65
Para. 37
We know that the parasite must enter the body from without ; but how,
exactly, it does so remains a mystery which will only be solved when we have
succeeded in finding its extra-corporeal stage and in causing experimental infec-
tion by means of these extra-corporeal organisms.
What makes us think that the disease is not communicable from the
sick ?
First, that people seem often to contract it in uninhabited wilds. Whether
they really do so may, I think, be open to question. But admitting the
plea, this gives only one mode of infection—namely, from the elements of
external nature. It does not, necessarily exclude other modes of infection,
such as communication from the sick. A traveller enters a particular jungle
or terai and acquires fever. He may certainly have acquired it from external
nature ; but what about the fever-stricken villages on his route, what about the
sick servants he probably takes with him ? Had he been carried alone in a
balloon to the particular jungle, and been left there for some nights and had
then been attacked by fever, the experiment had been of scientific value. A
malarious locality generally implies malarious inhabitants. At the best, the
one mode of infection does not necessarily exclude the other. Malaria may
be communicable both from the elements of nature and from the sick.
Secondly, that patients in our hospitals and in their homes seem very
rarely, in our experience, to spread the disease round them. Certainly ; but
the same—at least as regards hospitals—may be said of cholera, typhoid, or even
of plague, which are yet, surely, communicable diseases. There is a question
of environment.
Communicability may be direct or indirect. Scabies and venereal diseases
afford examples of the former. But contagion, strictly speaking, is rarely the
mode of communication ; it is seldom that the infection is taken from the body
of one person and placed directly upon or into the body of another. There is
generally a medium—air, water, soil, excreta, insects, other animals. Each
disease appears to require its special medium. If this medium does not exist,
the disease cannot be propagated from the sick to the healthy.
As regards our malarious patients, they have probably been removed
from the spot where they acquired the disease to our hospitals or their own
homes, were the disease does not ordinarily exist. It is, now, no argument to
point to the absence of infection of contacts, since, from the original absence
of the disease from the place, we may reasonably infer that the conditions
necessary to its propagation have never been present there. It is possible
that if the observer had removed the case to another house in the malarious
spot itself, where these conditions are presumably present, he would have
found that communication does take place—that the patient's relatives and
friends are attacked—just as in kala-azar. I need only refer to the similar
difference of communicability of plague, cholera, or typhoid in sanitary or
insanitary surroundings, respectively.
Such arguments, however, only weaken the case against communicability
in malaria ; what can be said definitely in favour of it ? Three things.
(1) Paludism is a parasitic disease. Parasites must propagate them-
selves ; and it would, therefore, appear to be necessary for them to pass from
host to host. All parasites which have as yet been sufficiently studied do so.
Are the parasites of human malaria to be exceptional ?
(2) Malaria of birds (proteosoma) is communicable from sick to healthy
sparrows through the mosquito [1 and 2]. Every step in this evolution
necessary to the enquiry has been studied under the microscope, and numerous
birds have been actually infected (paragraph 1).
(3) Some steps in the similar evolution of one, or perhaps two, of the
human parasites have been made out; but the life-history has not been
completed.
It should be added here that there are indications that proteosoma may
be conveyed in a manner less direct than from an infected to a healthy bird [2],
namely through the "black spores."
Frankly considered then, the reasoning which makes against paludism
being communicable cannot be looked upon as amounting to the first order
of scientific proof. The fact is that the epidemiology of malaria has long
been in an extremely nebulous and unsatisfactory condition.
K