Researches on malaria Nobel Lecture, December, 12, 1902

were probably only cases of relapse.

These arguments were not strong enough to be conclusive on either side of

the question. I had done quite right in spending so much time over the grey

and brindled mosquitoes; there was enough prima facie evidence against them

to demand a full enquiry. But before spending more time over them it was

now advisable to see whether further light could be obtained by epidemiolog-

ical investigation. The towns in which I had worked hitherto could scarcely

be considered more than moderately malarious; I now proposed to visit an

intensely malarious spot, at the height, too, of the malarious season in order

to ascertain what kind of mosquitoes prevailed there at the time; and reason-

ably hoped that this kind would prove to be the guilty species.

Being a servant of Government I could not of course go where I pleased

without leave, and I therefore first attempted to interest Government in my

work. Owing to my representations, the United Planters’ Association of

Southern India took up the matter; and the Honourable Mr. Bliss, Member of

Council of the Madras Government, and also Surgeon General Sibthorpe,

head of the Madras Medical Service, were kind enough to give their warm

assistance - for which I shall always be much indebted. The result was that the

Government of Madras made a proposal to the Government of India that I

should now be placed on special duty to investigate malaria. Most unfortu-

nately, however, in addition to the plague, the Afridi war broke out just about

that time, and owing to the paucity of medical officers the Government of

India was obliged to reject the proposal - May 1896. But in the meantime I

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49

had determined to begin the enquiry at once at my own expense during two

months’ leave which was due to me; and accordingly, on the completion of

my duty in Bangalore, I went to the Nilgherry Hills for the purpose of study-

ing the point referred to in some of the intensely malarious plantations at the

foot of these mountains.

12. The Sigur Ghat (1897). I arrived at Ootacamund, the great hill station of

the Nilgherry Hills, at the beginning of April, 1896. This station which is

about 8,000 feet above sea-level, is surrounded by numerous tea and coffee

plantations, scattered here and there in the rich valleys of the hills, and even

for some distance out on the plains which encompass the hills like a sea. After

enquiry it was determined to begin the investigation in the Sigur Ghat, a long

natural trench which cuts at one stroke from the Ootacamund plateau right

down to the plain, and which had the worst reputation for malaria. A dâk

bungalow 

(rest house) and a small plantation existed near the top of the trench,

at a place called Kahutti about 5,500 feet above sea-level; and owing to the

fact that a single night spent lower down the valley was thought enough to

ensure a bad and perhaps fatal attack, I determined to lodge here and visit the

lower valley only during the day time. Nevertheless even at Kalhutti I found

almost everyone suffering from fever - which was ascribed to miasmata float-

ing up the ravine from the plains below; and I had been there only a few days

and had paid only one diurnal visit to the plain when I myself suffered a bad

attack of aestivo-autumnal infection, the diagnosis being confirmed by the

microscope.*

After two weeks’ energetic treatment with quinine I was well enough to

resume operations; and this time went direct to the plantations at the foot of

the Sigur Ghat. The owner of one of them, Mr. Kindersley, wise enough to

reside in the hills during the intensely malarious season of the year, very kindly

placed his house in the plantation at my disposal; so that I was able to make a

thorough survey of the locality. Both plantations are situated in the midst of

luxuriant forest and undergrowth close under the declivities of the mountains,

* This case was remarkable for the brevity of its incubation period. I had never suffered

before from malaria, and was not likely to have acquired the infection either at Bangalore

or Ootacamund. I had arrived at Kalhutti at 6 pm on the 22nd April, and my attack com-

menced at

 

10 p.m. on the 25th April. I ascribed it at the time to my visit to the plain made

on the 23rd April; but there is now little doubt that the infection was acquired at Kalhutti

itself, which was swarming with mosquitoes, and where the servant of the dâk bungalow

and all his family were ill. At the same time I do not remember to have been bitten by

mosquitoes, and said so in my published account.

50

   1 9 0 2  R .R O S S

and are copiously watered by irrigation channels. Almost all the native em-

ployees, as well as some families of aborigines, were suffering from severe

malaria - anaemia, emaciation, and enlarged spleen; and the parasites were

easily found in the blood of some of them. But I was not a little astonished

when I discovered that mosquitoes appeared to be almost absent in all the

houses. In spite of considerable rewards which were offered for their capture,

and in spite of the efforts of my trained servants and myself, scarcely any were

secured. I was informed indeed by some of the employees that they were often

bitten at night by insects which escaped in the morning; but these nocturnal

visitors were not procurable      

*. Later however, we were told of some insects

which haunted the jungle and bit in the daytime under the trees. I found these

to be a small kind of brindled mosquito, and strongly suspected that they

might be the culpable species; and accordingly examined them closely and

called them Culex silvestris.

A part of my mission here was to enquire whether the mosquitoes in this

highly malarious spot did not contain parasites which were not contained in

the mosquitoes of the less malarious Bangalore. If they did so these parasites

might reasonably be suspected of being the mosquito stage of the malaria

parasite; and the question could subsequently be tested by experiment. These

mosquitoes were at once found to contain two new kinds of parasites, namely

crowds of active swarm-spores in the intestine, and, secondly, clusters of

spores (each cluster containing eight bright oval spores) in the ventral nervous

system. A close study was made of these organisms; but they did not appear

in some of the jungle mosquitoes which had been fed on patients. Strangely

enough, however, a person who volunteered to swallow a number of the

swarm-spores in water was attacked subsequently with fever, the malaria

parasites, however, not being found in his blood; but I heard afterwards that,

contrary to his statements, he had had fever just previously.

It will be remembered that Manson’s secondary hypothesis suggested that

the motile filaments, after living for some time in the mosquito, pass from it

into the water, and thence by ingestion or inhalation into man. My experi-

ence, however, tended to convince me that if such infection of water takes

place at all it must be very limited - in other words, that after their escape

from the dead mosquito, the organisms can neither travel far in the water nor

live long there. For if they could do this, almost all water in India would be

infected, and the disease would be universal, instead of being confined, as it is,

to certain spots. For the same reason the miasmatic theory never appealed

* Judging by our present knowledge, these must have been the offenders.

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51

strongly to me. I thought it most likely that men became infected from small

stores of drinking water such as wells, cisterns, and even pots and ewers, into

which infected mosquitoes often fall and die while laying their eggs - a theory

which would easily account for the isolation of the malady, because, as I had

observed over and over again, mosquitoes seldom wander far from their

haunts. As, according to hypothesis, the organism escapes from the gnat into

the water in which she lays her eggs, it followed that water which contained

most larvae should contain most malaria parasites, and, conversely, that drink-

ing water free from larvae would probably be free from parasites. Now in

attempting to apply these considerations to the case of the Sigur plantations,

I found them at once opposed by many facts. Not only were there few adult

mosquitoes there, but the larvae could be found only in a few stagnant puddles

in the depth of the jungle, while the drinking water was obtained from rapid

streams just issued from pure mountain springs, in which larvae neither existed

nor were likely to exist.

These facts again forced me to reconsider the whole of Manson’s secondary

hypotheses, and to search for more plausible theories. Three such theories oc-

curred to me. I had long observed that while they are sucking blood, gnats

deposit minute drops of excretae on the skin every ten seconds or so; and I

had actually shown that these drops may contain the pseudo-navicellae of gre-

garines. It was therefore possible that they might contain the spores of the

parasites of malaria, which might then be able to work their way through the

skin and into the blood of the victim. Another hypothesis of mine was that

the malarial spores might be voided by the insects, not upon the skin, but upon

rotting vegetation or damp earth (e.g. the floor of the houses and huts of na-

tives), and might there possibly develop into some extracorporeal form cap-

able of infecting man by air-borne spores.* The third theory was that infected

mosquitoes could in some mysterious manner introduce the parasites directly

into the blood during the acts of puncture and haustellation. This view was

* This was by no means an idle conjecture, and was indeed strictly based upon the anal-

ogy of Cunningham’s life history of the Amoeba coli, which that observer stated was void-

ed from the intestines of cattle and afterwards formed pseudo-plasmodia in the exposed

dung-men and cattle being infected by the air-borne spores of these pseudo-plasmodia.

He thought that the organisms were related to the Mycetozoa and called them Protomyxo-

myces coprinarius

. His important statements have been ignored but not disproved by sub-

sequent writers. Similarly I thought that the parasites of malaria might possibly be ex-

tracted from the circulation by mosquitoes, be deposited by them upon the damp floors

of dwelling houses and there develop in a like manner. This hypothesis was at that time as

cogent as any other.

52

    1 9 0 2   R . R O S S

similar to that of King and Bignami, with this difference that while these ob-

servers thought that the mosquitoes derived the parasites from marshes, I held,

in consequence of Manson’s induction, that they derived them from patients.

In the account of my work in the Sigur Ghat which was published a few

months later

40 

it was stated that this was the hypothesis which I now held to

be the probable one.*

It was during these researches that I first noticed the "dappled-winged mos-

quitoes". While looking for mosquitoes in a vacant rest house at the foot of

the ghat I captured an insect resting in a peculiar attitude with the body-axis at

an angle to the wall (as I noticed at the moment). On examination, its wings

were found to have a series of black marks along the anterior nervure; but as

I saw no more individuals of the species, I did not think the observation to be

of sufficient importance to be included in my paper. Yet, had I only known it

at the time, this was the very species I was in search of!

Indeed the whole of this investigation afforded a clear example of the well-

known ambiguity of epidemiological work. Of the kind of insect which was

really causing the disease at the time, I saw but a single individual! The reason

is now quite apparent. Unlike the grey and brindled mosquitoes which rest in

the dark comers of dwellings by day in large numbers, many species of dap-

pled-winged mosquitoes fly out at daybreak. It is true that other species of

this genus have more domestic habits and can therefore be more easily found;

and if fortune had been my friend in those days she would have brought me

to a place where these species abound - such as places afterwards visited by me

in Assam and the Darjeeling Terai. Nor does it follow in any case that the

predominant species of mosquito in a locality must be the malaria-bearing

species there; there is no reason why the innocent species should not out-

number the dangerous species even in the most malarious spots: while lastly,

it is now known that the dangerous species may abound where there is no

* I said, "On the whole from a consideration of the epidemiological facts I should be in-

clined to favour the idea of contact being the mode of infection; and may add that one of

my servants who was employed in catching the adult silvestris by allowing them to settle

on his legs and arms was attacked five days afterwards by the quartan parasite". By con-

tact I meant contact of the mosquito with the skin as explained further on by the following

words : "Since the presence of a human being in the jungle at once causes a number of sil-

vestris 

mosquitoes to attack him on all sides, it is very clear that he may readily be infected

by their agency, either by injection of the parasite through the puncture, or by its deposi-

tion on the skin in the shape of spores contained in the insect’s faeces, which, observation

shows, are always discharged in quantity during the act of haustellation". My theories re-

garding infection are also referred to in my previous paper

30

.

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53

malaria at all. Hence, though I did not know it at the time, it is impossible to

indicate, much less to certify, the malaria-bearing species by its numerical rela-

tions with other species in malarious localities.

One of the principal results of my work in the Sigur Ghat was that it led me

to doubt the probability of infection by drinking water. I should have liked

to remain there much longer; but on the expiry of my leave was forced to

return to my regiment at Secunderabad, five hundred miles away, and was

never able to visit the place again.*

13. Secunderabad (1897). The fundamental discovery. 

On my return to Secunde-

rabad (July 1897), the first thing I noticed was that the malaria had continued

unabated during almost two years since I had left; if anything it was worse,

and many recruits who had recently joined the regiment had been attacked -

as they averred, for the first time. This clearly showed that these cases were not

merely relapses, and that some cause of infection was actually at work among

the troops. It was for me to discover the cause; and I determined to return to

my old method, and to test experimentally all the kinds of mosquitoes preva-

lent anywhere near the barracks. I had now been studying the subject almost

constantly for over two years, and had become so very familiar with the mi-

croscopical appearance of the various structures of the mosquito** that I felt

the mosquito stage of the parasite could no longer escape me if it existed at all.

Numerous "cases of crescents" suitable for the experiments were in my hos-

pital, and it was obvious from the number of fresh cases occurring that the

proper kind of mosquito must be somewhere about. If I failed it could only

be because there was some flaw in Manson’s induction.

At the same time a possible fallacy was detected in the logic of that part of

the theory which suggested that the motile filaments after their escape from

the parent cells in the mosquito’s stomach must take up their abode in the tissues

of the insect. The vital and inevitable part of the induction consisted only of

the reasoning which inferred that the stomach of the mosquito is the natural

locus

 for the escape of the motile filaments. It was only conjecture to say that

they must enter the tissues; because for all we knew it was possible that they

might remain in the intestine for some time and then be voided, probably in

some altered form, either upon the ground or upon the human skin (see my

* I had been offered an appointment in Berar, but had declined it in order to carry on

these researches in the Sigur Ghat. I suffered severely for this later on.

** 

This does not mean that I was equally familiar with the macroscopical anatomy of the

mosquito - a subject which has only recently been dealt with fully.

54

 

  1 9 0 2   R.R OS S

hypotheses in the previous section). It was therefore now necessary to examine

the evacuations as well as the tissues of my subjects.

I commenced work by making a careful survey of the various kinds of mos-

quitoes which were to be found in the officers’ quarters, in the regimental hos-

pital, and in the numerous little houses of the native soldiers, which consti-

tuted the barracks or "lines", as they were called. I found first, the insects with

which I was familiar during my previous studies here in 1895, namely (a)

several species of brindled mosquitoes, and (b) two species of grey mosqui-

toes. But at the same time I was astonished at observing that the whole place

was overrun by swarms of (c) a small and delicate variety of mosquitoes,

which were at once observed to rest with the body-axis at an angle to the

wall, and which had spotted wings. In fact they were evidently of the same

genus (though not of the same species) as the mosquito which had been previ-

ously found in the Sigur Ghat - a genus, or perhaps family, quite distinct from

those of the grey and brindled mosquitoes with which I had hitherto been

working.

It is now time to speak more particularly of all these mosquitoes. I had writ-

ten repeatedly to Manson, to various booksellers in England, and to several

persons in India who I thought might help me, for some literature on the sub-

ject; but could obtain nothing except a few notes by popular authors, such as

Thomas, who wrote on piscatorial subjects in India. I could not even obtain

any adequate works on the anatomy of insects in general. Of Ficalbi’s work

on European gnats - which would have helped me immensely - I was igno-

rant, and received no copy. Manson had found the name of one species of

mosquito which I sent to him; but this did not help me, for what I required

was a scientific work on the structure and classification of the mosquitoes as

a group. I was therefore obliged, as mentioned in section 10, to trust to my

own rough methods of classification; and these were based, not on the criteria

of entomologists, such as the structures of the mouth parts or the nervures of

the wings, but on the general appearance and markings, the eggs, the habits,

etc., of the insects. It was only the working classification of an amateur with-

out literature to guide him, and made for his own convenience; but, as events

have proved it was roughly correct. Up to July 1897 I recognized the two

following groups :

(a) Brindled mosquitoes (

                     now recognized as belonging to the genus Stegomyia,

Theobald). Body and legs boldly marked black and white, or brown and

white. Wings plain. Biting voraciously, mostly in the day-time. Resting with

abdomen hanging towards the surface of attachment, and the last pair of

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55

legs tilted on the back. Breeding mostly in pots of water. Larvae floating head

downwards and possessing short stumpy breathing tubes. Eggs black, oval,

and laid separately.

(b) Grey mosquitoes (now recognized as belonging to the genus Culex, Linn.

as defined by Theobald). Back barred with transverse brown and white stri-

pes. Legs and wings plain. Biting somewhat timidly, mostly at night. Resting

with abdomen hanging towards surface of attachment. Breeding mostly in

wooden tubs, ditches, garden cisterns, and drains. Larvae floating head down-

wards and possessing long breathing tubes. Eggs elongated and somewhat

lanceolate and laid simultaneously in "rafts".

I had found mosquitoes of the same genera, though possibly of different

species, at Bangalore and at several spots in the Nilgherry Hills; and also at

Bombay, Poona, and Madras during short visits made to these cities in con-

nection with my sanitary duties at Bangalore. I remembered also to have seen

similar insects in Burma and the Andamans; so that it was reasonable to sup-

pose that they constituted the common or ordinary kinds of mosquitoes in

India. The new mosquitoes which I now and subsequently met with, and

named dappled-winged mosquitoes, were evidently of quite another genus to

the foregoing, and were distinguished by me by the following characteristics:

(c) Dappled-winged or spotted-winged mosquitoes (now recognized as belong-

ing to the genus Anopheles, Meig.). Body, legs, and proboscis marked brown

and white, or dark and light brown. Wings with several dark blotches on or

near the anterior nervure. Resting with abdomen pointed outward from the

surface of attachment. Body more elegant, and shaped like that of a humming-

bird moth. Breeding mostly in natural pools of water on the ground. Larvae

floating flat on the surface of the water like sticks, and possessing no breathing

tube at all. Eggs laid singly; cohering in triangular patterns, and shaped like an

ancient boat with raised prow and stem, and surrounded with a membrane

which - when the egg is seen in profile - gives the appearance of a bank of