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BIOLOGICAL CONTROL OF ANIMAL PARASITES 
Everett E. Lund, Animal Disease and Parasite Research 
Division, Agricultural Research Service, 
U.S. Department of Agriculture, Beltsville, Md. 
The term "biological control" as here used 
means the employment of any naturally oc- 
curring phenomenon manifested by living things 
that affects the incidence of parasitism or the 
response incited in the host by the presence of 
parasites. The nature of the process initiating 
the occurrence of the biological phenomenon 
is of no consequence as long as the process 
immediately preceding the alteration of the 
degree of parasitism is biological and some- 
thing other than death. 
The term "animal parasite" is here used as 
meaning any organism of the animal kingdom 
that lives in or on another organism at the 
expense of the latter, I am limiting my dis- 
cussion to the nonarthropod parasites of 
animals of importance in agriculture. 
The purposeful use of biological phenomena 
to control animal parasites is not easy. In 
the eyes of Nature the parasites we would 
seek to destroy probably are as cherished as 
are the hosts we seek to preserve, Further- 
more, forms of biological regulation are 
going on constantly in all natural ecological 
systems, and any species must, to a con- 
siderable extent, be a product of its progeni- 
tors' responses to situations not too unlike 
some of those we might wish to use. Parasites 
throughout the ages have become accustomed 
to certain types of adversity and have de- 
veloped means of coping with them. It is un- 
likely that we shall baffle Nature by brandish- 
111 
ing weapons that she has been wielding for a 
billion years. But with sufficient skill we may 
enlist her cooperation to help achieve reason- 
able objectives. 
Our demands for enormous production of 
animals, the incredible ease with which we 
traverse natural barriers, and our almost 
complete disregard of the operation of natural 
selection all tend to favor parasitism. We 
have, of course, developed measures aimed 
at offsetting this. Some of the most successful 
involve the use of chemicals foreign to natural 
processes, 
The disadvantages and limitations of chemi- 
cal control have received much attention. 
Nature sometimes forewarns us of some of 
these limitations if we are attentive. For 
example, we should have suspected many years 
ago that the coccidia would probably rather 
quickly develop tolerances to drugs meeting 
the other requirements of acceptable coc- 
cidiostats. These parasites have been ex- 
tremely mutable in nature. Species belonging 
to the two genera, both of which are of im- 
portance in our domestic animals, parasitize 
organisms ranging from flatworms to man, A 
recent catalog lists 562 species of the genus 
Eimeria alone (1). In contrast to this, another 
genus of protozoan parasites, Histomonas, 
belonging to a group with an evolutionary 
history possibly longer than that of the coc- 
cidia, is today represented by only two known 
