1902-1903

 

Desenho – fêmea do mosquito Aedes aegypti. Acervo

DAD/COC/Fiocruz.

Drawing of a female Aedes aegypti. DAD/COC/Fiocruz.

 

Yellow Fever and the mosquito *

 

On the 18th of this month we sent the following letter to Dr. Adolpho Lutz, Director of the Bacteriological Institute at the Isolation Hospital

in the State of São Paulo: 1

Dear Sir,

Each of the two groups into which the hygienists of this country

are divided in regard to the proper prophylaxis against yellow

fever invokes your name in support of the doctrines defended by

them.

Those who deny that the striped mosquito2 is the transmitter of

yellow fever cite a report in which you consider as valid the

experiments carried out by Sanarelli that identify the ictheroid

bacillus as the pathogen of this terrible disease. Disputing this

argument, the others cite works in which you dispute Sanarelli's

theory and even the etiologic role of his bacillus, and in which

you praise the work done by the Commission in Havana, which

points to the mosquito as the sole known transmitter of icterical

tiphus.3

As you know, this question has become of the highest interest to

us since the prophylactic measures proposed by the Director

General of Public Health have been laid before Congress. It is

now essential to shed all light possible on this important matter.

As we are convinced that your opinion is of great value,

because of the position you occupy and because of your

knowledge of the subject, we appeal to you, requesting you to

put forward your current views on yellow fever prophylactics,

categorically yet succinctly.

Confident that you will not refuse to make this patriotic and

valuable contribution towards the extinction of our worst enemy,

we sign ourselves etc.

Owing to other duties, Dr. Adolpho Lutz was only able to answer

us yesterday [Oct. 25], with the following letter:

Sir,

I cannot refuse your repeated requests for my views on the

transmission of yellow fever by mosquitoes and I should very

much like to help rid the city in which I was born of this scourge.

I am sure that this will only be achieved by measures directed

against its transmission by Stegomyia, which I consider a

perfectly established fact.

I shall even go further and say that I have leaned towards the

idea of this mode of transmission since the first great epidemic

of yellow fever in Campinas (1889), during which I observed the

coincidental occurrence of an enormous quantity of striped

mosquitoes, extreme heat, and a pandemic of yellow fever. As a

medical man and a naturalist, I have always been greatly

interested in these mosquitoes and I know that they were very

abundant in Rio and in Santos but very scarce in the interior of

São Paulo. There they only became troublesome very rarely, in

the places in which I have been or where I have lived. Campinas

was the only place in which I was obliged to make use of a

mosquito-net even in day time, when I wanted to snatch a few

minutes rest in the house in which I lived and where some cases

of yellow fever had occurred.

At that time all the elements needed for explaining the

conditions of the transmission of yellow fever by mosquitoes

were lacking. The striped mosquito was considered

autochthonous and limited to the American countries, and to me

the main problem were the epidemics observed on the shores of

the Mediterranean and along the coasts of Africa. I was very

much surprised, a few months later, to observe the same

mosquito in the Sandwich Islands, not only in the capital but in

distant plantations. Mosquitoes did not even exist there at the

time of the discovery, and the date of their introduction is even

known. I then realized that this mosquito is spread by shipping

and should also be found in other hot countries, but I still did not

know of its occurrence in southern Europe. This fact only came

out later.

In 1887, the bacillus of Sanarelli was published and for a time it

looked as if the problem of yellow fever had been solved, at

least as to what organism caused it. (Speaking of Sanarelli's

bacillus, in 1897 I wrote: “The role that may be played by flies

and blood-sucking insects in the transmission of this disease

must not be overlooked.”) Sanarelli's bacillus did not succeed in

explaining the mode of transmission of yellow fever, and doubts

concerning its etiologic role also gradually increased.

In 1898, the well-known Joseph Chamberlain, one of the

Ministers of the British Empire, sent out a circular letter on

behalf of the government to the colonies and to the British

consulates abroad, asking that collection be made of all kinds of

Culicidae and other blood-sucking insects, so that they could be

studied at the British Museum.

This goes to show that clear-sighted men, far from considering

such studies as worthy of ridicule, realized their importance in

the investigation of the diseases of man and of domestic

animals. As I was engaged in similar studies at the time, I also

received an invitation to contribute to the project and responded

by sending all my Culicidae, which amounted to over forty

species. The cosmopolitan collection of the British Museum

served as a basis for a monograph on the Culicidae of the

world, in four volumes, by the British entomologist F. V.

Theobald. One of the most interesting results of his studies was

the discovery that our striped mosquito had occurred in Spain

and Portugal already at the beginning of the last century; this

furnished the explanation for the epidemics of Gibraltar and

Lisbon. Generally speaking, the species was observed in all the

countries where Yellow Fever has been rampant. It was often

described as a new species, receiving various names, which

have now been substituted by the generally accepted one of

Stegomyia fasciata.

The study of mosquitoes was given its first impetus by the

discoveries regarding filariasis and malaria. They were begun by

Manson, who proved that our common night-mosquito played an

important part in the propagation of filaria, which causes

elephantiasis and endemic chyluria. Only later did Bancroft and

others demonstrate that the transmission occurred during the

bite of the mosquito, which contained the larvae in its proboscis,

where they arrived after a lengthy evolution. The work done by

Manson, Ross, Grassi, and others, proven through experiments

of transmission in England, in Italy, and in some other countries,

showed that malaria is transmitted by certain mosquitoes of the

group of anophelids, which has at least five representatives in

Brazil. In this case also, inoculation can only take place after

prolonged incubation, during which the hematozoan has to pass

through several phases of its life-history in the body of the

mosquito. It ends by forming sporozoites, which are introduced

through the saliva of the mosquito, injected during its bite.

It was formerly believed that mosquitoes only sucked blood

once, then maturing and laying their eggs, and it was thought

that, like many other insects, mosquitoes died soon after. This

made it difficult to understand the transmission of parasitic

diseases by mosquitoes. More recent studies have made it clear

that many species of mosquitoes live for quite a while, sucking

blood and laying eggs several times.

The work of Finlay (who since 1881 had propounded the theory

of the transmission of yellow fever by mosquitoes) furnishes

good proof of this but it has been very little known. Of late, it has

been demonstrated many times that striped mosquitoes can

easily live through two or three months of summer, provided that

they are fed; it is even known that, not only here, but in cooler

countries than ours, they may last through the whole winter.

When the temperature is low, Stegomyia do not bite, and new

infections cease until the hot weather returns. If the temperature

rises a good deal in wintertime, they may come out of their

hiding places and suck blood, and this is why some cases may

be observed during that season. As a rule, the number of larvae

and of adult mosquitoes is very much reduced during the winter,

especially if it is long and cold, when they may be totally

extinguished. For this reason, yellow fever is not permanent in

São Paulo and does not spread through the entire city. The

Stegomyia infected during the summer are perfectly able to

multiply and thus to account for epidemics restricted to parts of

the city, which have occurred several times. If an epidemic dies

out at the end of summer and does not reappear the next year,

this will be due to the lack of survival of infected mosquitoes and

not to measures undertaken, which are unable to prevent the

spread of natural epidemics.

These observations make it easy to accept the transmission of

yellow fever by mosquitoes; they also supply an explanation for

the characteristics of the disease, such as: its limitation to hot

countries or cities, where there are long periods of hot weather;

its preference for seaports, and, here, for towns served by

railways; the cessation of epidemics when the cold weather

begins; the lack of direct contagion; the interval observed

between the first case introduced and the appearance of

subsequent cases; the immunity of certain localities; and the

inefficacy of prophylactic measures not directed against the

mosquitoes, such as the disinfecting of clothes.

Moreover, the transmission of yellow fever by Stegomyia is no

longer just a comprehensible and satisfactory theory, supported

by many analogous facts, but it has been demonstrated

experimentally. Twenty-five positive results have come from

published experiments. One must indeed be demanding not to

find these enough, all the more so because these experiments

were carried out in three different countries and in the presence

of many physicians. Their significance is gaining recognition day

by day; this is evident in the recent medical literature, especially

in the professional periodicals which deal with this subject.

Magnificent practical results were obtained in Havana, where

five months of work were enough to do away with an endemic

condition that had lasted for one and a half centuries. These

results have withstood all attempts to deny their worth. At the

Congress of Cairo they were not contested by anyone; likewise

not contested was the formal statement that these results were

due solely to the extermination of mosquitoes while the other

sanitary measures (which led to a general decrease in mortality)

proved useless against yellow fever.

For those who are familiar with parasitology, the transmission of

yellow fever by mosquitoes precludes any other mode of

transmission. The etiologic agent of yellow fever is so

specialized that it can not reproduce even in Culex fatigans, our

common night mosquito. This excludes the possibility that other

species, diverging further from Stegomyia, might serve as

intermediary hosts. It is still less possible to suspect

transmission by air or water, which is in disagreement with the

facts observed. The introduction of canalized water and sewers

in Campinas did not prevent the recurrence of a great epidemic,

and in Rio the number of cases increased instead of diminishing

after a disinfection-station was established.

I do not want to deny that there are other mosquitoes which are

also able to transmit yellow fever. It is probable that this property

is common to the whole genus Stegomyia and not peculiar to

one species of it only.

I know two other species of mosquito belonging to our fauna

which ought probably to be included in this genus. Fortunately,

they are sylvan species, not very abundant and able to cause

only small, disseminated foci.

Prophylactic measures should be directed firstly against all the

mosquitoes which breed in or around houses; this does not offer

insurmountable obstacles and may perfectly well lead to the

disappearance of Stegomyia fasciata. One need not fear that

yellow fever will only become extinct upon the death of the last

mosquito. In this case as in many other measures of hygiene, a

small effort may produce substantial results. Besides, the

protection of both patients and healthy persons by the use of

mosquito-nets will also help greatly to diminish danger.

If energy and money are not spared at the time of need, the

difficulties and expenses will soon be diminished by the

decrease in the number of victims. I feel certain that it is within

our scope to give the world another example like that of Havana.

We would like to register here our thanks to the distinguished

Dr. Adolpho Lutz, who responded to our request, in that it dealt

with an issue of major importance, and who did so without the

slightest intention of fueling controversies, for which he indeed

has no time.

 

* Article written by Adolpho Lutz and published in Gazeta de Noticias, Rio de Janeiro, 299, year XXX, 10.26.1903, p. 1. In BR. MN. Fundo Adolpho Lutz box 6, folder 212, bundle 1, one can find the handwritten original copy of the letter and its translation into English, which was probably done by either Bertha or Gualter Adolpho Lutz. For the present edition, the translation has been revised and undergone some small changes. [E.N.]

1 In fact, director of Instituto Bacteriológico do Estado de S. Paulo.

2 Name usually given to Stegomyia fasciata in Brazil. [E.N.]

3 Name usually given to yellow fever in Brazil. [E.N.]