NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
SLEEPING SICKNESS.
9
transmitted amongst human beings in the proved absence of tsetse flies.
One has to guard against deducing too much from laboratory
experiments, for in such cases all the conditions are made most
favourable for the success of the experiment and factors which in
nature come in to counterbalance or modify these conditions are
eliminated.
The French Commission described some localised epidemics of
Sleeping Sickness on the French Congo with the view of throwing
some light on this problem.
They say " It appears that the agent of infection must be an
insect biting in the night * when all the members of the family are
together, an insect which passing from one to the other may carry
the germ to everyone in the immediate surroundings of the patient
by its repeated bites. It is mosquitos especially which seem to
play the part particularly Stegomyia and Mansonia, which are so
common in the Congo."
They sum up as follows :—
" Glossina Palpalis is the principal agent of infection. Its
presence is absolutely necessary for the propagation of the disease
to a distance and for the maintenance of endemicity, but in addi-
tion mosquitos and probably other biting insects may be import-
ant auxiliaries, having a formidable role as epidemic agents,
exercising their action in the interior of houses and amongst native
families, sometimes even throughout villages."
Todd of the Liverpool School of Tropical Medicine referring to
the work of the French Commission writes : " Their conclusions
entirely harmonise with our own observations/ 5
On the other hand the late Professor Koch wrote regarding
a segregation camp where hundreds of patients were collected to-
gether :—·
" They were in a country in which Glossina Palpalis is al-
together wanting but other blood sucking insects which
might be concerned are very abundantly represented ;
there occur also in patches Glossina Fusca and Morsitans,
* Glossina Palpalis only bite during the day.
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SLEEPING SICKNESS.
In a district suoli as this the problem must inevitably
be settled whether other insects are able to transmit
the Trypanosoma Ganibiense from sick to healthy men.
To decide this very careful observations were made, but
not a single case could be discovered in which such
transmission had taken place by means of other insects
than Glossina Palpalis."
There is considerable doubt as to what is the transmitting
agent in the form of human trypanosomiasis now spreading in
Nyassaland and in Rhodesia. The tsetse Glossina Palpalis is said
to be absent from those regions, but Glossina Morsitans is common
and Glossina Fusca is also found.
It is believed that Glossina Morsitans will be incriminated as
it has already been proved to be the natural carrier of Nagana
(T. Brucei) and it is capable of carrying Trypanosoma Garribiense
by laboratory experiment. Some observers however doubt whether
the Rhodesian Trypanosoma is Trypanosoma Gambiense at all or
that the disease is identical with Sleeping Sickness as ordinarily
understood. At present the whole question must remain sub-
judice. The new commission going out under Bruce will doubtless
clear up these points.
There remains one more possible method of transmission, that is
either by contact or by sexual intercourse. Koch believed the
latter to be a probable mode of spread, and he brought some evi-
dence to prove his contention. This theory has not gained ground,
and does not accord with the experience of most observers though
there are experimental grounds for believing it to be possible.
A recent paper by Hindle (Parasitology, March 1911, page 24)
shows that Trypanosoma Gambiense can be absorbed through the
unbroken mucous membrane either of the alimentary tract or of
the female genital passages of the guinea pig.
He did not succeed in transmitting the disease in these animals
from infected male to healthy female or vice versâ by natural sexual
intercourse.
Application of infected material to the unbroken skin also gave
rise to infection, so it is possible that in man simple contact with
infected fluid without a skin lesion might suffice to transmit the
disease.
SLEEPING SICKNESS.
11
Trypanosomes may, therefore, be transmitted in very varying
ways—for the rat trypanosome (T. Lewisi) has been proved to be
transmitted by the rat flea or by a species of rat louse, the human
trypanosome disease of Brazil (ScMzotrypanum Cruzi) by a redu-
vial bug whilst dourine, an equine disease, is transmitted by sexual
intercourse through lesions on the genital organs.
The question has been examined at length because of its vast
importance to India and to other hitherto uninfected tropical
countries, but this part of the problem will be referred to later in the
paper.
THE DEVELOPMENT OF THE TRYPANOSOME IN THE
TSETSE FLY.
As a result of the previous work on Sleeping Sickness it was
supposed that the part which the fly played in transmission was
a passive one, in fact that it was no more than a mechanical act and
that the virus was carried on the proboscis of the fly just as vaccinia
is carried on the ivory points of the vaccinator's lancet.
Now we know that the tsetse fly plays a much more important
part and that is in reality a true intermediate host and that Try-
panosome Gambiense develops in the fly in somewhat the same way,
though without such a defined cycle of stages, as the malarial
organism does in the interior of the mosquito.
The credit for the discovery belongs to Kleine who was working
in German East Africa and who at the end of 1908 showed that
a trypanosome disease could be conveyed by tsetse flies some 50
days after they had fed on an infected animal. We were very
shortly afterwards able to confirm this discovery·—he had worked
with Glossina Palpalis and Trypanosoma Brucei whilst our experi-
ments were carried out with the same fly and with Trypanosoma
Gambiense.
The details of the first successful experiments which are recorded
in full in paper No. 2 (vide Summary, page 37) may be sum-
marised as follows.
The experiment was carried out with flies caught on the Lake
shore.
They were first fed once a day on two successive days on an
animal whose blood contained Trypanosoma Gainbiense. Then
12
SLEEPING SICKNESS.
they were starved for 72 hours to do away with any possibility
of " mechanical transference " which was still believed to take
place.
For the next five days the flies were fed on a healthy monkey
and hence forward changed on to a clean monkey every succeed-
ing five days.
The result was that the first two monkeys remained healthy,
but every subsequent monkey on which the flies fed, up to 75 days,
developed Sleeping Sickness and died.
If seven days be deducted for the incubation period then the
flies became infective 18 days after their first feed on an infected
animal.
There is no doubt that all the mischief was done by one fly,
because though all the flies which died during the course of the
experiment were carefully dissected and at the end all that re-
mained alive were dissected, yet only one fly contained trypanosomes
and that fly died on the 75th day. After its death no more monkeys
became infected. This fly was swarming with trypanosomes of a
Gambiense type and when a minute portion of the alimentary
contents of this fly was infected under the skin of a monkey it
promptly gave rise to Sleeping Sickness in the monkey.
The salivary glands of the fly contained involution forms of
Trypanosima Gambiense and polymorphic forms of the same
parasite swarmed in the fore-gut and mid-gut. (Fig. VII & VIII.)
After this success a large number of experiments were done on
similar lines not only using Trypanosoma Gambiense but also
Trypanosoma Pecorum, Trypanosoma Vivax and Trypanosoma
Nanum. In the first series of these wild or lake shore flies were
used and they involved the fallacy that some of the lake shore flies
were known to be infected in nature.
When Fraser discovered the true breeding grounds of Glossina
Palpalis large numbers of pupse \vere brought up and hatched out
in the laboratory and an abundant stock of " clean " flies was
available.
We were able also to ascertain that there was no hereditary
infection of tsetse flies so that experiments with laboratory bred
flies involved no fallacy.
SLEEPING SICKNESS.
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Full details of these experiments are to be found in paper
No. 8 which contains a mass of information of which the bare con-
clusions were as follows :—
" That Trypanosoma Gambiense multiplies in the gut of about
one in every 20 Glossina Palpalis which have been fed
on an infected animal.
" That the flies became .infected on an average 34 days after
their first feed.
" That a fly may remain infected for 75 days.
" That Trypanosoma Pecorum, Trypanosoma Vivax and
Trypanosoma Nanum may also multiply on Glossina
Palpalis which must be looked upon as a possible
carrier in these diseases.
" That multiplication in the tube of the proboscis is character-
istic of Trypanosoma Vivax."
MECHANICAL TRANSMISSION.
There were still some lingering doubts as to how far this late
infectivity of the fly was responsible for the spread of Sleeping
Sickness and what part, if any, was played by the hitherto believed
method of mechanical transmission.
Some experiments were, therefore, carried out with Laboratory
bred flies to prove or disprove the point.
In the first series of experiments a considerable number of flies
were used and the infected and the clean animal having been placed
side by side the box of flies was transferred rapidly from one to
the other until all the flies had fed.
In this way they were not given time to finish their feed on one
animal but were constantly changed from one to the other. This
method was called " interrupted feeding " and was carried on on
twelve successive days. Five experiments of the sort were done
and in two of them the experimental animal developed Sleeping
Sickness which showed that the disease could be transmitted in
this way.
The second method consisted in allowing a variable time to
elapse between the feeds and times were chosen from half an hour
pnwards to 48 hours.
14
SLEEPING
SICKNESS.
When even such a short interval as half an hour was allowed to
elapse between the feed on the infected and the clean monkey, no
result was obtained and all the monkeys remained healthy.
The conclusions we arrived at were that :—
(1) The mechanical transmission of Sleeping Sickness by means
of Glossina Palpalis can take place if the transference of the flies
from the infected to the healthy animal is instantaneous—that is
by " interrupted feeding. "
(2) This mechanical transmission does not take place if an
interval of time comes between the feedings.
(3) Mechanical transmission plays a much smaller part, if any,
in the spread of Sleeping Sickness than has been supposed.
WHAT HAPPENS TO TEYPANOSOMA GAMBIENSB WHEN
IT GETS INTO THE BODY OF THE TSETSE ELY?
Something was said about this question in Paper 2 (see Sum-
mary, page 37) which has already been reviewed, but from the
time of the first successful experiment there detailed right up till
the time the Commission left Africa, an enormous mass of informa-
tion was collected bearing on the question of morphological change
in the parasite of Sleeping Sickness.
It was important to trace the fate of the trypanosoma from the
very first hour it was infested by the tsetse fly right up to the time
that the fly became infective to monkeys and indeed as long as
the infectivity lasted.
It was thought that perhaps the trypanosomes would be seen to
differentiate themselves into male, female and in different forms,
that sexual union might be witnessed, and that a long cycle of
changes as a result of the union might be recognised much as may be
seen in the case of the malarial organism in the anopheles mosquito.
One must emphasise at the outset that no such changes could be
traced and there was no evidence to show that any sexual differen-
tiation took place.
To make the experiment a complete one cages full of tsetse
flies were taken and fed on infected monkeys in such a way that a
different batch of flies was dissected every day from one day on-
ward to 56 days after the infected feed.
SLEEPING SICKNESS.
15
Sometimes it happened that when the flies came to be dissected
on a particular day out of the 40 or 50 flies examined none were
found to be infected—in this case that particular feeding had to be
repeated and so in the end the series was made complete from 1 to
56 days which was as far as the experiment was taken.
When an infected fly was found, a careful dissection was made
and all its various organs isolated, carefully washed to exclude
contamination, smeared on slides, stained and subsequently studied
and drawn under a magnification of 2,000 diameter. In all cases,
the organ specially studied were the proboscis, proventriculus, fore-
gut, mid-gut, hind-gut, proctodseum and salivary glands.
More than six hundred selected drawings* were studied to
form the basis of our conclusions.
When these drawings were placed on a wall the horizontal rows
represented pictorially the changes which the trypanosoma under-
went, say, in the fore-gut from the 1st to 56th day, whilst those in a
vertical direction represented the trypanosomes found in the whole
length of one fly on a particular day.
So that, as the original paper adds, " it seems impossible that
any important form can have been left out."
One must remember that it is only in a certain proportion of
flies—8 per cent, in this series—that any development of trypano-
somes takes place. What happens to them in the other 92 per cent,
of flies is not known, nor why it is that the 8 per cent, are alone
susceptible to infection.
It was shown not to be a matter of chemical re-action, nor of the
presence or absence of intestinal bacteria nor yet of the sex of the
fly—it must be something more subtle.
The original paper is already so condensed that it is difficult
to do more than to emphasise the most important point and to refer
those interested in the question to the paper itself.
It was found that in the 92 per cent, of the flies referred to,
the trypanosomes begin to degenerate a few hours after having been
ingested and die out entirely in a few days never to re-appear.
In the remaining 8 per cent, the parasites seem to degenerate
and become fewer and at a later period to re-appear and undergo
* All the work of Lady Bruce.
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SLEEPING SICKNESS.
enormous multiplication. The appearance under the low power
of such a fly is astounding—the whole alimentary canal is dis-
tended with a seething mass of trypanosomes and the furthest
confines of the dissection fluid quiver with them.
Trypanosoma Gambiense does not undergo any development
in the tube of the proboscis (a striking difference to Trypanosoma
Vivax which in the same tsetse fly seems to infect the proboscis
alone and to develop there).
It is in the fore-gut, mid-gut and hind-gut that the chief
multiplication takes place, and the type of parasite met here
is bewildering in its variety. None of them are typical of those
met with in the circulatory blood of the final hosts but there
is one form so common as to justify it being looked upon as the
" healthy normal developing type in the intestine of the fly.'*
It is seen at all stages and in all flies. The other trypanosomes
are made up of small forms, slender and crithidial forms ; resting
forms (some of them not unlike Leishmania) and giant types—
huge bloated masses of protoplasm full of chromatin grannules
and waving with flagella.
The behaviour of the trypanosome in the salivary glands merits
special attention.
They are not found in these organs until the 25th day after
an infected feed ; it is only here that the trypanosome reverts to
the normal blood type ; and before the 25th day the bite of the
fly is not infective but it becomes infective synchronously with
the appearance of mature trypanosomes in the salivary glands.
To make the above facts more striking one may add that
in all the salivary glands from flies which gave a positive result
trypanosomes of a mature blood type were found.
There is one important hiatus in the above description. How
do the trypanosomes get from the alimentary canal to the salivary
glands ? The analogy between trypanosomes and malaria is tempt-
ing ; whilst the malarial parasite is developing in the stomach wall
of the mosquito the insect is non-infective, its infectivity coincides
with the presence of ripe sporozoites in the salivary glands which
find their way there by the rupture of the oocyst into the body
cavity of the mosquito. No one has yet witnessed this change
in the life history of trypanosome. Very recently however Minchin
SLEEPING SICKNESS.
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and Thomson have described the development of the rat trypano-
some in the epithelial cells of the alimentary canal of its carrier,
the rat flea.
The conclusion arrived at may be summarised as follows :—
1. In the course of the development of Trypancsoma Gam-
biense in Glossina Palpalis the proboscis does not be-
come involved as in the case of some other species.
2. A few days after an infected feed the trypanosoma disappear
out of the great majority of the flies, but in a small per-
centage this initial disappearance is followed by a renew-
ed development.
3. After a very short time the flies which have fed on an in-
fected animal become incapable of conveying the in-
fection by their bites and their non-infectivity lasts for
some 28 days when a renewed or late infectivity takes
place.
4. A fly in which the renewed or late infectivity occurs can
remain infective for at least 96 days.
5. An invasion of the salivary glands occurs at the same time
as the renewal of infectivity and without this invasion
of the salivary glands there can be no infectivity.
6. The type of trypanosoma found in the salivary glands when
the fly is infective is similar to the short stumpy form
found in vertebrate blood and it is believed that the
reversion to the blood type is a sine quâ non in the
infective process.
Some more experiments of a different class were done to ascer-
tain the question of the return of infectivity of tsetse flies.
They were fed on animals infected with Sleeping Sickness, but
instead of being allowed to bite clean animals or being subjected
to microscopic examination their infectivity was tested by braying
them in a mortar and injecting the resulting intestinal fluid with a
hypodermic syringe into healthy monkeys.
A number of these experiments were done and it was proved
that the Trypanosoma Gambiense retains its virulence for about
two days after ingestion by flies when administered in this way.
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SLEEPING
SICKNESS.
After two days have elapsed they lose their virulence again, but
it returns in a very small proportion of flies in about 22 days.
From this it appears that the virulence of the trypanosome
in the fly is lost for just about the same period that the fly is non-
infective to animals when allowed to bite them in the ordinary
way.
In other words when the fly is able to transmit Sleeping Sickness
by biting an animal, its alimentary and salivary contents are also
infective when injected with a hypodermic syringe.
IV.—THE EESERVOIE OF THE VIRUS OF SLEEPING
SICKNESS.
Eeference has already been made to the fact that it was believed
that the way to stamp out the infectivity of the epidemic areas was
to remove all the human population from the fly zone on the islands
in and on the shores of the great Lake. Acting on this advice the
Administration had done this, and when we began to work on the
subject the lake shore had been deserted for about two years and
most of the islands had been cleared of human population. No
one was allowed within two miles of the water edge or on its
water under pain of penalty. One of our first experiments was
designed to ascertain how far these measures had succeeded in
sterilizing the fly population and we certainly were not prepared for
the result.
We found that when batches of tsetse flies were caught at various
parts of the deserted lake shore and were brought up and fed on
healthy monkeys that these monkeys invariably became infected
with Sleeping Sickness.
The surprising conclusion was " that the Glossina Palpalis on
the uninhabited shores of Victoria Nyanza can retain their infecti-
vity for a period of at least two years after the native population
has been removed."
There were four ways in which this might be accounted for :—
Firstly, the flies may occasionally get access to stray natives
passing from the islands to the main land or others who
have evaded the regulations.
SLEEPING SICKNESS.
19
Secondly, our own fly boys and canoe boys who go to catch the
flies might have been suffering themselves from Sleeping
Sickness.
Thirdly, the life of the Glossina Palpalis may be longer than two
years and the flies which were now proved to be infective
may have become so two years or more ago.
Fourthly, it is possible that some animals or birds on the lake
shore are infected and so constitute a "reservoir "
of the disease.
We were able to satisfactorily exclude the first two possibilities,
we could not deny the third though it seemed improbable.
We thought that the reservoir theory was most likely to be the
explanation and work was commenced in a variety of ways to prove
this. As a result of these experiments it is not too much to say that
a new chapter has been opened in the epidemiology of Sleeping Sick-
ness as the succeeding pages will show.
The proof of the existence of a " reservoir " is not scientifically
complete, but there is little doubt of its accuracy.
The important part which is played by wild animals in keeping
up local infectivity to trypanosome disease was discovered by Bruce
in Zululand and will be found detailed in his classical researches
into Nagana or the tsetse fly cattle disease of South Africa.
He found that the infectivity of the tsetse fly (Glossina Morsi-
tans) was kept going by wild animals, particularly buffaloes, which
frequented the cattle grazing grounds and which harboured in their
blood the Trypanosoma Brucei and which every now and again broke
out in virulent epidemic form amongst the domestic stock.
The phenomenon of " reservoirs " is comparable with that of
carriers of entericj diphtheria and other diseases, a problem which
now-a-days is so prominently occupying the attention of epidemiolo-
gists in many parts of the world. It amounts to this that certain
individuals who themselves show no symptom of disease are carry-
ing about a supply of virus wherewith epidemics may be lit up in
appropriate surroundings.
In trypanosome reservoirs the animals which carry the virus are
probably racially immune, whereas as in the above disease of man
the immunity is only a personal one and acquired as a result oi
an attack of the disease from which the individual has recovered.
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