MURRAY VALLEY ENCEPHALITIS ALERT in NE Victoria. .(per DPI bulletins March 2011) Introduction

1. MURRAY VALLEY ENCEPHALITIS ALERT in NE Victoria. .(per DPI bulletins March 2011) Introduction Murray Valley encephalitis (MVE) virus is a type of arbovirus (flavivirus) spread by mosquitoes. It has the capacity to cause severe human disease, with encephalitis being the most notable clinical feature. MVE virus can be demonstrated in a wide range of animals, including horses, pigs, marsupials, poultry and wild birds. Recently cases of neurological disease in horses (depression, changes in temperament incoordination, etc), in conjunction with high levels of MVE antibodies, have however been observed sporadically in North East Victoria and the Ballarat areas. As transmission relies on the bite of infected mosquitoes, humans are not at risk of infection from direct contact with, or eating meat from, MVE positive animals. Veterinary practitioners should consider MVE virus infection as a differential diagnosis when investigating any horse displaying neurological signs, particularly those cases suggestive of viral encephalitis. Although MVE is not a notifiable animal disease in Victoria, the Victorian Department of Primary Industries (DPI) is interested in the occurrence of the disease to assist the Department of Health. Occurrence MVE virus infection is endemic in northern Australia where sporadic human cases or small outbreaks of MVE occur every few years. This is usually at the end of the wet season. Several outbreaks of MVE have occurred at irregular intervals in south-eastern Australia since 1917. There were 58 human cases reported from all mainland states in 1974, of which approximately half were in northern Victoria. Cases of neurological disease in horses in association with the 1974 outbreak were recorded and MVE virus was implicated. During these times there was heavy rainfall leading to widespread flooding which promoted large increases in water bird and vector mosquito populations. Since Jan 1st 2011, horses on over 90 properties in Victoria have been affected. A smaller number of cases have been observed in South Australia and New South Wales. Cases have been located throughout the northern and western parts of Victoria with two apparent clusters – one in the Murray/Goulburn Valley area and one extending south and west of Ballarat. There are two clinical syndromes dominating within these clusters; a neurological syndrome in the northern cluster and a musculoskeletal syndrome in the Ballarat cluster. The Ballarat cluster horses have been mildly affected and have generally recovered uneventfully. More severe signs have been observed in the northern cluster, and at least 12 deaths have been recorded.

2. Epidemiology Transmission involves the bite of infected mosquitoes and the primary vector during epidemics is the mosquito Culex annulirostris. Other mosquito species may be involved in other aspects of MVE virus ecology. The primary hosts in Victoria of MVE virus during years of high virus activity are believed to be wild water birds. Monitoring of MVE in Victoria For more than 20 years, DPI has run an arbovirus monitoring program on behalf of the Department of Health. This program provides an early warning of MVE incursions into Victoria. Every year as part of the program, sentinel poultry are placed at ten locations along the Murray River and monitored from the beginning of November to the end of March. Blood samples are collected weekly and tested for the presence of flavivirus antibodies. The DPI also participates in the National Arbovirus Monitoring Program (NAMP) which involves examining blood samples from five sentinel cattle herds at a number of sites around the state for the presence of antibodies to the important arboviruses of ruminants: bluetongue, akabane and bovine ephemeral fever viruses Terry Lowis 25th March 2011