This study was undertaken to define the nature and extent of arbovirus activity in the environment of Tucson, Arizona; especial interest was centered in the prevalence of arthropod-borne encephalitis. Field collections of mosquitoes, ticks, birds, and small mammals were made in the selected study areas along the valleys of the Santa Cruz River and the Rillito in or near Tucson. Flocks of sentinel chickens were maintained during the hot seasons in the same areas. The virus of St. Louis encephalitis (SLE) was isolated 12 times from mosquitoes in three separate summer seasons. The virus of Western encephalitis (WE) was recovered in the summer of 1966 in three different lots of mosquitoes. The results of serologic tests on the chickens maintained as sentinels in the study areas confirmed that active transmission of SLE and WE viruses had occurred during the summers that the isolations had been obtained. There was a relative scarcity of Culex tarsalis in the areas studied. The explanation for this is not clear. As a result of this study it appears clear that arthropod-borne encephalitis poses a continuing threat to the population of southern Arizona.
Present address: Department of Internal Medicine, University of Utah Medical Center, Salt Lake City, Utah 84112.