2012 ARBOVIRUS SURVEILLANCE IN VERMONT

Alan C. Graham

Vermont Agency of Agriculture

Plant Industry Section

322 Industrial Lane

Barre, VT 05641

Alan.Graham@state.vt.us

ABSTRACT

2012 Mosquito surveillance in Vermont was restricted to two counties with towns organized into mosquito control districts. The summer months of July and August were very dry, drastically reducing the number of floodplain mosquito species. The trapping began on June 14 and ended on October 22. CDC light traps were set at long-term trap sites in 13 locations. An additional 19 trap sites were added during the season. Resting box trap sites, a line of 10 boxes, were set up at 7 locations. After arbovirus positives were found, and additional 3 sites were added. Two gravid trap sites, consisting of 3 gravid traps at one and 6 at the other, where EEE activity in emus was present in 2011.

This year a total of 251 mosquito pool samples were tested, representing 4676 mosquitoes and 13 species. Samples were tested using RT-PCR analysis at the Wadsworth Laboratory in New York. Vermont reported 10 EEE virus isolations and 1 WNV isolation. All EEE isolations came from a single species, Culiseta melanura. This was the first isolation of EEE from a mosquito pool in Vermont. Three sites tested positive for EEE in Vermont, with early positives first reported in the two towns of Whiting and Brandon on July 24. The acidic hardwood swamp site in Whiting had continuous EEE activity for several weeks: week numbers 30, 31, 32, 33, 35 and 36. Week 34 was not sampled. A CDC light trap site located in Brandon, adjacent to where 19 out of 100 emus died last year, had a single EEE positive mosquito pool. Expanded light trap sampling at another acidic hardwood swamp 3.5 miles from the Whiting site, resulted in another positive on September 9. Two human deaths were reported in Rutland County, dates of onset August 13 and August 27. The two horse EEE cases were in the towns of Whiting (onset September 6, survived), and Proctor (onset September 16, died). This was the first year that EEE was reported in humans and horses in Vermont.

The single WNV isolation from a mosquito pool sample came from Ochlerotatus canadensis taken from a light trap at the Whiting site on July 3. This date represents the earliest WNV detection in Vermont since the surveillance began in 2001. Three human cases of WNV in Vermont were reported this year and all survived. The counties and dates of onset were as follows: Franklin County on August 24, Essex County on September 2 and Rutland County on October 19. WNV was implicated in the deaths of two alpacas in Vermont. One was in the town of Highgate on August 29 and the other was in Morristown on September 25.

During the 2012 season 2534 acres were treated with larvicide. Most of the treatments in the three Vermont mosquito districts utilized VectoBac BTI, but VectoLex BS was used on 144 acres in the Leicester Junction wetland system adjacent to Otter Creek. Treatment dates were April 28, May 10 and 11, May 14 and May 17. The dry season reduced the need for additional treatments.

An aerial adulticide treatment of 19,543.8 acres in two towns was done on the nights of September 6 and 7 to reduce the EEE risk to humans. Anvil 10+10 was used at 0.62 oz/acre in Whiting and Brandon, Vermont. The BLSG mosquito district did ground-based adulticiding in June, but due to the dry weather, no treatment was done in July and August. They resumed truck spray operations once EEE positive mosquito pools samples were reported in late August.

The Connecticut Agricultural Experiment Station is analyzing 800 individual blooded mosquitoes for arbovirus activity and the host feeding preferences of mosquitoes near the arbovirus foci in Vermont. These samples represented 12 species, predominantly Culiseta melanura (427), Anopheles quadrimaculatus (194), Culex territans (90) and Anopheles punctipennis (63).

This was the third year for the Vermont cervid EEE antibody study. This year’s serosurvey of 49 moose and 290 deer during the opening of hunting season in Vermont resulted in 336 blood samples which will be tested by the CDC and then geocoded to location.