53
Para. 29
There is, however, this notable difference between halteridium and human
malaria (or even proteosoma). In the first, the numerosity of the infection
remains fairly constant ; in the latter, it varies within considerably wide limits.
In the former, it is always above the vanishing point. In the latter, it is above
it only during the febrile periods, below it during the apyrexial ones—there
are alternating periods of profusion and paucity corresponding with the alter-
nating periods of conflagration and quiescence of the fever and its incidents.
A case of paludism removed to Europe may have occasional attacks of fever
for years afterwards ; during the attacks the parasites are found ; between
them none can be observed. By the hypothesis just given we may assume
that during the apyrexial periods the parasites are really propagating them-
selves all the time in comparatively small numbers below the vanishing point.
But what allows the occasional increase, and what compels the subsequent
return to the normal condition ?
We now approach the application of these remarks, which I fear have been
tedious. The question is connected with several others. Since the parasites are
capable of indefinite multiplication, what prevents them from destroying the
host in every case of untreated infection straight off ? On the other hand, what
produces spontaneous recovery in so many cases ?
These facts demand a reply. There must be some natural force which
restrains the multiplications of the parasites in cases where death does not
occur ; and there must be some natural force which finally exterminates the
invasion in spontaneous recovery. The existence of such a natural force is
not a hypothesis ; we are forced to admit it.
In short, we are confronted by the same problem as we find in the case
of many bacterial infections. Like the bacteria, these protozoa demand our
recognition of some force which makes head against them.
We can only ask, what is the nature of this force ? It must be either some
quality inherent in or produced by the parasites themselves, which checks their
over-production ; or some power generated in the body of the host which opposes
them. Asexual modes of reproduction, such as we believe these parasites to possess
in the human and avian hosts, are thought to tend toward spontaneous extinction ;
this may afford one explanation of the facts. Or the parasites may fabricate some
chemical product noxious to themselves. Or the tissues of the host may acquire
some principle of immunity. Or all these forces may act simultaneously.
On the whole, spontaneous decay of an invasion cannot be the only cause
for its restraint or extinction. It is difficult to explain on this hypothesis
such cases as the one referred to, where the infection lasted for years
after the patient removed to England. The hypothesis implies also that all
infections, severe or mild (i.e., numerically severe or mild) will recover at about
the same time, and that the condition of the host will not be a factor in the
recovery. This can scarcely be admitted.
As a matter of fact, the total sickness produced by a parasitic invasion
must depend on several factors, such as the virulence or harmfulness of the
species of parasite, their numbers, and the resisting and healing powers of
the host. We can easily attempt to represent this relation numerically in a
simple manner. The total sickness (s) may be considered as the difference
between the mischief (m) caused by the parasites and the extent to which
this mischief is healed by the system (h) ; or s = m —h. Again, the mis-
chief caused by the parasites (m) varies directly as their virulence (v) and
the number in which they exist (n) ; or m = vn. Lastly, the number of the
organisms (n) varies directly as their prolificness (p), and, inversely, as the
resistance (r) which they experience in living : or n = p/r. Combining these
equations we obtain—
s = m — h = v p/r — h = vp - hr /r.
γ
Here vp represents the total morbific energy of the invasion, while hr
represents the total resistant energy of the host—the power of healing (h) mul-
tiplied by the power (r) which checks the multiplication of the parasitas. If
vp increases to excess while hr remains constant, s becomes correspondingly
large, and death ensues. If, however, vp exactly equals hr, the sickness
vanishes—the point of toleration is arrived at.