Links between dryland salinity, mosquito vectors, and Ross River Virus disease in southern inland Queensland--an example and potential implications.
The extent and importance of secondary salinity (salinity induced by the activities of man) in Australia has been widely discussed for many decades, and summarised at the national scale in documents such as the National Land and Water Resources Audits (NLWRA 2001). Large funding initiatives, such as the National Action Plan for Salinity and Water Quality and the Murray Darling Basin Salinity Management Strategy, are currently aimed at reducing both the extent and impact of secondary salinity, particularly in agricultural areas. While the impacts of salinity on agricultural production, and to a lesser extent infrastructure, have been described around Australia, little linkage has been made to public health issues. In 2004, Warwick Shire Council (WSC), in southern Queensland (Fig. 1), recorded a high incidence of Ross River Virus (RRV) disease, with an apparent relationship to a peri-urban dryland salinity expression. This short communication highlights the apparent relationship, and the need for further investigations to validate the role of dryland salinity expressions as potential habitat for RRV vector species in southern Queensland.
Dryland salinity is a significant problem in several key landscapes in Queensland, particularly in south-eastern Queensland--from Bundaberg to Beaudesert and inland to the Darling Downs and Burnett. Extensive secondary salinity expressions are particularly associated with the sandstones of the Marburg Subgroup, the Walloon Coal Measures, and overlying basalts (Salcon 1997). These sandstones occur in both the Lockyer Valley and the Darling Downs (Cranfield and Schwarzbock 1971). Many authors, including Hughes (1979, 1986), Biggs and Power (2003), and Searle et al. (2007) have previously described the general nature and extent of salinity outbreaks on the Darling Downs, with the latter two making specific reference to dryland salinity located on the sandstones of the Marburg Subgroup around the town of Warwick. They described an extensive valley-floor outbreak on the western side of the town (Little Warner/Newby Streets), commenting that it was the largest of a number in Warwick.
Ross River Virus is a non-fatal, but potentially debilitating mosquito transmitted virus, endemic to Australia and in particular Queensland. Hosts include macropods, and the major proven vectors include the salt marsh mosquito (Aedes vigilax syn. Ochlerotatus vigilax, Skuse), common banded mosquito (Culex annulirostris, Skuse), and Aedes camptorhynchus syn. Ochlerotatus camptorhynchus (Dale and Knight 2006; Lindsay et al. 2007). Ae. vigilax and C. annulirostris are common vectors of RRV in the Brisbane area (Hu et al. 2006). Linkages between RRV and various natural resource variables, including climate, have been explored by several authors (Gatton et al. 2004, 2005; Kelly-Hope et al. 2004; Ryan et al. 2006). There has, however, been little exploration of potential linkages between secondary salinity and RRV vector species which prefer saline habitats.
The relationships between mosquito management and management of certain soil types has been discussed by authors such as Hey (2000), who described mosquito problems associated with acid sulfate soils in coastal areas of Queensland. Like dryland salinity outbreaks, acid sulfate soils are a function of human disturbance of the landscape, and can lead to bare, saline areas with ponded water. Dale and Knight (2006) studied the effectiveness of different drainage treatments to reduce mosquito numbers in coastal salt marsh areas near Brisbane. Most recently, Lindsay et al. (2007) raised an apparent correlation between the prevalence of Ae. camptorhynchus and salt affected areas. The implication is that the increasing extent of salinity is leading to a reduction in freshwater mosquito species, with replacement by salt water species.
[FIGURE 1 OMITTED]
The site at Little Warner/Newby Streets is a long, narrow salinity expression located in a v-shaped valley. The outbreak is sporadic along the valley floor and in the adjacent footslopes. The catchment is heavily cleared, with a history of clearing/cultivation extending back to the 1800s. The hillslopes were originally used for small-crop/grain farming and dairying. In recent years, agricultural uses have declined, with the area becoming rural residential, and it now lies adjacent to an area of more densely settled urban development (Fig. 2). While not a large salt expression (approx. 5 ha), it has a considerable catchment area (approx. 500ha), and the site exhibits near continual groundwater seepage. Soils are Red to Yellow Chromosols in upper slopes, grading to mottled Yellow and Grey Sodosols in lower slopes (Biggs et al. 2001).
Groundwater baseflow is generally 20 000-30 000 [micro]S/cm. Water depth is <0.1 m in low-lying areas. The affected area varies spatially and temporally from bare scald, to a mixed cover of green couch (Cynodon dactylon), saltwater couch (Sporobolus virginicus), Rhodes grass (Chloris gayana), and salt-tolerant forbs such as Plantago coronopus. The core of the outbreak lies just upstream of a road that crosses the catchment, suggesting the road is partly the cause of the problem, by limiting the passage of shallow groundwater. The Little Warner/Newby St site is one of 6 known sites within the Warwick town area, but the only one to exhibit near-permanent groundwater seepage.
Entomological investigations and RRV incidence
In 2004, WSC recorded a high incidence (38 cases) of RRV disease (1), with 6 cases centred on the dryland salinity outbreak at Little Warner/Newby St. Statistics regarding incidence of RRV disease for the area (Table 1) indicate the 2 peaks in the last 10 years have been 1996 and 2004 (186 and 173 cases/100 000, respectively). A smaller peak occurred in 2006 (88 cases/100 000). The site of interest lies in the Warwick West SLA, but very close to the boundary of the Warwick Central SLA. In most other years, the Warwick West SLA recorded no cases.
[FIGURE 2 OMITTED]
Investigation by WSC and Queensland Health started in March 2004, following the higher than usual notification of cases of RRV disease, and complaints (regarding mosquitoes) from residents who lived near Little Warner/Newby St regarding mosquito problems. Light trapping and surveying of the areas surrounding the suburb complainants led investigators to the salt expression, which contained 2 large breeding sites (which were continuously inundated with baseflow) and several shallow depressions fringed by saltwater couch. Investigations confirmed !he presence of all stages of the salt marsh mosquito (Ae. vigilax) m a breeding area of approximately 3000 [m.sup.2] (Mottram and Fraser 2005). High numbers of Ae. vigilax larvae were found throughout the warmer months in the large breeding sites and eggs were collected under saltwater couch in most shallow depressions and drainage channels.
Larvicide was applied to the area immediately following initial investigations, as a short-term control to reduce the risk of RRV. Following 1 year of larval surveys (larvae and eggs) and treatment, the Council reduced the breeding areas by filling the 2 large breeding sites and surrounding depressions with waste soil. The remaining area which cannot be filled is treated with larvicide if Ae. vigilax larvae are found following heavy rain. It is worth noting that the impetus for contact between the authors was in fact the earthmoving activities at the site, which prompted the senior author to contact WSC, and thus discover that the stimulus for these activities was mosquito management.
A drain has been constructed through the centre of the landfill to increase the flow of water out of the discharge zone. While the drain is a short-term solution, this may have some long- term consequences. Increased flow of saline water out of the site is likely to cause downstream impacts. Furthermore, in-filling of the site will most likely create a dam effect, similar to the road, simply pushing the problem further up-catchment. The best long-term solution requires a decrease in groundwater recharge within the catchment, necessitating some strategic revegetation. As a result of discussion with all parties, council and landholders are commencing a plan of action to progress revegetation in the valley.
[FIGURE 3 OMITTED]
As suggested by Lindsay et al. (2007), there is a logical link between increasing presence of dryland salinity, increased habitat for saltwater mosquito species, and increased likelihood of transmission of RRV. Kelly-Hope et al. (2004) and Gatton et al. (2005) have demonstrated linkages between climatic variables such as rainfall, and prevalence of RRV. Such climatic variables also have strong relationships with the prevalence of dryland salinity in Queensland (Salcon 1997; Biggs and Power 2003).
The size and number of salt expressions in southern Queensland increases during wet (La Nina) phases, and decreases during dry (El Nino) phases. Large rainfall events (e.g. 300-400mm experienced in May 1996 on the Darling Downs) and associated flooding can lead to rapid recharge of landscapes, and trigger the development of salinity expressions, or the expansion of existing sites. Larger baseflow fed sites like the Little Warner/Newby Street site often have a considerable time lag between the commencement of wet phases and decline/cessation of discharge, due to the transmission times of groundwater through the landscape. For example, groundwater levels at a salinity expression at Umbiram in the central Darling Downs took 5 years to decline following the 1996 wet period. This can result in these sites actively discharging during lower rainfall periods, a time when smaller sites often cease discharging. Recharge/discharge time lags for the Warwick site have not been specifically measured, but field observations suggest it could be as long as 12 months. Comparison of rainfall records with RRV disease notifications for the area (Fig. 3) indicate there is no clear correlation between rainfall and RRV disease incidence, suggesting factors such as recharge/discharge lag and others are important.
While predictions regarding the increase in secondary salinity are highly varied across Queensland, the co-location of existing salinity expressions with rural-residential/urban populations may be a critical factor in exposure to RRV--the prevalence of RRV in coastal suburbs of Brisbane is clearly linked to the associated habitat for vector species. The description of Ae. vigilax at the Warwick site is the first record that the species has fully established at a southern Queensland inland town, about 160 km from the nearest coastal salt marshes, and on the western foothills of the Great Dividing Range. Ae. vigilax usually has a maximum flight range of 50 km, but it may be carried further by prevailing winds. A single specimen of Ae. vigilax was trapped at Tara, 292 km inland of Brisbane (Marks 1982).
[FIGURE 4 OMITTED]
Figure 4 shows the occurrence of known secondary salinity expressions in Queensland. It is worth noting that south-east Queensland contains many hundreds of saline expressions in current and ex-agricultural lands. Many of these sites are hectares in size, with semi-permanent to permanent ponded saline water. Like the Darling Downs, south-east Queensland has a long history of settlement and land-use change, and dryland salinity outbreaks have been observed for many decades. Expression size is primarily controlled by wet/dry climate phases. Salinity outbreaks occur in/adjacent to towns such as Crow's Nest, Marburg, Laidley, Hattonvale, Kingaroy, and Boonah, often (but not always) in the peri-urban outskirts where there has been recent expansion into old agricultural lands. Most of these locations are also on the eastern side of the Great Dividing Range, and closer to the coast than localities such as Warwick. Importantly, urban populations in southeast Queensland are expanding rapidly, and are predicted to continue expanding, resulting in further urbanisation of old agricultural areas containing existing salinity sites. Given the well-documented prevalence of RRV and associated vectors in Brisbane, it is suggested that there may be an increased exposure to potential habitat for mosquito vectors of RRV, as urban populations expand from Brisbane and Ipswich.
An established population of Ae. vigilax, a known RRV vector, was confirmed in southern inland Queensland at a secondary salinity expression during 2004. During that year, the SLA recorded the highest incidence of RRV disease for 10 years. While short-term control of the population has occurred, long-term solutions are required to reduce the potential habitat area. It is suggested that future analyses of environmental factors influencing RRV incidence should consider the presence of secondary salinity outbreaks in conjunction with climatic variables, and that further field investigations should occur into mosquito species associated with secondary salinity expressions in south-east Queensland.
The authors wish to acknowledge the assistance of Warwick Shire Council, Shawn Darr (NRW), Mark Silburn (NRW), Dan Brough (NRW), Louis van Slobbe (WSC), Matthew Fraser (WSC) and Kathy Piotrowski (Qld Health).
Manuscript received 10 May 2007, accepted 30 November 2007
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(1) Reported as the Warwick Statistical Local Area (SLA).
A. J. W. Biggs (A,B,D) and P. Mottram (C)
(A) Queensland Department of Natural Resources and Water, PO Box 318, Toowoomba, Qld 3150, Australia.
(B) School of Land, Crop and Food Sciences, University of Queensland, St Lucia, Qld 4067, Australia.
(C) Queensland Health, Communicable Diseases Branch, 147-163 Charlotte Street, Brisbane, Qld 4000, Australia.
(D) Corresponding author. Email: email@example.com
Table 1. RRV cases per 100 000 people for the Warwick Statistical Local Areas Warwick Warwick Warwick Warwick Warwick Central East North West total 1996 240 73 128 183 186 1997 27 0 42 0 19 1998 45 48 0 0 34 1999 18 0 41 0 14 2000 44 24 0 0 28 2001 62 0 40 31 42 2002 0 23 0 0 5 2003 26 0 0 0 14 2004 191 69 275 171 173 2005 8 0 0 55 13 2006 101 22 191 53 88
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|Title Annotation:||Short Communication|
|Author:||Biggs, A.J.W.; Mottram, P.|
|Publication:||Australian Journal of Soil Research|
|Date:||Feb 1, 2008|
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