HOW INSECTS CAUSE OR CARRY DISEASE
It has been estimated that there are about four thousand species or
kinds of Protozoans, about twenty-five thousand species of Mollusks,
about ten thousand species of birds, about three thousand five hundred
species of mammals, and from two hundred thousand to one million species
of insects, or from two to five times as many kinds of insects as all
other animals combined.
Not only do the insects preponderate in number of species, but the
number of individuals belonging to many of the species is absolutely
beyond our comprehension. Try to count the number of little green aphis
on a single infested rose-bush, or on a cabbage plant; guess at the
number of mosquitoes issuing each day from a good breeding-pond;
estimate the number of scale insects on a single square inch of a tree
badly infested with San José scale; then try to think how many more
bushes or trees or ponds may be breeding their millions just as these
and you will only begin to comprehend the meaning of this statement.
As long as these myriads of insects keep in what we are pleased to call
their proper place we care not for their numbers and think little of
them except as some student points out some wonderful thing about their
structure, life-history or adaptations. But since the dawn of history we
find accounts to show that insects have not always kept to their proper
sphere but have insisted at various times and in various ways in
interfering with man's plans and wishes, and on account of their
excessive numbers the results have often been most disastrous.
Insects cause an annual loss to the people of the United States of over
$1,000,000,000. Grain fields are devastated; orchards and gardens are
destroyed or seriously affected; forests are made waste places and in
scores of other ways these little pests which do not keep in their
proper places are exacting this tremendous tax from our people.
These things have been known and recognized for centuries, and scores of
volumes have been written about the insects and their ways and of
methods of combating them.
But it is only in recent years that we have begun to realize the really
important part that insects play in relation to the health of the people
with whom they are associated. Dr. Howard estimates that the annual
death rate in the United States from malaria is about twelve thousand,
entailing an annual monetary loss of about $100,000,000, to say nothing
of the suffering and misery endured by the afflicted. All this on
account of two or three species of insects belonging to the mosquito
genus _Anopheles_.
Yellow fever, while not so widespread, is more fatal and therefore more
terrorizing. Its presence and spread are due entirely to a single
species of mosquito. Flies, fleas, bedbugs, and many other insects have
been shown to be intimately connected with the spread of several other
most dreaded diseases, so it is no wonder that physicians, entomologists
and biologists are studying with utmost zeal many of these forms that
bear such a close relation not only to our welfare and comfort but to
our lives as well.
It would be out of place to try to give here even a brief outline of the
classification of insects, such as may be found in almost any of the
many books devoted to their study.
The most generally accepted classification divides the insects into
nineteen orders; as the Coleoptera, containing the beetles; the
Lepidoptera, containing the butterflies and moths; the Hymenoptera
containing the bees, ants and wasps, etc. Four or five of these orders
will be of more or less interest to us.
The order Diptera, or two-winged flies, is the most important because to
this belong the mosquitoes which transmit malaria and yellow fever, and
the house-fly that has come into prominence since it has been found to
be such an important factor in the distribution of typhoid and other
diseases.
FLIES
The order Diptera is divided into sixty or more families, many of which
contain species of considerable economic importance. For our present
consideration the flies may be divided into two groups or sections:
those with their mouth-parts fitted for piercing such as the mosquito
and horse-fly, and those with sucking mouth-parts such as the house-fly,
blow-fly and others.
Some of the species belonging to the first group are among the most
troublesome pests not only of man but of our domestic animals as well.
Next to the mosquitoes the horse-flies (Fig. 22) are perhaps the best
known of these. There are several species known under various names,
such as gad-fly, breeze-fly, etc. They are very serious pests of horses
and cattle, sometimes also attacking man. Their strong, sharp, piercing
stylets enable them to pierce through the toughest skin of animals and
through the thin clothing of man. The bite is very severe and
irritating, and as the flies sometimes occur in great numbers the
annoyance that they cause is often very great indeed. It has often been
claimed that these flies as well as the stable-fly and others carry the
anthrax bacillus on their proboscis from one animal to another, and
although this may not have been definitely proven the evidence is strong
enough to make a very good case against the accused. It is interesting
to note in this connection that anthrax, a very common disease among the
domestic animals and one which may attack man also, was the first
disease to be shown to be of bacterial origin. It was only about
thirty-five years ago that the investigations of Koch and Pasteur
demonstrated that the presence of this particular germ (_Bacillus
anthracis_) was the cause of the disease, and it was early recognized
that such biting flies may be important factors in the spread of the
disease.
[Illustration: FIG. 22--Horse-fly (_Tabanus punctifer_).]
[Illustration: FIG. 23--Stable-fly (_Stomoxys calcitrans_).]
[Illustration: FIG. 24--A Black-fly (_Simulium sp._). (From Kellogg's
Amer. Insects.)]
[Illustration: FIG. 25--Screw-worm fly (_Chrysomyia macellaria_).]
[Illustration: FIG. 26--Blow-fly (_Calliphora vomitoria_).]
The stable-fly (Fig. 23) (_Stomoxys calcitrans_) which looks very much
like the house-fly and, as will be noted later, frequently enters
houses, is often an important pest of horses and cattle. Its
blood-sucking habit makes it quite possible that it too may be concerned
in carrying anthrax and other diseases.
In a later chapter it will be shown how the tsetse-fly, which is
somewhat like the stable-fly, is responsible for the spread of the
disease known as the sleeping sickness. This disease is caused by a
Protozoan parasite, a trypanosome, which is transmitted from one host to
another by the tsetse-fly.
In Southern Asia and in parts of Africa there is a very serious disease
of horses known as surra which is caused by a similar parasite
(_Trypanosoma evansi_). This parasite attacks horses, mules, camels,
elephants, buffaloes and dogs, and has been recently imported into the
Philippines. It is supposed that flies belonging to the same genus as
the horse-fly (_Tabanus_ and others), and the stable-fly (_Stomoxys_)
and the horn-fly (_Hæmatobia_) are responsible for the spread of the
disease.
Nagana is one of the most serious diseases of domestic animals in
Central and Southern Africa. In some sections it is almost impossible to
keep any kind of imported animals on account of this disease which is
caused by a parasite (_Trypanosoma brucei_) similar to the one causing
surra. This parasite is to be found in several different kinds of
native animals which seem to be practically immune but are always a
source of danger when other animals are introduced. Two or three species
of tsetse-flies are responsible for the transmission of this disease.
Another group of flies much smaller but more numerous and much more
insistent are the black-flies or buffalo-gnats (Fig. 24). For more than
a century these little flies have been recognized as among the most
serious pests of stock, particularly in the south where, besides the
actual loss by death of many animals yearly, the annoyance is so great
as to sometimes make it impossible to work in the field. Human beings
are often attacked, and as the bite is poisonous and very painful great
suffering may result and cases of deaths from such bites have been
reported.
Belonging to another family, and smaller, but much like the buffalo-gnat
in habits, are the minute little "punkies" or "no-see-ums" which
sometimes occur in great swarms in certain regions where they make life
a burden to man and beast. While it has not been shown that either the
buffalo-gnats or the punkies are responsible for the transmission of any
disease, their habits of feeding on so many different kinds of wild and
domestic animals as well as on man makes it possible for them to act as
carriers of parasites that might under proper conditions become of
serious importance. Then, too, the irritation caused by the bites of
these insects usually causes scratching which may result in abrasions of
the skin that open the way for various harmful germs, particularly those
causing skin diseases.
Coming now to the group containing the house-flies and related forms we
find a number that are of interest on account of the suffering that they
may cause, particularly in their larval stages.
The screw-worm flies (_Chrysomyia macellaria_) are among the most common
and important of these (Fig. 25). These "gray flies," as they are
sometimes called, lay a mass of three or four hundred eggs on the
surface of wounds. The larvæ which in a few hours hatch from these make
their way directly into the wound where they feed on the surrounding
tissue until full grown when they wriggle out and drop to the ground
where they transform to the pupa and later to the adult fly. Of course
their presence in the wounds is very distressing to the infected animal,
and great suffering results. Slight scratches that might otherwise
quickly heal often become serious sores because of the presence of these
larvæ.
Many cases are recorded of these flies laying their eggs in the ears or
nose of children or of persons sleeping out of doors during the day.
Especially is this apt to occur if there are offensive discharges which
attract the fly. In such cases the larvæ burrow into the surrounding
tissues, devouring the mucous membranes, the muscles and even the bones,
causing terrible suffering and usually, death. The larvæ in such
situations may be killed with chloroform and, if the case is attended to
before they have destroyed too much of the tissues, recovery usually
occurs.
The blow-flies (Fig. 26) (_Calliphora vomitoria_) and the blue-bottle
flies (Fig. 27), (_Lucilia spp._) and the flesh-flies (Fig. 28)
(_Sarcophaga spp._) all have habits somewhat like the screw-worm fly.
Any of them may lay their eggs in wounds on man or animals with the same
serious results.
The flesh-fly instead of laying eggs deposits the living larvæ upon meat
wherever it is accessible, and as these develop with astonishing
rapidity they are able to consume large quantities of flesh in a
remarkably short time. In this way they may be of some importance as
scavengers, but it is better to get rid of the waste in other ways than
to leave it for a breeding-place for flies that are capable of causing
so much damage and suffering.
Not infrequently the larvæ of certain flies are to be found in the
alimentary canal where as a rule they do no particular damage.
Altogether the larvæ of over twenty different species of flies have been
found in or expelled from the human intestinal canal. In Europe, the
majority of these larvæ belong to a fly which looks very much like the
house-fly except that it is somewhat smaller and so is often known as
"the little house-fly" (Fig. 29) (_Homalomyia canicularis_). The same
species is very common in the United States, frequently occurring in
houses. Under certain conditions it may even be more abundant than the
house-fly. It is believed that the larvæ in the intestinal canal come
from eggs that have been deposited on the victim while using an outdoor
privy where the flies are often very abundant. Instances are also on
record where these larvæ have been discharged from the urethra.
Another fly (_Ochromyia anthropophaga_) occurring in the Congo region
has a blood-sucking larvæ which is known as the Congo floor-maggot. The
fly which is itself not troublesome deposits its eggs in the cracks and
crevices of the mud floors of the huts. The larvæ which hatch from these
crawl out at night and suck the blood of the victim that may be sleeping
on the floor or on a low bed.
BOT-FLIES
Another group of flies known as the bot-flies (Fig. 30) have their
mouth-parts rudimentary or entirely wanting so of course they themselves
cannot bite or pierce an animal. Nevertheless they are the source of an
endless amount of trouble to stockmen and sometimes even attack man.
Although these flies cannot bite, the presence of even a single
individual may be enough to annoy a horse almost to the end of
endurance. Horses seem to have an instinctive fear of them and will do
all in their power to get rid of the annoying pests.
The eggs of the house bot-fly are laid on the hair of the legs or some
other part of the body. The horse licks them off and they hatch and
develop in the alimentary canal of their host. Sometimes the walls of
the stomach may be almost covered with them thus of course seriously
interfering with the functions of this organ. When full grown the larvæ
pass from the host and complete their transformation in the ground.
[Illustration: FIG. 27--Blue-bottle fly (_Lucilia sericata_).]
[Illustration: FIG. 28--Flesh-fly (_Sarcophaga sp_).]
[Illustration: FIG. 29--"The little house-fly" (_Homalomyia
canicularis_).]
[Illustration: FIG. 30--Horse bot-fly (_Gastrophilus equi_).]
[Illustration: FIG. 31--Ox warble-fly (_Hypoderma lineata_).]
[Illustration: FIG. 32--Sheep bot-fly (_Gastrophilus nasalis_).]
The bot-flies of cattle or the oxwarbles (Fig. 31) gain an entrance
into the alimentary canal in the same way, that is, by the eggs being
licked from the hairs on the body where they have been laid by the adult
fly. But instead of passing on into the stomach they collect in the
esophagus and later make their way through the walls of this organ and
through the tissues of the body until they at last reach a place along
the back just under the skin. Here as they are completing their
development they make more or less serious sores on the backs of the
infested animals. The hides on such animals are rendered nearly
valueless by the holes made by the larvæ. When fully mature they drop to
the ground and complete their transformations.
The sheep bot-flies (Fig. 32) lay their eggs in the nostrils of sheep.
The larvæ pass up into the frontal sinuses where they feed on the mucus,
causing great suffering and loss. Many other species of animals are
infested with their own particular species of bots. Several instances
are recorded where the oxwarble has occurred in man, always causing much
suffering and sometimes death.
One or more species of bot-flies occurring in the tropical parts of
America frequently attack man. The early larval stage soon after it has
entered the skin is known as the _Ver macaque_. Later stages as _torcel_
or _Berne_. The presence of the larvæ produces very painful and
troublesome sores. It is supposed that the adult flies (one species of
which is _Dermatobia cyaniventris_) lay their eggs on the skin which
the larvæ penetrate as soon as they hatch. It has also been suggested
that they might reach the subcutaneous tissue by migrating from the
alimentary canal as do some of the other bot-flies. A very serious eye
disease, _Egyptian opthalmia_, is known to be spread by the house-flies
and others. These flies are often abundant about the eyes, especially of
children suffering from this disease. It is suspected that certain small
flies (Oscinidæ) in the southern part of the United States are
responsible for the spread of disease known as "sore eye."
FLEAS
The fleas used to be considered as degenerate Diptera and were placed
with that group but they are now classed as a separate order
(Siphonaptera). Within recent years these little pests have come into
special prominence on account of their importance in connection with the
spread of the plague. The fact that they are so abundant everywhere and
that they will so readily pass from one host to another makes the
possibility of their spreading infectious diseases very great. Besides
the kinds that are concerned in the transmission of plague, which are
discussed in another chapter, there are many other kinds infesting
various wild and domesticated animals and a few attacking birds.
One of the most important of these is the jigger-flea or chigoe
(_Dermatophilus penetrans_, Fig. 33). Various other names such as
chigger-flea, sand-flea, jigger, chigger are also applied to this insect
as well as to a minute red mite that burrows into the skin in much the
same way as the female of the flea. So although they are entirely
different creatures you can never tell from the common name, whether it
is the flea or the mite that is being referred to. Both the male and
female jigger-fleas feed on the host and hop on or off as do other
fleas, but when the female is ready to lay eggs (Fig. 34), she burrows
into the skin. Her presence there causes a swelling and usually an ulcer
which often becomes very serious, especially if the insect should be
crushed and the contents of the body escape into the surrounding tissue.
These little pests are found throughout tropical and subtropical America
and have been introduced into Africa and from there have spread to India
and elsewhere. They attack almost all kinds of animals as well as many
birds, being of course a source of great annoyance and no inconsiderable
loss. They are more apt to attack the feet of men, especially those who
go barefooted. Sometimes they occur in such numbers as to make great
masses of sores.
On account of being such general feeders they are difficult to control,
but some relief may be obtained by keeping the houses and barns as free
as possible from dirt and rubbish and by sprinkling the breeding-places
of the pest with pyrethrum powder or carbolic water. Those that gain an
entrance into the skin should be cut out, care being taken to remove the
insect entire.
BEDBUGS
In the order Hemiptera, or the true "bugs" in an entomological sense, we
find a few forms that may carry disease. The bedbug (Fig. 35) (_Cimex
lectularis_) has been accused of transmitting plague, relapsing fever
and other diseases. Very recent investigations show that the common
bedbug of India (_Cimex rotundatus_) harbors the parasite that causes
the disease known as _kala azar_, and there is no doubt that it
transmits the disease.
LICE
The sucking lice (Fig. 36) which also belong to this order are suspected
of carrying some of these same diseases. It is thought that the common
louse on rats (_Hæmatopinus spinulosus_) is responsible for the spread
from rat to rat of a certain parasite. (_Trypanosoma lewisi_), which,
however, does not produce any disease in the rats, but if they are
capable of acting as alternative hosts for such parasites, it is quite
possible that they may also carry disease-producing forms.
[Illustration: FIG. 33--Chigo or jigger-flea, male (_Dermatophilus
penetrans_). (After Karsten.)]
[Illustration: FIG. 34--Chigo, female distended with eggs. (After
Karsten.)]
[Illustration: FIG. 35--Bedbug (_Cimex lectularis_).]
[Illustration: FIG. 36--Body-louse (_Pediculus vestimenti_). (From
drawing by J.H. Paine.)]
HOW INSECTS MAY CARRY DISEASE GERMS
Insects may carry the germs or parasites which cause disease in a purely
mechanical or accidental way, that is, the insect may in the course of
its wanderings or its feeding get some of the germs on or in its body
and may by chance carry these to the food, or water, or directly to some
person who may become infected. Thus the house-fly may carry the typhoid
germs on its feet or in its body and distribute them in places where
they may enter the human body.
Several other flies as well as fleas, bedbugs, ticks, etc., may also
carry disease germs in this mechanical way. While this method of
transmission is just as dangerous as any other, and possibly more
dangerous because more common, another method in which the insect is
much more intimately concerned is more interesting from a biological
standpoint at least and will be discussed more fully in the chapters on
malaria, yellow fever and elephantiasis.
In these cases the insect is one of the necessary hosts of the parasite,
which cannot go on with its development or pass from one patient to
another unless it first enters the insect at a certain stage of its
life-history.
[Illustration: FIG. 37--One use for the house-fly.]
BABY-BYE.
1. Baby-Bye,
Here's a fly;
We will watch him, you and I.
How he crawls
Up the walls,
Yet he never falls!
I believe with six such legs
You and I could walk on eggs.
There he goes
On his toes,
Tickling Baby's nose.