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Page 44
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]
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