Animals P by: Geoffrey LaPage Published by ... - PSSurvival.com

Animals Parasitic in Man
sheep or a cow or a rabbit or a human being - is called the
firi& or dejhitive host. It follows that the @asite cannot pass
directly from one host in which it produces fertilized eggs to
another host in which it can do this; it ca!i only pass indirectly
from one to another of these hosts, namely, by way of the intermediate
host in which its larval phases must develop. For
this reason its life history is said to be indirect.
The distinction between the fmal and the intermedijte host
is important both to the parasitb animal and to m&n, who
wishes to prevent infection of hi&self and other animals with
the parasitic animal. It is important to the parasitic animal because
it forces this animal to be parasitic in two different hinds
of animals, one the final host in which its fertilized eggs are
produced and another, the intermediate host, in which its
larval phases must develop. The distinction between the final
and the intermediate host is important from the point of view
of the control of parasi-Gc infections, because infections of the
final host can only come from larval phases ofkhe parasitic
animal produced inskie the body of the intermer iate host and
infections of the intermediate host can only comf from the products
of sexual Teproduction, which must oc ur inside the
body of the fin74 host. It follows that, when I :i an is the final
host of any spcdies of parasitic animal with an indirect life his-
tory, he cannot infect himself with phases of #this I species present
inside his own body, as he can when t b e life history is
direct; he can be infected only by larval phas+s of the parasitic
animal coming from the intermediate host; and, if he can avoid
these, he can avoid the infection altogether. The same principle
applies when man is the intermediate host. He can then
be infected, not by phases of the parasitic animal present in his
OB;~ body, but only by the products of its sexual reproduction,
which must occur inside the body of the final host. Control of
infections with parasitic animals with indirect life histories
usually, therefore, involves the protection of man from either
the intermediate or the final hosts from which the infections
may come.
The species described in chapters 5, 6,7, and 8 all have indirect
life histories. The species described in chapter 7, how-
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