of material through the digestive tract, and 
other processes related to feeding schedules 
or other activities controlled by management 
practices. 
Intentionally introducting or encouraging the 
proliferation of other organisms to combat a 
specific parasitism may be only of theoretical 
interest, at least at present. To establish 
harmless organisms in a given region and 
to keep them there in sufficient numbers to 
have a predictable effect on a specific parasite 
has not proven to be easy. Possibly this is 
because we do not yet know well enough the 
circumstances that favor their proliferation. 
For purposes of orientation, it may be use- 
ful to outline categorically the possible ap- 
proaches to the biological control of parasites 
outside the body of the host and to consider 
for each some of the means operative in nature, 
These approaches include (1) the partial or 
complete destruction of potentially infective 
stages of the parasite as a result of (a) 
predation, (b) hyperparasitism, and (c) de- 
pletion of stored energy resources; (2) the 
partial or complete elimination or exclusion 
of important or essential vectors through 
agencies similar to those enumerated above; 
and (3) the partial or complete elimination of 
other biological influences that favor the 
persistence of a parasite or enhance its 
chances of being acquired by the host. 
As indicated previously, about 92 percent 
of the parasites of our domestic animals are 
acquired through contract with the soil or its 
appurtenances. Both are vast and extremely 
complex, as well as varying greatly from 
region to region or even from place to place 
in a relatively restricted area. Moreover, 
Nature does not recognize the boundaries of 
man's arbitrary categories. Furthermore, the 
protective devices employed by the develop- 
mental stages of parasites vary widely in 
structure, chemical composition, and effect 
on the parasite's ability to respond to change 
in the environment or the necessity for its 
making a response. Consequently, I shall select 
as examples certain situations as they occur 
in nature, and point out some of the areas of 
opportunity for biological control, and at the 
same time, I hope, convey some concept of 
the difficulties we encounter. 
Many of the protozoan parasites and a few 
of the helminths are transmitted directlyfrom 
113 
one vertebrate host to another by arthropods, 
and no stage of the parasite resides in or on 
the soil (4). Discussions of the control of 
arthropod vectors, whether a mosquito, black- 
fly, midge, or tick, I shall leave to the 
entomologists. 
Little is known concerning the destruction 
of protozoan cysts in nature by biological 
means. In closed systems in the laboratory, 
such cysts are often destroyed by the un- 
checked growth of bacteria or molds or con- 
sumed by organisms such as rotifiers, small 
crustacea, and insect larvae. Even the rela- 
tively resistant oocysts of at least some 
species of coccidia are rapidly destroyed by 
some molds, Freshly passed rabbit droppings 
kept in tightly stoppered test tubes at 4° C, 
have been observed to become overgrown with 
mold in as little as 4 to 6 days. Hyphae also 
penetrated the pellets. On examination, only 
empty odcyst shells or shells containing a 
sundry assortment of globules instead of 
protoplasts remained as evidence of the 
myriads of o6cysts originally present. The 
results of subsequent tests were not always 
the same, but neither were the molds (11). 
The problem needs study. 
But we must not Suppose that what occurs 
so conspicuously in a closed system can be 
made to operate to an appreciably greater 
degree on the soil than is occurring without 
our intervention. From time to time, Brain 
(12) and others have reviewed the abundant 
literature concerning the importance of anti- 
biotics and other forms of biological antago- 
nism under natural conditions in the soil. 
When such antagonisms do occur under soil 
conditions, their effects are likely to be highly 
localized and quite transitory. To make some 
of them otherwise seems presently out of 
reach. Some do afford considerable promise, 
especially if we consider that the deposition of 
potentially infective stages of parasites is 
also discontinuous with regard to both time 
and space, 
From the moment a cow discharges its 
feces, populations from at least three ecological 
sources start to mingle competitively for the 
most part. There are those organisms passed 
as part of the manure, those of the soil, and 
those deposited by aerial creatures attracted 
to the newly found situation. Physical, chemi- 
cal and biological factors all start changing, 
