What is MVE?

Murray Valley encephalitis (MVE) is a rare disease caused by the Murray Valley encephalitis virus. It is spread to humans by infected mosquitoes. Most people with this infection remain completely well while others may only develop a mild illness with fever. A small proportion of those infected develop a severe brain infection called encephalitis.

MVE usually occurs in remote north-western Australia. In south eastern Australia, MVE is occasionally seen when heavy rainfall, flooding and hot weather favour bird and mosquito breeding.

In NSW, the pattern of disease over the last century has been outbreaks occurring decades apart, with no or very few cases identified in between. People who have visited or live in or around wetlands or rivers are most likely to be at risk. The Murray-Darling basin is sometimes affected but mosquitoes with the virus may also live in other rivers and wetlands around NSW.

Symptoms

Most MVE infections in people cause no symptoms.

Some people with MVE infections experience an illness with fever, headache, nausea, vomiting and loss of appetite, diarrhoea and muscle aches.

Rarely, the MVE virus can cause a severe brain infection known as encephalitis. The danger signs of MVE encephalitis include the following symptoms:

• severe headache
• neck stiffness
• sensitivity to bright lights (photophobia)
• drowsiness and confusion.

This can progress to cause trouble with coordination and speech, seizures, loss of consciousness, coma and even death. Some people who recover from MVE encephalitis are left with permanent neurological complications.

Spread

• The MVE virus is spread by the common banded mosquito, Culux annulirostris. This mosquito breeds in fresh water and tends to be found in spring, summer and autumn around rivers, natural wetlands and irrigation waters and along coastal areas of NSW. The mosquito is especially common around the Murray Darling River basin areas in NSW during summer.
  
  
• This mosquito tends to be most active at sunset and in the first few hours of the evening and again around dawn.
  
  
• The virus infects some water bird species such as the Rufous Night Heron. Mosquitoes become infected by feeding on infected birds. An infected mosquito can then bite a human and transmit the infection.
  
  
• People with Murray Valley encephalitis infection do not transmit the infection to other people or to mosquitoes and people with MVE infection develop long lasting immunity that is probably life-long.
  
  
• The virus can infect animals such as horses, kangaroos and non-water birds. MVE cannot be passed from these animals to humans.

Who is at Risk?

People most at risk include people who have recently been bitten by mosquitoes in areas where MVE is circulating in local water birds and in the mosquitoes that bite them. Many people who have lived for a long time in MVE-affected areas will be protected (immune) because they have been infected in the past.

When MVE is in local mosquitoes, some people are more likely to develop MVE infection because they have not been exposed before. This includes:

• Babies and young children
• People who are visiting or have recently moved to MVE-affected areas.

In NSW, the biggest risk for MVE appears to be in the warm months after there has been heavy rainfall or flooding. The greatest risk is in and around wild bird habitats where infected mosquitoes are most likely to live. This means that people who are bushwalking, camping, boating, fishing and bird-watching in MVE affected areas are at increased risk of infection.

NSW regularly test flocks of chickens located near known bird breeding sites for MVE during the mosquito breeding season. Detection of the virus in these "sentinel" chicken flocks or in mosquitoes is an early warning system that indicates that humans may be at increased risk of infection with MVE if they are bitten by mosquitoes.

Prevention

Avoid being bitten by mosquitoes. Mosquitoes that carry the virus are usually most active in the hours after sunset and again around dawn.

• When outside cover up as much as possible with light-coloured, loose-fitting clothing and covered footwear.
  
  
• Use an effective repellent on all exposed skin. Re-apply repellent within a few hours, as protection wears off from perspiration, particularly on hot nights. The best mosquito repellents contain Diethyl Toluamide (DEET) or Picaridin.
  
  
• The stronger the concentration of an insect repellent, the less frequently it will need to be applied to stop mosquito bites. Repellents containing low concentrations of DEET or Picaridin provide shorter periods of protection and need to be reapplied more frequently so it's important to read the product information.
  
  
• Topical repellents are not recommended for use on children under three months. Use of physical barriers such as netting of prams, cots and play areas is preferred. Repellents containing less than 10 per cent DEET or Picaridin are safe for older children if applied according instructions. Parents or carers should apply repellent.
  
  
• Light mosquito coils or use vapourising mats indoors. Devices that use light to attract and electrocute insects are not effective.
  
  
• Cover all windows, doors, chimneys, vents and other entrances with insect screens.
  
  
• When camping, use flyscreens on caravans and tents or sleep under mosquito nets.

Diagnosis

MVE infection is usually diagnosed from measuring levels of antibody in samples of blood or cerebro-spinal fluid or occasionally from detecting the virus nucleic acids in these samples. It can sometimes be difficult to distinguish recent infections from old infections from testing one specimen. Two samples of blood taken a week apart usually need to be tested to see if there has been an increase in the levels of antibody against the virus suggesting a recent infection.

Treatment

There is no specific treatment available for MVE. People with encephalitis require treatment in hospital, sometimes in intensive care.

Public Health Response

Laboratories diagnosing cases of Murray Valley encephalitis must notify the local public health unit. Once the infection is confirmed, public health unit staff will collect detailed information about where the person has recently travelled and where they are likely to have been exposed to infected mosquitoes. The main public health response is to give people advice about avoiding mosquito bites and to search for new cases of human infection. Sometimes planned outdoor events in or near high-risk areas may need to be cancelled or postponed.