Nervous Disease In Horses – NSW, Victoria, SA

Since February there have been an increased number of reports of NSW horses displaying unusual nervous signs. Similar cases have also been reported in Victoria and South Australia.

Laboratory testing suggests that a majority of cases are due to infection with one or more Australian strains of mosquito-borne Flaviviruses that includes viruses like Murray Valley encephalitis virus and Kunjin virus. All testing to date has ruled out Japanese encephalitis virus. NSW cases are widely distributed, mostly west of the Great Divide from Mungindi in the north to the Murray River, and including a significant cluster in the Hawkesbury Valley west of Sydney and also the Upper Hunter Valley. Early signs of infection may include depression or mild colic. These initial signs are followed by nervous signs include lack of coordination, high stepping in front, weakness in the hind quarters, muscle twitching and increased responsiveness to touch and sound. In some cases there has been facial paralysis or twitching, especially of the lips. Severely affected horses may fall repeatedly or develop convulsions. Good husbandry under veterinary supervision will assist in recovery. Testing And Results
As of 3 May 2011 samples from 252 affected horses have been received from NSW veterinarians at the virology laboratory at the Elizabeth MacArthur Agricultural Institute. The number of samples being submitted each week from new cases appears to be trending downwards. This condition is not related to Hendra. All NSW cases tested for Hendra virus have returned negative results. Flaviviruses are one of a number of groups of arboviruses (viruses carried by insects) that occur naturally in Australia. The very wet season across much of Australia has resulted in high insect numbers and an increased risk of infection. It is likely that many horses may be infected but only a small proportion of infected horses will become ill. This means that a positive blood test must be carefully interpreted. Repeat testing will be needed to show that antibody levels are rising. Samples are being tested at the Virology Laboratory at the Elizabeth Macarthur Agricultural Institute, Camden. Horse owners should try to reduce the exposure of their animals to insect bites to minimise the risk of infection and ensure they are in good condition so that they have a strong immune system. Both registered chemical treatments and physical barriers such as fly veils and rugs will assist to minimise insect bites. The horse is usually a 'dead-end' host for mosquito-borne arbovirus infections and is not considered a likely source of new infection for people or other horses. Owners who notice horses displaying unusual signs should contact their private veterinarian. Source: DPI NSW Murray Valley Encephalitis (MVE) Virus
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. Whilst antibodies against MVE virus can be demonstrated in a wide range of animals, including horses, pigs, marsupials, poultry and wild birds, there is no definitive evidence that MVE virus causes disease. Serological evidence of MVE virus has been detected in sentinel poultry in northern Victoria, as well as New South Wales, in early 2011. Cases of neurological disease in horses (depression, changes in temperament, incoordination, etc), in conjunction with high levels of MVE antibodies, have been observed sporadically in areas where MVE virus is known to have been active. As transmission relies on the bite of infected mosquitoes, humans are not at risk of infection from direct contact with, or eating meat from, sero-converted animals. 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. 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. Source: DPI Victoria Mosquito Borne Neurological Disease in Horses
Biosecurity SA is continuing to receive reports of horses showing neurological signs in South Australia. The number of cases reported has been reducing in the last few weeks with only five reported cases in the past 10 days. This is most likely due to the cooler weather and reduced mosquito numbers. Horses are being tested for a mosquito borne group of viruses called ‘flaviviruses’ that includes Kunjin Virus and Murray Valley Encephalitis virus (MVEV). Since the middle of February a total of 136 cases have had blood samples submitted for testing. Of these, 89 horses have presented with neurological signs and 43 horses with a more generalized illness. No more horses have been reported as being euthanased with neurological signs since mid-April. In all, seven horses have been euthanased which were suspected of flavivirus infection between February and May in SA. {sidebar id=3}Neurological cases are continuing to occur sporadically across SA. Maps showing the geographical distribution of these cases and their test results can be found on the Biosecurity SA Animal Health Website at the following address. http://www.pir.sa.gov.au/biosecuritysa/animalhealth/disease_surveillance Laboratory results so far have indicated that the majority of positive flavivirus serology is due to Kunjin Virus with a few cases of MVEV. The laboratory results also indicate that recent exposure to Ross River virus has also occurred in some horses. Laboratory testing on four horses that were euthanased and had the brains collected have returned results indicating Kunjin Virus infection. Horse owners should contact their local veterinarian if their horse is unwell. The vet can then work with Biosecurity SA to undertake a disease investigation on cases with neurological signs. Source: PIR SA