A case comparison study and arboviral surveillance of LaCrosse encephalitis in eastern Tennessee

The eastern region of Tennessee has experienced an increase in the occurrence of LaCrosse encephalitis (LACE) since 1996. The virus that causes the disease (LACV) is known to be endemic in West Virginia and North Carolina. Due to the recent increase in the number of cases of LACE it is suspected that a new endemic focus has been established in eastern Tennessee. This increase has occurred at the same time that Aedes albopictus (Suske) became established in the region. Aedes albopictus has been shown to be capable of transmitting the virus under laboratory conditions, and LACV has been isolated from Ae. albopictus reared from wild collected eggs from a known LACE site in Anderson County, Tennessee. This finding indicates that Ae. albopictus may be playing a role in the transmission of LACV. Surveys of the residences and other areas frequented by the 38 potential LACE patients were conducted from May-October, 2000. Data about the habitat, home and yard maintenance, distance from the tree line and road, and numbers of containers present at each of the sites were collected. Larval mosquitoes and eggs were collected and reared, and host seeking adult mosquitoes were captured using CO² -baited Centers for Disease Control (CDC) light traps. The number of male Ae. albopictus males collected with CDC traps were significantly higher at LACE sites than at control sites. The combined numbers of host seeking Ae. albopictus adults and larvae collected from LACE sites were significantly higher than at control sites. No other factor was found to be significantly different between LACE and control sites. Continued surveillance of a known LACE site in Anderson County, Tennessee was conducted from June - October, 2000. Three groups often oviposition traps were placed in the woods at the site and the eggs were collected weekly. Significantly fewer Ae, triseriatus eggs were found the further the traps were placed into the woods. No differences were found with the number of Ae. albopictus eggs collected at different distances into the woods. Adult host seeking female mosquitoes were collected weekly using a CO^{2-baited Centers for Disease Control (CDC) light trap and dissected to determine parity rates. Parity rates for Ae. albopictus, as determined by examination of the tracheoles in the ovaries, ranged from 28% - 75% parous. The survival rate as calculated increased later in the summer, from 73% in June to 93% in September, but still did not reach levels found in similar studies.Aedes triseriatus parity rates ranged from 33% - 66%, but there were not enough Ae. triseriatus captured to accurately calculate survival rates. The parity rate for both species were lower than those found in similar studies.}