Fact sheets
In Western Australia, the mosquito-borne diseases of most public health concern are the debilitating Ross River virus (RRV) disease and Barmah Forest virus (BFV) disease, as well as the rare, but potentially fatal Murray Valley encephalitis (MVE). They are caused by viruses that are transmitted by the bite of an infected mosquito.
Local cycles of dengue fever and malaria do not currently occur in WA.
In nature, RRV, BFV and MVE are passed back and forth between wildlife and some species (types) of mosquitoes. Humans can only catch these diseases through the bite of an infected mosquito. There are no vaccines or cures for these diseases; medical treatment is aimed at easing symptoms.
A doctor who diagnoses one of these diseases in a patient is required under the Health Act to notify the Department of Health. This information is increasing our knowledge about the distribution of these viruses and the environmental triggers for virus outbreaks. It also allows public health action to be taken, such as mosquito control measures or the issuing of health warnings.
RRV and BFV result in symptoms in fewer than one in three people bitten by an infected mosquito. Symptoms vary from person to person, but include painful and/or swollen joints, sore muscles, aching tendons, skin rashes, fever, tiredness, headaches and swollen lymph nodes. Less common symptoms include sore eyes, a sore throat, nausea, and tingling in the palms of the hands or soles of the feet. Because the symptoms may be similar to some rheumatic diseases or other viral diseases, a specific blood test organised by a doctor is the only reliable method of diagnosis.
MVE virus causes symptoms in approximately one in a thousand people that is bitten by an infected mosquito. Most do not develop any symptoms at all. In adults and older children, symptoms include fever, drowsiness, bad headache and stiff neck, nausea, muscle tremors and dizziness. In young children the symptoms include fever, floppiness, irritability, drowsiness and fits. The disease can be mild or severe, but in severe cases can lead to coma and death. People with suspected MVE should be taken to the nearest hospital.
Local governments carry out mosquito control programs in some areas, but it is not always possible when breeding areas are large or inaccessible. Therefore it is important for people to take the following measures to reduce the risk of contracting mosquito-borne disease:
- avoid being outdoors at dawn, dusk and at night when mosquitoes are active
- ensure insect screens are installed and use mosquito nets and mosquito-proof tents
- wear insect repellent containing diethyl toluamide (DEET) or picaridin, as well as loose, light-coloured protective clothing when outdoors
- ensure infants and children are protected against mosquito bites, with suitable clothing, bed nets or other forms of insect screening.
RRV and BFV can occur anywhere in WA when conditions are warm enough for the viruses to be active and wet enough for the breeding of mosquitoes. In the northern half of WA, it is warm enough at any time of the year, whenever heavy rainfall or unusually high tides occur. Generally the risk is greater during and just after the wet season.
In the south of WA, RRV and BFV can be active between September and May if sufficient rainfall or unusually high tides occur. The period of activity is slightly shorter in the colder areas along the south coast and slightly longer in the warmer areas around Geraldton and Kalbarri.
In the southern half of WA the area of greatest risk for RRV and BFV disease is the coastal plain between Mandurah and Busselton during spring and summer. In some years this risk can extend to other areas within the southern half of the state, including Perth.
The incubation period (the time between being bitten by an infected mosquito and becoming sick) for RRV and BFV diseases varies from 3 to 21 days, but is normally 7 to 14 days. It may be possible for humans to pass the virus back to mosquitoes that bite them, but only during the incubation period.
MVE virus occurs in the northern half of Australia, during and in the months following heavy wet season rains. February to April is the season of most risk, but the risk can commence as early as December and extend to June or July in very wet years. MVE virus is active in the Kimberley and parts of the Pilbara during every wet season. It is occasionally active in the Gascoyne, Murchison, northern Goldfields and Midwest regions of WA, as well as in the Northern Territory, north Queensland and south-eastern Australia.
The incubation time for MVE varies from 5 to 15 days, but symptoms usually appear within 8 to 10 days after being infected.
People living, camping or recreating within 3-5km of saltmarshes, estuaries, tidal rivers and freshwater wetlands are at greater risk of RRV and BFV infection than people living further away. MVE infection is a greater risk near swamps, floodplains, river systems, irrigation areas and major dams.
For more information about mosquito management contact the Environmental Hazards Unit of the Environmental Health Directorate on (08) 9383 4244 or email eninfo@health.wa.gov.au
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Last Updated: 12 March 2008
