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Journal of Experimental Medicine

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E T I O L O G Y OF Y E L L O W FEVER.
IX.

MOSQUITOES IN RELATION TO YELLOW FEVER.
BY HIDE¥O NOGUCHI, M.D.

(From the Laboratories of The Rockefeller Institute for Medical Research.)(Received for publication, July 3, 1919.)
That a certain species of mosquito serves as a carrier of yellow fever has long
been suspected. FinlayI supported this theory as early as 1881 and went so far
as to advise conferring immunity upon non-immune persons by causing them to be
bitten by mosquitoes (Stegomyia calopus) which had previously fed on a yellow
fever patient. Thenceforth Stegomyia calopus became the center of attention inyellow fever investigations, the theory being experimentally confirmed finally byReed, Carroll, Agramonte, and Lazear,2 who successfully transmitted yellow fever
to non-immune human subjects by allowing the infected stegomyias to bite them.
The successful eradication of yellow fever by Gorgas3 from various endemic centersin the western hemisphere through auti-stegomyia campaigns amply attests thecorrectness of this important discovery. Reed and his coworkers established thefact that the virus exists in the peripheral blood during the first 3 days of theillness and that the mosquitoes, which have fed on a yellow fever patient during
this period, become capable, after about 10 days, of transmitting the infection
by their bite to a normal person. Hence they assumed that the virus requireda period of at least 10 to 12 days of extrinsic incubation during which it must
pass through a life cycle, as was known to be the case with the parasite of malaria.
The necessity for an extrinsic incubation of about 12 days was pointed out by
Carter 4 in his epidemiological studies of yellow fever. This theory, coupled with
the improbability of a bacterial origin of the disease, brought about the general
impression that the virus was a protozoan, although nothing definitely objective
was adduced in support of this view.
1 Finlay, C., El mosquito hipot~ticamente considerado como agente de trasmisi6n de la fiebre amarilla, An. r. Acad. cien. m~d. Habana, 1881-82, xviii, 147.
2 Reed, W., Carroll, J., Agramonte, A., and Lazear, J. W., The etiology ofyellow fever, Am. Pub. Health Assn., 28th Meeting, 1900; also Senate Doc. No.
822, 61st Congr., 3rd Sess., 1911, 56.
a Gorgas, W. C., Sanitation in Panama, New York and London, 1915.
4 Carter, H. R., Note on the spread of yellow fever in houses. Extrinsic incubation, Med. Rec., 1901, lix, 933.
401

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ETIOLOGY O FYELLOW FEVER.

IX

Reference has already been made 5,6to the fact that the virus of yellow
fever passed through the pores of a Berkefeld or a Chamberland filter.
This point is important in excluding as the inciting agent of the disease
any organism incapable of passing through these filters which might
be found incidentally in the blood or tissues of yellow fever patients.
Some persons believe that the virus is ultramicroscopic, at least
with the magnification possible at the present time.
The characteristics of the organism isolated from the yellow fevercases in the present investigation conformed with all the other known
characteristics of the yellow fever virus, and it became necessary to
determine whether or not it behaved like the latter in relation tomosquito transmission. That it does, under certain conditions, isshown in the experiments to be described.
Material and Mode of Experiments.

Larva~ of Stegomyia calopus collected from houses in Guayaquil
were brought to the laboratoryY They varied in stage from thevery young to the pupal form.
For the purpose of maintaining the imagos which emerged fromthese larva~ special cages were constructed consisting of a square
wooden frame with wire net on all but two opposing lateral walls,
which were solid pieces of wood with a round opening in the centerfor the insertion of the arm or a glass of water from either side of the
cage. The opening was covered by a well fitting sliding door which
could be drawn to one side and slipped back in place. A long glass
cylinder about 8 inches long and 2 inches in diameter with one end
closed was used for handling individual mosquitoes. When thespecimens were to be killed for examination a piece of cotton saturated
with chloroform was put at the bottom of the cylinder. For histological study the mosquitoes were placed in fixing fluid such as sub6 Reed, W., and Carroll, J., The etiology of yellow fever; a supplemental note,
Senate Doc. No. 822, 61st Congr., 3rd Sess., 1911, 149.
6 Marchoux, Salimbeni, and Simond, La fi~vre jaune, Ann. Inst. Pasteur,
1903, xvii, 665.
These were obtained through the cooperation of the officialsof the Municipal Board of Health of Guayaquil.

mD~YO ~OGVCm

403

limate-alcohol, Zenker's fluid, or 10 per cent formaldehyde, after thewings and legs were trimmed off. Mosquitoes engorged with blood• usually sink to the bottom in a short time, but those which are unfedor have already digested the blood do not sink for many hours or evendays, and it was necessary to keep them within the fixing fluid by
forcing them down with a piece of absorbent cotton. In some instances a minute needle puncture into the abdomen or thorax, or both,facilitated the prompt penetration of the fixative agents. The
specimens fixed in sublimate-alcohol or Zenker's fluid were washedin water for 48 hours and then transferred to 50 per cent alcohol, in
which they were kept until worked out later for serial sections.
The larvae showed a surprising resistance to phenol (3 per cent),
formaldehyde (10 per cent), sublimate-alcohol (two parts of saturated
aqueous solution of mercuric chloride and one part of absolute
alcohol), and Zenker's fluid, being capable of surviving for at least
15 minutes. Some lived as long as 30 minutes in 10 per centformaldehyde.
Monkeys and infected as well as normal guinea pigs, when notallowed to defend themselves, are eagerly bitten by the stegomyias.
Rats, however, are seldom bitten by these mosquitoes, and bats havenever been seen to be attacked when placed in a cage with hungry
stegomyia females.

Transmission Experiments with Stegomyia calopus from Man to
Animals.
Attempts were made to reproduce the appearances of yellow feverin the guinea pig by permitting the stegomyias to bite the animals
after having fed upon the blood of yellow fever patients during theearly stage of the disease. The procedure consisted in putting one
arm of the patient into a cage containing 100 to 300 mosquitoes which
had been hatched from the larv~ in captivity, and allowing the mosquitoes to feed until most of the females were engorged with blood.
The cage containing the insects was then carefully kept in the shadow;sometimes the females were transferred to a separate cage. A glassof water containing a pea and a green twig was put into the cage,and each morning a slice of banana or papaya was placed upon thewire wall.

4:04

ETIOLOGS~ OF "¥-ELLOW ~'EVER. IX

Since an enormous a m o u n t of labor would be required to t e s t theinfectivity of each female mosquito, s u m m a r y experiments werem a d e ; t h a t is, a dozen or more mosquitoes were allowed to bite one
and sometimes two animals a t the same time. T h e animals weret h e n placed in s e p a r a t e cages and k e p t under daily o b s e r v a t i o n for aperiod of a t least 1 m o n t h , in case no infection t o o k place within ashorter time. T h e tests for infectivity were usually m a d e 2 weeks
after the feeding on the patients, b u t sometimes earlier.
As the protocols show, positive leptospiral transmission was obtained in one of the following experiments.

Experiment 1 (Negative).--Aug. 5, 1918. 40 recently hatched females of
Stegomyia calopus were allowed to become engorged by feeding on the right armof Case 9 (G. C.), a severe case of yellow fever, admitted on the 3rd day of disease
to the hospital. The patient recovered. Aug. 10. Sixteen females surviving.
Eggs were found on the wet leaves in the glass. The sixteen mosquitoes surviving were allowed to feed on a very young normal guinea pig on that day; thatis, 5 days after feeding on the patient's blood. All became engorged within the
period from 12 m. to 2 p.m. The guinea pig was removed for observation intoanother cage. I t remained well with normal temperature (38-39.5°C.) for 11days, when it died of an intercurrent disease. There was no jaundice or change
in the liver or kidney.
Experiment 2 (Positive Transmission).--Aug. 15. Twelve of the sixteen mosquitoes used in the above experiments still surviving. Larvm in the glass. Aug.
28. Eight still surviving (23 days after feeding on the patient). A normal
guinea pig was placed in the cage with these eight mosquitoes, which immediately
attacked the animal, all becoming engorged. The guinea pig was removed toanother cage and kept under observation. Temperature 38°C. Aug. 29, temperature 38°C.; Aug. 30, 40°; Aug. 31, 38.6°; Sept. 1, 39.3°; Sept. 2, 38°; Sept.
3, 37.5°; Sept. 4, 36 °. Death occurred in 6 days.
Autopsy.--Moderately marked general jaundice. The lungs showed several
small ecchymoses; the liver was yellowish brown and mottled; the kidneys werecongested and showed ecchymoses; the stomach contained blood-stained food;the intestines showed numerous serous and mucous ecchymoses, and the stool
was mixed with blood; the bladder was half full of yellowish brown urine whichcontained an abundance of albumin and casts; the adrenals were congested; thespleen, pancreas, and testes were apparently unchanged; the heart showed a few
minute ecchymoses on the surface.
Mivroscopic Examina$ion.--Examinations of emulsions of the liver, kidney,
and blood for the leptospira were negative. The infectivity of the liver andkidney was tested on another guinea pig by injecting organ emulsions into thesubcutaneous tissue. This animal died under the same symptoms in 7 days.

mDEYO NOGVCm

405

The leptospira was demonstrated in the kidney in small numbers, but not in theliver or blood.
E x p e r i m e n t 2 shows t h a t the mosquitoes which sucked the bloodof a yellow fever p a t i e n t on the 3rd d a y of the disease were infective
to the guinea pig a f t e r a period of 23 days. T h a t t h e y were unable
to produce a typical infection when tested 3 days after the feedingis shown in E x p e r i m e n t 1.

Experiment 3 (Negative).--Aug. 5, 1918. Twenty-eight females were allowed
to become engorged on the right arm of Case 22 (A. M.), a fatal yellow fever case,admitted to the hospital on the 4th or 5th day of the disease. The patient diedon the 10th day with typical symptoms. Aug. 10. Twelve surviving. Aug. 12(7 days after feeding). The mosquitoes were placed on a normal guinea pig.
Several became engorged, and several were killed by the animal. This guinea
pig showed no symptoms within the period of observation and was discarded.
Aug. 28. The surviving females were allowed to bite a normal guinea pig.
The temperature rose to 39.9°C. on the 5th day but soon returned to normal andthe animal showed no symptoms afterwards.
Experiment 4 (Doubtful).--Aug. 14, 1918, 4.15 p.m. A large number of stegomyias were placed on the left arm of Case 10 (M. N.), admitted on the 2nd dayof the disease. 60 females which had become fully engorged were put into anothercage. The patient died of typical yellow fever on the 7th day. Aug. 15, 9.30
a.m. Another feeding by the same mosquitoes on the same patient. Aug. 20.
34 surviving. Aug. 27 (13 days after feeding). 29 surviving. These wereallowed to bite a normal guinea pig, and all became engorged. The guinea pig
showed the following symptoms: Aug. 28, temperature 37.6°C.; Aug. 29, 380;
Aug. 30, 38.2°; Aug. 31, 39.5°; Sept. 1, 39.8°; Sept. 2, 40°; Sept. 3, 38.4°; Sept. 4,
38.1°; Sept. 5, 37.8°; Sept. 6, 38°; Sept. 7, 38.2°; Sept. 8, 38.1°. There was a
trace of icterus in the scleras on the 8th and 9th days, which faded during thefollowing day.
As the animal returned quickly to n o r m a l w i t h o u t further s y m p t o m s ,a positive diagnosis was n o t possible, b u t it is p r o b a b l e t h a t there wasa mild infection.

Experiment 5 (Negative).--Aug. 14, 1918, 5 p.m. The right arm of Case 8(R. V.), admitted on the 2nd day of disease, was put into a cage containing
about 50 mosquitoes. Only five females sucked the blood within about 30 minutes.
The case was severe but recovered. Aug. 15, 12 m. The feeding was repeated,
32 females becoming engorged immediately. Aug. 25. Many minute larvae were
found in the vessel containing water. Aug. 28, 10 a.m. The surviving females,
thirteen in number, were allowed to feed on a normal guinea pig. All eagerly

406

ETIOLOGY OF YELLOW F E V E R .

IXengorged. The guinea pig was placed in a separate cage for observation. As nosymptoms developed within a month it was discarded.
Experiment 6 (Negative).--Aug. 28, 1918, 12 m. 80 female stegomyias wereallowed to engorge on Case 52, a girl of 10 years, whose brother had died in thehospital 4 days previously with yellow fever, but who was showing only a suspicion of icterus in the scleras which disappeared on the following day. The casewas so mild that the patient never was confined to bed and left the hospital in 7
days. The temperature did not exceed 39°C. and remained at that point for the
1st day only. If the case was one of yellow fever it was extremely mild. Sept.
11. 23 females surviving. These were allowed to feed on a normal guinea pig;
all became fully engorged immediately. The guinea pig was put into a separate
cage for observation for 10 days. No symptoms developed and the animal was
discarded after 3 weeks.

Transmission Experiments with Stegomyia calopus from Animal to
Animal.
I n earlier experiments 8 on Leptospira icterotmmorrhagice transmission of the disease from one guinea pig to another b y means ofCulex pipiens was unsuccessful. T h e present s t u d y with Leptospiraicteroides, however, brought out the fact t h a t Stegomyia calopus iscapable of transmitting the experimental disease resembling yellow
fever from an infected to a normal guinea pig. T h e positive results
are few, notwithstanding the numerous a t t e m p t s made, b u t are
sufficiently conclusive to establish the main point, t h a t this mosquito
m a y serve as an intermediary host of Leptospira icteroides. T h e termintermediary host, however, is not used here in the sense u n d e r s t o o din the case of certain p r o t o z o a n organisms, which require an extrinsic
host in which to pass their life cycle, b u t denotes t h a t a certain lengthof time is necessary for multiplication of the organisms to such numbers t h a t the mosquito m a y transmit enough to produce infection.
F r o m the biological and cultural properties of the organism thishypothesis seems reasonable, though the possibility of a stage of development in the mosquito has not been excluded. T h e following
experiments indicate the positive transmission of Leptospira icteroides from animal to animal b y the bite of the stegomyia mosquito.
8 Noguchi, H., The survival of Leptospira (Spiroch~ta) icterohcemorrhagi~innature; observations concerning microchemical reactions and intermediary hosts,
J. Exp. Med., 1918, xxvii, 609.

xmEYo N O O U C H I

407

Experiment 7 (Negative).--Aug. 4, 1918. 50 female stegomyias were engorgedwith the blood of a guinea pig which had been infected with the Case 1 strain of
Leptospira icteroides 6 days previously. The animal was showing the organisms inthe blood and had all the characteristic symptoms. Aug. 8. The mosquitoes fed
on another guinea pigwith beginning feverand albuminuria. The blood contained
occasional leptospiras. Aug. 10. The mosquitoes which were used later in theexperiments recorded below were allowed to bite a normal guinea pig on the
6th day after the first feeding. Only five mosquitoes attacked the animal.
No symptoms followed the biting, and the guinea pig was discarded as negativeon Sept. 13.
Experiment g (Positive).--Aug. 16 (12 days after the first and 8 days after thesecond feeding). Two normal guinea pigs were placed in the cage with themosquitoes. Guinea Pig 544 was bitten by nineteen mosquitoes. The highest
temperature was 39.6°C. on the 5th day, and the animal became mildly ictericon the 8th day. On the 12th day the temperature dropped to 37° and deathensued.
Autopsy.--Fatty degeneration of the liver, recent hemorrhagic .spots in thelungs, semidigested bloody contents in the stomach and intestines, acute parenchymatous nephritis, and some ecchymoses in the lymph glands. The icterns
was very slight. The leptospira was demonstrated under the dark-field microscope.
Guinea Pig 545 was bitten by nine other mosquitoes on the same day. The
highest temperature was reached on the 6th day, but the animal gradually returned to normal without any icterns. It remained well for 24 days and wasthen subjected to a second infection with the same strain of the organism on
Sept. 7. It proved refractory to the infection.
Experiment 9 (Negative).--The mosquitoes used in Experiment 8 were kept
for another experiment, but within 5 days only eight of the twenty-eight still
survived. These were allowed to feed on another normal guinea pig (No. 571)
On Aug. 21. The animal showed no symptoms after the biting and was discarded
as negative on Sept. 7.
Experiment 10 (Positive).--Aug. 13, 1918. In this series four guinea pigs
(Case 1 strain) infected at different stages of the disease, 6th, 7"th, 8th, and 9th
d a y s after the inoculatfon, all showing the symptoms, some intensely icteric,and some showing the leptospiras in the blood, were used to infect the mosquitoes
in four separate cages. Three of the guinea pigs died subsequently, and one recovered. 142 engorged females in all were collected and put together in one cagefor further experiments.
Aug. 21 (8 days after feeding on the infected guinea pigs). There were 83
mosquitoes surviving, and these were allowed to bite a normal guinea pig. All
the mosquitoes sucked the blood eagerly. The protocol of this animal follows.
Guinea Pig 570.--Aug. 21. Bitten by 83 stegomyias. Temperature Aug. 22,
38.2°C.; Aug. 23, 38°; Aug. 24, 38.1°; Aug. 25, 39.8°; Aug. 26, 38°; Aug. 27, 37.2°;
Aug. 28, 36.8°; Aug. 29, 37.6% Aug. 30, 37% Aug. 31, 36.2°; Sept. 2, 37.2°; Sept 3,
36.5°. Death occurred during the night 13 days after the animal had been bitten.

408

ETIOLOGYOF

YELLOW

FEVER.

IX

Autopsy.--Jaundice general but mild; hemorrhages in the lungs, stomach, andintestines; liver yellowish brown; kidney intensely congested, containing someminute ecchymoses, yellowish; bladder filled with yellowish brown urine with
albumin and casts. Ecchymosis in the serosa and mucous membranes in general.
Very few of the leptospiras in the kidney, none in the liver or blood.
Experiment 11.--To supplement the foregoing experiments, on Aug. 23, 1918,
that is 2 days later, the same mosquitoes, twenty-five in all, were crushed in amortar and emulsified in Ringer's solution. The emulsion was examined for the
leptospira under the dark-field microscope and also smeared over the scarified
surface of the skin of a normal guinea pig. Occasional specimens of leptospirawere found in the emulsion after long search.
The guinea pig, No. 544 A, came down with the typical symptoms on Aug. 31;that is, 8 days after the inoculation. In the liver and kidney the leptospira wasdemonstrated in small numbers under the dark-field microscope.
This experiment conclusively proves t h a t the leptospira was present
in the b o d y of the s t e g o m y i a mosquitoes which h a d been fed on theinfected guinea pig and were capable of t r a n s m i t t i n g infection b y theirbite to a n o t h e r animal. T h e course of the infection p r o d u c e d b ysmearing the m o s q u i t o emulsion over the skin was m o r e rapid t h a nt h a t caused b y the bite.

Experiment 12 (Negative).,Some of the mosquitoes used in Experiment 10
which had caused a positive transmission of Leptospira icteroides after 8 days
from the time of feeding on infected guinea pigs were kept another week. As
already stated, twenty-five of these 83 mosquitoes were crushed on Aug. 23 for a
supplementary confirmation (Experiment 11) of Experiment 10. Subsequently
most of the remaining females laid eggs, and some perished. On Aug. 28, 7 days
after Experiment 10, only thirteen were left. On that date a normal guinea pig(No. 633) was placed in the cage, and all the mosquitoes became engorged. The
guinea pig was kept under daily observation for 13 days, but there was no suspicion of infection, and it was discarded on Sept. 10.
Experiment 13.--Aug. 11, 1918, 11 a.m. An infected guinea pig (Case 1 strain)
showing the typical symptoms was placed in a mosquito cage. Twenty-four
engorged females were collected and put into another cage. Aug. 14. Sixteen
additional engorged females were also put into the cage, making the total 40.
Aug. 21 (10 days later). A normal guinea pig was placed in the mosquito cage.
The mosquitoes fed eagerly, all becoming engorged within 10 minutes. This
animal (Guinea Pig 572) remained apparently well for 11 days. It was founddead on Sept. 2. At no time did the morning temperature exceed 38.4°C., which
was about normal for this animal.
Autopsy.--A trace of jaundice throughout the body; liver highly degenerated
and pale yellow; a few ecchymoses in the lungs; kidneys much congested. There
was too scanty a quantity of urine in the bladder to examine for albumin. Diagnosis: probably a mild infection with Leptospira icteroides.

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SUMMARY.

The foregoing experiments show that symptoms and lesions closely
resembling those of yellow fever in man may be induced in guinea pigs
by the bite of female stegomyias that have previously sucked the bloodof a yellow fever patient or of an animal experimentally infectedwith Leptospira icteroides. With mosquitoes infected directly froma yellow fever patient the infectivity seems to become manifest after
a longer period of incubation than with those infected with the animal
blood. In the former, at least 12 days are said to be necessary before
they become infectious, and this hypothesis seems to be borne outby the present experiment. On the other hand, the mosquitoes whichwere engorged with the infected blood of the guinea pig were foundto be capable of transmitting the disease within 8 days after thefeeding. This discrepancy may be explained by the fact that the
number of leptospira existing in experimentally infected guinea pigs
is far greater than that in human blood.
The frequency with which positive transmission by the stegomyia
was obtained in both instances was very small indeed, in view of thenumber of mosquitoes employed. It appears that even under natural
circumstances the percentage of mosquitoes that eventually become
infected with the yellow fever microbe by sucking the blood may bevery small. It has already been shown by previous investigators 2,~that to transmit yellow fever from a patient to a non-immune person
requires from 0.1 to 2 cc. of blood at the height of disease. According to my estimate a female stegomyia may take up 0.01 cc. or evenless. Apparently a mosquito occasionally becomes infectious bytaking up the one or two organisms which happen to be circulating in
the peripheral blood of man, and it is these occasionally infected few
which carry the disease. It is not difficult to realize the extent ofever increasing danger from a constant supply of the microbic virus
which an endemic center or an epidemic of yellow fever can provide.
One infected mosquito may mean many patients, and the life of sucha mosquito is usually longer than that of the persons whom it fatally
infects.
Finally, it is of interest to note that the development and maintenance of Leptospffa icteroides are indispensably associated with theblood constituent, the serum, and this is amply supplied by the blood-410

ETIOLOGYOF YELLOW,EVER.

IX

sucking insect. The organism is one of the most fragile of all thepathogenic parasites and cannot survive the concurrence of other lessfastidious organisms such as bacteria. The comparatively aseptic
body cavity of the stegomyia 9 furnishes a secure shelter for theparasite, which Undoubtedly penetrates the zone of safety as soon as
it is taken into the stomach of the insect. Unlike many other parasites
this organism is capable of penetrating the intact skin or a bacteriaproof filter, and hence it is probably an easy matter for it to pierce
the tissue of the visceral organs of the mosquito. Whether or not
Leplospira icteroides can survive and multiply only in the body ofStegomyia calopus and not in other varieties or genera is yet to be
determined.
Another interesting fact with regard to the extrinsic life of thisorganism is that it can multiply steadily at a temperature from 18-37°C. The optimum temperature, at which it remains viable formany months, is 260 . The climate in most of the tropical countries
offers optimum conditions both for Leptospira icteroides and for themosquito which carries and nourishes it.
9 This refers to the presence of bacteria and not certain higher plant parasites(yeast, moulds, etc.) or protozoa which have been occasionally found in stegomyia
mosquitoes. These non-bacterial organisms may exert no adverse influence upon

Leptospira icteroides.