Ronald Ross Nobel Lecture

    R E S E A R C H E S   O N   M A L A R I A

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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.

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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

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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

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