R E S E A R C H E S O N M A L A R I A
41
days’ residence in the mosquito - to become more visible, and, if they were to
pass into the water as we thought they would do, to take a definite form of
resistance which ought to be easily recognizable. My new method then was to
give up the attempt to follow the newly escaped and almost invisible motile
filaments, and to dissect the mosquitoes, not at once, but after the lapse of
some days, during which time the motile filaments should by hypothesis de-
velop into something more tangible. I proposed then to feed the mosquitoes
as before on cases with crescents in the blood; to keep them alive for some
days; and then search their tissues for any parasites which might occur in them.
The parasites found, it would be easy to determine whether or not they are
derived from the motile filaments, simply by ascertaining whether or not they
also occur in mosquitoes of the same kind fed on healthy blood. Throughout
the investigation it was of course necessary to employ only what I called in
bacteriological parlance "sterile mosquitoes", that is mosquitoes freshly hatch-
ed from the larvae in captivity and therefore not contaminated by previous
feedings.
Such was the procedure now adopted; but the difficulties involved even in
it were very great. As the situation of the sought-for parasites could not be
indicated with any certainty, it became necessary to search for them through
all the tissues of each insect examined - to scrutinize by a powerful micro-
scope, one by one, all the minute cells composing the huge aggregate of which
the insect consists.* To investigate a single insect thoroughly in this manner
required at least two hours’ exhausting and blinding work. Added to this dif-
ficulty, I had no clue as to the form and appearance of the object which I was
seeking for; nor was I even sure that the kind of insect under examination was
amenable to the infection at all. I was looking for a thing of which I did not
know the appearance in a medium which I did not know contained it. In short
it was a mere blind groping for some clue which I trusted fortune would give
in the end. As an instance of the difficulty of such work I may mention that
neither the organisms of yellow fever, which is now known to exist in a par-
ticular kind of mosquito, nor the Pyrosoma of Texas cattle-fever, which is
known to exist in a tick, have yet been found in these animals, though long
searched for by competent observers. Nevertheless, I am confident that, hope-
less as the method may appear, it was the only one capable of solving this
difficult problem.
At the outset of the investigation it was necessary for me to become thor-
oughly acquainted with the normal histology of the mosquito - for which I
* Under a magnification of a thousand diameters a mosquito appears as large as a horse.
42
1 9 0 2 R .RO S S
had again to trust to my own observation; in spite of all efforts no literature
on the subject could be obtained by me. It was also necessary to note and study
the ordinary parasites of these insects - of which I found a number during the
ensuing years. Indeed, at the commencement of the work I found one which
required careful working out. It was a pseudo-navicella occurring in the mal-
pighian tubes of the brindled mosquitoes (Stegomyia). After a little study it
was ascertained that pseudo-navicellae have no connection with the parasites
of malaria, being the sporocysts of a species of gregarine. Next year Manson
published an account of these interesting organisms taken from my letters to
him
26
. I refer to them also in my publication at the end of the year
24
.
The second method alluded to above was based on the following considera-
tions. According to Manson’s
more advanced hypothesis, the motile fila-
ments, after living some days in the mosquito, would probably pass from it
into the water, on the surface of which we then supposed it usually died after
laying its eggs. Such water then ought to be infective to human beings, either
when ingested, or perhaps when inhaled as a vapour. It would be easy to test
this speculation by experiment. I caused a number of mosquitoes, both of the
brindled and grey varieties, to feed on a selected patient, and then kept them
in large jars containing water at the bottom, until they died one by one. The
water was then exposed to sunlight and otherwise allowed to remain in the
condition of marsh water. Different batches of fed mosquitoes were intro-
duced into the jars from time to time so as to make sure that the water should
indeed contain the parasites which by hypothesis should escape from the in-
sects. In May 1895 I gave draughts of this water to three natives who volun-
teered themselves for the experiment. All of them declared that they had not
suffered from fever for years.* Strangely enough one of the men developed a
mild but marked attack of fever in eleven days, the parasite being found in his
blood. I was naturally much pleased with the success of the experiment and
began to hope that the mode of infection had been found; but the failure of
many subsequent attempts of the same kind forced me later to reject any defi-
nite conclusion on the point (section 11).**
11. Bangalore (1895-1897). Progress of
work. Possessing abundance of material
* The experiment was justifiable owing to the slight degree of illness usually produced
by malarial fever in natives when properly treated.
** Owing to the interest of Surgeon Major Owen, the Maharajah of Patiala had at this
time offered the government of the Punjab to employ me at his own expense to study ma-
laria in his dominions. The government of the Punjab, however, refused the offer.