Agriculture accounts for about 75 percent of the pesticides used in the United States. In 1987, approximately 407,000 tons of pesticides were applied in agricultural settings throughout the nation, of which about 89,500 tons were insecticides. About 10 percent of the pesticides used worldwide are applied for public health purposes, mainly to control malaria, filariasis, schistosomiasis, onchocerciasis, and trypanosomiasis (Moses, 1992).

This high volume of pesticide use for agricultural purposes has contributed to the development of resistance in infectious disease vectors, particularly mosquitoes. Public health use of insecticides has also played a role in the emergence of resistance, although not in the United States, where public health use of insecticides is not sufficiently regular to elicit the development of resistance.

Resistance, in the field, to a number of pesticides belonging to organochlorine, organophosphate, and other insecticide groups has developed in vector arthropods. In addition, recent evidence from laboratory studies points to the emergence of resistance in mosquito larvae to the delta endotoxins of the commercialized microbial control agents Bacillus thuringiensis israeliensis (Georghiou, 1990) and B. sphaericus (Rodcharoen and Mulla, in press). Although resistance to these agents has not yet been demonstrated in a field situation, the laboratory finding illustrates the strong potential for the development of such resistance.

New Understandings: Microbes as Cofactors in Chronic Disease

Although medical science has been able to discern the causes of many diseases, the etiology of some that have a significant impact on the health of the U.S. population is still speculative, even after decades or more of research. The recognition that an ''old" disease, with heretofore unknown causes, is associated with an infectious agent is one of the more interesting ways infectious diseases emerge. A number of diseases are now thought to be caused by microbial infection or to involve microbes as cofactors in pathogenesis. These include, but are not limited to, atherosclerosis, rheumatoid arthritis, insulin-dependent diabetes mellitus, Reye's syndrome, Kawasaki disease, systemic lupus erythematosis, and Alzheimer's disease. The final chapters on these diseases have not been written; it remains to be seen which, if any, will be determined with certainty to involve microbial agents. The examples below illustrate the relationships between some of these diseases and the infectious agents associated with them.

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