Sydney University Department of Medical Entomology Westmead Hospital
Ross River & Barmah Forest
Overview Natural History
Symptoms Laboratory Diagnosis
Treatment Prevention
Further Reading

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Overview

Traditionally known as Epidemic Polyarthritis, both Ross River (RR) and Barmah Forest (BF) disease are caused by viruses which are transmitted to humans through the bite of mosquitoes. A wide variety of symptoms may occur from rashes with fevers, to arthritis that can last from months to years with RR virus infection. There are no specific treatments and actions which reduce mosquito bites are the best form of prevention against these debilitating diseases.


Natural History

RR disease is the most commonly transmitted mosquito-borne viral disease to humans in Australia. The number of cases has averaged >5,000 per annum during 1991-1997. The virus appears to be endemic in most rural areas, and there has been an increasing incidence near major cities. BF disease is less common, but the number of cases appears to be increasing annually, with several outbreaks occurring during the1990's. (For annual number of cases, visit the National Notifiable Diseases Surveillance System web page:
http://www.health.gov.au/pubhlth/cdi/nndss/nndss2.htm).

For most of Australia, peak incidence of the diseases is through the summer and autumn months, particularly from January through to March, when the mosquito vectors are most abundant. However, in southwestern Australia and eastern Victoria, RR activity often begins in the spring months and peaks in early summer. Areas under intensive irrigation and localities close to saltmarshes, are most productive for mosquito populations and hence tend to result in the highest number of human cases of disease. Outbreaks occur when local conditions of rainfall, tides and temperature promote vector abundance.

Serological studies and laboratory investigations have indicated that native mammals, most likely kangaroos and wallabies, are natural hosts for RR virus but little is known about the hosts of Barmah Forest virus.

RR virus transmission from human to mosquito to human (thus occurring without the involvement of an animal) has been proposed, and there is now little doubt that such a cycle involving only humans and mosquitoes occurs during periods of intense virus activity.

RR and BF viruses have been isolated from many mosquito species, indicating wide susceptibility among mosquitoes. In inland regions, the major vector is Culex annulirostris which breeds in freshwater habitats, especially in irrigated areas. Along coastal regions, saltmarsh mosquitoes represent the major threat, including Aedes vigilax and Ae. camptorhynchus in northern and southern coastal regions respectively. There is some evidence that 'floodwater' Aedes species such as Ae. normanensis play an important role in transmission in inland regions following heavy rains or floods, and Coquillettidia linealis is a secondary vector in areas with established wetlands. In the domestic urban situation, there is evidence to suggest that Ae. notoscriptus may be a vector, while Cx. quinquefasciatus is not.


Symptoms

Human infection with RR virus or BF virus, may result in the clinical condition known as polyarthritis. The effects range from a symptomless condition, through a transient rash and mild illness with fever, to polyarthritis affecting chiefly the ankles, fingers, knees, and wrists, but other joints may be affected. The disease is not fatal. For RR virus, symptoms become evident from 3-21 days (average 9 days) after infection, and mild cases may recover in less than one month but many persist for months to years. Recent studies have indicated that the rash may be more florid with BF virus infections but that the arthritic symptoms are greater with RR virus infection. People of working age are most likely to be afflicted with the diseases, whilst symptoms are rare in children.


Laboratory Diagnosis

A variety of blood tests are used to demonstrate the presence of specific antibodies to RR and BF virus. Blood samples should be taken during the acute and convalescent phases of the illness, and a fourfold rise in antibody levels will confirm the clinical diagnosis.


Treatment

Specific therapies do not exist to treat the disease, rather it is the symptoms that are alleviated. This includes various analgesics to reduce the pain and fevers, and anti-inflammatory agents for the arthritic symptoms.


Prevention

Prevention of mosquito borne viral diseases is mainly accomplished through reducing the threat of bites from mosquitoes. This can be achieved either through undertaking active mosquito control or by the use of personal protective measures. A variety of active mosquito control measures are available including habitat modification in order to reduce water availability for breeding of the larval stage, through to the use of appropriate insecticides for controlling the larval or adult stage. These are large scale control measures which can only be undertaken by government bodies, generally local councils. On a small scale, householders can ensure that their own backyard does not contain water holding containers which can provide suitable mosquito larval habitats (e.g. undrained pot plants, blocked gutters, disused bottles, old tyres, etc). Personal protective measures include: avoiding known mosquito infested areas, especially at dawn and dusk when mosquitoes are most active; ensuring that houses are adequately screened; using insect repellents that contain the chemical DEET, and reapplying it regularly; and wearing long sleeved shirts and pants.

Other preventative measures include government based programs that undertake mosquito monitoring and virus surveillance from mosquitoes. These programs aim to act as an early warning system for virus activity by monitoring mosquito populations, viruses such as Ross River or Barmah Forest and weather patterns. In New South Wales, such a program has been running for several years at the Department of Medical Entomology at Westmead Hospital and is funded by the NSW Health Department.

 
Further Reading

Boughton, C.R. (1996). Australian Arboviruses of Medical Importance. Royal Australian College of General Practitioners, Melbourne, pp 67.

DOGGETT, S., RUSSELL, R., Cloonan, M., Clancy, J. and Haniotis, J. (1995). Arbovirus and mosquito activity on the south coast of New South Wales, 1994-95. Communicable Diseases Intelligence, 19: 473-475.

Kay, B.H. and Aaskov, J.G. (1988). Ross River Virus Disease (Epidemic Polyarthritis). in Monath, T. (ed). The Arboviruses: Epidemiology and Ecology, Volume IV. CRC Press, Florida, pg: 93-112.

RUSSELL, R.C. (1995). Arboviruses and their vectors in Australia: an update on the ecology and epidemiology of some mosquito-borne arboviruses. Review of Medical and Veterinary Entomology, 83: 141-158.

 Assoc. Prof. Richard C. Russell and Stephen L. Doggett

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