immobile, the nucleus divides again
and again and the protoplasm collects around these nuclei, forming a
number of small cells or spores, as they are called. In about
forty-eight or seventy-two hours, depending on whether the parasite is
_vivax_ or _malariæ_ the wall of the corpuscle bursts and all these
spores with the black pigment and the waste products that have been
stored away within the cell are liberated into the blood-plasm.
[Illustration: FIG. 99--Diagram to illustrate the life-history of the
malarial parasite. 1 is a red blood-corpuscle, 2 to 7 shows the
development of the parasite in the corpuscle, _a_ _b_ _c_ _d_ and _a´_
_b´_ _c´_ and _e_ the development of the parasite in the stomach of the
mosquito, _f_ _g_ _h_ _i_ the development in the capsule on the outer
wall of the stomach of the mosquito, _k_ in the salivary gland.]
[Illustration: FIG. 100--Malarial mosquito (_A. maculipennis_) on the
wall.]
[Illustration: FIG. 101--Malarial mosquito (_A. maculipennis_) standing
on a table.]
These spores are round or somewhat amoeboid and are carried in the
blood for a short time. Very soon, however, each one attacks a new red
corpuscle and the process of feeding, growth and spore-formation
continues, taking exactly the same time for development as in the first
generation, so every forty-eight hours in the case of the _vivax_, and
every seventy-two hours in the case of the _malariæ_ a new lot of these
spores and the accompanying waste products are thrown out into the
blood. Thus in a very short time many generations of this parasite occur
and thousands or hundreds of thousands of the red-blood corpuscles are
destroyed, leaving the patient weak and anemic. It will be seen, too,
that the recurrence of the chills and fevers is simultaneous with the
escaping of the parasites from the blood-corpuscles, together with the
waste products of their metabolism.
These waste products are poisonous, and it is believed that this great
amount of poison poured into the blood at one time causes the regular
recurring crisis. Zoölogists well know that this process of asexual
reproduction, _i. e._, reproduction without any conjugation of two
different cells, cannot go on indefinitely, and those who were studying
the life-cycle of these parasites were at a loss to know where the
sexual stage took place. In the meantime studies of other parasites more
or less closely related to _Plasmodium_ showed that the sexual stage
occurred outside the vertebrate host. The remarkable work of Dr. Smith
on the life-history of the germ that causes the Texas fever of cattle
had a strong influence in directing the search for this other stage of
the malarial parasite. Another thing that indicated that this sexual
generation must take place outside the body of the vertebrate host was
the fact that the investigators found that the parasites in certain of
the cells did not sporulate as did the others. When these individuals
were drawn from the circulation and placed on a slide for study it was
found that they would swell up and free themselves from the inclosing
corpuscle and some of them would emit long filaments which would dart
away among the corpuscles.
Many men have worked on this problem, but perhaps the most credit for
its solution will always be given to Sir Patrick Manson, the foremost
authority on tropical diseases, and to Ronald Ross, a surgeon in the
English army. There is no more interesting and inspiring reading than
that which deals with the development of the hypothesis by Manson and
the persistent faith of Ross in the correctness of this theory, and his
continuous indefatigable labors in trying to demonstrate it. It was an
important piece of scientific work, and shows what a man can do even
when the obstacles seem insurmountable.
THE PARASITE IN THE MOSQUITO
Briefly stated again, the problem was this: We have here a parasite in
the blood which behaves as do many other forms of life. Some of these
parasites do not go on with their development until they are removed
from the circulation. Now, how are they thus removed from the
circulation under normal conditions? This must first be solved before
the still greater and more important problem of how the parasite gets
from one human host to another can be taken up. In studying this over
Manson reasoned that certain suctorial insects were the agencies through
which blood was most commonly removed from the circulation and he
ventured the guess that this change in the parasite that may be seen
taking place on the slide under the microscope, normally takes place in
the stomach of some insect that sucks man's blood. Ross was greatly
impressed with the theory and began his long and apparently hopeless
task of finding these parasites in the stomach of some insect. When we
remember that they are so minute that they can only be seen by the use
of the highest power of the microscope we can realize something of the
magnitude of the task. Ross, who was at that time stationed in India,
selected the mosquito as the most likely of the insects to be the host
that he was looking for. For over two and one-half years he worked with
entirely negative results, for after examining thoroughly many thousands
of mosquitoes he found no trace of the parasite.
Practically all his work was done on the most common mosquito of the
region, a species of _Culex_. But one day a friend sent him a different
mosquito, one with spotted wings, and in examining it he was interested
to note certain oval or round nodules on the outer walls of the stomach.
On closer examinations he found that each of these nodules contained a
few granules of the coal-black melanin of malarial fever. Further
studies and experiments showed that these particular cells could always
be found in the walls of the stomach of this particular species of
mosquito a few days after it had bitten a malarial patient. This
epoch-making discovery was made in 1898. Ross was detailed by the
English government to devote his whole time to the further solution of
the problem, and after two years more of careful experimentation and
study was able to give a complete life-history of this parasite. His
experiments have been repeated many times, and the conclusions he
arrived at are as undeniable as any of the known facts of science.
The whole life-history as we now know it can be summed up as follows:
The parasites develop within the circulation but certain of them seem to
wander about and do not go on with their development there. When these
particular parasites are taken into the stomach of most mosquitoes they
are digested with the rest of the blood. But when they are taken into
the stomach of a mosquito belonging to the genus _Anopheles_ or other
closely related genera they are not digested but go on with their
development, conjugation and fertilization taking place, resulting in a
more elongated form which makes its way through the walls of the stomach
on the outside of which are formed the little nodules discovered by Ross
on his mosquitoes. Within these nodules further division and development
takes place until finally the nodule is burst open and many thousand
minute rod-like organisms, sporozoites, are turned loose into the
body-cavity of the mosquito. Owing to some unknown cause these little
organisms are gathered together in the large vacuolated cells of the
salivary glands of the mosquito, and when the mosquito bites a man or
any other animal they pour down through the ducts with the secretion and
are thus again introduced in the circulation.
The nodules or cysts on the walls of the stomach of the mosquito may
contain as many as ten thousand sporozoites, and as many as five hundred
cysts may occur on a single stomach.
It takes ten, twelve or more days from the time the parasites are taken
into the stomach of the mosquito before they can go through their
transformations and reach the salivary gland, the time depending on the
temperature. So it is ten or twelve days or sometimes as much as
eighteen or twenty days from the time an _Anopheles_ bites a malarial
patient before it is dangerous or can spread the disease. On the other
hand, the sporozoites may lie in the salivary gland alive and virulent
for several weeks. It does not give up all the parasites at one time, so
that three or four or more people may be affected by a single mosquito.
It is well known that two parasites may often be seen in the same
corpuscle. This is often simply a case of multiple infection, but Dr.
Craig has very recently shown that under certain conditions two
individuals may enter the same corpuscle and conjugate and the resulting
individual will be resistant to quinine and may remain latent in the
spleen or bone marrow for a long time. Under favorable conditions it
may again begin the process of multiplication and the patient will
suffer a relapse.
SUMMARY
Now let us sum up some of the reasons why we believe that the malaria
fever can be transmitted only through the agency of mosquitoes. First,
we know the life-history of the parasite, it has been studied in both of
its hosts. Attempts have been made to rear it in other hosts but without
avail, and we know from the general relations of the parasite that it
must have this sexual as well as the asexual generations. Second, in
some regions which would seem to be malarial, that is, where the
miasmatic mists arise, no malaria occurs. Why? Usually it can be
definitely shown that no _Anopheles_ occur there. Other mosquitoes may
be there in abundance, but if no _Anopheles_, there is no malaria. In
certain regions this is well demonstrated. The west coast of Africa is
one of the worst pest-holes of malaria and _Anopheles_. The east coast
has no malaria and no _Anopheles_. In many islands the same condition
exists. On the other hand, the Fiji Islands have _Anopheles_ but no
malaria. No malaria has ever been introduced there to infect the
mosquitoes. In the same way _Stegomyia_ occurs in some of the South Sea
islands and yet there is no yellow fever there.
EXPERIMENTS
We may review, too, a few of the classic experiments that have served to
show that malaria can be contracted in no other way than through the
bite of the mosquito.
For many years Grassi, an Italian, devoted almost his whole time to the
study of malaria. In 1900 he received permission from the government to
experiment on the employees of a piece of railroad that was being built
through a malarial region. This was divided for the purpose of the
experiment into three sections, a protected zone in the middle and an
unprotected zone at each end.
Those working in the protected zone had their houses completely screened
and no one was allowed out of doors after sunset except they were
protected with veils and gloves. Early in the season they were all given
doses of quinine to prevent auto-infection. In the unprotected zone no
screens were used and every one was allowed to go without special
protection. The result for the summer was that there were no new cases
of fever in the protected zone. In the unprotected zones practically all
had the fever as usual.
[Illustration: FIG. 102--Salt-marsh mosquito (_O. lativittatus_)
standing on a table.]
[Illustration: FIG. 103--Anopheles hanging from the ceiling.]
In the same year two English physicians, Sambon and Low, went to Italy
where they built a cabin in one of the marshes noted as being a malaria
pest-hole. The house was thoroughly screened so that no mosquitoes could
enter, but the windows were always open so as to admit the air freely
day and night. Here they lived for three months, out of doors as much as
they pleased during the day but inside where they were protected from
the mosquitoes at night. No quinine was used and no fever developed,
although all about them other people were having the fever as usual.
Another English physician who had not been in malarial regions allowed
himself to be bitten by infected mosquitoes sent from a malarial
locality. In due time he developed the fever. Many other experiments
made in various places might be cited. The results have all been
practically the same. To-day the soldiers of many civilized nations are
required to protect themselves from mosquitoes because it has been found
that it pays. Disease has always been a worse terror than bullets in any
war, and we are fast learning that the great loss from diseases
heretofore considered unavoidable may be very largely eliminated by
proper sanitary arrangements and protection from noxious insects.