applicable to certain compounds, such as parathion, diazion, malathion, and Bayer 
L 13/59. With these materials a rapid drop of blood cholinesterase to 10 percent or so of 
normal is usually accompanied by symptoms of poisoning. 
For repeated exposures, the method loses significance. With parathion we have re- 
duced blood cholinesterase activity to zero with 2 doses of 20 mg./kg. in cattle, but 
poisoning as manifest by symptoms, did not become evident until after 78 additional daily 
doses of 20 mg./kg. each. 
Zero activity levels repeatedly with Co-Ral (Bayer 21/199) and with other insec- 
ticides, which persisted for several weeks without the appearance of symptoms, have 
been observed. 
Several of the new experimental insecticides produce cholinergic symptoms and 
even death while cholinesterase activity in blood is near 50 percent of normal. This is 
not helpful in reaching a diagnosis of poisoning. Hazelton pointed out similar observa- 
tions (59). 
There is safety in using cholinesterase activity to indicate exposure to organic 
phosphorus compounds or carbamates, but we believe this is the only firm conclusion 
that can be drawn. 
Residues in Diagnosis.--The use of chemical or biological determinations in the 
diagnosis of poisoning by insecticides is vastly complicated by their behaviour. 
I doubt that sufficient emphasis can be given to this problem. It is not readily under- 
stood because the necessary explanations are apparently contradictory of our usual 
contept of the significance of the presence of a foreign chemical in animal or human 
tissues, This lack of understanding has caused us particular difficulty when toxicologists, 
after finding high residues, flatly state that these residues indicate death due to the 
material found. 
Let me carry you through some examples. In feeding tests at Kerrville with hep- 
tachlor, using a diet containing 60 p.p.m. of heptachlor, 52 p.p.m. of heptachlor epoxide 
could be recovered from the fat of cattle at the end of a 16-week feeding period. The 
cattle at that time were in excellent health and condition. One of the cattle, when 
slaughtered immediately after the feeding period, contained, as stated, 52 p.p.m. in the 
fat, 1.2 p.p.m, in the thymus, and 1.5 in the liver. Other tissues, including brain, kidney, 
muscle, adrenal, heart, lung, and spleen contained none. 
By contrast, a calf poisoned and killed by a single large dose of heptachlor, revealed 
a residue of only 2.2 p.p.m. in its fat. 
In our feeding trials with lindane, using a level of 100 p.p.m. in the feed for 10 weeks, 
a residue of 100 p.p.m. existed in the fat at the end of the feeding period. This was re- 
duced to 50 p.p.m, in 4 weeks on control feed. 
By contrast, three cattle of similar breed, age, and condition, deliberately poisoned 
by high concentrations of lindane indips, revealed only 23 p.p.m. 1 week later in their fat. 
Let us assume that either of these animals with high residues had been found dead on 
pasture treated with the particular insecticide, no history other than this available and 
with the animals having been seen in good health a few days earlier. At necropsy, no 
lesions are observed other thana few petechiae on the heart and cloudy swelling of various 
organs. Because of the prior use of insecticides, tissues are taken for analysis, with the 
results being at high levels. Unfortunately, many veterinarians would accept this as def- 
inite evidence of death caused by the insecticide and so render their diagnosis. Some 
have, only to have others prove the presence of rabies, Aujesky’s disease, or other 
diseases. 
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