[Illustration: The life history of the malarial parasite. This cut of the
malarial parasite shows parts of the body of the mosquito and of man. To
understand the life history begin at the point where the mosquito injects
the crescent-shaped bodies into the blood of man. Notice that after the
spores are released from the corpuscles of man two kinds of cells _may be
formed_. These are probably a sexual stage. Development within the body of
the mosquito will only take place when the parasite is taken into its body
at this sexual stage.]
The Cause of Malaria.--The study of the life history and habits of the
Protozoa has resulted in the finding of many parasitic forms, and the
consequent explanation of some kinds of disease. One parasitic protozoan
like an amoeba is called _Plasmodium malariae_. It causes the disease known
as malaria. When a mosquito (the _anopheles_) sucks the blood from a person
having malaria this parasite passes into the stomach of the mosquito. After
completing a part of its life history within the mosquito's body the
parasite establishes itself within the glands which secrete the saliva of
the mosquito. After about eight days, if the infected mosquito bites a
person, some of the parasites are introduced into the blood along with the
saliva. These parasites enter the corpuscles of the blood, increase in
size, and then form spores. The rapid process of spore formation results in
the breaking down of the blood corpuscles and the release of the spores,
and the poisons they manufacture, into the blood. This causes the chill
followed by the fever so characteristic of malaria. The spores may again
enter the blood corpuscles and in forty-eight or seventy-two hours repeat
the process thus described, depending on the kind of malaria they cause.
The only cure for the disease is _quinine_ in rather large doses. This
kills the parasites in the blood. But quinine should not be taken except
under a physician's directions.
[Illustration: How to distinguish the harmless mosquito (_culex_), _a_,
from the malarial mosquito (_anopheles_), _b_, when at rest. Notice the
position of legs and body.]
The Malarial Mosquito.--Fortunately for mankind, not all mosquitoes harbor
the parasite which causes malaria. The harmless mosquito (_culex_) may be
usually distinguished from the mosquito which carries malaria (_anopheles_)
by the position taken when at rest. Culex lays eggs in tiny rafts of one
hundred or more eggs in any standing water; thus the eggs are distinguished
from those of anopheles, which are not in rafts. Rain barrels, gutters, or
old cans may breed in a short time enough mosquitoes to stock a
neighborhood. The larvae are known as wigglers. They breathe through a tube
in the posterior end of the body, and may be recognized by their peculiar
movement when on their way to the surface to breathe. The pupa,
distinguished by a large thoracic region, breathes through a pair of tubes
on the thorax. The fact that both larvae and pupae take air from the surface
of the water makes it possible to kill the mosquito during these stages by
pouring oil on the surface of the water where they breed. The introduction
of minnows, gold fish, or other small fish which feed upon the larvae in the
water where the mosquitoes breed will do much to free a neighborhood from
this pest. Draining swamps or low land which holds water after a rain is
another method of extermination. Some of the mosquito-infested districts
around New York City have been almost freed from mosquitoes by draining the
salt marshes where they breed. Long shallow trenches are so built as to tap
and drain off any standing water in which the eggs might be laid. In this
way the mosquito has been almost exterminated along some parts of our New
England coast.
[Illustration: Swamps are drained and all standing water covered with a
film of oil in order to exterminate mosquitoes. Why is the oil placed on
the surface of the water?]
Since the beginning of historical times, malaria has been prevalent in
regions infested by mosquitoes. The ancient city of Rome was so greatly
troubled by periodic outbreaks of malarial fever that a goddess of fever
came to be worshiped in order to lessen the severity of what the
inhabitants believed to be a divine visitation. At the present time the
malaria of Italy is being successfully fought and conquered by the draining
of the mosquito-breeding marshes. By a little carefully directed oiling of
water a few boys may make an almost uninhabitable region absolutely safe to
live in. Why not try it if there are mosquitoes in your neighborhood?
Yellow Fever and Mosquitoes.--Another disease carried by mosquitoes is
yellow fever. In the year 1878 there were 125,000 cases and 12,000 deaths
in the United States, mostly in Alabama, Louisiana, and Mississippi. During
the French occupation of the Panama Canal zone the work was at a standstill
part of the time because of the ravages of yellow fever. Before the war
with Spain thousands of people were ill in Cuba. But to-day this is
changed, and yellow fever is under almost complete control, both here and
in the Canal zone, where the mosquito (_stegomyia_) which carries yellow
fever exists.
[Illustration: Notice the difference in the number of yearly deaths from
yellow fever before and _after_ the American occupation of Havana.]
This is due to the experiments during the summer of 1900 of a Commission of
United States army officers, headed by Dr. Walter Reed. Of these men one,
Dr. Jesse Lazear, gave up his life to prove experimentally that yellow
fever was caused by mosquitoes. He allowed himself to be bitten by a
mosquito that was known to have bitten a yellow fever patient, contracted
the disease, and died a martyr to science. Others, soldiers, volunteered to
further test by experiment how the disease was spread, so that in the end
Dr. Reed was able to prove to the world that if mosquitoes could be
prevented from biting people who had yellow fever the disease could not be
spread. The accompanying illustration shows the result of this knowledge
for the city of Havana. For years Havana was considered one of the pest
spots of the West Indies. Visitors shunned this port and commerce was much
affected by the constant menace of yellow fever. At the time of the
American occupation after the war with Spain, the experiments referred to
above were undertaken. The city was cleaned up, proper sanitation
introduced, screens placed in most buildings, and the breeding places of
the mosquitoes were so nearly destroyed that the city was practically free
from mosquitoes. The result, so far as yellow fever was concerned, was
startling, as you can see by reference to the chart. Notice also the rise
in the death rate when the young Cuban Republic took control. How do you
account for that? We all know what American scientific medicine and
sanitation is doing in Panama and in the Philippines.
[Illustration: Stegomyia, the carrier of yellow fever. (After Howard.)]
Other Protozoan Diseases.--Many other diseases of man are probably caused
by parasitic protozoans. Dysentery of one kind appears to be caused by the
presence of an amoeba-like animal in the digestive tract which comes
usually through an impure water supply. Smallpox, rabies, and possibly
other diseases are caused by protozoans. Smallpox, which was once the most
dreaded disease known to man, because of its spread in epidemics, has been
conquered by _vaccination_, of which we shall learn more later. The death
rate from rabies or hydrophobia has in a like manner been greatly reduced
by a treatment founded on the same principles as vaccination and invented
by Louis Pasteur.
Another group of protozoan parasites are called _trypanosomes_. These are
parasitic in insects, fish, reptiles, birds, and mammals in various parts
of the world. They cause various diseases of cattle and other domestic
animals, being carried to the animal in most cases by flies. One of this
family is believed to live in the blood of native African zebras and
antelopes; seemingly it does them no harm. But if one of these parasites is
transferred by the dreaded tsetse fly to one of the domesticated horses or
cattle of the colonist of that region, death of the animal results.
Another fly carries a species of trypanosome to the natives of Central
Africa, which causes "the dreaded and incurable sleeping sickness." This
disease carries off more than fifty thousand natives yearly, and many
Europeans have succumbed to it. Its ravages are now largely confined to an
area near the large Central African lakes and the Upper Nile, for the fly
which carries the disease lives near water, seldom going more than 150 feet
from the banks of streams or lakes. The British government is now trying to
control the disease in Uganda by moving all the villages at least two miles
from the lakes and rivers. Among other diseases that may be due to
protozoans is kala-agar, a fever in hot Asiatic countries which is probably
carried by the bedbug, and African tick fever, probably carried by a small
insect called the tick. Bubonic plague, one of the most dreaded of all
infectious diseases, is carried to man by fleas from rats. In this country
many fatal diseases of cattle, as "tick," or Texas cattle fever, are
probably caused by protozoans.
[Illustration: Life history of house flies, showing from left to right the
eggs, larvae, pupae, and adult flies. (Photograph, about natural size, by
Overton.)]
The Fly a Disease Carrier.--We have already seen that mosquitoes of
different species carry malaria and yellow fever. Another rather recent
addition to the black list is the house fly or typhoid fly. We shall see
later with what reason this name is given. The development of the typhoid
fly is extremely rapid. A female may lay from one hundred to two hundred
eggs. These are usually deposited in filth or manure. Dung heaps about
stables, privy vaults, ash heaps, uncared-for garbage cans, and fermenting
vegetable refuse form the best breeding places for flies. In warm weather,
the eggs hatch a day or so after they are laid and become larvae, called
maggots. After about one week of active feeding, these wormlike maggots
become quiet and go into the pupal stage, whence under favorable conditions
they emerge within less than another week as adult flies. The adults breed
at once, and in a short summer there may be over ten generations of flies.
This accounts for the great number. Fortunately relatively few flies
survive the winter. The membranous wings of the adult fly appear to be two
in number, a second pair being reduced to tiny knobbed hairs called
balancers. The head is freely movable, with large compound eyes. The mouth
parts form a proboscis, which is tonguelike, the animal obtaining its food
by lapping and sucking. The foot shows a wonderful adaptation for clinging
to smooth surfaces. Two or three pads, each of which bears tubelike hairs
that secrete a sticky fluid, are found on its under surface. It is by this
means that the fly is able to walk upside down, and carry bacteria on its
feet.
[Illustration: The foot of a fly, showing the hooks, hairs, and pads which
collect and carry bacteria. The fly doesn't wipe his feet.]
[Illustration: Colonies of bacteria which have developed in a culture
medium upon which a fly was allowed to walk.]
The Typhoid Fly a Pest.--The common fly is recognized as a pest the world
over. Flies have long been known to spoil food through their filthy habits,
but it is more recently that the very serious charge of spread of diseases,
caused by bacteria, has been laid at their door. In a recent experiment two
young men from the Connecticut Agricultural Station found that a single fly
might carry on its feet anywhere from 500 to 6,600,000 bacteria, the
average number being over 1,200,000. Not all of these germs are harmful,
but they might easily include those of typhoid fever, tuberculosis, summer
complaint, and possibly other diseases. A recent pamphlet published by the
Merchants' Association in New York City shows that the rapid increase of
flies during the summer months has a definite correlation with the increase
in the number of cases of summer complaint. Observations in other cities
seem to show the increase in number of typhoid cases in the early fall is
due, in part at least, to the same cause. A terrible toll of disease and
death may be laid at the door of the typhoid fly.
[Illustration: Showing how flies may spread disease by means of
contaminating food.]
Recently the stable fly has been found to carry the dread disease known as
infantile paralysis.
[Illustration: There were 329 typhoid cases in Jacksonville, Florida, in
1910, 158 in 1911, 87 in first 10 months of 1912. 80 to 85 per cent of
outdoor toilets were made fly proof during winter of 1910. Account for the
decrease in typhoid after the flies were kept out of the toilets.]
Remedies.--Cleanliness which destroys the breeding place of flies, the
frequent removal and destruction of garbage, rubbish, and manure, covering
of all food when not in use and especially the _careful_ screening of
windows and doors during the breeding season, will all play a part in the
reduction of flies. To the motto "swat the fly" should be added, "remove
their breeding places!"
[Illustration: Flea which transmits Bubonic plague from rat to man.]
Other Insect Disease Carriers.--Fleas and bedbugs have been recently added
to those insects proven to carry disease to man. Bubonic plague, which is
primarily a disease of rats, is undoubtedly transmitted from the infected
rats to man by the fleas. Fleas are also believed to transmit leprosy
although this is not proven.
To rid a house of fleas we must first find their breeding places. Old
carpets, the sleeping places of cats or dogs or any dirty unswept corner
may hold the eggs of the flea. The young breed in cracks and crevices,
feeding upon organic matter there. Eventually they come to live as adults
on their warm-blooded hosts, cats, dogs, or man. Evidently destruction of
the breeding places, careful washing of all infected areas, the use of
benzine or gasoline in crevices where the larvae may be hid are the most
effective methods of extermination. Pets which might harbor fleas should be
washed frequently with a weak (two to three per cent) solution of creolin.
Bedbugs are difficult to prove as an agent in the transmission of disease
but their disgusting habits are sufficient reason for their extermination.
It has been proven by experiment that they may spread typhoid and relapsing
fevers. They prefer human blood to other food and have come to live in
bedrooms and beds because this food can be obtained there. They are
extremely difficult to exterminate because their flat body allows them to
hide in cracks out of sight. Wooden beds are thus better protection for
them than iron or brass beds. Boiling water poured over the cracks when
they breed or a mixture of strong corrosive sublimate four parts, alcohol
four parts and spirits of turpentine one part, are effective remedies.
How the Harm done by Insects is Controlled.--The combating of insects is
directed by several bodies of men, all of which have the same end in view.
These are the Bureau of Entomology of the United States Department of
Agriculture, the various state experiment stations, and medical and civic
organizations.
The Bureau of Entomology works in harmony with the other divisions of the
Department of Agriculture, giving the time of its experts to the problems
of controlling insects which, for good or ill, influence man's welfare in
this country. The destruction of the malarial mosquito and control of the
typhoid fly; the destruction of harmful insects by the introduction of
their natural enemies, plant or animal; the perfecting of the honeybee (see
Hodge, _Nature Study and Life_, page 240), and the introduction of new
species of insects to pollinate flowers not native to this country (see
_Blastophaga_, page 43), are some of the problems to which these men are
now devoting their time.
All the states and territories have, since 1888, established state
experiment stations, which work in cooperation with the government in the
war upon injurious insects. These stations are often connected with
colleges, so that young men who are interested in this kind of natural
science may have opportunity to learn and to help.
The good done by these means directly and indirectly is very great.
Bulletins are published by the various state stations and by the Department
of Agriculture, most of which may be obtained free. The most interesting of
these from the high school standpoint are the Farmers' Bulletins, issued by
the Department of Agriculture, and the Nature Study pamphlets issued by the
Cornell University in New York state.
[Illustration: This diagram shows how bubonic plague is carried to man.
Explain the diagram.]
Animals Other than Insects may be Disease Carriers.--The common brown rat
is an example of a mammal, harmful to civilized man, which has followed in
his footsteps all over the world. Starting from China, it spread to eastern
Europe, thence to western Europe, and in 1775 it had obtained a lodgment in
this country. In seventy-five years it reached the Pacific coast, and is
now fairly common all over the United States, being one of the most
prolific of all mammals. Rats are believed to carry bubonic plague, the
"Black Death" of the Middle Ages, a disease estimated to have killed
25,000,000 people during the fourteenth century. The rat, like man, is
susceptible to plague; fleas bite the rat and then biting man transmit the
disease to him. A determined effort is now being made to exterminate the
rat because of its connection with bubonic plague.
Other Parasitic Animals cause Disease.--Besides parasitic protozoans other
forms of animals have been found that _cause_ disease. Chief among these
are certain round and flat worms, which have come to live as parasites on
man and other animals. A one-sided relationship has thus come into
existence where the worm receives its living from the host, as the animal
is called on which the parasite lives. Consequently the parasite frequently
becomes fastened to its host during adult life and often is reduced to a
mere bag through which the fluid food prepared by its host is absorbed.
Sometimes a complicated life history has arisen from their parasitic
habits. Such is seen in the life history of the liver fluke, a flatworm
which kills sheep, and in the tapeworm.
[Illustration: The life cycle of a tapeworm. (1) The eggs are taken in with
filthy food by the pig; (2) man eats undercooked pork by means of which the
bladder worm (3) is transferred to his own intestine (4).]
Cestodes or Tapeworms.--These parasites infest man and many other
vertebrate animals. The tapeworm (_Taenia solium_) passes through two stages
in its life history, the first within a pig, the second within the
intestine of man. The developing eggs are passed off with wastes from the
intestine of man. The pig, an animal with dirty habits, may take in the
worm embryos with its food. The worm develops within the intestine of the
pig, but soon makes its way into the muscle or other tissues. It is here
known as a bladderworm. If man eats raw or undercooked pork containing
these worms, _he_ may become a host for the tapeworm. Thus during its
complete life history it has two hosts. Another common tapeworm parasitic
on man lives part of its life as an embryo within the muscles of cattle.
The adult worm consists of a round headlike part provided with hooks, by
means of which it fastens itself to the wall of the intestine. This head
now buds off a series of segmentlike structures, which are practically bags
full of sperms and eggs. These structures, called _proglottids_, break off
from time to time, thus allowing the developing eggs to escape. The
proglottids have no separate digestive systems, but the whole body surface,
bathed in digested food, absorbs it and is thus enabled to grow rapidly.
[Illustration: _Trichinella spiralis_ imbedded in human muscle. (After
Leuckart.)]
Roundworms.--Still other wormlike creatures called roundworms are of
importance to man. Some, as the vinegar eel found in vinegar, or the
pinworms parasitic in the lower intestine, particularly of children, do
little or no harm. The pork worm or _trichina_, however, is a parasite
which may cause serious injury. It passes through the first part of its
existence as a parasite in a pig or other vertebrate (cat, rat, or rabbit),
where it lies, covered within a tiny sac or _cyst_, in the muscles of its
hosts. If raw pork containing these worms is eaten by man, the cyst is
dissolved off by the action of the digestive fluids, and the living
trichina becomes free in the intestine of man. Here it reproduces and the
young bore their way through the intestine walls and enter the muscles,
causing inflammation there. This causes a painful and often fatal disease
known as _trichinosis_.
The Hookworm.--The discovery by Dr. C. W. Stiles of the Bureau of Animal
Industry, that the laziness and shiftlessness of the "poor whites" of the
South is partly due to a parasite called the _hookworm_, reads like a fairy
tale.
The people, largely farmers, become infected with a larval stage of the
hookworm, which develops in moist earth. It enters the body usually through
the skin of the feet, for children and adults alike, in certain localities
where the disease is common, go barefoot to a considerable extent.
A complicated journey from the skin to the intestine now follows, the larvae
passing through the veins to the heart, from there to the lungs; here they
bore into the air passages and eventually work their way by way of the
windpipe into the intestine. One result of the injury of the lungs is that
many thus infected are subject to tuberculosis. The adult worms, once in
the food tube, fasten themselves and feed upon the blood of their host by
puncturing the intestine wall. The loss of blood from this cause is not
sufficient to account for the bloodlessness of the person infected, but it
has been discovered that the hookworm pours out a poison into the wound
which prevents the blood from clotting rapidly (see page 315); hence a
considerable loss of blood occurs from the wound after the worm has
finished its meal and gone to another part of the intestine.
[Illustration: A family suffering from hookworm.]
The cure of the disease is very easy; thymol is given, which weakens the
hold of the worm, this being followed by Epsom salts. For years a large
area in the South undoubtedly has been retarded in its development by this
parasite; hundreds of millions of dollars and thousands of lives have been
needlessly sacrificed.
"The hookworm is not a bit spectacular: it doesn't get itself
discussed in legislative halls or furiously debated in political
campaigns. Modest and unassuming, it does not aspire to such dignity.
It is satisfied simply with (1) lowering the working efficiency and
the pleasure of living in something like two hundred thousand persons
in Georgia and all other Southern states in proportion; with (2)
amassing a death rate higher than tuberculosis, pneumonia, or typhoid
fever; with (3) stubbornly and quite effectually retarding the
agricultural and industrial development of the section; with (4)
nullifying the benefit of thousands of dollars spent upon education;
with (5) costing the South, in the course of a few decades, several
hundred millions of dollars. More serious and closer at hand than the
tariff; more costly, threatening, and tangible than the Negro problem;
making the menace of the boll weevil laughable in comparison--it is
preeminently the problem of the South."--_Atlanta Constitution._
Animals that prey upon Man.--The toll of death from animals which prey upon
or harm man directly is relatively small. Snakes in tropical countries kill
many cattle and not a few people.
The bite of the rattlesnake of our own country, although dangerous, seldom
kills. The dreaded cobra of India has a record of over two hundred and
fifty thousand persons killed in the last thirty-five years. The Indian
government yearly pays out large sums for the extermination of venomous
snakes, over two hundred thousand of which have been killed during a single
year.
[Illustration: A flesh-eating reptile, the alligator.]
Alligators and Crocodiles.--These feed on fishes, but often attack large
animals, as horses, cows, and even man. They seek their prey chiefly at
night, and spend the day basking in the sun. The crocodiles of the Ganges
River in India levy a yearly tribute of many hundred lives from the
natives.
Carnivorous animals such as lions and tigers still inflict damage in
certain parts of the world, but as the tide of civilization advances, their
numbers are slowly but surely decreasing so that as important factors in
man's welfare they may be considered almost negligible.