NSW Arbovirus Surveillance & Vector Monitoring Program
1998 - 1999 Annual Report

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For the season of 1998-99, the NSW Arbovirus Surveillance Program provided monitoring of mosquito vector populations and surveillance of arbovirus activity on the NSW western slopes and plains, coastal regions and metropolitan Sydney. The majority of sites operated between November and April.

The drought conditions, which dominated the 1990's, were broken with heavy rainfall across much of the state through winter and spring. This resulted in some extraordinary mosquito collections from the inland, with Culex annulirostris numbers up to 34 times the average. There was a high level of virus activity in the mosquitoes, with several Ross River (RR) isolates during November; this has never occurred before during the history of the Program. All up there were a record 104 isolates (4 RR, 68 SIN, 1 EH, 3 KUN, 1 KOK, 27 unknowns) from the inland. There were also record numbers of notifications of human arbovirus infections during November, December and January. These were concentrated in the Greater Murray Area Health region and were mostly RR virus infections.

Kunjin virus reappeared for the first time since 1990-91, with mosquito isolates from Leeton and Griffith. In response to this, public health warnings were intensified, a new chicken flock placed at Leeton, and the chicken surveillance season extended until the end of April. Three notifications of Kunjin infection in humans were reported.

Sentinel chicken flocks were operated at Bourke, Deniliquin, Forbes, Griffith, Leeton, Menindee and Macquarie Marshes. There were no flavivirus seroconversions.

Both the Forbes' and Nicholls' models are suggesting possible Murray Valley Encephalitis virus activity (MVE) for 1999-2000 [note that the Nicholl's model is no longer suggesting possible MVE activity - SD]. This, and the fact there has been extensive MVE activity in northern Australia, means that environmental conditions will be closely monitored over the next few months.

Mosquito numbers on the north coast were below average during November to January. High localised rainfall in mid summer resulted in several large collections in February and March, with unusually high numbers of freshwater mosquitoes. Thirteen viruses (6 RR, 7 unknowns) were isolated from the north coast. Record numbers of human notifications of Barmah Forest (BF) and RR were reported.

The south coast had very dry conditions with mosquito numbers well below average. Three viruses (2 RR, 1 unknown) were isolated, all from Batemans Bay. There were a number of notifications of RR and BF infections in humans.

Sydney collections also yielded low mosquito numbers, however there were six isolates (2 RR, 4 STR), the majority from Aedes notoscriptus. There were human RR cases in rural Sydney and a RR seroconversion in a horse prior to the human cases.

Overall, 1998-99 had the highest number of human notifications ever recorded for one season in NSW, with 1,219 RR and 238 BF infections reported.

The major innovation for the current season was the introduction of the NSW Arbovirus Surveillance Web Site <http://www.arbovirus.health.nsw.gov.au/>. While the main function of the site is to rapidly disseminate surveillance results, a wide range of information on mosquitoes and arboviruses is included.

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