species (2). The definitive hosts of both of 
these are gallinaceous birds. One of these two 
species, Histomonas meleagridis, causes a 
serious disease of turkeys, chickens, and 
certain game birds. The first two drugs 
that were really effective in the control of 
Histomonas were marketed about 1950 (3). 
They can still be used to aid in the control of 
this relatively immutable parasite. 
Because some parasites can adapt to un- 
favorable conditions, it may become neces- 
sary to develop and utilize biological control 
measures or forever be committed to a 
continuing search for new drugs. Some of the 
time-tested natural procedures that operate 
to maintain a balance within nature, although 
difficult to come by, may not lose their ef- 
fectiveness as rapidly 4s manmade ones. 
To establish guidelines as to where to seek 
means of biological control that could pos- 
sible be fruitful, we may consider, cate- 
gorically at first, how the parasites of domestic 
animals get from one host to another. About 
8 percent are transmitted primarily or ex- 
Clusively by contact or by mobile stages of 
arthropods, About 92 percent are acquired by 
the host through contact with the substratum, 
which is usually the soil (4). Intermediate 
hosts or other vectors on or in the soil or 
the vegetation or water overlying it may be 
involved. 
Obviously we may direct our attack on any 
given parasite at those stages within the body 
of the vertebrate host or at those stages as- 
sociated with vectors or the substratum, or, 
of course, at both. 
The principal means of biological control of 
parasites that operate within the body of the 
host are as follows: (1) Immune responses, 
broadly classified as natural immunity, ac- 
tive acquired immunity, and passive immunity 
and (2) the existence of conditions unfavorable 
to the establishment, development, or propaga- 
tion of the parasite because (a) the host has 
not yet developed all of attributes essential to 
the welfare of the parasite or (b) some other 
parasite or some physiological disturbance has 
created conditions not compatible with the 
normal host-parasite relationship. 
It is clearly impossible to elaborate on all 
phases of biological control of parasites. 
Natural immunity within a given host species 
can often be enhanced by selective breeding, 
112 
but all too frequently it is lowered as the at- 
tention of the grower is focused on conforma- 
tion, a more favorable feed conversion ratio, 
or some other feature. 
Active acquired immunity to internal para- 
sites has received a great deal of attention in 
recent decades, and it is perhaps in this area 
that the greatest strides have been made. A 
substantial part of each of several recent re- 
views has been devoted to the accomplish- 
ments and the literature in this field (5, 6, 7). 
However, even these reviews have usually 
dealt only with certain groups of parasites 
because of fundamental differences in their 
life histories, potentialities as antigens, and 
other factors. For example, protozoan para- 
sites multiply within an individual, whereas 
helminths do not. Some parasites migrate 
extensively in the host, but others are known 
only in very restricted sites. Any such dif- 
ferences as these, and there are many, serve 
to make the use of potential immunological 
responses very complex. 
In nature, passive immunity is imparted to 
the young of a host species through the placental 
circulation, the milk or colostrum, or the 
yolk of eggs. Obviously this area of biological 
control has interesting possibilities, too com- 
plex to detail here. 
At present, little practical use is en- 
visioned for employing the principle of curtail- 
ing parasitism by preventing the host's de- 
velopment of attributes essential to the welfare 
of the parasite. These attributes usually con- 
tribute also to the welfare of the host, although 
there are probably exceptions. For example, 
the fate of a parasite in the gnotobiotic animal 
is often quite unlike that in the conventional 
host (8, 9, 10). There can be many reasons. 
This field holds much of interest for the 
researcher, but little as yet for the grower. 
I mentioned earlier that under natural con- 
ditions the course of a parasitism is some- 
times affected by the presence of other orga- 
nisms or by physiological disturbances. As 
might be supposed, most physiological dis- 
turbances that are unfavorable to the parasite 
are also unfavorable to the host if they 
persist. However, in many instances the suc- 
cess of a parasite depends on the careful 
timing of the progress of its stages in the 
host, and this timing can depend on the secre- 
tion of certain enzymes, the rate of passage 
