Maintenance and Transmission of Keystone Virus by Aedes atlanticus (Diptera: Culicidae) and the Gray Squirrel in the Pocomoke Cypress Swamp, Maryland

Ecological studies were done in the Pocomoke Cypress Swamp, Worchester Co., Md., to clarify the role of Aedes atlanticus and gray squirrels as enzo¨tic hosts of Keystone (KEY) virus. Among 43 seronegative squirrels, 65% were infected with KEY virus during July, August, and September 1975, coincident with the emergence and activity of KEY virus-infected Ae. atlanticus . Minimum infection rates for this mosquito increased from 2.7 to 11.2/1,000 from July to October, but we could not conclude that KEY virus-infected squirrels served as an amplifying host. Although Ae. atlanticus was infected readily following per os exposure to KEY virus, only 1 of 83 infected mosquitoes transmitted virus to a suckling mouse. In contrast, 33 of 46 (72%) parenterally infected Ae. atlanticus transmitted virus to mice. KEY virus also was transmitted vertically by Ae. atlanticus to F1 progeny, and one of two vertically infected females transmitted virus to suckling mice by bite. Preliminary data suggested that the per os-infected mosquitoes failed to transmit KEY virus because of a midgut escape, salivary gland barrier(s), or both.