Effects of ultra-low volume applications of malathion in Hale County, Texas. I. Western encephalitis virus activity in treated and untreated towns.

Abstract

Ultra-low volume (ULV) aerial applications of technical malathion at the rate of 214 g/ha (3 oz per acre) were applied routinely in 3 Hale County, Texas towns (Plainview, Abernathy, Hale Center) to reduce arbovirus transmission by mosquitoes during the summer of 1967. One untreated town, Petersburg, and 3 rural premises sites were selected for comparison. Used to evaluate the effectiveness of treatment were vector population indices before and after treatment, western encephalitis (WE) virus infection rates in mosquitoes and nestling house sparrows, WE antibody conversion rates in sentinel chicken flocks, and subclinical infection rates in school children. A significant reduction in the Culex tarsalis Coq. population level was evident only during the 48 hr immediately following treatment. Infiltration of adult mosquitoes from outside the treated areas was postulated to account for this rapid posttreatment population build-up. Seasonal WE virus infection rates in mosquitoes collected in treated and untreated towns did not differ significantly. Infiltration of infected adults into the treated areas may have contributed to the uniform infection rates. The WE virus infection rate was significantly higher in nestling house sparrows from untreated Petersburg as compared to Plainview and Hale Center, but not to Abernathy. Sparrow infection rates also were significantly lower at untreated rural sites as compared to Petersburg; therefore, factors other than ULV treatment must have been operative in keeping the infection rates at a low level. The WE antibody conversion rate in sentinel chicken flocks was significantly lower in Plainview than in Petersburg; however, observed differences in conversion rates between Petersburg and the 2 other treated towns and 3 rural sites were not significant. There was no confirmed human case of arboviral encephalitis in Hale County during 1967; however, few human cases were reported elsewhere in Texas or other parts of the United States during the year. The effectiveness of the ULV treatments in reducing the number of sub-clinical human infections was suggested, but the data were not conclusive. In order to control C. tarsalis in Hale County towns sufficiently to reduce WE virus activity, it was concluded that it would be necessary to make barrier treatments around the populated areas to prevent infiltration of infected mosquitoes.