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limited data for the greater Townsville area (Kay et al.1996). Based on the prevalence of key vector species and their abundance and that of the viruses recovered, it was concluded that Big Bay, originally recommended as a prime site for recreational development by the Department of Local Government in 1985, actually presented lower risk than any other locality. Antill Creek also proved relatively safe in terms of mosquito-borne infections, whereas Toonpan during the wet season was a place to be avoided. Both Ross River and the environs of Townsville offered intermediate risk, the latter due to large numbers of saltmarsh mosquitoes breeding in intertidal wetlands. 9.5 Snails and swimmer’s itch Schistosome dermatitis, known as swimmer’s itch, is a common global problem for users of recreational swimming areas in water resource developments. The rash is caused by free living larvae called cercariae (Figure 9.4) of parasitic flukes which burrow into exposed parts of the body. Normally the life-cycle involves water birds such as ducks and pulmonate snails, so infection of humans is accidental. A large number of cercariae may penetrate the skin where they die but cause a localized allergic reaction in sensitized persons. In northern Australia, swimmer’s itch (Trichobilharzia) has been traditionally associated with Austropeplea (= Lymnaea) lessoni (= vinosa) although two planorbid snails, Amerianna carinata and Gyraulus stabilis, have also been identified as intermediate hosts in Lake Moondarra near Mt Isa, Queensland. Our recent data implicates Gyraulus gilberti at the Ross River dam. Snails are also commonly infected with other trematode cercariae, mainly echinostomes, strigeids/diplostomids and clinostomids.
Because of previous isolated reports of swimmer’s itch at the stage 1 lake, periodic surveys by sweep netting commenced in July 1990. For each of six localities covering three distinct habitat types (open bays within the lake, along the margins of permanent creeks, temporary ponds), 1 m quadrats containing all vegetation types were scoured for snails. Austropeplea snails were not present in the lake proper until February 1991, but in November 1990 they were first located in ponds along the north-east, east and south-western shorelines. Egg masses were often found attached to the undersides of nardoo (Marsilea mutica) and sometimes wrapped around the stalks and ventral surfaces of the water lily, Nymphaea gigantea. Thus its absence from the lake was attributed to the lack of established vegetation in the stage 2A lake, and from this we developed a working hypothesis that host snails were possibly vegetation-specific. Thus to facilitate recreational use, control of infected Austropeplea could be achieved by simply clearing the appropriate water plant. By July to August 1991, however, schistosome-infected Austropeplea were collected from various types of vegetation along the margins of Ross River, close to the lake. A few Amerianna and Gyraulus gilberti were found in Ti-Tree Bay contiguous with Big Bay and Round Island, which were still negative for snails. By February 1992, planorbids were present in all three habitat types, with Austropeplea in two, i.e. ponds and creeks around the lake. Until 1993, 2,365 snails were dissected to detect both patent and pre-patent Trichobilharzia infection. Four different species of snails were identified, size classed according to shell length or width using vernier calipers. Snails were crushed on a microscope slide or in a Petri dish with a few drops of water under a warm light. A heavy infection of cercariae is evident to the naked eye, but any worm-like animals were removed on to another slide, stained with two to three drops of 0.1 per cent neutral red dye, covered with a coverslip and examined microscopically. Schistosome cercariae are distinctive with their eye spots, forked tail and presence of oral and ventral suckers (see Figure 9.4). Schistosomes were recovered from 4.5 per cent and 1.7 per cent of Austropeplea and Gyraulus gilberti snails, but not from Amerianna nor Thiara. In terms of management solutions, several questions seemed paramount: • Which habitat types presented the greatest (and lowest) risk? • Which time of the year presented the greatest hazard? • Can certain indicators be used to predict infection? Statistical analysis of the presence and abundance of Austropeplea snails did not correlate with any particular vegetation type (Hurley et al. 1995) but was connected with vegetation generally. There was no clear-cut relationship with snail density and physicochemical parameters including temperature, biomass of periphyton or with percentage surface coverage. However, highest densities of Austropeplea lessoni (45/m and Amerianna
the dosages used to kill mosquitoes, non-target organisms are safe. Both of these products, especially Bacillus, break down quickly and should therefore present no hazard to water quality. Given that a site such as Big Bay may become a mecca for those wishing to swim, sail or even fish, a surveillance programme and some environmental modifications are necessary. The deeper open waters of this bay coupled with a vegetation-free foreshore as a buffer zone, perhaps 400 m on either side of a swimming zone, should minimize or even negate swimmer’s itch. Adjacent Hydrilla and other macrophytes will require clearing as these will also present a physical hazard to swimmers and watercraft. The monitoring programmes could ideally be done three to four weeks prior to extensive public usage to allow time for any remedial action. The prevalence of key mosquito species and of large Austropeplea (and Gyraulus and Amerianna) snails can be established quickly as can cercarial infection in the snails. If it is found necessary to establish infection rates in mosquitoes, the newly developed Ross River and Barmah Forest virus testing procedures using mosquito cell cultures and enzyme immunosorbent assay (Oliveira et al. 1995) would require six days processing time. This offers considerable economy over previous methods using intracerebral inoculation of baby suckling mice. We would suggest that prior to selected recreational events, especially those from March to May, the Water Supply Board should initiate the action shown in Figure 9.6. The information supplied in response to a request should be communicated to recreational users to ensure that they are aware of the risks. Perhaps mosquito, arbovirus, and schistosome status could be displayed in the same way as fire hazard status is commonly indicated. It would be remiss of us to generate the impression that we had all the answers to the Ross River dam. The stage 2A lake and its surrounds are undergoing a process of ecological change and realization of this must remain paramount. There are issues relating to mosquito biology and behaviour and to do with snail ecology generally that would repay further study. Thus further selective monitoring and research should not be forsaken. References
Virus isolations Mosquito collections obtained during most field trips to the north-west of Western Australia have been processed for virus isolation. Until 1985, virus isolation was undertaken by intracerebral inoculation of suckling mice, but this was then replaced by cell culture using C6/36 mosquito, PSEK, BHK and Vero cells. The use of cell culture has significantly reduced the overall virus isolation rate by largely excluding arboviruses, rhabdoviruses and most bunyaviruses, but is as effective as suckling mice for the isolation of flaviviruses and alphaviruses. MVE virus has been isolated every year that significant numbers of adult mosquitoes have been processed except 1983 (Broom et al. 1989; Broom et al. 1992; Mackenzie et al. 1994c). Isolations of MVE, Kunjin and other flaviviruses are shown in Table 8.2. There was a strong correlation between the number of virus isolates in any given year and the prevailing environmental conditions. Thus those years with a heavy, above average wet season rainfall and subsequent widespread flooding yielded large numbers of virus isolates (1981, 1991, 1993) compared with years with average or below average rainfall and with only localized flooding. Although most MVE virus isolates were obtained from Culex annulirostris mosquitoes, occasional isolates were also obtained from a variety of other species, including Culex quinquefasciatus, Culex palpalis, Aedes normanensis, Aedes pseudonormanensis, Aedes eidvoldensis, Aedes tremulus, Anopheles annulipes, Anopheles bancroftii, Anopheles amictus and Mansonia uniformis (cited in Mackenzie et al. 1994b; Mackenzie and Broom 1995), although the role of these species in natural transmission cycles has still to be determined. Virus carriage rates in Culex annulirostris mosquitoes are shown in Table 8.3 for the Ord River area (Kununurra–Wyndham) and Balgo and Billiluna in south-east Kimberley. Very high mosquito infection rates were observed in those years with above average rainfall. Virus spread and persistence Stanley (1979) suggested that viraemic waterbirds, which are often nomadic, may generate epidemic activity of MVE in south-east Australia and in the Pilbara region. In an attempt to understand the genesis of epidemic activity better, our laboratory initiated a long-term study in the arid south-east Kimberley area at Billiluna and Balgo, two Aboriginal communities on the northern edge of the Great Sandy Desert. Occasional cases of Australian encephalitis had occurred in both communities (1978, 1981). The studies have clearly shown that MVE virus activity only occurs following very heavy, widespread rainfall both locally and in the catchment area of the nearby watercourse, Sturt Creek, which results in extensive flooding across its floodplain (Broom et al. 1992). Localized flooding is insufficient to generate virus activity. Two possible explanations can be proposed to account for the reappearance of MVE virus activity when environmental conditions are suitable: either virus can be reintroduced into the area by viraemic waterbirds arriving from enzootic areas further north; or virus may
