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theoretical concepts . However, any particular state is true absolutely, not
comparatively, for any individual worm.
This scheme accordingly proposes the recognition of five concepts in
the relation between host and helminth parasite.
Compatibility and incompatibility, which refer to the environment as it
exists before the invasion by the parasite; resistance, which refers to the
reaction of the environment to the presence of the helminth ; and tolerance
and intolerance, which refer to the reaction of the environment to the effects
of the helminth.
The intricate host-parasite relationship which exists is one of fundamental
importance in pathology and medicine. From the point of view of control
and prevention, however, the most important fact is that there is, in nature,
a host specificity which, because of unsuitability of the host or the resistance
of the host, renders many species of animals incapable of infection with certain
parasites.
So far as our present conception of the internal parasites of domesticated
animals is concerned it is probably true to say that the ideal of control is their
complete extermination. This implies a degree of knowledge which we do
not yet possess as well as a whole-hearted and complete cooperation between
all interested parties. Accordingly, our present aim must be the reduction
in numbers to such an extent that the parasites do not cause any appreciable
economic loss. Simultaneously, of course, laboratory and field investigations
on the details of life-cycles and bionomics of these parasites must be prose-
cuted with the utmost vigour and an intensive educational campaign carried
out among all those interested in the health of domestic animals. There
are signs that both of these aims will be kept well in sight in the future ; it
is regrettably true that neither has received its due attention in the past.
It is true also that our real knowledge is still extremely meagre. We do know
that various animals parasites cause application of what little knowledge
we do have, could even now produce results of incalculable value to farmers
and to all who depend, directly or indirectly on animal husbandry.
Before the application of control measures can be discussed, it is desirable
to recall several fundamental principles.
I. Under natural conditions, parasites are more or less specific to a
single species or a group of closely related species of animals. In general,
it may be taken that parasites of ruminants are not transmissible to horses,
pigs and poultry nor those of the latter to each other. However, many
parasites of wild ruminants are transmissible to domesticated ruminants,
of wild carnivores to dogs, cats and foxes, of wild birds to domestic ones,
and so on. The important exceptions to this generalization include the
flukes (many of which inhabit a wide variety of hosts), the Trichina, some
members of the genus Trichostrongylus and some of the larval stages of the
cestodes of carnivores.
II. Metazoan parasites cannot complete their entire life-cycle within
the same animal, any more than they can live a free independent existence.
At one stage or another they must leave the host to undergo some form of
development outside of it—either free, or in an alternate or intermediate
host. In effect, this means that a single young parasite develops into a single
adult only; there is no multiplication as in the base of bacteria. In both