6 October 2000

Patients suffering from Ross River virus disease do not relapse, as is commonly thought. The results come from the PhD project of University of Queensland researcher Dr Dave Harley.

"There is currently no well understood mechanism by which Ross River virus disease could relapse. While my study only extended over about seven months, it showed progressive resolution of symptoms and signs over time. It is also consistent with other studies recently carried out in Brisbane, Townsville and Dubbo," Dr Harley said.

The study has also identified a series of risk factors that contribute to the likelihood of people in tropical environments contracting the disease. "The results show that repellents, citronella candles and mosquito coils all decrease risk," Dr Harley said.

Ross River virus is a mosquito-borne virus that is transmitted from other vertebrates (reservoir hosts) to humans. The virus has a complex ecology that differs in different geographical areas. There were many uncertainties regarding the importance of different vectors (the mosquitoes that transmit the virus) and reservoir hosts, Dr Harley said.

Ross River virus disease can cause joint pain (arthralgia) and sometimes joint swelling (arthritis). Several thousand people are infected every year in Australia. Studies conducted during the 1990s suggest that a significant proportion of people may have symptoms for a year or more.

"We trapped about 60,000 mosquitoes and processed them for virus isolation. We discovered five species of mosquitoes carrying the virus that had not previously been identified as carriers," Dr Harley said.

"There is also evidence that the spectacled flying fox may be a reservoir host for the virus, though its restricted distribution in Queensland means it is not likely to propagate the virus over wide geographical areas."

Forty-seven general practice patients suffering from Ross River virus disease were reviewed in their homes for periods of up to 197 days from symptom onset. The reviews included the administration of standard health questionnaires to track progress of the disease.

"Patients showed a consistent improvement in symptoms and signs of the disease over the period of follow-up rather than a relapsing, or indolent, course. This is in conflict with the received wisdom and folk mythology about Ross River virus," Dr Harley said.

In the first controlled study of risks for Ross River virus disease, Dr Harley identified a variety of factors that enhance or diminish the chance of people contracting the disease. As might be expected, camping does increase the risk. So does the presence of bromeliads on the coastal plain where they can hold water in which mosquitoes can breed.

"Wearing light-coloured clothing reduces the risk of catching the disease as those mosquitoes which bite during the day use visual clues and are attracted to dark colours," Dr Harley said.

Dr Harley is currently working in the Tropical Public Health Unit for Queensland Health.

For more information, contact Dr Dave Harley (telephone 07 4050 3605) or Shirley Glaister at UQ Communications (telephone 07 3365 2339) or email: communications@mailbox.uq.edu.au.