Masters Theses

Date of Award

12-2004

Degree Type

Thesis

Degree Name

Master of Science

Major

Entomology and Plant Pathology

Major Professor

Reid R. Gerhardt

Committee Members

Carl J. Jones, Lisa I. Muller

Abstract

La Crosse (LAC), a California (CAL) serogroup bunyavirus, is the most prevalent pediatric arboviral disease in the United States and accounts for virtually all encephalitis cases associated with CAL serogroup arboviruses. In Tennessee, USA, prior to the drastic increase of confirmed La Crosse (LAC) encephalitis cases in 1997, the sum of documented cases reported to the Tennessee Department of Health totaled 9. During subsequent years in Tennessee (1997 to 2003), an average of 12 cases has been reported annually. Approximately 62% of these reported cases have occurred in the months of July and August. The native, container-inhabiting mosquito Ochlerotatus triseriatus (Say) is a proven vector of the LAC. Recently, the LAC virus was isolated from Aedes albopictus (Skuse) in Tennessee and North Carolina and it has been postulated that an increase in both incidence and distribution of LAC virus throughout Tennessee coincided with an expansion of the exotic and ubiquitous Ae. albopictus (Gerhardt et al. 2001, Erwin et al. 2002).

Weekly collections of container-inhabiting mosquitoes were conducted from 19 May to 24 November, 2003, in an attempt to quantify differences in seasonal patterns of Oc. triseriatus and Ae. albopictus populations between a high (Knox County) and low (Blount County) prevalence area for reported cases of LAC encephalitis in eastern Tennessee. When eggs found in ovitraps (n = 108,973) were utilized as an index of population density, the data showed that Oc. triseriatus populations were highest from 26 May to 23 June and are abundant during the early onset dates of LAC encephalitis cases in eastern Tennessee. Collections of Oc. triseriatus eggs declined throughout July with a smaller peak in August. In contrast, Ae. albopictus egg collections gradually increased in density from May to July and sustained high levels until September. Egg densities of both Oc. triseriatus and Ae. albopictus were abundant during July and August and increased the opportunities for vertical and horizontal transmission throughout the mosquito season.

Adult females of Oc. triseriatus and Ae. albopictus were collected weekly with host-seeking and gravid traps from each sites in both Knox and Blount County. A total of 5,187 adult Oc. triseriatus and Ae. albopictus was collected from all adult trapping techniques. The proportion of parous females for each species was determined via dissection. Monthly parity averages were above 40% and based upon the general increase from May to October, opportunities for vertical and horizontal transmission by Oc. triseriatus and Ae. albopictus should have increased throughout the mosquito season.

The blood-engorged females (n = 8) successfully tested by polymerase chain reaction (PCR) contained mammalian blood. Furthermore, each site contained sufficient populations of sciurids and potentially allowed for LAC virus amplification. This research suggests that there are no differences between the high and low prevalence area for LAC encephalitis in regards to egg or adult collections, parity rates, host selection preferences.

Seasonal abundance, occurrence, and diversity of all mosquito species were evaluated between 19 May and 17 November, 2003, and 24 May and 5 July, 2004, at the two forested sites in Knox and Blount County. Nineteen species representing 7 genera were identified between the 2 collection seasons with an accumulation of 4,961 host-seeking and 296 gravid females. The overall abundance of adult mosquitoes collected and identified to species was similar between Knox (n = 2590) and Blount (n = 2667) counties. Diversity of collected species was highest in June. Aedes vexans (Meigen), a potential bridge vector of West Nile virus (WNV), was the most commonly collected species in 2003 and accounted for approximately 28% of all host-seeking and gravid females collected. In 2004, Ae. vexans numbers were extremely low at both sites and was likely affected by lower amounts of precipitation in the early portion of the mosquito season. Neither a high abundance nor diversity of Culex species were found in gravid traps in either year. The proven and potential LAC vectors, Oc. triseriatus and Ae. albopictus, constituted 59.3% and 73.8% of all adult species collected in the 2003 and 2004 season, respectively. In Knox County, Oc. japonicus japonicus (Theobald) was collected during both seasons and was the first reported occurrence of this species in the state of Tennessee.

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