Murray Valley Encephalitis Virus
The Murray Valley encephalitis (MVE) virus, (a flavivirus), is a type of arbovirus spread by mosquitoes. It has been associated with neurological disease in horses and humans. Although clinical cases are unusual in horses, it is important to recognise and seek professional assistance if you suspect infection.
Murray Valley encephalitis virus infection is known to occur in northern Australia where sporadic human cases or small outbreaks of MVE are seen every few years; usually at the end of the wet season.
Several outbreaks of MVE have occurred at irregular intervals in south-eastern Australia since 1917. Cases of neurological disease in horses and humans were detected most notably in 1974 and 2011, linked to heavy rainfall and subsequent flooding. MVE was implicated in both events, along with Kunjin and Ross River viruses in 2011.
Seasonal conditions in Victoria during the winter and spring of 2016 are again favourable for increased mosquito activity and potential increases in arboviral disease, so horse owners are asked to be vigilant.
Murray Valley encephalitis virus infection has been detected in a wide range of animals including horses, pigs, poultry, marsupials and wild birds. Whilst most infections do not cause clinical illness, some horses (and humans) have shown signs of depression, incoordination and other neurological signs.
The clinical signs of MVE also mimic a number of other illnesses, including notifiable diseases such as Hendra. It is therefore important to get an accurate diagnosis and wear suitable personal protective equipment when sampling or handling horses displaying neurological signs.
All Hendra investigations must be reported to Agriculture Victoria on the Emergency Animal Disease Watch Hotline: 1800 675 888.
Resources on the safe management of suspected Hendra virus cases are available from the Queensland Government.
Transmission of MVE virus involves an infected mosquito biting a suitable host. The primary vector during epidemics is the mosquito Culex annulirostris. Other mosquito species may be involved in other aspects of MVE virus ecology. As transmission relies on the bite of infected mosquitoes, humans are not at risk of infection from direct contact with infected animals.
The primary hosts of MVE virus in Victoria are believed to be wild water birds.
Avoiding contact with mosquitoes can be very challenging for horses in a stable or paddock environment. The following measures may assist:
- Remove sources of stagnant water such as old buckets, tubs, tyres, etc. to reduce breeding sites for mosquitoes.
- Use suitable repellents or protective coverings such as summer rugs/fly veils.
- Stable animals during peak periods of mosquito activity (i.e. during dusk and dawn).
- Use commercial mosquito traps.
Monitoring of MVE in Victoria
For more than 20 years, Agriculture Victoria has run an arbovirus monitoring program on behalf of the Department of Health and Human Services. Every year as part of the program, sentinel poultry are placed along the Murray River and monitored by regular blood testing for arboviral disease. These chickens are a valuable early warning system for MVE virus incursions into Victoria.
Murray Valley encephalitis is not a notifiable disease of horses in Victoria; however owners should discuss the health of their animals with their private veterinarian to determine a cause when illness is observed.
Agriculture Victoria is interested in the occurrence of MVE as part of our work to assist the Department of Health and Human Services in their human health activities. To facilitate this, private veterinarians, in some circumstances, can access assistance to undertake disease investigations. Further information and submissions forms are available under the Equine Arborvirus disease project section on the Arbovirus in Horses webpage.
Please contact your private veterinary practitioner, the District Veterinary Officer or Animal Health Officer at your local Agriculture Victoria office or the Customer Call Centre on 136 186.
For further information on MVE and related arboviral diseases in animals, please contact the District Veterinary Officer or Animal Health Officer at your local Agriculture Victoria office or call the Customer Call Centre on 136 186.