National Wetlands Update 2012
Issue No. 20, February 2012
Wetlands and mosquitoes - reducing the public health risks to visitors
Cameron E Webb and Richard C Russell, Department of Medical Entomology, University of Sydney and Westmead Hospital
Mosquito (Culex annulirostris) (Stephen Doggett).
Mosquitoes are a natural part of Australian wetlands and while the ecological role of these small, but occasionally abundant, insects has yet to be fully understood, there is strong evidence to suggest that birds, bats, frogs, fish and invertebrates make a meal of mosquitoes from time to time. Notwithstanding the impacts of nuisance-biting, the risks associated with the transmission of disease-causing pathogens are a major concern. Balancing the management of mosquito populations, wetland health and the subsequent impacts on people within and around wetlands can be a challenging issue. Do wetland managers have a duty of care to protect tourists and recreational visitors from the risks of mosquito-borne disease?.
Ross River virus(RRV) and Barmah Forest virus(BFV) are responsible for approximately 5000 cases of human disease reported every year in Australia. However, as the symptoms (e.g. fever and rash, arthritic pain in the ankles, fingers, knees and wrist) can vary greatly between individuals, many infections go undiagnosed, resulting in a potentially greater impact on human health than the statistics report.
While neither (RRV) nor (BFV) is fatal, human illness caused by infection by the Murray Valley encephalitis virus (MVEV) can be potentially fatal. Only a small proportion of (MVEV) infections result in symptoms (e.g. fever, headache, vomiting, nausea, diarrhoea and dizziness) but in severe cases, encephalitis can occur and amongst those who survive, some residual mental or functional disability has been reported. For the closely related Kunjin virus (KUNV), there are fewer human cases reported, the disease is milder and there are no known fatalities resulting from the infection.
From an ecotourism perspective, the flooding of wetlands and increased bird activity that attract visitors can also increase local risks of mosquito-borne disease. In the case of (MVEV), the primary reservoir host is generally thought to be wading birds, with the freshwater mosquito Culex annulirostris considered the major vector. The activity of (MVEV) is generally considered to be endemic in the north-west of Australia while in the south-east, epidemics occur in association with the flooding of wetlands in the Darling/Murrumbidgee/Murray river systems of NSW, Victoria and South Australia that provides suitable conditions for both birds and mosquitoes.
Fortunately, there have only been a small number of human cases of disease caused by (MVEV) in south-eastern Australia since the last large epidemic in 1974. The re-emergence of (MVEV) following flooding during the summer of 2010-11 has raised specific concerns regarding the potential impact on communities close to wetlands as well as visitors to the regions drawn by the prospect of visiting flooded wetlands and abundant bird life.
Historically, the incidence of disease in many areas is incredibly low due to the very low density of human populations and also because many of those individuals are lifelong residents and have developed immunity to these pathogens. Tourists visiting these areas, however, may be at a potentially higher risk given their lack of previous exposure to mosquito-borne pathogens and the immunity that brings, not to mention a lack of knowledge regarding mosquito-borne disease risk.
While there is a range of environmentally sensitive mosquito control agents registered for use in Australia, the broad scale nature of control activities that would be required to reduce mosquito populations is not sustainable. Visitors to these wetlands, particularly during periods of abundant mosquito and bird activity, should be made aware of the risks and provided advice on suitable personal protection strategies. The use of bed nets when camping, wearing suitable clothing (i.e. long sleeved shirts, long pants and covered shoes) that provides physical protection from biting mosquitoes and the use of an effective mosquito repellent (i.e. a registered repellent containing the active ingredients diethyltoluamide or picaridin) will all assist in minimising the risks of mosquito-borne disease.
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