Compounds capable of destroying one form of life must be suspected of being capable 
of at least injuring other forms. Stopping life processes, whether in an animal or a plant, 
involves the immobilization of essential enzymes or in some other way blocking the 
essential functions of individual cells. When the essential cells of the organism die or 
stop functioning, then so must the organism. 
Life-destroying compounds are generally somewhat selective in action, permitting 
us, by intelligent choice of material, to destroy one form of life without harming those 
forms we consider desirable. 
The classes of pesticides are not so well-defined as might be desired and their 
members could be discussed under several classes. Most herbicides have some bac- 
tericidal or fungicidal activity, although none are used for these purposes. Similarly, 
many fungicides are herbicidal, but we can select many that can be safely applied to 
plants. One of the first systemically active animal insecticides 2-pizalyl-1,3-indanedione 
(Pival) found its better place as a rodenticide. Several insecticides, such as Bayer 
21/199, Bayer L 13/59, Ruelene, and ronnel have been shown to have an anthelmintic 
action (5,36,49,51,61,62,72,101). 
Ultimately we must select those compounds which destroy these undesirable forms 
of life without creating a hazard to desirable plants, animals, or to humans, 
To cite the specific data that have been obtained concerning the toxicity of pesticides 
by scientists of the Agricultural Research Service alone would require many hours; to 
include those from industry and the various state and commercial laboratories would 
require again as much time. Our references to published literature have been selected 
to establish principles rather than to provide a complete bibliography of this subject. 
INSECTICIDES 
The discovery of DDT as an insecticide by Muller in 1939 was a great advancement 
of our potential for insect control, and for it he received a Nobel prize. The low acute 
toxicity of DDT for most mammals suggested there would be no problems associated with 
its use. Several developments caused a revisionof scientific attitudes toward the safety of 
DDT and toward several similar compounds then under development. 
Appearance in Meat and Milk 
Howell, et al, (64) found DDT in the milk of'‘cattle sprayed with DDT. This was taken 
in many quarters to represent mechanical contamination of the udders by the insecticide, 
because at that time it was not an acceptable theory that such compounds could pass 
through the unbroken skin to the blood and be circulated. It was necessary for us, at 
Kerrville, to conduct similar studies and to devise closed systems for collection of milk 
from within the udder to prove beyond doubt that excretion was taking place. It was then 
argued that the cattle were licking themselves and absorbing the DDT from their digestive 
tracts, The authors and others at the Kerrville Laboratory then kept sprayed cattle in 
stanchions--muzzled except for a few minutes feeding time--and collected milk in sealed 
systems. Again, DDT was found in the milk, DDT was found in milk by a large number 
of workers (1,9,16,19,20,26,28,39,42,111,116,122). 
From the beginning, none of us dealing with the toxicity of these compounds for live- 
stock had any doubts concerning the speedy absorption of insecticides from any of the 
body surfaces, either external or internal. One does not doubt when animals are poisoned 
and die within 30 minutes of treatment without having licked themselves. 
Although the evidence was overwhelming, even the Council on Pharmacy of the: 
American Medical Association insisted that the tests musthave been done by inept people 
and should be repeated by competent persons without delay. As we now realize, these 
early studies were sound and have stood the test of time. 
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