Ronald Ross Nobel Lecture

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

35

that in his original investigations on the development of filaria embryos in

mosquitoes he had failed to ascertain that the insects live for more than a few

days; he had thought that after a single meal of blood the mosquito lays her

eggs on the surface of water and dies in the act of doing so. Consequently,

when he had traced the development of the embryos to the stage which they

reach in the insect’s thorax he inferred that that was the whole development,

and that after the death of the host on the surface of the water the embryos

escaped into the latter and finally infected man by the digestive tract.* And he

now applied the analogy to the malaria parasites, and thought that, similarly,

after the insect’s death, they enter the water and infect man either by drinking

water, as he assumed for the filariae, or by the old machinery of the aerial

miasma. Thus Manson’s hypothesis suggested a clue only to the departure of

the parasite from the human host; it did not attempt to define the route

of entry, the exact mode of infection. In these points he admitted that his

speculation was looser, and research has shown that he was wrong - certainly

with regard to the malarial parasite and probably even with regard to filariae.

In another point also he was wrong - the motile filaments are not flagellate

spores, as he thought they were. I remember mentioning to him at the time

that they might possibly be of the nature of sperms - an idea which was sug-

gested to me by Lewis, who conjectured that his trypanosoma might be of

that nature

6

. We shall see later what they really are. But these errors were

immaterial. The fundamental part of his hypothesis was the close and power-

ful argument to the effect that the motile filaments and the parent cells from

which they spring must be meant to infect the mosquito in some manner. This

was more than a hypothesis; it was a great and illuminating induction. It gave

the required clue to further research; and without it I am convinced the ma-

laria problem would not have been solved at all and we should still be engaged

in a laborious and hopeless search for the parasites in water and air.

Cogent as Manson’s arguments appeared to me, they were far from con-

vincing to most other students of malaria; and in fact no one else took the

trouble to investigate the matter in spite of its immense importance to hu-

manity. In 1896 indeed, Bignami wrote a long and dexterous article attacking

the whole induction

29

. He still refused to believe that the motile filaments

were anything but the result of death in vitro, and added that if the induction

were true, malarial fever could be propagated by patients living in the pres-

ence of mosquitoes - which he refused to consider possible. At the same time

he evolved a theory of his own to the effect that mosquitoes become infected

* Or possibly "by piercing the integuments".

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    1 9 0 2   R.R OS S

with the parasites when in the larval stage in water, and then inoculate them

into man during puncture. Thus while Manson thought mosquitoes carry the

parasite from man to the marsh, Bignami thought that they carry it from the

marsh to the man. The latter view does not appear to me a very philosophical

one, since it presupposes the possibility of organisms living normally partly a

saprophytic and partly a parasitic existence in the mosquito, and then being

suddenly transferable on exceptional occasions to man. Bignami’s theory,

however, was exactly that of King given much more forcibly thirteen years

earlier. He mentioned that he had previously attempted to infect men by gnats

brought from malarious localities, but that the attempts had failed. He also

referred to experiments made by Calandruccio, who had failed to observe

any development of the parasites in the stomach of mosquitoes fed on malarial

blood. Lastly he cites another most valuable analogy in favour of the mos-

quito theory of malaria, namely that of the Pyrosoma bigeminum, a parasite of

cattle allied to the parasites of malaria, and known by the brilliant researches of

Theobald Smith and Kilborne to be carried by ticks

19

. Koch also had used this

analogy, but I think that both Manson and myself had overlooked it some-

what unduly.* Curiously enough all this time it seems to have occurred to no

one that the mosquito may act in both roles imagined by King and Manson

severally - that it may both take the parasite from the patient and also inocu-

late it into healthy persons. I traversed Bignami’s criticisms in an article which

will be referred to later (section 11).

In considering the merits of these various hypotheses we must always re-

member that all of them have found no little support from Manson’s original

discovery of the development of Filaria bancrofti in mosquitoes.

7. Nature of proposed investigation. 

We must now return to my own labours.

As already mentioned, directly I became acquainted with Manson’s induc-

tion, I determined to continue my investigation of the malaria problem en-

tirely on this new basis.

Before my departure for India I discussed with Manson the best method of

* The development of the Pyrosoma in ticks is still unknown, though the second host has

long been recognized. It should be understood that the history of this organism, and of

the filaria in mosquitoes, while adding great force to the mosquito theory of malaria, gave

us no information regarding the form and position of the malaria parasites in mosquitoes,

nor of the species of insect concerned. The Pyrosoma is not very nearly related to the ma-

laria parasites, and the filaria not at all. I was not aware of the work of Smith and Kilborne

until much later.

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