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Page 41
It has been shown that in coughing and speaking very fine particles of
spray are formed by the intermingling of air and saliva, which may be
projected a considerable distance and remain floating in the air for
some time. These particles are so fine as to be invisible; they may be
inspired, and their presence in the air forms an area of indeterminate
extent around the infected person within which such infection is
possible. Such spray formation is also an important means of the
extension of infection in the sick individual, for it is continually
formed and inspired. It is in this way that the extreme prevalence of
broncho-pneumonia in infants and young children is to be explained. No
matter what the essential disease, an almost constant finding in young
children after death is small areas of inflammation in the lungs in
and around the terminations of the air tubes. The situation renders it
evident that the organisms which caused the lesions entered the lung
by the air tubes. The mouth of the child is unclean and harbors
numbers of the same sort of organisms as those causing the lung
inflammation; but in the absence of such a mode of infection as is
given by spray formation it is difficult to see how the extension from
the mouth to the lungs could take place. The weakened condition of the
body in these cases favors the secondary infection.
If the disease be located in the intestines, as in typhoid fever and
dysentery, the organisms are contained in the fecal discharges, and by
means of these the infection is extended. In typhoid fever, dysentery
and cholera massive infections of the populace may take place from the
contamination of a water supply and the disease be extended over an
entire city. One of the most striking instances of this mode of
extension was in the epidemic of cholera in Hamburg in 1892. There
were two sources of water supply, one of which was infected, and the
cases were distributed in the city in the track of the infected
supply. Many such instances have been seen in typhoid fever. Certain
articles of food, particularly milk, serve as sources of infection.
This is more apt to happen when the organism causing the infection
grows easily outside of the body. A few such organisms entering into
the milk can multiply enormously in a few hours and increase the
amount of infectious material. In all these cases the sick individual
remains a source of infection, for it is almost impossible to avoid
some contamination of the body and the immediate surroundings with the
organisms contained in the discharges.
Transmission by air plays but little part in the extension of
infection. In such a disease as smallpox, where the localization is on
the surface of the body, the organisms are contained in or on the thin
epithelial scales which are constantly given off. These are light, and
may remain floating in the air and carried by air currents just as is
the pollen of plants. There seem to have been cases of smallpox where
other modes of more direct transmission could be excluded and in which
the organisms were carried in the air over a considerable space. All
sorts of intermediate objects, both living and inanimate, such as
persons, domestic animals, toys, books, money, etc., can serve as
conveyors of infection.
Insects play a most important part in the transmission of disease, and
in certain cases, as when a disease is localized in the blood, this is
the only means of transmission. There are three ways in which the
insect plays the r�le of conveyor.
1. The insect may play a purely passive part in that its exterior
surface becomes contaminated with the discharges of the sick person,
and in this way the organisms of disease may be conveyed to articles
of food, etc. The ordinary house fly conveys in this way the organisms
of typhoid and dysentery. Flies seek the discharges not only for food,
but for the purpose of depositing their eggs, and the hairy and
irregular surface of their feet facilitates contamination and
conveyance. When flies eat such discharges the organisms may pass
through the alimentary canal unchanged and be deposited with their
feces; they also often vomit or regurgitate food, and in this way also
contaminate objects. Flies very greedily devour the sputum of
tuberculous patients, and the tubercle bacilli contained in this pass
through them unchanged and are deposited in their feces.
[Illustration: FIG. 19.--TRYPANOSOMES FROM BIRDS. All the trypanosomes
are very much alike. They contain a nucleus represented by the dark
area in the centre and a fur-like membrane terminating in a long
whip-like flagellum. They have the power of very active motion within
the blood.]
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.
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