THE CAUSE AND THE MENACE OF PLAGUE
The foregoing facts are quite sufficient to make us realize both the
possibility and the danger of a world-epidemic; a danger which has
existed for some years and which recently has been especially menacing
to the United States.
CAUSATION OF THE DISEASE.--Plague is an acute infectious epizoötic
disease, caused solely by _Bacillus pestis_, a bacterial organism. The
disease is common to man and to a number of the lower animals and
fowls.
Prominent among the animals susceptible to the disease is the rat, and
from this animal, through the intermediation of the flea, by far the
most cases of human plague arise. In California the ground squirrel
(_Citellus beecheyi_), a rodent closely related to the marmots of Asia,
plays a similar rôle. Of the Asian marmots, the tarbagan, a large
rodent, also commonly suffers from subacute chronic plague, which is
transmissible to man as an acute disease by the fleas which the animal
harbors.
ITS CONVEYANCE.--Although conveyance of plague through rats by contact
alone--that is to say without the medium of the flea--is denied by
modern experimenters, it is perhaps wiser and safer to consider the
disease infectious, inoculable and contagious in the common medical
meaning of these terms. While it is usually conveyed to man by the
flea, it may be acquired by the inhalation of plague bacilli and,
according to some authorities, by ingesting or swallowing the bacilli.
When infection takes place through the digestive tract, or in other
words, by the ingestion of bacilli, either the flesh of plague-infected
animals or fowls, or food superficially contaminated with plague
bacilli by rats, cockroaches or other carriers, serves as the medium.
Speaking practically, the possibility of infection through ingestion is
nearly negligible. Indeed, the conclusion of Simpson in regard to this
possibility has been disputed and denied. However, the recent
occurrence of plague in a cat in Manila, in my own experience, observed
with me and carefully worked out by Dr. Otto Schöbl, points strongly to
the possibility of ingestion plague, the cat in this case apparently
having acquired plague from eating rats dead from plague.
A full account of this case appears in the bacteriologic observations
of Dr. Schöbl and in my recital of the history of the Manila epidemic.
TYPES OF PLAGUE.--Plague in man may be of several types and these are
designated by names descriptive of the symptoms or of the regions of
the body most affected. Thus we have bubonic, septicæmic and pneumonic
types. As both mild and virulent cases occur, we also use terms
descriptive of the severity and course of the cases. Thus we describe
certain cases as ambulant, abortive, larval and fulminant. In the rat
the evidences of plague are less striking in life than they are at the
post-mortem table. Indeed plague-stricken rats, either naturally or
artificially (experimentally) infected, often show very slight
evidences of disease before death. Chronic plague in rats and a
relative immunity to inoculation in certain wild rats are fairly well
recognized phenomena.
FLEA CONVEYANCE OF PLAGUE BACILLI.--Both male and female fleas convey
plague, but the exact method of carrying the plague bacilli from
diseased rats to man, while fairly well determined, is of such recent
decision as to leave room for further experimentation. At present it is
believed that the flea deposits plague bacilli, at the time of biting,
upon the skin, by ejecting the contents of its rectum and by
regurgitation of its stomach contents. At least the flea is known to
perform these acts at the time of biting, and the rubbing or scratching
of the flea bite with the hand may easily introduce the bacilli into
the skin at this spot.[1]
[1] Acknowledgment is hereby made to the Contributors to "The Rat and
Its Relation to Public Health" by various authors, prepared by
direction of the Surgeon-General, P. H. and M. H. S., for numerous
facts utilized in the preparation of this article. The particular
contributors whose valuable chapters have been drawn upon for
information are D. E. Lantz, C. W. McCoy, D. H. Currie, Carrol Fox,
Rupert Blue, W. C. Rucker, R. H. Creel, M. J. Rosenau, V. C. Heiser,
W. C. Hobdy, and J. W. Kerr.
The possibility that the flea introduces the plague bacilli upon his
mandibles, or the skin-piercing armament with which he is provided, is
also to be considered. However, the following facts support the first
proposition. It has been experimentally shown that the average capacity
of a flea's stomach is about one-half of a cubic millimetre and that
thousands of plague bacilli may be ingested by the flea during the
biting of a plague-diseased rat; that the plague bacilli multiply
enormously and for many days in the flea's stomach and that the bacilli
are found only in the insect's digestive tract; that plague bacilli are
regurgitated from the stomach and are voided from the rectum with the
digested blood.
It has also been proved that almost all varieties of rat fleas, under
favorable circumstances, will bite man and that the most common human
flea (_Pulex irritans_) is frequently found upon rats, the flea,
generally speaking, being much less particular in his choice of hosts
and in his permanence of residence than most insects and ectoparasites
in general.
Of the rat fleas, _Pulex pallidus_ (_Lœmopsylla cheopis_) is common
under various names in India, the Philippines, Australia, Italy, Brazil
and in tropical countries generally. It bites both rat and man.
_Ceratophyllus fasciatus_, the common rat flea of Great Britain and the
United States, also bites both rat and man. In North America and
elsewhere certain other fleas of the genus _Ceratophyllus_ have been
found upon ground squirrels, cats, rats, sparrows and in chicken yards.
Dog fleas and cat fleas (genus _Ctenocephalus_) also infest rats, and
fleas of other genera are found upon mice, rats and ground squirrels
rather indiscriminately.
The significance of these facts in connection with prevention of
plague is apparent and it is plain that our warfare against fleas must
be made upon _all_ fleas and not upon a single variety. In this
connection the possibilities of the conveyance of plague bacilli by
other suctorial parasites and by insects which are not parasites, must
be borne in mind.
Thus the bed-bug, the louse, the tick and the mosquito must be
suspected as possible intermediaries and the fly and the cockroach as
possible food contaminators. Indeed, laboratory experiments have
already incriminated bed-bugs, flies and lice as potential vectors of
plague bacilli.
Experiment and observation have demonstrated, however, that above all
other parasites and insects, the flea is most likely to convey the
plague germ from rat to man, by reason of his frequent excursions from
rat-host to human-host, his taste for blood from either host, his
enormous activity and his ability to jump. After a searching inquiry
into the plague question the Indian Plague Commission came to the
conclusion that contagion plays a very minor part in the spread of the
disease, less than three per cent of human cases being so acquired.
This commission also decided that infection is conveyed from rat to
rat and from rat to man solely through the agency of fleas. While these
conclusions are probably true--and therefore of the utmost importance
from the standpoint of practical prevention--I should question whether
the other possibilities, however remote, are entirely negligible.
Seasonal conditions may affect the course of an epidemic in various
ways. (a) By effect upon flea prevalence, cold weather greatly
lessening the number of insects. (b) By effect upon rats, cold weather
and rains either driving them from overground to underground, or vice
versa, or from their principal avenues of travel in cities (the
sewers), into houses and buildings. (c) By effect upon the plague germ,
_Bacillus pestis_. The resistance of this organism is very variable,
sunlight and drying being its greatest enemies, while darkness and
dampness are its chief allies. So far as temperature is concerned, the
plague bacillus is not likely to be seriously affected by natural
temperatures, as it is not destroyed by heat below 150 degrees
Fahrenheit, nor by cold measured by zero Fahrenheit, which means that
it survives freezing, generally speaking.
It is probable that the periods of greatest seasonal prevalence of
plague will be found to correspond generally with increased prevalence
of rat fleas. During the periods when rat fleas are absent or least
prevalent, the disease is perpetuated in the form of chronic (subacute)
rat plague in a small number of the rodents. The India Plague
Commission made and verified this observation.
Cholera epidemics often abate spontaneously and this is believed to be
due in part to attenuations of virulence and changes in the cholera
organism which may be demonstrated in the laboratory. We can hardly
hope for such spontaneous abatements in plague epidemics, as it has
been found difficult to attenuate or to intensify cultures of plague
bacilli permanently in laboratory experiments with animals. If it is
true that plague epidemics are often marked by a preponderance of mild
cases in the early days and a gradual subsidence of intensity of the
cases as the epidemics wane, we probably will have to look to the
susceptibility of our patients for our explanation of this phenomenon,
rather than to variations in the virulence of the plague bacilli. If
plague bacilli continue to be distributed to susceptible people the
disease should continue with a general stability of virulence.
STABILITY OF VIRULENCE OF _B. Pestis_.--According to Strong, stability
of virulence is a marked characteristic of _B. pestis_, it having been
shown by him that it is difficult to increase the virulence of a very
virulent strain or to intensify an attenuated one in laboratory
animals, working with monkeys, rats and guinea-pigs.[2] If his
observations are correct (and they seem to correspond with the findings
of other observers), the oft-recorded occurrence of a preponderance of
mild cases of plague in the early days of an epidemic and the gradual
subsidence in intensity of the disease as the epidemic approaches its
close will have to be explained upon other grounds than those of
variability of virulence by attenuation of virulent strains alone.
While he admits that _B. pestis_ may become attenuated under certain
conditions many times during the course of an epidemic, it may also
regain its virulence, he contends, under other conditions.
[2] "Studies in Plague Immunity," R. P. Strong, Philippines Journal
of Science, June 1907, No. 3. Frequent reference has been made to
these studies in the preparation of this article, for which
acknowledgment is hereby made.
With these facts concerning the cause and the manner of extension of
plague and its menace before us, we are in position to approach the
problem of prevention intelligently, and in the case of plague
prevention is preëminently preferable to cure, as well as decidedly
more practicable.
I think we may be permitted here to sum up the problem of plague
prevention thus: Without fleas, without rats, or without human plague
cases, there can be no extension of plague, practically speaking.
Therefore the destruction of both rats and fleas, the isolation of
human plague cases, and the exclusion from them of all suctorial
parasites and insects, will provide practical security for mankind
generally.
A word concerning pneumonic plague may be permissible. This form of
plague occasionally occurs in epidemics of great fatality, as, for
example, the epidemic in Manchuria, North China, a few years ago.
The mystery of this outbreak was largely dispelled by the work of the
Americans, Strong, Teague and Barber, of the Bureau of Science of
Manila.
The occurrence of secondary pneumonia in bubonic or septicæmic plague
is rather common and it is likely that such secondary plague pneumonias
are the starting points of epidemics of pneumonic plague, _i.e._, of
cases of primary plague pneumonia, the point of infection being in the
respiratory organs and the infection being acquired through the
inspiration of plague bacilli.
The principal prerequisites seem to be an extremely moist atmosphere
under confined conditions and a low temperature; conditions most
unfavorable to evaporation and ventilation. Under these conditions the
pneumonic patient sprays plague bacilli into the air while coughing and
droplet infection follows.
It is therefore apparent that epidemic pneumonic plague is controllable
by sanitary and hygienic measures and, furthermore, that in the absence
of original cases of bubonic and septicæmic plague, with secondary
plague pneumonias which give rise to primary plague pneumonia in the
manner explained, respiratory plague in epidemic form will not occur.
There is no evidence pointing to the conveyance of respiratory plague
by insects or other carriers.