The Rhode Island Office of Mosquito Abatement Coordination (MAC) conducts a yearly surveillance program for the early detection of Eastern Equine Encephalitis virus (EEE). During the summer and fall of 1996 an unprecedented amount of EEE was isolated from various mosquito species within the state. Mosquitoes were collected on a weekly basis using CDC light traps baited with dry ice. Initially, 18 traps were set in 10 towns. As the season progressed, additional traps were set such that 60-70 traps were placed in all 39 towns weekly. Trapping began on 17 June 1996 and continued through the final week of October 1996. Collected mosquitoes were sorted on dry ice by species and sex for each trap location and date. All male mosquitoes were discarded. Female mosquitoes were identified to species level, with the exception of Anopheles (An.), Culex (Cx.) and Culiseta (Cs.) species which were only identified to genus. Mosquitoes were pooled in vials of up to 100 individuals by species, site and date. Viral presence was determined by the Yale Arbovirus Research Unit, New Haven, CT.

The first isolation of EEE was obtained from a pool of 41 Cx. spp. collected on the night of 14 August 1996, in southern Westerly. In response to this isolation, surveillance was intensified and the extent of the outbreak in Westerly was determined. Refer to Healy (1997, same issue) for more details.

Concurrently, Highlands J virus (HJ) was isolated in several other towns throughout the state and surveillance was increased in those locales. Additionally, the MAC office was under pressure from town legislators and citizens to increase trapping statewide. As a result, beginning in early September, 60-70 traps were set on a weekly basis to include all 39 towns in Rhode Island. During the first week of September, the first EEE isolations outside the town of Westerly were detected. There were 11 additional isolations of EEE in Westerly; a single isolation (47 Cs. spp.) from Burlingame State Park in Charlestown; a single isolation (18 Coquilletidea perturbans) near the University of Rhode Island in South Kingstown; 2 isolations (12 Aedes canadensis, 67 Cx. spp.) at the Little Compton Elementary school; and an isolation (13 Cs. spp.) in Tiverton (Figure 1). Additionally, HJ was isolated in many of these same towns.

The following week (9 September 1996) EEE was isolated within the town of Hopkinton in a pool of 19 Cs. spp. Four additional isolations of EEE occurred in Westerly at two different sites. Mosquitoes (16 Cx. spp.) from the town of Richmond first tested positive for EEE on 12 September 1996. The final EEE isolation of the season occurred during the final week of October. A pool of 3 Cs. spp. trapped in North Smithfield was positive for the virus.

In addition to the mosquito surveillance, the state veterinarian collected brain tissue from deceased birds throughout the state. Several birds (emus, pheasants, turkeys) were tested. Two emus from a farm in Hopkinton tested positive for EEE virus. These two birds died on 5 September and 12 September 1996.

In summary, EEE infected mosquitoes were recovered in a total of 8 towns throughout the state during the 1996 season. The majority of the viral activity was in the southern portion of the state with Westerly providing 42 of the state's 56 EEE isolations. Additionally, 44 isolations of HJ was isolated in 6 towns throughout the state. A total of 612 trap nights collected 44,792 mosquitoes of which 35,486 mosquitoes were tested consisting of 1,489 pools. At least six mosquito species were found to be carrying EEE virus (Ae. canadensis, Ae. vexans, An. spp., Cq. perturbans, Cu. spp. and Cs. spp). No human cases of EEE were diagnosed; however, two emu deaths in the town of Hopkinton were associated with the outbreak.

ACKNOWLEDGMENTS

Al Gettman, Sean Healy and Mary Jane James-Pirri participated in all aspects of the surveillance. The personnel of the Division of Agriculture were instrumental in the state's response.

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