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Emerging Infections: Microbial Threats to Health in the United States
debated. There may well be instances in which the limited application of pesticides, such as DDT, to deal with a public health emergency may be acceptable—as long as the overall burden on the environment is not excessive. The committee believes that the current EPA contingency plan that addresses this issue is ineffective: the approval process for emergency use of pesticides is so cumbersome that approval would likely come after the critical period in which application of the pesticide could avert the outbreak. Under emergency circumstances, a tradeoff must be made, so that the process can be more expedient.
Several arboviruses (St. Louis, western, and eastern equine encephalitis) are examples of diseases that could erupt suddenly into emergency proportions that might require pesticide use. These arboviruses are enzootic in North America and are maintained in a cycle of infection between wild birds and vector mosquitoes, with little or no transmission to humans. Periodically, however, excessive rain or snow, followed by high summer temperatures, favors the emergence of increased vector populations, which may lead to the rapid spread of infection to humans.
These events can occur in both urban and rural communities, and when they do, there is an immediate need to implement a control program. The primary goal at the onset of mosquito-borne disease epidemics is to eliminate the infective mosquitoes as quickly as possible. Transmission can only be stopped by the effective application of a pesticide that kills adult mosquitoes. A control program directed against the preadult aquatic and adult stages of the vector would not have an immediate effect on virus transmission but might be valuable for preventing a prolonged epidemic.
St. Louis encephalitis (SLE) exemplifies the above scenario. It has frequently reemerged as an epidemic infection in the United States (Monath, 1980), most recently in Florida and Texas in 1990 (Centers for Disease Control, 1990d). In 1966, an effort was made, in the middle of an epidemic in Dallas, Texas, to evaluate the effectiveness of controlling populations of adult mosquitoes that transmit this disease. There were 545 suspected and 145 confirmed cases of SLE in a period of a few weeks (Hopkins et al., 1975). In an eight-day period, 475,000 acres of the area were aerially sprayed with 12,000 gallons of malathion in an ultra-low-volume, high-concentration mist. Observations made before and after the application indicated that there was a significant reduction in the vector population and its infection rate. Few new cases were detected during the two to three weeks after the spraying. This is one of the few epidemics of a reemerging infection for which a study was conducted on its economic impact. It was estimated that the SLE outbreak cost the community $796,500, of which almost $200,000 was spent on vector control (Schwab, 1968). The economic and public health consequences would certainly have been greater had pesticides not been available.