USE OF CHEMICALS FOR CONTROL OF PARASITES OF FOOD ANIMALS 
A. O. Foster, F. D. Enzie, and D. K. Mc Loughlin! 
Some 300 species of animal organisms are economically significant parasites of food 
animals. They are the insidious protozoa and helminths, commonly known as internal 
parasites, and the ubiquitous arthropod parasites which, for convenience, are often called 
external parasites. In 1954 the Department estimated that internal and external parasites 
were about equally responsible for losses that exceeded $1 billion annually at the farm 
level. Developments of the intervening 6 years have indicated that the estimate was con- 
servative, Arthropod parasites, from the standpoint of chemicals used against them, are 
adequately covered by other papers of this symposium. Internal parasites, or just 
‘‘parasites,'' are, therefore, the primary subject of this discussion, 
Our position with respect to parasite control is extremely acute and vulnerable, It is” 
so because foreign chemicals are the most powerful aids available, They must yet be 
relied upon to roughly the same degree that immunological measures are employed for 
controlling pathogenic viruses, rickettsiae, and bacteria. From standpoints of other and 
broader alternatives to chemical control, there is little for use against parasitic diseases 
that is not also applicable to other diseases, 
In general, problems of parasitism are intensified by shifts toward better agriculture 
and land usage. Changing agricultural practices, suchas grassland conversion, irrigation, 
reclamation of barren lands, concentrated production, free movement of stock, and 
geographical relocations within the industry are causing ever-increasing alarm about 
parasite loss and a mounting urgency about combative measures, Other factors are con- 
tributory, such as better diagnosis and fuller cognizance of the economic impact of 
parasitic disease, more farm animals and higher inventory value per unit, and keener 
competition in production and marketing. 
Against this background, one must weigh today’s antiparasitic situation, Available 
chemicals, as useful as they are, arenot satisfactory; even the best are quite short of the 
ideal, Despite great progress in the last 45 years, we still lack chemical measures for 
controlling about two-thirds of our injurious helminths of food animals, The situation 
with protozoan infections is only comparatively better. The kinds of parasitisms against 
which chemical agents can be used with evenmodest efficiency are, therefore, a minority 
of the important ones. Also, while not surprising, it is nevertheless unfortunate that 
parasites are unusually nimble at circumventing chemical attack. Chemotherapy is 
traditionally close to parasitology and there are few, even in this day of enlightenment, 
who do not recognize an enormous agricultural and human debt to progress in this field, 
yet ever since the classical work of Paul Erlich, populations of organisms exposed to 
chemicals have demonstrated their capacity to develop tolerance. Added to these con- 
siderations is the matter of toxicity, which has always been a gremlin in the development 
of antiparasitic agents, yet there would probably not be the pressure for alternatives to 
chemical control that is now manifest if events had not created a mass fear of insidious 
chemical poisoning and legislation had not curtailed the number and uses of available 
agents and, at the same time, made thecost of new developments almost prohibitive. It is 
well, and undoubtedly wise, that we recognize the seriousness of our situation in parasite 
control, 
There is no question about soundness of the position on residues, yet food animals 
and food crops are hard hit by it, with one of the heaviest blows falling on parasite control. 
No defense of antiparasitics, however justifiable, can or should alter in any way or mea- 
sure the requirements imposed, ‘‘Residue’’ legislation has caused less concern than 
embarrassment to scientific research. The latter is rarely confronted so clearly with an 
1Parasitologists, respectively, Animal Disease and Parasite Research Division, Agricultural Research Service, U, S. De- 
partment of Agriculture, 
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