51
Para. 80
number, continuing to propagate themselves indefinitely and without causing
any or much discomfort to the host. I suppose that they are so innocuous that
they are, so to speak, permitted to reside permanently in the blood in numbers
sufficiently large to be always detectable by the microscope. This is especially
the case with pigeons, where the organisms are frequently extremely numerous
without, apparently, doing any harm whatever. In crows they are usually
much less numerous and, I think, occasion a certain amount of sickness.
These facts may be expressed otherwise ; a point of toleration between the host
and the intruders appears to be arrived at, which is higher, permitting the
parasites to be more numerous the less virulent they are. But, however this
may be, we shall perceive that no theory regarding encapsulated forms is
required in this species of hæmocytozoa ; the explanation that they propagate
themselves indefinitely by ordinary sporulation suffices here to account for
their continuance.
With proteosoma of sparrows, larks and crows, I have observed much the
same thing ; but the parasites are usually far more scarce and they seem to pro-
duce much more illness (as Labbé has already remarked). In the warm weather
in Calcutta, with a noon temperature of about 85° F., only once out of a large
number of birds observed, did I note a disappearance of the parasites—and that
was just before death from diarrhoea. While, however, the organism seldom
vanished entirely, they often showed considerable fluctuations in number.
Thus, the birds infected by the bites of mosquitoes (paragraph 1) had
at first such severe infections that "from ten to sixty or even more"
parasites could be counted in a single field ; but in the few which re-
covered these numbers ultimately declined to a more reasonable limit, at which
the " point of toleration " seemed to be reached. In the cold weather in
in Calcutta, however, (noon temperature about 70° F.), Dr. Daniels and Riven-
burg and myself have noted a much more rapid recovery in the case of birds
which we have infected by the bites of mosquitoes, and, frequently, even an
apparently complete disappearance of the invaders.
Hence, in the case of proteosoma we may say that, owing to the greater
virulence of the parasite, the point of toleration is lower than in halteridium,
and may indeed be so low that the parasites become too few to be easily detected
by ordinary microscopic examinations. In the cool weather it may be lower
still than in the warm season, perhaps because the vital forces of the host are
more braced and vigorous. At any rate, in proteosoma as in halteridium, we
have absolutely no justification for calling in any theory of encapsulation ; we
are justified in holding, until anyone proves the reverse, that the parasites con-
tinue to propagate themselves in small numbers, perhaps even in numbers too
small for detection, so long as they continue to exist at all.
With human malaria it is obvious that the parasites are much more malign
than halteridium and proteosoma appear to be to birds, and, therefore, probably
cannot be endured in such large numbers—in other words, the point of tolera-
tion must be still lower than even in proteosoma, and must usually be below
the vanishing point. We know by long experience that when the typical fever
is present, we can generally find some of the parasites in the small drop of
blood examined, but that when the febrile period has passed and has given place
to a long quiescent period they cannot be detected (except when the crescents,
which do not cause fever, are present). But this does not imply that the para-
sites have become extinct or that they have entered an encapsulated phase ; we
have no justification for thinking so ; all we know is that they have become too
few to be detected in ordinary specimens and apparently too few to cause any fever.
As a matter of fact, we know that the degree of illness generally depends,
cæteris paribus, on the number of parasites present—not necessarily found. We
know that if we find one parasite, say, to fifty corpuscles, we may expect a very
severe fever ; whereas if we detect only one parasite (say of the quartan species,
which does not generally accumulate in the organs) in a whole specimen or
two, we may expect scarcely any fever. Hence, we may reasonably infer that
if the invaders are still less numerous than this, no appreciable fever at all will
be produced.
This is just what is continually observed. If there be fever we find the
parasites in one or two specimens ; if there be no fever, we do not find them
(excepting crescents, of course). In other words, taking a case of quartan for
H 2