Disease and Its Causes by William Thomas Councilman

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2. Diseases which are localized in the blood are transmitted by biting
flies. The biting apparatus becomes contaminated with the organisms
contained in the blood, and these are directly inoculated into the
blood of the next victim. The trypanosome diseases form the best
example of this mode of transmission. The trypanosomes are widely
distributed, exclusively parasitic, flagellated protozoa which live in
the blood of a large number of animals and birds (Fig. 19). They may
give rise to fatal diseases, but in most cases there is mutual
adaptation of host and parasite and they seem to do no harm. One of
the most dangerous diseases in man, the African sleeping sickness, is
caused by a trypanosome, and the disease of domestic cattle in Africa,
nagana, or tsetse fly disease, is also so produced. In certain regions
of Africa where a biting fly, the _Glossina morsitans_, occurs in
large numbers, it has long been known that cattle bitten by these
flies sickened and died, and this prevented the settling and use of
the land. In the blood of the sick cattle swarms of trypanosomes are
found. The source from which the tsetse fly obtained the trypanosomes
which it conveyed to the cattle was unknown until it was discovered
that similar trypanosomes exist in the blood of the wild animals which
inhabit the region, but these have acquired by long residence in the
region immunity or adaptation to the parasite and no disease is
produced. With the gradual extension of settlement of the country and
the accompanying destruction of wild life the disease is diminishing.
Some of the inter-relations of infections are interesting. The
destruction of wild animals in South Africa which, by removing the
sources of nagana, rendered the settlement of the country possible was
due chiefly to the introduction of another infectious disease,
rinderpest, which not only destroyed the wild animals but produced
great destruction of the domestic cattle as well.

The _sleeping sickness_ has many features of interest. In the old
slavery days it was found that the negroes from the Congo region in
the course of the voyage or after they were landed sometimes were
affected with a peculiar disease. They were lethargic, took little
notice of their surroundings, slept easily and finally passed into a
condition of somnolence in which they took no food and gradually died.
There was no extension of the disease and it was attributed to extreme
homesickness and depression. A similar disease has been known for more
than one hundred years on the west coast of Africa, and attracted a
good deal of interest and curiosity on account of the peculiar
lethargy which it produced and from which it has received the name of
"sleeping sickness." Although apparently infectious in its native
haunts, it lost the power of spreading from man upon removal to
regions where it did not prevail. At first confined to a very small
region on the Niger river, it gradually extended with the development
of trade routes and the general increase of communications which trade
brings, until it prevails in the entire Congo basin, in the British
and German possessions in East Africa, and is extending north and
south of these regions. The cause of the disease and its mode of
conveyance was discovered in 1903. The fly _Glossina palpalis_
which conveys the disease is a biting fly about the size of the common
house fly and lives chiefly in the vicinity of water. When such a fly
bites an individual who has sleeping sickness its bite can convey the
disease to monkeys, on whom the transmission experiments were made.
After biting the fly is infectious for a period of two days. After
this it is harmless, unless it again obtains a supply of living
trypanosomes. There is quite a period in which there are no symptoms
of the disease, although trypanosomes are found in the blood and in
the lymph nodes, and the individual is a source of infection. The
peculiar lethargy which has given the disease its name does not appear
until the nervous system is invaded by the parasites. It is impossible
to compute accurately the numbers of deaths from this disease--in the
region of Victoria Nyanza alone the estimates extend to hundreds of
thousands.

3. In the third mode of insect conveyance the insect does not play a
merely passive role, but becomes a part of the disease, itself
undergoing infection, and a period in the life cycle of the organism
takes place within it. In all these cases quite a period of time must
elapse before the insect is capable of transmitting the disease; in
malaria, which is the best type of such a disease, this period is ten
days. Malaria is due to a small protozoan, the _Plasmodium
malariae_, which was discovered by Lavaran, a French investigator,
in 1882. The organism lives within or on the surface of the red blood
corpuscles. It first appears as a very minute colorless body with
active amoeboid movements, and increases in size, attacks a succession
of corpuscles, and finally attains a size as large as or larger than a
corpuscle. The corpuscles attacked become pale by the destruction of
haemoglobin, swell up and disintegrate, the haemoglobin becoming
converted into granules of black pigment inside the parasite. Having
attained a definite size the organism forms a rosette and divides into
a number of forms similar to the smallest seen inside the corpuscles;
these small forms enter other corpuscles and the cycle again begins.
This cycle of development takes place in forty-eight hours, and
segmentation is always accompanied by a paroxysm of the disease shown
in a chill followed by fever and sweating which is due to the effect
of substances liberated by the organism at the time of segmentation. A
patient may have two crops of the parasite developing independently in
the blood, and the two periods of segmentation give a paroxysm for
each, so that the paroxysms may appear at intervals of twenty-four
hours instead of forty-eight (Fig. 20). This cycle of development may
continue for an indefinite time, and there may be such a rapid
increase in the parasites as to bring about the death of the
individual; but with him the parasite would also perish, for there
would be no way of extending the infection and providing a new crop.
The disease has been transmitted by injecting the infected blood into
a normal individual.

[Illustration: FIG. 20.--PART OF THE CYCLE OF DEVELOPMENT OF THE
ORGANISM OF MALARIA, _a-g_, Cycle of forty-eight hour development, the
period of chill coinciding with the appearance of _f_ and _g_ in the
blood. The organisms _g_, which result from segmentation, attack other
corpuscles and a new cycle begins. _h_, The male form or
microgametocyte, with the protruding and actively moving spermatozoa,
one of which is shown free. _i_ and _j_ are the macrogametes or female
forms. _k_ shows one of these in the act of being fertilized by the
entering spermatozooen. The differentiation into male and female forms
takes place in the blood, the further development of the sexual cycle
within the mosquito.]

If a mosquito of the species _anopheles_ bites the affected
person, it obtains a large amount of blood which contains many
parasites. Within the mosquito the parasite undergoes a further
development into male and female sexual forms, which may also form in
the blood, termed respectively _microgametocyte_ and _macrogamete_.
From the microgametocyte small flagellate bodies, the male sexual
elements _microgametes_ or _spermatozoa_, develop and fertilize the
_macrogametes_; after fertilization this develops into a large body,
the _ooecyst_ which is attached to the wall of the stomach of the
mosquito. Within the ooecyst, innumerable small bodies, the
sporozoites, develop, make their way into the salivary glands and are
injected into the individual who becomes the prey of the mosquito,
and again the cycle of development begins. The presence of the
parasite within the mosquito does not constitute a disease. So far as
can be determined, life goes on in the usual way, and its duration in
the insect is not shortened.

The nature of the parasite which produces yellow fever is unknown, for
it belongs to the filterable viruses; the infectious material,
however, has been shown by inoculation to exist in the blood, and the
disease is transmitted by a mosquito of another species, the
_stegomyia_. The development cycle within this takes a period of
twelve days, which time must elapse after the mosquito has bitten
before it can transmit the disease. Here again the mutual
interdependence of knowledge is shown. Nothing could have seemed less
useful than the study of mosquitoes, the differentiation of the
different species, their mode of life, etc., and yet without this
knowledge discoveries so beneficial and of such far-reaching
importance to the whole human race as that of the cause and mode of
transmission of malaria and yellow fever would have been impossible;
for it could easily have been shown that the ordinary _culex_
mosquito played no role. The role which insects may play in the
transmission of disease was first shown by Theobald Smith in this
country, in the transmission by a tick of the disease of cattle known
as Texas fever. The infecting organism _pyrosoma bigenimum_ is a
tiny pear-shaped parasite of the red corpuscles. Smith's
investigations on the disease, published in 1893, is one of the
classics in medicine, and one of the few examples of an investigation
which has not been changed or added to by further work.

One of the most interesting methods of extension of infection, showing
on what small circumstances infection may depend, is seen in the case
of the hookworm disease, which causes such devastation in the Southern
States. The organism which produces the disease, the _Uncinaria_,
belongs to the more highly developed parasites, and is a small round
worm one-third of an inch long. The worms which inhabit the intestines
have a sharp biting mouth by which they fasten themselves to the
mucous membrane and devour the blood. The most prominent symptom of
the disease is anaemia, or loss of blood, due not only to the direct
eating of the parasite, but to bleeding from the small wounds caused
by its bite. Large numbers of eggs are produced by the parasite which
are passed out with the feces, which becomes the only infectious
material. In a city provided with water-closets and a system of
sewerage there would be no means of extension of infection. The eggs
in the feces in conditions of warmth and moisture develop into small
crawling larvae which can penetrate the skin, producing inflammation of
this, known in the region as the ground itch. The larvae enter the
circulation and are carried to the lungs, where they perforate the
capillaries and reach the inner surface; from this they pass along the
windpipe, and then by way of the gullet and stomach reach their
habitat, the small intestine. Unfortunately, the habits and poverty of
the people in every way facilitate the extension of the infection.
There is no proper disposal of the feces, few of the houses have even
a privy attached to them, and the feces are distributed in the
vicinity of the houses. This leads to contamination of the soil over
wide areas. Most of the inhabitants of the country go barefoot the
greater part of the year, and this gives ready means of contact with
the larvae which crawl over the surface of the ground. The disease is
necessarily associated with poverty and ignorance, the amount of blood
is reduced to a low point, and industry, energy and ambition fall with
the blood reduction; the schools are few and inefficient; the children
are backward, for no child can learn whose brain cells receive but a
small proportion of the necessary oxygen; and a general condition of
apathy and hopelessness prevails in the effected communities. The
control of the disease depends upon the disinfection of the feces, or
at least their disposal in some hygienic method, the wearing of shoes,
and the better education of the people, all of which conditions seem
almost hopeless of attainment. The infection is also extended by means
of the negroes who harbor the parasite, but who have acquired a high
degree of immunity to its effects and whose hygienic habits are even
worse than those of the whites. The organism was probably imported
with the negroes from Africa and is one of the legacies of slavery.

The diseases of animals are in many ways closely linked with those of
man. In the case of the larger parasites, such as the tapeworms and
the trichina, there is a direct interchange of disease with animals,
certain phases of the life cycle of the organisms are passed in man
and others in various of the domestic animals. A small inconspicuous
tapeworm inhabits the intestine of dogs and seems to produce no ill
effects. The eggs are passed from the dog, taken into man, and result
in the formation of large cystic tumors which not infrequently cause
death. Where the companionship between dog and man is very close, as
in Iceland, the cases are numerous.

Most of the diseases in animals caused by bacteria and protozoa are
not transmitted to man, but there is a conspicuous exception. Plague
is now recognized as essentially an animal disease affecting rats and
other small rodents, and from these the disease from time to time
makes excursions to the human family with dire results. The greatest
epidemics of which we have any knowledge are of plague. In the time of
Justinian, 542 B.C., a great epidemic of plague extended over what was
then regarded as the inhabited earth. This pandemic lasted for fifty
years, the disease disappeared and appeared again in many places and
caused frightful destruction of life. Cities were depopulated, the
land in many places reverted to a wilderness, and the works of man
disappeared. The actual mortality cannot be known, but has been
estimated at fifty millions. Plague played a large part in the
epidemics of the Middle Ages. An epidemic started in 1346 and had as
great an extension as the Justinian plague, destroying a fourth of the
inhabitants of the places attacked; and during the fifteenth and
sixteenth and seventeenth centuries the disease repeatedly raised its
head, producing smaller and greater epidemics, the best known of
which, from the wonderful description of De Foe, is that of London in
1665, and called the Black Death. Little was heard of the disease in
the nineteenth century, although its existence in Asia was known. In
1894 it appeared in Hong Kong, extended to Canton, thence to India,
Japan, San Francisco, Mexico, and, in fact, few parts of the tropics
or temperate regions of the earth have been free from it. Mortality
has varied greatly, being greatest in China and in India; in the last
the estimate since 1900 is seven million five hundred thousand deaths.
The disease is caused by a small bacillus discovered in 1894 which
forms no spores and is easily destroyed by sunlight, but in the dark
is capable of living with undiminished virulence for an indefinite
time. The disease in man appears in two forms, the most common known
as bubonic plague, from the great enlargement of the lymph nodes,
those of the groin being most frequently affected. The more fatal form
is known as pneumonic plague, and in this the lungs are the seat of
the disease.

In the old descriptions of the disease it was frequently mentioned
that large numbers of dead rats were found when it was prevalent, and
the most striking fact of the recent investigations is the
demonstration that the infection in man is due to transference of the
bacillus from infected rats. There are endemic foci of the disease
where it exists in animals, the present epidemic having started from
such a focus in Northern China, in which region the _Tarabagan_,
a small fur-bearing animal of the squirrel species, was infected. Rats
are easily infected, the close social habits of the animal, the vermin
which they harbor, and the habits of devouring their dead fellows
favor the extension of infection. The disease extends from the rat to
man chiefly by means of the fleas which contain the bacilli, and in
cases of pneumonic plague from man to man by means of sputum
infection. The disease once established in animals tends to remain,
the virus being kept alive by transmission from animal to animal, and
the persistence of the infection is favored by mild and chronic cases.

CHAPTER IX

DISEASE CARRIERS.--THE RELATION BETWEEN SPORADIC CASES OF INFECTIOUS
DISEASE AND EPIDEMICS.--SMALLPOX.--CEREBRO-SPINAL
MENINGITIS.--POLIOMYELITIS.--VARIATION IN THE SUSCEPTIBILITY OF
INDIVIDUALS.--CONDITIONS WHICH MAY INFLUENCE SUSCEPTIBILITY.--RACIAL
SUSCEPTIBILITY.--INFLUENCE OF AGE AND SEX.--OCCUPATION AND
ENVIRONMENT.--THE AGE PERIOD OF INFECTIOUS DISEASES.

We have seen that insects serve as carriers of disease in two ways: in
one, by becoming contaminated with organisms they serve as passive
carriers, and in the other they undergo infection and form a link in
the disease. The more recent investigations of modes of transmission
of infectious diseases have shown that man, in addition to serving
while sick as a source of infection, may serve as a passive carrier in
two ways. For infection to take place not only must the pathogenic
organism be present, but it must be able to overcome the passive and
active defences of the body and produce injury. Pathogenic organisms
may find conditions favorable for growth on the surfaces of the body,
and may live there, but be unable to produce infection, and the
individual who simply harbors the organisms can transmit them to
others. Such an individual may be a greater source of infection than
one with the disease, because there is no suspicion of danger. The
organisms which thus grow on the surfaces have in some cases been
shown to be of diminished virulence, but in others have full
pathogenic power. Such passive carriers of infection have been found
for a number of diseases, as cerebro-spinal meningitis, diphtheria,
poliomyelitis and cholera. In all these cases the organisms are most
frequently found in those individuals who have been exposed to
infection as members of a family in which there have been cases of
disease. The other sort of carrier has had and overcome the disease,
but mutual relations have been established with the organism which
continues to live in the body cavity. Diphtheria bacilli usually
linger in the throat after convalescence is established, and until
they have disappeared the individual is more dangerous than one
actually sick with the disease. Health officers have recognized this
in continuing the quarantine against the disease until the organism
disappears. In typhoid fever bacilli may remain in the body for a long
time and be continually discharged, as in the well-known case of
"typhoid Mary."[1]

Single cases of certain infectious diseases may appear in a community
year after year, and at intervals the cases become so numerous that
the disease is said to be epidemic. Such a disease is smallpox. This
is a highly infectious disease, towards which all mankind is
susceptible. Complete protection against the disease can be conferred
by Jenner's discovery of vaccination. The disease becomes modified
when transferred to cattle, producing what is known as cowpox, in
which vesicles similar to those of smallpox appear on the skin. The
inoculation of man with the contents of such a vesicle produces a mild
form of disease known as vaccinia, which protects the individual from
smallpox. This protection is fully as adequate as that produced by an
attack of smallpox, and we are warranted in saying that if thorough
vaccination, or the inoculation with vaccinia, were carried out
smallpox would disappear. There are great difficulties in the way of
carrying out effective vaccination of the whole population, which are
accentuated by the active opposition of people who are ignorant and
wilfully remain so. There exists in every state a number of people
unprotected by vaccination, and among these single cases of smallpox
appear. The unprotected individuals gradually increase in number,
forming an inflammable material awaiting the spark or infection which
produces a conflagration in the one case and an epidemic in the other.

Cerebro-spinal meningitis is another example of a disease which exists
in sporadic and epidemic form. This disease is caused by a small
micrococcus, the organisms joined in pairs. The seat of the disease is
in the meninges or membranes around the brain and spinal cord. The
micrococci enter the body from the throat and nose, and either pass
directly from here into the meninges, or they enter into the blood and
are carried by this into the meninges. The organisms are easily
destroyed and cannot long survive the conditions outside the body, so
that for infection to take place the transmission must be very direct.
Carriers who have the organisms in the throat, but who do not have the
disease, are the principal agents in dissemination. The mortality is
high, and even in recovery permanent damage is often done to the brain
or to the organs of special sense. Sporadic cases constantly occur in
small numbers, and it is difficult or impossible to trace any
connection between these cases. At varying intervals, often twenty
years intervening, an epidemic appears which sometimes remains local
in a city or state, sometimes extends to adjoining cities or states,
and may even extend over a very large area. In the epidemics the
mortality is much higher than in the sporadic cases. The same
explanation given for smallpox cannot apply here, for there is not a
similar accumulation of susceptible material. We know there is a great
deal of variation in the virulence of the different pathogenic
organisms, and the virulence can be artificially increased and
diminished. In epidemics of meningitis the virulence of the organisms
is increased, as is shown by the greater mortality. It is highly
probable that such epidemics are due to changes which arise in the
organisms from causes we do not know and which increase their capacity
for harm. It is possible that such a change would convert a carrier
into a case of disease, the organism acquiring greater powers of
invasion. Such a strain of organisms arising in one place and
producing an epidemic could be transported to another locality and
exert the same action, or similar changes in the organisms could arise
simultaneously in a number of places. Analogies to such conditions are
given in plants. In certain plants it has been shown that from unknown
causes there appears a tendency to the production of variations. A
very beautiful herbaceous peony known as "Bridesmaid" after having
grown for a number of years in single form, in one year wherever grown
suddenly became double. The peculiar thing with the lower unicellular
organisms is that the changes which so arise do not tend to become
permanent, the organism reverts to its usual character, the disease to
its sporadic type.

A very fatal form of poliomyelitis has for a number of years prevailed
in Sweden. In the United States there have been continually a number
of single cases of the disease, and it is not impossible that a more
pathogenic strain of the organism has developed in Sweden and has been
imported into this country, giving rise to the much greater extension
of the disease in a number of places.

The most cursory study of the infectious diseases shows that there is
great variation in the susceptibility of individuals. Even in the most
severe epidemics all are not equally affected, some escape the
infection, others have the disease lightly, others severely, some die.
Chance enters into this, but plays a small part, for the same varying
individual susceptibility is shown experimentally. If a given number
of animals of the same species, age and weight, even those from the
same litter, be inoculated with a given number of bacteria shown to be
pathogenic for that species, the results differ. If the dose be
necessarily fatal, death will take place at intervals; if a dose
smaller than the fatal be used, some animals will die, others will
recover. The defences of the organism being centred in the activity of
the living tissue, any condition which depresses cell activity may
have an effect in increasing susceptibility to infection. Animals
which ordinarily are not susceptible to infection with a certain
organism may be made so by prolonged hunger, or fatigue, by the
influence of narcotics, by reduction of the body temperature, by loss
of blood. In man prolonged fatigue, cold, the use of alcohol to excess
and even psychic depression increases susceptibility. It has been
shown that such conditions are accompanied by a diminution in the
power of the blood to destroy bacteria.

There is variation in the susceptibility to infection in the different
races of man. If a race be confined to one habitat with close
intercourse between the people, such a race may acquire a high degree
of immunity to local diseases by a gradual weeding out of the
individuals who are most susceptible. A degree of comparative harmony
may be gradually established between host and parasite, as is the case
in wild animals. These have few diseases, the weak die, the resistant
breed; they harbor, it is true, large numbers of parasites, but there
is mutual adjustment between parasite and host. Diseases in animals
greatly increase under the artificial conditions of domestication.
Certain highly specialized breeds of cattle, as the Alderneys, are
much more susceptible to tuberculosis than the less specialized. The
high development of the variation which consists in a marked ability
to produce milk fat is probably combined with other qualities, shown
in diminished resistance to disease, and under natural conditions the
variation would not have persisted. The introduction of a new disease
into an isolated people has often been attended with dire
consequences. It is much the same thing with the introduction of
disease of plants. In Europe the brown-tail moth and the gypsy moth
produce continuously a certain amount of damage to the trees, but
their parasitic enemies have developed with them and check their
increase. These pests were brought to this country in which there were
no conditions retarding their increase and have produced great damage.

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