Sydney University Department of Medical Entomology Westmead Hospital
Murray Valley Encephalitis virus & Kunjin virus
  
Overview Natural History
Symptoms Laboratory Diagnosis
Treatment Prevention
Further Reading

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Overview

‘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. Symptoms are variable, from mild to severe with permanent impaired neurological functions, to sometimes fatal. Mosquito avoidance measures are the best form of prevention.
 

Natural History

The term 'Australian encephalitis' (AE) has been used to indicate encephalitis induced by infection with either Murray Valley encephalitis (MVE) and/or Kunjin (KUN) virus. However, they are different viruses, with slightly different clinical symptoms and so the infections should be named after the causative virus (i.e. MVE disease/KUN disease).

The first reports of disease that might be attributed to MVE infection in humans in Australia occurred in southeastern Australia in 1917, 1918 and 1925 (114, 67 and 10 cases respectively), and were described under the title of "Australian 'X' disease". A virus, designated Murray Valley encephalitis virus, was later isolated from fatal cases in an epidemic in 1951, when there were 48 cases (and 19 deaths), and this virus has been accepted as the causal agent of the earlier Australian 'X' disease outbreaks. The next major outbreak occurred in 1974; although 42 cases were recorded in the southeast (primarily in the Murray Valley region), human infections occurred in all mainland states, with a national total of 58 cases and 13 fatalities. Serological studies indicated that some non-fatal cases were due to KUN virus infection, although only MVE virus was associated with fatal cases.

Since 1974, cases have occurred sporadically in northern Australia and in the northwest of WA in particular, but there have been no cases of MVE recorded in southeastern Australia. However, in 1984 there was a case of non-fatal KUN encephalitis in northeastern Victoria and KUN infections were reported from two other areas in the Murray Valley, and in 1991 there were two KUN infections reported in southwestern NSW.

It is now generally acknowledged that both MVE and KUN viruses have a natural endemic cycle, which involves water birds as the vertebrate host and Culex annulirostris (which breeds in freshwater environments) as the major vector, in northern regions of Australia. The proposal that the virus is endemic in northern Australia is supported by serological studies in Queensland and in northern Western Australia. MVE virus activity has been shown to be continuous in the Kimberley region of Western Australia, and it appears that foci of MVE and KUN exist at least in northern Western Australia, and possibly in the NT and northern QLD. 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. It is still uncertain whether the virus/es are introduced to the southeast prior to outbreaks, or whether they are endemic there at undetectable levels and only become evident with periods of intense bird and mosquito breeding following extensive flooding. Certainly, there is an association of previous outbreaks in the southeast with extraordinary rainfall and widespread flooding in the eastern Australian watersheds and the Murray-Darling basin in particular.
 

Symptoms

For MVE virus infection, there is a high subclinical rate and perhaps only 1 in 500 or more infections becomes noticeably ill. Cases vary from the mild to severe and 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.
 

Laboratory Diagnosis

A variety of blood tests are used to demonstrate the presence of specific antibodies to MVE and KUN 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

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).
 

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 to control both the larval and adult mosquito stages. These are large scale control measures which can only be undertaken by government bodies, generally local councils. 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; 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 MVE or KUN 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.

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.

RUSSELL, R.C. (1993). Mosquitoes and Mosquito-Borne Disease in Southeastern Australia. Revised edition. Published by the Department of Medical Entomology, Westmead Hospital. xii + 310pp.

Marshall, I.D. (1988). Murray Valley and Kunjin Encephalitis. in Monath, T. (ed). The Arboviruses: Epidemiology and Ecology, Volume III. CRC Press, Florida, pg: 151-190.

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


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