In 2002, Tasmania reported the largest number of Ross River virus (RRV) infections ever recorded for the state. Of the 117 cases, 37 lived in, or had visited, the Sorell Municipal Area, east of Hobart. In early 2002, a combination of spring tides and high summer rainfall produced extensive saltmarsh habitat in the Sorell region, resulting in unseasonably high densities of the mosquito Ochlerotatus camptorhynchus, recognised vector of RRV. Four isolates of RRV were identified from collections of adult mosquitoes. All four isolates were from Oc. camptorhynchus, collected near the Carlton River. This is the furthest south that RRV has been identified in Australia and the first identification from south-east Tasmania. The virus carriage rate in the mosquito vector populations were very high, with successive weekly minimum infection rates of 17.1, 3.0 and 11.1 per 1,000 Oc. camptorhynchus at Carlton River from mid-February to early March. The first isolation of RRV from mosquitoes coincided with the onset dates of the first human cases of RRV infection. Commun Dis Intell 2004;28:261-266.

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Introduction

The number of cases of Ross River (RRV) disease have fluctuated in Tasmania between 8 and 117 during the period 1995-2002. The highest notifications were reported in 1996, 1999 and 2002, with 74, 67 and 117 notifications, respectively.¹

Investigations into the dynamics of arboviral diseases within Tasmania have been limited to two previous studies. Both were undertaken on the east coast, largely focusing on the Coles Bay to Scamander region, and identified Ochlerotatus camptorhynchus and Oc. flavifrons as vectors of RRV for that region.^2,3

Despite the presence of human cases of arbovirus disease further south in the Hobart region, there have been no investigations of the mosquito vectors and the relative disease risks in these south-eastern communities.

The Sorell Municipal Area is 25 km east of Hobart, and its population of 10,800 is located mostly in the coastal townships of Midway Point, Sorell, Dodges Ferry and Primrose Sands. Sorell Council has received notifications of cases of RRV infection since 1995. In 1999, following 30 cases of RRV and complaints of excessive local mosquito activity, Sorell Council commenced surveys of mosquito larval habitats and began adult trapping in several areas.

This surveillance was subsequently expanded throughout the municipal area, and in the summer of 2002, the Council decided to test field-collected adults for the presence of arboviruses to determine which local mosquitoes were likely RRV vectors. In order to determine the origin of the RRV infections, all local human cases were interviewed to establish the location of their residence and if they had travelled to, or outside of, the Sorell area in the 2-3 weeks before the onset of symptoms.

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Methods

Case investigation

Notifications of RRV were included in this study if they satisfied the Tasmanian case definition of RRV which required notification to the Director of Public Health, i.e. laboratory analysis of a blood sample satisfying one of the following criteria:

1. isolation of RRV from a sample;

2. detection of RRV by nucleic acid testing;

3. IgG seroconversion or a significant increase in antibody level, or a fourfold or greater increase in titre to RRV; or

4. detection of RRV specific IgM antibody titre.

Notifications of RRV infection in residents of Sorell Municipal Area, and in individuals that had travelled to the area within two weeks before the onset of symptoms were interviewed. The possible site of infection was determined by comparing residential address with proximity to known larval habitats. Demographic details of local cases were examined to determine if any particular group was at a greater risk. Infections acquired on the western side of Pittwater were not included in this study as that area is within the City of Clarence (which reported 50 of the 117 cases notified state-wide). The City of Clarence is located between Hobart and Sorell. Pittwater is an estuary extending from Richmond (City of Clarence) to Lewisham (Sorell Municipal Area). Pittwater contains the coastal towns of Midway Point and Sorell (Figure 1).

Figure 1. Adult and larval sampling locations

Habitat investigation and mosquito surveillance

The purpose of the study was to determine which local mosquito species were the likely vectors of RRV.

Aquatic habitats were explored in the municipal area (west to east) at Penna, Midway Point, Lewisham, Dodges Ferry, Carlton, Primrose Sands, Connellys Marsh, Dunalley and Marion Bay (Figure 1). Mosquito larvae were collected and quantified by dip sampling techniques⁴ and identified to species using larval identification keys.⁵

Adult mosquito populations were sampled with dry ice-baited EVS light traps⁶ at seven sites; indicated in Figure 1. Traps were set late in the afternoon and collected early the following morning on 21 January, 20 February, 25 February, and 5 March 2002. Trapping days were selected considering climatic conditions and transport logistics for sending the mosquitoes to Sydney for virus isolation. All mosquitoes collected were sent for virus isolation testing.

Traps were set within coastal towns with the exception of the Carlton River site, which was set on the edge of the saltmarsh habitat. Areas where complaints about excessive numbers of mosquitoes had been received from local residents were also selected. Trapping focused around Carlton River where larval surveying revealed larger saltmarsh habitat areas. Trapping expanded to Dunalley, Connelly's Marsh, Dodges Ferry and Midway Point to establish the extent of distribution into residential areas.

Arbovirus isolation

The mosquito collections were transported live to the Institute of Clinical Pathology and Medical Research, Sydney, for processing. Mosquitoes were sorted and identified^4,5,7,8 on a refrigerated table, into pools of up to 25, according to species, sex, bloodfed, date and site.⁹ Each pool was placed into a sterile plastic 5 mL tube containing 5 x 5 mm glass beads and 5 mL of cell growth media, macerated in a grinder/shaker for 20 minutes and centrifuged at 4,000 rpm and 4° C for 20 minutes, and a 50 UL aliquot of supernatant from each tube was inoculated into the cell lines C6/36, BHK and PSEK, as previously described.^9,10

The presence of viruses was indicated by cell death. Presumptive viral isolates were tested by Fixed Cell Enzyme-Linked Immunosorbent Assay (FCE)¹¹ using specific monoclonal antibodies to detect alphaviruses (Ross River, Sindbis, Barmah Forest viruses) and flaviviruses (Murray Valley encephalitis, Kunjin, Stratford, Alfuy, Edge Hill, Kokobera). As RRV was detected in the mosquito samples, a RRV FCE screen was performed on later samples to rapidly identify virus. Minimum infection rates (MIRs) per 1,000 were calculated¹² per trap for the species that yielded virus.

Meteorological conditions

Rainfall data were recorded at the Park Beach Weather Station (Dodges Ferry), the closest weather station to Carlton River. Temperature data at Hobart Airport were obtained from the Bureau of Meteorology.

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Results

Human cases

In 2002, a total of 117 cases of RRV were notified in Tasmania, with 19 of these resident in the Sorell Municipal Area.¹³ Another 18 had travelled to the municipal area within two weeks prior to onset of symptoms and may have been infected during this period. The Department of Health and Human Services reported that more than 90 per cent of all cases notified were from single sample IgM results. Single sample IgM serology does not provide absolute certainty that infections occurred recently and cases notified should be regarded as presumptive.¹⁴ However, in combination with clinical symptoms and RRV isolates from local mosquitoes, it is unlikely that a large number of previous infections are included within the 37 cases.

Of the 37 cases that had some association with the Sorell Municipal Area, 23 lived in, or had visited, the Carlton River, eight were from Pittwater, four were from Marion Bay and the other two cases were Sorell Municipal Area residents that had links with sites in the City of Clarence. RRV infection onset dates ranged from 22 February 2002 to 10 May 2002. Generally, infections linked to the Carlton River area occurred first and continued throughout the period, with cases from the Penna area occurring in late-March and those at Dunalley/Marion Bay in mid-April.

There were 20 males and 17 females with infections that could have been acquired locally. The average age of patients was 47 years (range 8 to 81); the majority (21) were in the 30-49 years age range, with three under 30 years and three over 70 years.

In addition to the cases reported in the Sorell Municipal Area another 50 RRV notifications were reported from the residents of the City of Clarence. Of these 50 cases, 22 lived in, or travelled to, the western Pittwater towns of Seven Mile Beach, Acton, Cambridge or Richmond.

Of the 117 cases in Tasmania, 105 lived in So