Comparison of mosquito abundance, distribution and parity between a high and a low prevalence site for La Crosse encephalitis in Eastern Tennessee

by 1982- Scheffel, Sabra Lee

A three-year investigation of the seasonal distribution, abundance and diversity of mosquitoes at a high and a low prevalence area for La Crosse (LAC) encephalitis was conducted in eastern Tennessee, USA. We identified a high LAC prevalence site (Knox County) from which two cases of LAC encephalitis were confirmed, one in 1997 and the other in 2000, and an ecologically similar low prevalence site (Blount County) with no confirmed LAC cases. Mosquitoes were collected at each site using 2 Center for Disease Control (CDC) miniature light traps baited with carbon dioxide, 1 Omni-directional Fay trap baited with carbon dioxide, 2 gravid traps and 25 oviposition traps. At both sites, mosquitoes were collected weekly between late May and early November 2003-2005. The traps that attracted host-seeking and gravid adults were operated for a 24-hour time period each week, while the oviposition traps were left out for the entire week. Throughout the 2003, 2004, and 2005 collection periods, a total of 7,593 adult female mosquitoes were collected and identified to species (Knox County (n=3,646), Blount County (n=3,946)). Aedes albopictus, Ochlerotatus triseriatus, and Ae. vexans were the most commonly collected mosquitoes at both sites. The proven and suspected LAC vectors, Oc. triseriatus and Ae. albopictus, comprised 19.1% and 46.6% of all adult female mosquitoes collected and identified, respectively. Ochlerotatus triseriatus was collected most often in the early summer (June) with fewer numbers collected in the late summer, whereas Ae. albopictus collections tended to be largest in the late summer to early fall (August and September). In 2003, egg and adult collections fluctuated in a similar manner between sites, but not among species. Aedes vexans comprised 26.6% of all adult female mosquitoes collected in 2003, but only 8.3% and 14.5% in 2004 and iii 2005, respectively. There were no significant differences in egg or adult collections of Ae. albopictus and Oc. triseriatus between the high and low prevalence LAC sites. Weather patterns also appeared to be similar between the two sites. The total average monthly parity rate for Ae. albopictus and Oc. triseriatus at each site was between 40- 48% parous in 2003, between 35-49% parous in 2004 and between 8-24% parous in 2005. Parity rates did not significantly differ between sites or species. The carbon dioxide baited CDC light traps collected most of the adult mosquitoes used in parity determination. Small mammals were sampled multiple times at each site with live traps. Each site contained sufficient populations of squirrels for LAC virus amplification, but no chipmunks were collected from the low prevalence area for LAC encephalitis. La Crosse antibodies were found at both sites, but the Knox County site had a higher prevalence of 2.65% compared to the Blount County site with a prevalence of 0.44%. Out of a total of 226 serum samples tested from both sites, 5 squirrels and 2 chipmunks tested positive for LAC antibodies. iv