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Significant Increase in Ross River Virus (RRV) in Rural Areas

NSW Health has recently issued a health warning for people to protect themselves against mosquitoes, following a rise in the number of cases of Ross River Virus in parts of NSW. Ross River Virus, also known as epidemic polyarthritis, is an arbovirus spread by mosquitoes. It is thought that the recent rains and warm weather have contributed to an increase in mosquito numbers, and being holiday season means people are spending more time outside.

Transmission

Serological and laboratory studies have shown that native mammals, such as kangaroos and wallabies, are natural hosts for the virus, which is then spread to humans via mosquito bites. It has been proposed that human to human transmission via mosquitoes may also occur during periods of high activity. There are many mosquito species capable of spreading the virus, with the most common in coastal areas being the saltmarsh mosquitoes, including Aedes vigilax and Aedes camptorhynchus in northern and southern coastal regions respectively. In the urban environment, there is evidence that Aedes notoscriptus may carry the virus.

Symptoms

Whilst not life-threatening, RRV can cause persistent and debilitating symptoms in some people. These include joint aches and pain, fever, chills, headaches and sometimes a rash, which usually disappears after 7-10 days. Symptoms may continue to reappear for several months after the initial onset of illness. There is no specific treatment but people with symptoms should see their doctor. Most people fully recover in time. Ross River Virus has shown seasonal increases over the last few years, with January to March usually having the largest number of cases, although in some areas this period can be extended from November to April.

graph/chart: Ross river virus infection notification in NSW residents by month of disease onset January 2002 to February 2006

In January, 167 cases were reported across NSW, an increase of 133 cases compared to January last year, and 68 more than December 2005. Cases in the Greater Southern, Greater Western and Hunter New England Areas have accounted for 133 of these cases. The South Eastern Sydney/Illawarra Area reported 15 cases of RRV in January this year, four from the Northern Sector (South Eastern Sydney) and 11 from the Southern Sector (Illawarra). In addition there were three cases of Barmah Forest Virus (a similar virus) reported in the Illawarra area. There were no cases reported for the same period last year.

Prevention

Mosquitoes may be attracted by factors such as body odour, lack of body hair, skin temperature, the amount of carbon dioxide we excrete and chemicals produced by microbes on our skin. It has also been suggested they may be attracted by perfume and deodorants, which may account for more females than males being bitten. Pregnant women also seem to be more attractive to mosquitoes.

The best way to avoid RRV is to avoid being bitten by mosquitoes, which are most active for two hours or so either side of dawn and dusk. During these times, if you cannot avoid being outside, wear loose fitting light coloured clothes, and use an insect repellent that contains DEET or picaridin. DEET is the common name for N, N-Diethyl-m-toluamide, a chemical which disrupts the ability of biting insects to detect the source of carbon dioxide - the gas naturally given off by our skin and in our breath - which is what attracts mosquitoes and other insects to us. DEET does not kill the insects, it simply repels them. Picaridin (also known as KBR3032 or Bayrepel) is an effective alternative to DEET, which works by blocking the insect’s ability to detect you by forming a barrier on your skin. Both DEET and Picaridin offer good protection against mosquitoes if the directions are followed, and have been shown to have little or no adverse effects. Pesticide products designed to kill insects should NEVER be applied to the skin. However, if you have mozzies inside, use a knockdown insecticide, always follow the instructions, and ensure window and door flyscreens are in good repair. Check around your home for places mozzies can breed, such as saucers under flowerpots, blocked gutters, overgrown ponds and old tyres. Mosquitoes can quickly breed in very small amounts of water so regular checking is crucial for mosquito control.

Monitoring and Surveillance

The NSW Health Department funds an ongoing mosquito monitoring and virus surveillance program through the Department of Medical Entomology at Westmead Hospital. This program acts as an early warning system for virus activity by monitoring mosquito populations, viruses and weather patterns.

Local Councils are also involved in management of mosquitoes, through trapping and local monitoring of species to gain information on the potential spread of mosquito borne diseases. Some councils produce brochures and pamphlets offering information and advice on mosquito control, and these are available by phoning your local council.

More information

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