Ronald Ross Nobel Lecture

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Fig. 6. Salivary gland of mosquito. From letter of Ross to Manson, dated 6th July, 1898.

the throat of the mosquito. It consisted of three lobes on each side. The ducts

of each lobe unite together like the midrifs of a trefoil. The duct so formed

runs forward and meets the similar duct of the other side under the chin - so to

speak - of the mosquito. The common duct advances still further and enters

through the round base of the central stylet or stabbing weapon of the mosqui-

to’s proboscis. It was easy now to recognize the nature of the gland; it was the

salivary gland, 

which secretes the irritating fluid which the mosquito injects in

the wound made by her in the skin, perhaps to dilate the vessels, perhaps to

prevent speedy coagulation of the blood.*

The exact route of infection of this great disease, which annually slays its

millions of human beings and keeps whole continents in darkness, was reveal-

ed. These minute spores enter the salivary gland of the mosquito and pass with

its poisonous saliva directly into the blood of men. Never in our dreams had

we imagined so wonderful a tale as this.

But still all this was inference only; the last proof was demanded. If the in-

fection can be given in this way, give it. I had long possessed in the laboratory

five old birds - four sparrows and one weaver-bird - which had been kept

there for my "control" experiments, because they had never been found to

contain Proteosoma, even after several examinations. On the 25th June, as soon

as I began to suspect the destination of the thread-like bodies, these birds were

all examined again, and were found to be still quite healthy. On that and the

following nights, a large number of grey mosquitoes which had been long

previously fed upon infected birds and many of which had been found to con-

* This gland had been discovered in 1888 by Macloskie

5

, but I did not know it at the time

and still had received no literature on the subject.

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tain the thread-like bodies in their salivary glands, were released within a mos-

quito-net in which the five healthy birds were placed. On the following

mornings I satisfied myself that the infected mosquitoes had gorged them-

selves freely on the birds; and then, fascinated by the study of the parasites in

the salivary glands of mosquitoes, I forgot all about even this important ex-

periment.

Now only a small percentage of birds in Calcutta are infected with Proteo-

soma. 

Out of 111 wild sparrows examined by me I found the parasites only in

15, or 13.5 per cent. Moreover, even in infected birds, the parasites were scarce,

seldom more than one being found in each field of the microscope. On the

9th July I suddenly remembered my experiment and examined the previously

healthy birds. All of them without exception were now found swarming with

Proteosoma, 

as many as twenty or even more being found in each field.

But not content even with this I repeated the experiment over and over

again; and within the next few weeks I succeeded in infecting 22 out of 28

healthy sparrows (79 per cent), and also a crow and four weaver-birds, and,

moreover, gave a more copious infection to four sparrows which previously

contained only a few parasites. At the same time I kept as controls a number

of healthy birds in mosquito-nets, safe from the bites of mosquitoes, and found

that none of them became infected (with one exception probably due to an

error).

Manson, to whom I had sent full details, told me that he would expound all

Fig. 7. Thread-like bodies (rods, sporozoids) in salivatory glands of mosquito. Published

from Ross’s drawing by Manson, British Medical Journal, 24th September, 1898, p. 852.

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my results, with demonstrations of my specimens, at the meeting of the Brit-

ish Medical Association to be held at Edinburgh at the end of July. I now an-

nounced the successful infection of birds to him by a telegram, which reached

him just as he was setting out (though ill at the time) for the meeting; and he

was therefore able to communicate the complete life-history of the parasites

in his address.

His exposition as Dr. Charles said, "created quite a furore", and was quickly

made known everywhere. His paper was published on the 24th September

43

and gave a full account of the subject up to the infection of healthy birds, to-

gether with several drawings of the thread-like bodies, both free and in the

salivary glands, taken from my letters.

It was interesting during these researches to watch the gradual invasion of

the birds by the parasites. From five to eight days after they were bitten by in-

fected mosquitoes no parasites could be found in their blood; then a few ap-

peared; then many; and at the last large numbers. The first five birds all died,

and so did some of the others; and their liver was found to be full of the char-

acteristic pigment of malaria. But many recovered, the parasites gradually

decreasing in number.

At the same time I was temporally not a little delayed by finding inside the

mature pigmented cells certain large brown or black bodies which I provi-

sionally thought might be connected with their life-history. As proved by the

researches just described, malaria could be carried by mosquitoes from the

sick to the healthy, but as we know malaria clings intensely to location. It

therefore seemed not at all unlikely that these black bodies, occurring as they

did actually within the pigmented cells, might be of the nature of sporocysts

meant in some way to infect other mosquitoes - so that the infection might

not only be carried from man to man by the mosquito, but from mosquito to

mosquito; or they might be meant to infect man, as Manson had thought,

through the water. It was of course necessary, for sound science, to examine

these bodies; and I therefore tried to infect both birds and the larvae of mos-

quitoes by feeding them on insects containing these black spores; but the re-

sults were negative. Subsequently I saw reason to doubt whether the black

spores really had any connection with the parasites (section 20).

But there was little time for such researches, necessary as they seemed at the

moment. Although there could be no doubt that the human parasites have the

same history as Proteosoma, still it was a necessary formality to complete the

partial demonstration of this fact which had been already attained, if only to

persuade Government to take active measures against the disease; and I was at

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