Ronald Ross Nobel Lecture

68

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rows, and larks were then immediately procured, and experiments com-

menced on them without delay.

The malarious parasites of birds are exceedingly closely related to those of

men, and together with these and the malaria parasites of bats and monkeys

form a group which is quite distinct from the intracorpuscular protozoa of

some mammalia, such as the Pyrosoma bigeminum of cattle, and of reptiles, such

as Drepanidium. The true malaria parasites (namely the intracorpuscular proto-

zoa of man, birds, bats, and monkeys) are distinguished by their generally

amoeboid character, by their possession of the characteristic black or brown

pigment (melanin), and by an identical life-history as regards the production

and appearance of the spores within the corpuscles, and of the motile filaments

shortly after the blood containing them is drawn from the host. The parasites

of birds differ from those of man only in some very small morphological de-

tails; and are so similar that in the earliest sub-classification of the group by

Grassi, one of the parasites of birds commonly called Proteoroma is placed with

two of the human species, the quartan and tertian, in one genus; while the

other parasite of birds, commonly called Halteridium, is placed in another

genus together with the remaining parasite of man, that of the pernicious,

remittent, or aestivo-autumnal fevers. The latter part of Grassi’s classification

was wrong; and we now recognize that both the parasites of birds must be

placed in one group with the quartan and tertian parasites of man; while the

third human species must be placed in a group by itself, owing to the distinct

shape of its gametocytes (crescents). Thus zoologically, the avian species are

actually more nearly related to two of the human species than these are to the

third human species. Anyone who had actually studied all these parasites,

moreover, would have little doubt that they would be found to possess prac-

tically identical life-histories outside the vertebrate hosts, or at least life-histo-

ries which, if not identical, would be closely similar. It did not of course follow

with certainty that the carrying agents of the avian parasites would be the

same as those of the human species; but we could safely assume that they would

be some kind of blood-sucking arthropod. At all events it was certain that the

discovery of the life-history of the avian parasites would immediately open up

that of the human organisms; while the practical difficulties of working with

birds and infecting them would be less than with men. In fact I should have

been wise to have begun my researches with birds in 1895. I therefore deter-

mined to employ birds at once pending the subsidence of the plague-scare,

when I purposed of course to return to the human parasites; and there is no

doubt that this was the right course.

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69

It was first advisable to see whether mosquitoes would not carry one or both

of the avian parasites. A number of crows and pigeons had been found to con-

tain Halteridirium; but without waiting to examine the other birds, I placed one

crow, two pigeons, four larks and six sparrows in several cages all within the

same mosquito netting, and then in the evening released within the net a num-

ber of grey and brindled mosquitoes bred from the larva in captivity. Next

morning many of the grey mosquitoes were found gorged and were collected

and kept for several days according to my rules. On the 13th and 14th March

I dissected them one by one. When thirteen had been examined with negative

results I began to fear that I had committed myself to another tedious search

for the proper kind of host of the avian parasites. But fortune was kinder on

this occasion; the fourteenth mosquito had pigmented cells precisely similar

to those which I had found in the dappled-winged mosquitoes fed on patients

with crescents.

Next I examined the larks and sparrows used in this experiment together

with the crows and pigeons, and found that they contained not Halteridium

but Proteosoma; so that it was doubtful from which kind of parasite the pig-

mented cell had been developed. Consequently I now put the birds with Halteri-

dium 

in one net and those with Proteosoma in another, and released within both

nets numbers of grey mosquitoes bred in the same bottle. Of thirty-four of

these fed on the birds with Halteridium all were negative; but out of nine fed

on the birds with Proteosoma, no less than five contained pigmented cells.

This result was obtained on the 20th March and practically proved the mos-

quito theory of malaria. Out of hundreds of grey mosquitoes previously ex-

amined none had contained pigmented cells except one which had been

caught feeding on a case of tertian (section 13), and one which may have bit-

ten one of the birds with Proteosoma in the experiment of the 14th March.

Now, however, no less than five out of nine fed on birds with Proteosoma

contained them. Mathematically therefore the probabilities were enormous

(amounting almost to certainty) in favour of the view that the pigmented

cells in this experiment had been derived from the Proteosoma.

The cells were in the tissues of the insect; the parasite must therefore be

able to make its way into and live in mosquitoes; precisely similar cells had

been found in mosquitoes fed on men with malaria - and the chain of proof

was complete.

But the fact that the pigmented cells in the mosquitoes are indeed derived

from the parasites in the birds was of such fundamental importance that it re-

quired the most formal and rigid proof - especially as no life-history of a pro-

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