MOSQUITO-TRANSMITTED VIRUSES

Saltwater environments

Ross River disease

Ross River 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. 

For most of Australia, peak incidence of the disease 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, Ross River 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 Ross River virus RR 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. quinquefasciatusis not. 

Human infection with RR 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. 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. People of working age are most likely to be afflicted with the diseases, whilst symptoms are rare in children. 

A variety of blood tests are used to demonstrate the presence of specific antibodies to Ross River 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. 

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. 

Barmah Forest disease

Barmah Forest disease is similar to Ross River disease in terms of symptoms, vectors, peak incidence, laboratory diagnosis and treatment. Barmah Forest disease is less common than Ross River disease, but the number of cases appears to be increasing annually, with several outbreaks occurring during the 1990'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). Little is known about the hosts of Barmah Forest virus. Recent studies have also indicated that the rash may be more florid with Barmah Forest virus infections but that the arthritic symptoms are greater with Ross River virus infection.

Freshwater environments

Australian/Murray Valley encephalitis

‘Australian encephalitis’, or ‘Murray Valley encephalitis’ are synonyms for a clinical syndrome caused by infection with Murray Valley encephalitis (MVE) virus or Kunjin (KUN) virus. The major mosquito vector is Culex annulirostris(which breeds in freshwater environments in northern regions of Australia). Epidemic activity in the southeast has been associated with excessive rainfall which increases bird and mosquito populations and leads to a virus overflow infecting humans. 

Symptoms are variable, from mild to severe with permanent impaired neurological functions, to sometimes fatal. Symptoms almost invariably include a sudden onset of fever; anorexia and headache are common, while vomiting, nausea, diarrhoea and dizziness may also be experienced. Brain dysfunction may be experienced after a few days with lethargy, irritability, drowsiness, confusion, convulsions and fits; neck stiffness can be expected, and both coma and death may ensue. It is rare for recovery from the encephalitic syndrome to occur without some residual mental or functional disability. As there are no specific therapies to treat the disease or control the virus, supportive treatments are used (such as respiratory support in severe disease). Mosquito avoidance measures are the best form of prevention. 

Malaria

This disease in humans results from infection with a protozoan blood parasite transmitted by  a  species  of  the mosquito  genus  Anopheles.   Malaria is characterised clinically by fever (usually periodic), varying degrees of anaemia and splenic enlargement, and a range of syndromes resulting from the physiological and pathological involvement of certain organs, including the brain, liver and the kidneys. In Australia, malaria has been endemic, but the malaria was declared eradicated from the country in 1981. Little is known of local vectors because few outbreaks were studied. Anopheles farauti is a major vector of malaria in Papua New Guinea, and it is presumed to be the species of greatest concern in the north of Australia. In southern Australia, An. annulipes has apparently been the vector where occasional cases of malaria have been contracted. The region of northern Australia above 19^oS latitude is the receptive zone for malaria transmission and vigilance is required to prevent reestablishment of the infection in this region.