67
Paras. 37 & 38
It should be added here, however, that I failed to propagate any of these
parasites in Nowgong, although several species of mosquito were employed and
much time was spent on the efforts. But there were few suitable cases for
experiment, and most of these refused to allow it. Later, in October, the cold
weather set in ; and, in short, the conditions were not at all favourable for the
work. The discovery of the particular species of insect required for each
species of gymnosporidium will probably require special research in different
countries or even localities, while it practically constitutes only a detail of the
work on proteosoma. I have, therefore, preferred the attempt to follow out the
life-history of this parasite, as a type for all the other varieties, to wasting my
time over efforts to find the hosts of each one in turn. There is, I think,
little doubt that all will have the same development in insects, that all will
produce " germinal threads " and " black spores," and that all will be commu-
nicated by the bites of the definitive hosts. Moreover, I think it very likely
that the infection of paludism is acquired only through the bites of mosquitoes
(or perhaps other insects). Injection of the poison straight into the blood by
means of the veneno-salivary gland of suctorial insects is a means of infection
which, devised by nature herself, far exceeds in certainty, as well as in its
beautiful adoption of the simplest route, the coarse and clumsy methods of
propagation which human imagination has yet been able to suggest for malaria.
38. The Special Virulence of Kala-Azar.—If kala-azar be paludism,
how explain its excessive gravity ? I do not know whether we possess any
satisfactory explanation ; but we know that many diseases are specially virulent
sometimes, less so at other times. Two factors may enter into the product
which causes the heightened gravity of the disease — the specific virulence of
the variety of parasite concerned, or its numerosity. In any epidemic of
paludism we may imagine either that a new and peculiarly virulent species of
parasite is passing through a country ; or that the epidemic is occasioned only
by one of the old parasites, which, however, now infects people in larger num-
bers than it formerly did. I need remark only on the latter supposition.
I have shown [1] that with mosquitoes fed, respectively, on sparrows with
many and with few parasites, the former insects, as is to be expected, con-
tained the greater number of organisms. It is to be presumed then that, on
biting healthy birds, the former would also inject a larger number of
" germinal threads " and produce severer infections than the latter could.
Similarly, I know as a fact that mosquitoes which have been fed over and over
again on infected birds finally acquire a vast number of parasites and give
the most terrible infections to birds. In this way I can artificially increase,
not exactly the virulence of an infection, but its numerosity. In fact, it might
be possible to start a local epidemic of severe " proteosomiasis " by this means
among sparrows. I should have to load a large number of mosquitoes
with parasites by feeding them over and over again on few infected birds and
should then release them. Every bird next bitten by these insects would, I
know for a fact, suffer from very severe infections, and each bird would simi-
larly propagate its severe infection to others. An epidemic of sparrows'
kala-azar would possibly sweep through the place.
It is evident that with all parasites we cannot expect the number of them
found in patients to be constant. In some localities, apart from sanitary
differences, they are few ; in others very numerous. This merely follows the
general law of chance or probability regulating the occasional accumulation
of phenomena in general. Thus, we may have epidemics of round-worms or
ankylostomes. If these happen to accumulate, so to speak, in large numbers
in any given locality, they will be propagated in all directions from that
locality in larger numbers than from localities where there were originally only
a few of them.
But, however we may attempt to explain the phenomenon, the fact re-
mains (paragraph 36) that epidemics of paludism, or of a disease closely simi-
lar to it, have spread over various countries from time to time ; so that
there is nothing at all unusual, as some appear to think, in the present out-
break in Assam. The discussion then returns to the point from which it
started—the pathological question— what is the real nature of kala-azar, and,
by implication, of these frequent epidemics so similar to it ?
K 2