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Parasitic Environment and Plan of Control.
extent may stop short at full maturity; the parasite may never be able to
reproduce. On the other hand, it may become mature but not fully so;
the adults may be small, the egg output reduced and so on. To enable these
to be distinguished from each other, the first is called ' partial incompatibility ',
the second ' partial compatibility '. None of these conditions has any con-
nexion with a host reaction; they merely define the type of environment
that the helminth finds in the host.
In complete incompatibility and complete compatibility, no host reaction
can occur, but in both partial incompatibility and partial compatibility,
a reaction may or may not be present, and this reaction may be directed
towards the effects of the parasite or towards the presence of the parasite.
The reaction of the host to the presence of the parasite is termed ' re-
sistance '. When the resistance is sufficiently high to prevent the parasite
from reproducing (i.e. producing eggs or larvae) it is termed ' absolute re-
sistance '. This may act at any part of the parasite's life, prior to the actual
production of eggs or larvae and it is possible to conceive of it being so pro-
nounced as to destroy the parasite immediately on entry. This theoretical
conception, which may be referred to as ' virtual resistance', has the same
result as complete incompatibility, but differs from it in that it assumes a
reaction on the part of the host; complete incompatibility on the other hand,
assumes an unsuitability of environment, not necessarily inimical to the para-
site—it may be the absence of something. When the parasite is able to
reproduce, in spite of the host reaction, the resistance is termed ' partial
resistance'; the life of the parasite must in consequence be, to some extent
at least, abnormal.
The reaction towards the effect of the parasite is essentially a repair
process, whereas resistance involves an active alteration in the parasite's
environment. In our present state of knowledge, it seems desirable to keep
these two concepts separate. Where the damage done is of a purely me-
chanical nature for example, it is possible that the reaction is directed entirely
towards the repair of this damage and does not affect the worm itself. If
the reaction towards the effects is sufficient to counteract and repair them it
is referred to as ' tolerance '; if not, then it is called ' intolerance '. These
terms are comparative and depend on a great number of varying factors
and they must grade into each other to some extent; intolerance postulates
clinical disease-tolerance, sub-clinical disease with no obvious symptoms.
Compatibility, incompatibility and resistance may all be natural or ac-
quired, but an intermediate form of acquired resistance may exist called
' premunition'. In this form some parasites may develop—either successfully
or unsuccessfully—but so long as they remain alive, resistance is offered to
further infection; as soon as they die or pass out of the host, the resistance
will disappear.
These terms have been defined as though they were hard and fast divi-
sions ; it is improbable that they are. Thus too, many worms in a com-
pletely compatible host would induce a state of incompatibility through
lack of food. Tolerance and intolerance obviously grade into each other
and both absolute and partial resistance can exist side by side in the same host
towards the same species of worm. Moreover, it is at least doubtful whether
complete compatibility, incompatibility or resistance are more than