Wildlife Damage Management, Internet Center for


Date of this Version

November 1979


The Ohio Department of Health (ODH) has maintained an arbovirus surveillance pro- gram since 1964, when the Vector-borne Disease Unit was formed to survey and study California encephalitis epidemiology in Ohio. Since 1975, a major part of the surveillance program has been devoted to St. Louis encephalitis. Both of these diseases have viruses as the causative agent of illness. Both have specific mosquito vectors, and both are classified as zoonotic diseases in that they are primarily diseases of wild vertebrates, transmissible to man -- in this case, only by the bite of an infected mosquito. The vertebrate reservoirs of California encephalitis are members of the squirrel family, which includes, besides the familiar fox, red, and grey squirrels, chipmunks, flying squirrels, and woodchucks (or ground hogs). The vertebrate reservoirs of St. Louis encephalitis have not been as neatly identified, but they are definitely birds and possibly, also bats. My remarks hereafter will deal primarily with St. Louis encephalitis and its bird reservoir, which are of major concern to the Ohio arbovirus surveillance program. The natural cycle of St. Louis encephalitis (SLE) virus, as we understand it at present, involves birds, the mosquito species Culex pipiens, and SLE virus. Normally, this virus is transmitted from bird to bird by Culex pipiens, which prefers to feed on birds and needs a blood meal in order to produce eggs. During the latter part of the summer, however, its feeding preference changes, or its preferred host availability is changed, resulting in a higher percentage of blood meals being taken from mammals, including man. It is this change in host-feeding pattern which sets the stage for transmission of SLE virus to man.