particles could coalesce. In some cases, the coalescense was followed by complete 
separation of the insecticide from the solvent or emulsifier. When this occurred, there 
was no reemulsification, resulting in total failure of parasite control, but usually in 
complete safety for the dipped animals. The intermediate stages of this deterioration 
were extremely dangerous for livestock. We learned that adhesion of emulsion droplets 
to animal hair was related directly to the size of the droplets (112). Greatest uniformity 
in treatment was obtainable with droplets less than 8 microns in diameter (same diameter 
as our red blood cells). Indeed, best work could be done with near-colloidal emulsions. 
A change in particle size from 4 to 8 microns increases deposition twofold. Particles 
at 60 to 150 microns increase deposition as much as 30 times. 
Thus it was that a dipping vat could be charged with a dilution containing a certain 
percentage of insecticide. If the particles were of 1 to 5 microns diameter, 1 unit of 
insecticide would be deposited on the hair, while if the particle size increased to 8 
microns, 2 units would be found on the hair. At 60 to 120 microns, 30 units would be 
deposited. Using a compound wherein 1 unit deposition was safe, but 2 dangerous, one 
can readily see that the increase in size was a serious danger to livestock. 
The particle size effect was greatest in dipping operations, because the emulsions 
were held for months at a time and the dipped animals were exposed to hundreds or 
thousands of gallons of emulsion. Spraying usually involved the application of 1 or 2. 
gallons per head, and each few hours of operation meant the preparation of new mate- 
rial. Deterioration with time was unimportant unless the operator was an unduly thrifty 
man who tried to save material for future treatments. 
Efforts have been made by manufacturers to produce the finer quality emulsifiables. 
Most materials marketed for livestock use at the present time have been carefully for- 
mulated. Most of these are recommended only for spray use because few manufacturers 
have been able to develop formulations that can stand without deterioration in dipping 
vats. 
The early recognition of these effects prevented many unfortunate accidents and in- 
sured uniform parasite control. 
Misunderstanding apparently still exists about the selection of proper formulations 
for animal treatment. Our manufacturers produce certain materials for animal use and 
others for plant use. These are by no means interchangeable from plants to animals. 
Occasionally a material is made for both purposes, but these are of the minority. It is 
important that users of animal insecticides read the labels on these materials and 
assure themselves of proper usage. 
Effect of Emaciation and Lactation 
We found that emaciation and lactation, singly or in combination, predisposed sheep, 
(89), and probably cattle, to poisoning by certain of the chlorinated hydrocarbons, notably 
the gamma isomer of BHC, but including toxaphene and chlordane, 
Effect of Stress 
While emaciation and lactation produce an increased susceptibility to poisoning, the 
opposite effect was noted with Bayer L 13/59 (Dipterex) during our studies of that com- 
pound as a systemic insecticide. One demonstration was inadvertant. We selected an 
intractable sheep to demonstrate kill of screwworms. Following treatment, the sheep 
(restrained by a neck rope) struggled extensively to escape. Although the dosage of 
Dipterex was twice that ordinarily required for screwworm kill, none of the worms were 
destroyed. 
144 
