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Murray Valley Encephalitis (MVE) in a stock horse

Unusual presentation:

This was the second horse on a station to have persistent neurological signs in 3 weeks.

The investigation was approved as an SDI as: several notifiable diseases of horses can present as encephalitis; zoonoses were on the differential list; and there were another 80 plant horses potentially at risk.

Time and location: May 2018, Kimberley WA.

Case definition: Horses with initial signs of colic that become lethargic and develop neurological signs including head tremor, muscle twitching and stiffness.

Gross findings: The first horse had signs of colic then developed neurological signs typical of tetanus and died two days later (no post mortem). A vaccination program for tetanus was instigated for horses on the station. Three weeks later a 7 year old stock horse became significantly depressed and showed persistent neurological signs. The field diagnosis was a viral encephalitis (with flavivirus high on the list) or tetanus. The vet contacted the NABS Vet Adviser to do the investigation as an SDI.

Laboratory findings: Blood samples were submitted from the second horse to DDLS and forwarded on to AAHL to test for agents that cause viral encephalitis, specifically: Flaviviruses (MVE, JE, West Nile), Alphaviruses (Ross River Virus, WEE, VEE, EEE) and Hendra virus. The bloods contained antibodies for MVE and West Nile virus. The lab asked the vet to collect a second blood sample, which was taken 3 weeks later. The paired sample confirmed the diagnosis of MVE, demonstrating a four-fold increase in the antibody titre (in contrast to the other flavivirus titres). The vet was asked to contact the station to let them know of the MVE virus activity in their area.

Animal / management / environment risk factors: The MVE virus cycles between wild birds (especially water birds) and mosquitoes, with activity more likely in the wet season in parts of the north between December and June.

Horses and people only catch MVE after being bitten by an infected mosquito. Many arbovirus infections are asymptomatic. None of the other 80 plant horses around the station complex developed clinical signs. The affected horse made a gradual recovery in the weeks after the clinical examination.