Report: Mosquitoes Considered As Transmitters of Yellow Fever and Malaria, by Carlos J. Finlay, May 27, 1899 |
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MOSQUITOES CONSIDERED AS TRANSMITTERS OF YELLOW FEVER AND
MALARIA.
By Charles J. Finlay, M.D.
Havana. May 27th 1899- ---------
The despised mosquito, denounded by me since 1881 as
the agent of transmission of yellow fever, is now attract-
ing considerable attention among distinguished and sagacious
observers who attribute to that insect an important role in
the etiology and propagation of the malaria infection. To those
who are familiar with the biological conditions and the ha-
bits of the mosquito this will not be a matter for surprise;
rather should we wonder how, considering the special apti-
tudes of the insect, other inoculable diseases are not trans-
mitted by it, specially such as are due to germs in the blood
or in the tissues that lie within reach of its sting. Much
light, however, has been thrown upon this singular eclecticism
by modern ideas concerning the process by which some blood-
sucking insects convey certain diseases to warm blooded ani-
mals. No are induced by them to regard as one of the essen-
tial conditions that the transmitting insect should itself
experience a true infection, which may not endanger its life
nor greatly disturb its physiological functions, but must
always require, on the part of the insect, pathogenous sus-
ceptibility for the specific germs which it is called upon
to transmit. It will thus be readily understood why the same
insect may transmit only certain germs and not others, as
also that, among insects of the same kind, some species may
possess that faculty while others do not. Among the publications that have appeared concerning
the transmission of malaria by mosquitoes, the most important
one, and that which has caused most sensation, has been the
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lecture delivered, a few months ago, by Robert Koch, in which
he declares himself decidedly in favor of the mosquito theory
as the one which most plausibly accounts for the propagation
of the said disease. In support of his idea he cites a very
appropriate precedent, the Texas fever, a cattle disease the
etiology abd propagation of which more so ably cleared up, in
1892, by Dr. Theobald Smith, chief of the Division of animal
parthology in the Bureun of Animal Industry, United States of
America. Dr. Smith proved that the germ of the disease in a
blood parasite, and that it is propagated by the cattle tick.
The germs sucked in which the blood of diseased cattle reach
the eggs of the tick, and the new generation of ticks, devel-
oped from the infected eggs, convey the Texas fever to the
sound cattle upon which they are applied. His experiments
were repeated and confirmed in Eastern Africa by Koch, who,
in view of the relations which seem to connect the presence
of mosquitoes with the transmissibility of malaria, does not
hesitate to make those insects responsable for the propaga-
tion of the malaria infection. He does not think, however,
that the latter can be communicated by so simple a process
as that of a mosquito first stinging a malaria patient and
afterwards a sound person, such as I have, in my theory, con-
sidered capable of causing the transmission of yellow fever.
The grounds for this distinction, however, are not very ap-
parent. In the case of the tick, which is supposed not to
attack a second animal after parting from its first host, the
exclusive transmission by the second generation, infected
through the eggs, may be considered a necessity; but it is
otherwise with mosquitoes, at any rate, with those which I
have observed in Havana. After an interval of two or more
days, which they require to digest the blood and empty them-
selves, they are ready to sting the next victim that offers,
and may do so as many as ten or twelve times, during the
thirty or more days that I have been able to keep them alive.
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It is, therefore, quite admissible that, when the mosquito
becomes contaminated, not only its eggs but also its saliva-
ly and venom glands may be invaded by the pathogenous germs,
so that the latter may be discharged with the secretion of
those glands along the track of the wound, and into the capil-
lary vessel entered by the sting when the insect sttacks its
next victim. Indeed, on some rare occasions I have seen mos-
quitoes die withing twenty four hours after they have stung
a patient with severe yellow fever, without assignable cause,
for they still retained some of the blood which they had
sucked; whence it might be surmised that the yellow fever
germ is pathogenic for the Havana mosquitoes, though the in-
fection seldom proves for those insects. In August last, during may stay in the field hospitals
on the hills near Santiago, I witnessed a fact which, as far
as it went, agreed with my theory about yellow fever, inas-
much as there were neither mosquitoes, mosquito eggs nor lar-
vae, to be found in my encampments, and not a single case of
yellow fever occurred among the one hundred and fifty men who
came under my observation, notwithstanding the daily communi-
cations with the city. It was otherwise, however, in regard
to malaria, for this constituted the prevalent cause of sick-
ness in all those camps. It assumed various types: the quo-
tidian or tertain intermittent, the remittent, irregular, or
subcontinuous; but in most of the cases it was accompanied
by diarrhea (sometimes mixed with blood). This instance, at any
rate, shows that Koch!s assertion, that "where there are no
mosquitoes there is no malaria" is altogether too absolute.
In those camps, I believe the propagation must have been ef-
fected through the flies (of which there was a great abundan-
ce). These insects, in spite of all precautions, had ample
opportunities of picking up, from the discharges of the ma-
maria patients, not only the malaria parasites contained in
the extravasated blood but also some infections intestinal
germ,
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germs, with both of which organisms they may have contaminated
the food and beverages used by the men who subsequently showed
signs of the double infection. A yellow fever epidemic occur-
ring under similar circumstances, in the absence of yellow
fever mosquitoes, night not be so readily reconciled with
my theory about that disease, which is founded upon more de-
finite and more exclusive arguments than those recorded in
connection with the malaria infection. The following instants
may serve to illustrate my meaning. In the capital of Mexico,
and in other districts of similar altitude above the sea le-
vel, Mexicans who never have visited the lowlands have no in-
munity whatsoever against yellow fever, a sure proof that no
epidemics of that disease ever occur in that part of the coun-
try. It sometimes happens, however, that a resident of the
capital takes the infection by going to Vera Cruz, though the
disease may nor declare itself untill his return to the capi-
tal. In such cases, the yellow fever will run its usual course
with the same symptoms and prognosis as if the patient had
remained at Vera Cruz; with this difference only;, that in
Vera Cruz other suscoptible persons night readily have caught
the infection from him, whereas in Mexico the disease is ne-
ver propagated. If the infection could be transmitted through
contact with the patient or his secretions, by inhaling his
emanations in the sick room, or by the use of contaminated
food or beverages, there would be no imaginable reason why
the disease should not be transmitted at Mexico as well as
at Vera Cruz. Such not being the case, we must infer: first,
that a factor which is necessary for the transmission is pre-
sent at Vera Cruz, but is absent from Mexico; and second from
the circumstances of the disease not being transmissible
through the forms of exposure enumerated above, that the yel-
low fever germ is pathogenous only when introduced in a less
trivial manner, probably by inoculation under the epidermis
or even directly into a blood vessel , . Hence my theory of the
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mosquito. NEW MOSQUITO THEORY.- My original mosquito theory, however
in viwe of the facts brought to light by Dr. T. H. Smith, in
his admirable demonstration of the transmission of the Texas
fever through the agency of the cattle tick, required now to
be somewhat modified, so as to include the important circum-
stance that the faculty of transmitting the yellow fever germ
need not be limited to the parent insect, directly contami-
nated by stinging a yellow fever patient (or perhaps by con-
tact with or feeding from his discharges), but may be like-
wise inherited by the next, generation of mosquitoes issued
from the contaminated parent. With this new development, in-
deed, the theory seems to cover the whole ground of conditions
which are known to govern the transmission or non-transmission
of the disease, and to account for every well-authenticated
fact on record. There are different species of mosquitoes which are pe-
culiar to certain localities, even within the yellow fever
zone, though we are not able to account for their preferences
in this respect, nor for the circumstances which determine
their permanency in such limited areas. It is a fact, however,
that the "domestic mosquito" (by which term I mean such spe-
cies as constitute a parasite and commensal of man) shows no
inclination whatever for extensive excursions so long as the
female insects find at hand convinient subjects from whom they
can draw the warm life-blood which they apparently require,
not so much for their own nourishment as for the purposes of
ovulation and for the reproduction of their species, That the
tropical species cannot well establish themselves in tempe-
rate or cold climates, will be readily understood by repeat-
ing the following experiments: Let a mosquito of the small
diurnal species found in Havana be introduced into a test
tube provided with a thermometer; if the temperature be low-
ered, it will be seen that, when it falls to between 15° and
19° C (59° and 62.2° F) the insect becomes benumbed, and un-
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less it finds some object upon which it can fasten its claws
it will drop to the bottom of the tube, in a condition of ap-
parent death, in which it will remain so long as the tempera-
ture is kept between 15° and 0° C. (59° and 32° F). On allow-
ing the temperature to rise again, when it reaches above 15°
to 19° C. the mosquito will gradually revive and soon recovers
its normal agility and the power of stinging. On the other
hand, if the mosquito is confined in a closed tube and the
air gradually rarefied, the insect appears to lose the power
of supporting itself on its wings, and also that of stiffen-
ing its proboscis for the purpose of stinging. With tempera-
ture below 25° C. (77° F.), the time required by the culex
mosquito to digest the blood and get ready for another bite
is prolonged to several days; and, according to my observa-
tions, the pupae of that species, if kept at temperature 25°
C. give five males to one fomale, whereas at 25°--30° C. the
proportions are reversed. Hence it follows that during the
cold season in Havana there are comparatively few mosquitoes
of that kind in a condition to propagate the yellow fever.
At see mosquitoes will save themselves from being blown into
the water only by avoiding the deck, and other exposed parts
of the vessel, and in travelling towards cooler regions they
will have a additional motive for secking refuge in the
warmer and more sheltered parts of the ship. Hence the like-
lihood of their gathering in the holds of vessels, in which
the source of yellow fever infections has been, many a time,
distinctly located. Once boxed up inside the hold of a vessel,
the contaminated mosquito may be reduced to the necessity of
drawing its blood-supply (faute de mieux) from lower animals ,
such as rate etc. and to lay its eggs in any collection of
fresh water that may have found its way through the chinks
or otherwise. On land, mosquitoes will instinctively frequent
the basement or ground floor of houses in preference to the
upper ones, and they seldom seek the open air of their own
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accord, while their usual frunctions can be fulfilled under
sholter, except when they are ready to lay their eggs, This
is in accordance with the maternal instinct which teaches
them to procure undisturbed possession of some stagnant wa-
ters for their larvas during the two or more weeks required
for the complete development of the winged insect; a condi-
tion seldom satisfied within inhabited dwellings. On the ap-
proach of its natural death, the parent insect returns to the
same water where the eggs have been laid, and its cadaver re-
mains floating on the water, to be devoured by its own lar-
va. Entrapped during the unconscious act of person putting
on his hat a contaminated locality, mosquitoes have been
conveyed to distant houses; and inside of boxes, trunks, par-
cels, etc. provided that a sufficient degree of moisture and
particles of available food exist in their place of confine-
ment, that can be conveyed to any distance that may be reach-
ed within the natural term of the insect's life (which some-
times lasts as many as thirty or thirty-five days). My experiments upon yellow-fever mosquitoes have al-
ready been published; their results may be thus briefly re-
corded; first: Reproduction of the disease, an a mild form,
within five to twenty five days after having applied centa-
minated mosquitoes to susceptible subjects; second, partial
or complete immunity against yellow-fever, obtained even
when no pathogenous manifestation, had followed those inocu-
lations; third, finally, the coincidence of cultures made
with the heads and proboscider of contiminated mosquitoes
giving the identical micrococcus in tetrads (M. tetragenus
febris flavae; M. tetragenus versatilis, Sternberg; tetracoc-
cus versatilis) previously dicovered by me, in colaboration
with Dr. C. Delgado, in the blood and secretions of yellow
fever patients. With such as array of evidence (presumptive or other-
wise) as to the role of the mosquito in the propagation of
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yellow fever, and the concurrence of Koch, Manson, and other
experts of the highest order in their advocacy, of a similar
doctrine from the transmission of malaria, the time seems at
last come when decided measures of protection against mosqui-
toes should be seriously considered; the more so as the ener-
getic spirit of the Anglo-Saxón race is about to replace the
fatalistic apathy of former rules in Cuba, and Porto Rico.
The suggestion of Koch, calling for dwellings from which mos-
quitoes could be barred out, in order that the German colo-
nies of Eastern Africa night be freed from Malaria, ought
surely to be acted upon in countries where it is not only ma-
laria that has to be contended with, but also the dreadful
yellow fever, aptly calles "the plague" in the early spanish
chronicles of America, from its analogies with the Oriental
disease of that name. Why should not the houses in yellow fe-
ver countries, be provided with mosquito blinds, such as are
used in the United States as a more matter of confort, where-
as here it might be a question of life or death. The mosqui-
to larvas might be destroyed in swamps, pools, privies, sinks,
street-sewers and other stagnant waters, where they are bred,
by a methodical use of permanganate of potassium or other
such substances, in order to lessen the abundance of mosqui-
toes. But the most essential point must be to prevent those
insects from reaching yellow fever patients, and to secure a
proper desinfection of all suspicious discharges, in order to
forestall the contamination of those insects. Well-ventilated
hospitals should be built on high grounds, with no atagnant
waters nor marches in their vecinity, the doors and windows
protected by mosquito blinds, a good system of drainage and
sewerage, with facilities for desinfecting all suspicious
discharges, and for destroying such mosquitoes and larvas as
sight be found within the buildings. Only the upper stories
should be occupied by the sick, and none but yellow fever
patients, and such malaria patients as are immune against
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yellow fever, should be admitted. The examination for admit-
sion might be corried out in a separate department devoted
to suspicious cases under observation. With such hospitals at hand, and an efficient board
of health that would see to the proper errangements for pa-
tients who could be left in their homes, and general sanitary
improvements in and around the principal cities, there can
be little doubt that yellow fever might be stamped out of
Cuba and Porto Rico, and malaria reduced to a minimum. It
would then be the business of the port and quarantine offi-
cers to prevent the introduction of fresh germs. ------------------ Published in the N. Y. Record of May 27
1899- Shown to Col. R. M. Reilly Ch. Surg.
Dis Cuba in Dec. 1898 & to Dr. H. R.
Carter M. H. L.- |
