Causes—simple—complicated: Proximate; Remote: Predisposing—race,
genus, family, heredity, individual, environment, food, age, sex,
temperament, idiosyncrasy, debility, plethora, interdependence of
organs, embolism, mechanical influence. Exciting causes, intrinsic,
extrinsic, inherent, acquired, heredity, dentition, heat, cold,
atmospheric conditions, electricity, moisture, dryness, dust,
darkness, light, soil, food, water, inaction, over-exertion,
mechanical causes, poisons,—mineral—vegetable—animal, microbes,
contagious, infectious, epizootic, enzootic, sporadic, panzootic,
zymotic, mediate contagion, bacterial poisons.
The causes of disease are simple or complicated, and in the latter case
a single factor may be altogether harmless unless associated with
another which also may have been innocuous alone. For example: the
infecting germ of glanders (Bacillus Mallei) is harmless to the ox which
lacks the predisposition to the disease:—feeding buckwheat is harmless
to the dark-skinned animal, but is injurious to the white-skinned, if
exposed to sunshine:—the chicken can bear with impunity exposure to cold
or to the bacillus anthracis, but it cannot endure these two etiological
factors combined. It follows that one cannot predict the same result
from the same cause in every case. Yet with all concurrent conditions
the same the result will follow with mathematical certainty. This will
serve to illustrate the value of thoroughness in etiological knowledge,
as the basis of a sound pathology.
Etiology is primarily divided into =proximate= and =remote=. _Remote_
causes are again divided into =predisposing= and =exciting=.
=Predisposing Causes= are such as induce a condition of the system or of
a particular organ or group of organs which renders them specially
susceptible to a disease. This may be a characteristic of the _race_ or
_genus_ of animal, thus the genus bovis alone suffers from lung plague,
the genus equus from dourine, and ruminants from Rinderpest. It may be a
_family trait_, (_hereditary_) hence we see certain families of both men
and cattle cut off by tuberculosis, while other adjacent ones largely
escape. It may be an _individual peculiarity_, thus some subjects have a
congenital insusceptibility to a given disease, from which others of the
same family suffer, and one who has passed through a self-limiting
disease like measles, cowpox or anthrax is rarely attacked a second
time. Again predisposition may be due to _environment_ as when we find
herds in damp and exposed localities obnoxious to rheumatism, and horses
in dark mines exposed to specific ophthalmia. It may be the result of
_food_ as when the flesh-fed fox or rat resists anthrax and the
farina-fed one falls a ready victim. _Age_ may predispose, early youth
being remarkably susceptible to parasitism and bacteridian infection,
and old age to fractures and degenerations. _Sex_ is inevitably a cause
of limitation of disease as the females and males can only suffer from
disease of their respective =sexual= organs. Again of diseases common to
both sexes certain nervous and digestive disorders are common in
connection with gestation, and certain calculous diseases in connection
with the long and narrow urethra of the male. _Temperament_ has a marked
influence, thus the sanguineous or nervous race horse or hound shows a
marked predisposition to diseases of the heart, lungs and brain, and to
a sthenic type of inflammation and fever, while the heavy lymphatic
draught horse has a proclivity to diseases of the lymphatics and skin.
_Idiosyncrasy_ is closely allied to temperament, but the condition may
be less manifest, and the peculiarity is only recognized by the results,
as when a man is poisoned by sound fish or raspberries. _Debility_
whether from deficiency or poor quality of food, on the one hand, or
from overwork, filth, dampness or disease on the other must be looked
upon as strongly predisposing to certain diseases, such as tuberculosis
and glanders. _Plethora_ which charges the blood and tissues in a
different way with effete organic products, lays the system especially
open to certain diseases like black quarter in young cattle, and
parturition fever in cows. _Disease of one organ_ often predisposes
another organ through interdependence of function, as when torpid or
congested liver leads to portal and intestinal congestion, diseased
teeth to digestive disorder, imperfect hæmatosis to kidney trouble; in
other cases blood clots or bacteria from one pathological centre may be
arrested in the blood vessels of a distant organ and start new foci of
disease (embolism, metastasis); in still other cases the impairment of
the healthy function in one organ acts injuriously on another, as when
emphysema or other disease of the lungs forces the blood back upon the
heart causing dilation with atrophy of the walls. _Previous disease in a
tissue_ leaves for a time an impairment of structure and function which
may become the essential predisposing cause of the effective operation
of a morbific factor. _Mechanical_ action on a part may predispose to
disease, as for example, by reducing its circulation and nutrition and
thereby directly impairing its power of resistance to other inimical
agencies. Not infrequently a pus microbe lies deep in the cuticle or
even in the tissues without harm, until there occurs a bruise, or a bony
fracture when it at once develops a focus of purulent infection
(abscess).
=Exciting Causes= are the immediate causes of particular diseases. Like
the predisposing causes they may be _intrinsic_ or _extrinsic_, and the
first may be =inherent= or =acquired=.
Among =inherent= causes are certain of those already named as
predisposing causes, but which have come to be forcible enough to
develop disease without the intervention of any other observable factor.
Thus a _hereditary_ monstrosity (redundancy or defect), will appear in
successive generations without any apparent additional cause. The
appearance of white calves in herds of black cattle, after the
whitewashing of their stables shows a similar hereditary operation
though the result is not in this case pathological. The birth of blind
foals from blind sires or dams, or of foals with distorted feet from
mares suffering from severe chronic foot lameness are true pathological
sequences, in which the exciting cause is hereditary and operates during
intrauterine life. _Dentition_, as an attendant on early life is often a
directly exciting cause, from direct injury by entangled or retained
teeth that should have been shed, by fever aroused by the active local
changes, or imperfect mastication or insalivation leading to consequent
indigestion; in puppies and kittens convulsions are not uncommon as a
result.
=Extrinsic Causes= are such as operate through the environment. _Heat_,
if excessive and prolonged, relaxes and exhausts the system and exerts a
direct influence on the process of sanguification so that it may become
the direct cause of a variety of diseases. As the result of extensive
burns, dangerous congestions of internal organs are liable to occur, and
even the prolonged heat of summer often superinduces hepatic and gastric
disorder, diarrhœa and dysentery. Fat cattle in uncovered cars or yards
under a hot sun and with no breeze suffer extensively from insolation,
the temperature of their bodies rising to 110° to 112° Fah. and even
higher. _Cold_ is equally potent. With a temperature below zero Fah.,
the iron bit will freeze the buccal mucous membrane, and cause extensive
erosion of the mouth. The cold of salted snow or ice will freeze the
feet, causing sloughing of the skin above and around the coronet and
shedding of the hoofs, and predisposing to fatal septic infection of the
wounds. On the system at large, cold causes retrocession of the blood
upon the internal organs, and endangers the occurrence of acute disease
in any structure which is already debilitated or otherwise susceptible.
The nervous effect of the chilling of the skin is often the unbalancing
factor which sways the scale in favor of disease, which the system was
able to resist until this disturbing element was introduced. The sudden
chill from passing out of the warm barn into the frosty air, from
plunging into icy water, from standing in cold rain or sleet, from
standing in a draught of cold air especially when perspiring, is a
fruitful source of many diseases. In the cow, lying with the udder on a
cold stone may be the starting point of mammitis. The effect of sudden
chill is well exemplified in the great prevalence of diseases of the
respiratory organs at the change of the seasons in spring and autumn
when the vicissitudes of temperature are greatest, and the system
unprepared by habit, to bear the sudden change. Again it must be noted
that exposure to cold has a tendency to cause disintegration and
solution of the red blood globules, and that certain animals are
especially susceptible to this influence. _The condition of the
atmosphere_ is often a direct cause of disease as when charged with
offensive or irritating gases, the result of decomposition of organic
matter, with sewer or cesspool emanations, with deleterious gases from
chemical works, telluric sources, or fires. A low state of health, a
local irritation in some part of the air passages, or even a speedy
asphyxia may be the outcome of such atmospheric conditions. Again the
presence of solid particles of a more or less irritant, septic or
infecting kind prove the starting point of various diseases. The stone
cutters’ phthisis, and the sand granule ophthalmia are familiar examples
of the irritant, which often acts through the dust of the highway. Of
the infecting particles we have the germs of cattle lung plague, of
infective ophthalmias, and of tuberculosis carried with the dry dust and
inhaled. Of toxic agents borne on the atmosphere we see the compounds of
arsenic, mercury and lead. Moisture and dryness of the air induce
respectively a lymphatic constitution and low tone of health, and a
nervous constitution and a tendency to neurosis, ophthalmias, and skin
diseases. The pressure of the atmosphere has a profound effect on animal
health as seen in the extreme troubles of the heart and circulation in
the diving bell, and the respiratory, hæmorrhagic and brain affections
of high attitudes. A low barometer is attended by nervous disorders
(neuralgia) (S. Weir Mitchell). Surgical operations do best with a high
or rising barometer (Adinell, Hewson). The electrical tension of the
atmosphere shows familiarly, in man, in the feeling of heaviness,
dullness and malaise that precedes the bursting of a thunderstorm and
the relief that follows its termination. To this influence many of the
domestic animals are incomparably more sensitive than man, as witnessed
in the disposition of swine to hide in their pens or under litter on the
approach of the storm, the nervous disturbance even to abortion of
certain ewes which are heavy in lamb, and the great discomfort and even
piteous cries of some domestic felines on such occasions. The greatest
electric tension is seen in the drier climates, where the air, robbed of
its moisture, proves a poor conductor and equalizer, and the positive
and negative electricity get stored up separately in air, cloud and
soil. The presence of ozone in the air, as a habitual concomitant of
electric discharges, has been supposed to be a disturbing influence,
since it is distinctly irritating to the mucous membrane when present in
excess, but such excess apart from its artificial production is highly
improbable. As habitually met with it is antiseptic and health giving.
_Darkness_ always deteriorates the general health, producing
bloodlessness and pallor. _Light_ is usually invigorating, yet bright
sunshine falling upon the eyes from a window in front of the stall, or
in the open air when the face is turned up by an overdraw check rein, or
reflected from white dust and, above all, from snow, will often induce
inflammation and blindness. _Soils_ are often potent etiological
factors. Dense, damp, cold, undrained soils, are habitually covered by a
stratum of cold air, saturated with moisture, which greatly lowers the
vital stamnia. Damp clays, and waterlogged soils of various kinds, rich
in organic debris, are the natural homes of various pathogenic microbes,
such as those of ague, anthrax, milk sickness, actinomycosis and yellow
fever. Well drained sandy or gravelly soils are usually healthy, unless
they contain a great excess of decomposing organic matter. Again soils
with an excess of alkaline or other mineral matter may prove
deleterious, and those on the magnesian limestone often harbor the
poison of goitre, and cretinism, and favor the occurrence of urinary
calculus. _Faulty food and feeding_ in the domestic animals are
chargeable with many diseases. Stock often fall off in condition, in the
hands of one feeder, when the same food given with regularity and
judgment by a more careful feeder would keep them in the highest health.
Hay and grain which is musty and filled with cryptogams and their
products, are common causes of disorder of the stomach, the kidneys, the
nervous system or of general nutrition. Smut and ergot at certain stages
of their growth or grown under given conditions cause nervous disorders,
abortions, and gangrene of the extremity. A long list of vegetable
poisons may mix with fodders, and animal poisons with the food of the
Carnivora. A number of standard fodders may be poisonous at certain
stages of growth, as partially ripened perennial rye grass, millet,
Hungarian grass, vetches, etc. _Water and deprivation of water_ are
fertile causes of illness. Ruminants cannot chew the cud when deprived
of water, hence impaction of the first and even of the third stomach
with fermentations, tympany and other disorders. Horses suffer more from
a full drink of water after a feed of grain, the unchanged albuminoids
being carried on into the intestines, and both gastric and intestinal
indigestion induced. Sheep suffer fatal fermentations after drinking the
alkaline water of the Plains; cattle have diarrhœa and dysentery from
selenitions, or from stagnant and putrid water; and the water from the
dolomite is the usual channel of the goitre poison. Certain germs like
the plasmodia of malaria, and comma bacillus have their natural home in
impure water, and others like anthrax bacillus survive in the mud and
silt at the bottom of wells, ponds, and rivers and enter the system in
the water. _Compulsory rest_ in a stall often induces torpor of liver
and bowels, general muscular debility, and fatty degeneration especially
of the liver and heart. A few months of the swill feed, hot atmosphere
and absolute rest in a distillery stable usually ruins cattle for stock
purposes. _Overexertion_ on the other hand is prolific of illness.
Exhaustion of the muscles, congestion, inflammation, cramps, congested
lungs, heart failure or rupture, apoplexies and other hæmorrhages are
among the resultant maladies. Auto-poisoning is another result shown in
equine hæmoglobinæmia, and the fever of leucomaines. The excessive
development of sarcolactic acid from muscular work may render an
insusceptible animal susceptible to the anthrax bacillus. _Mechanical
causes_ would include over-exertion, in the production of strains,
fractures, and other injuries. They would also include impaction by
foreign bodies, calculi, and ingesta, friction of folds of skin or by
harness and other objects and pressure which leads to absorption and
atrophy. To these must be added _poisons of vegetable, mineral and
animal origin_ and the _microörganisms_ which act as injurious ferments
within the animal body. These will be treated more fully later on. Of
the microörganisms it may be said here, that they are almost certainly
the cause of all transmissible diseases. These diseases are variously
named on the basis of different ideas. They are =enthetic=, that is
implanted as a seed is planted in the ground to grow and multiply. They
are =zymotic= or fermentative because the essential cause multiplies and
is propagated like a ferment. They are =contagious= because propagated
by contact mediate or intermediate. They are =infectious= when
transmitted, not alone by contact but through the atmosphere. They are
=epizootic= because they tend to attack animals generally or a given
genus or family of animals generally when these are exposed to the
infection. They are =enzootic= when confined to the animals in a given
locality, the soil or conditions of which are favorable to the
preservation of the germ in pathogenic potency, or to the production of
a special susceptibility in the animal system. They are =sporadic= when
each case occurs without any casual relation to another. They are called
=panzootic= when they attack all animals without apparent preference.
The term _panzootic_ is also used to describe those recrudescences of a
disease or cycles of exalted pathogenesis which are observed in
contagious diseases, which frequently last for years and again give
place to a period of benignancy. Such cycles, of malignancy and
benignancy, may be due to modified environment acting either on the
disease germ or the animal system, or on both simultaneously.
The terms =enthetic=, =zymotic=, and =contagious= best express modern
views of the nature of these maladies. The term =infectious= when used
to express a gaseous or otherwise intangible (unorganized) body, or
influence transmitted through the air, necessarily excludes the
particulate, living, self-propogating germ upon which the
transmissibility of the disease depends. A chemical, electrical, or
other body or influence generated outside the animal body, cannot well
be conceived of as reproducing itself within the animal body but must
act like any other ectogenous poison, according to the size of the dose
and the frequency of its exhibition. This might create an _enzootic_
disease but would lack all the qualities of a contagious affection since
it could not spread from a victim when taken elsewhere and turned among
animals which would prove equally susceptible if placed within the
infecting area. Suppose on the other hand we apply the term _infectious_
to diseases in which the levity of the particulate living germ allows of
its being inhaled into the body of the susceptible animal, the case
becomes one of simple _mediate contagion_, the air acting as the
intermediate bearer.
The term =zymotic= conveys a clear idea of the method of increase of the
disease germ in the body by the ordinary process of generation. The old
doctrine of fermentation by a continuous change, due to contact with
dead fermenting matter, as an inflammable body continues to burn by
contact with the incandescent portion, has been definitely disproved by
the investigations of Pasteur and others, and today we must recognize
that every fermentation is the result of the propogation and vital
activity of living organisms. This does not ignore that the chemical
products or enzymes which are constructed by the vital activity of the
microbes, will dissolve or transform organic matter, but in the absence
of the microbe no such enzyme can reproduce nor multiply itself and its
action must therefore be exactly limited by its amount. The living germ
itself is therefore the one effective factor, by which the contagious
disease may be maintained and propagated. In its turn the living germ
can only come from a pre-existing living germ. To the scientist of today
the doctrine of spontaneous generation is a thing of the past and the
aphorism _omnis ovum ex ovo_ is dominant. The argument drawn from the
saccharizing of starch in the germinating seed by the operation of
diastase is inapplicable, as the diastase is produced by the living
cells of the germinal part of the seed, which are thus the counterpart
of the disease germ. No such glycogenic action occurs in the seed that
has been boiled or otherwise robbed of its vitality. So with the
arguments drawn from the ptyaline of the saliva, the pepsin of the
gastric juice, and the trypsin of the pancreatic juice; each of these is
the product of the living cells of the gland by which it is secreted,
and cannot increase its own substance in the absence of these cells.
Like the enzyme of the bacteridian ferment, these gland products can
break down or digest certain organic matters, but in all alike, the only
source of the chemical solvent is the living bacterium or gland cell
from which the particular product is derived. The toxins of a virulent
liquid, after the sterilization of the latter may still produce most of
the lesions and morbid phenomena of the disease, but, although death
were to ensue, the body of the victim would not be infecting to other
susceptible animals. The parallel between the functions of the secreting
animal cells and the disease germs may thus be put in tabular form:
Living Source. Chemical Product. Result.
Salivary gland cells = Ptyaline = Starch changed to
Sugar.
Peptic gland cells = Pepsin = Albuminoids changed
to peptones in acid
solutions.
Pancreatic gland cells = Trypsin = Albuminoids changed
to peptones in
alkaline solutions.
Disease germ = Toxin: Enzyme = Morbid phenomena.
Disease germ = Contagious disease.
Further consideration of pathogenic microörganisms will be found in
connection with contagious diseases.