Vigilance is urged for horse owners with the current wet summer conditions leading to increased mosquito activity and a heightened risk of mosquito-borne diseases. Any unusual neurological signs, such as depression, changes in temperament, incoordination, etc. should be reported to your veterinarian promptly.
In 2011, also a wet summer, over 200 horses with neurological symptoms in NSW and Victoria were diagnosed with Kunjin virus. While Kunjin virus has always been present in northern Australia, it had not previously been known to cause disease. It appears that Kunjin virus has mutated to a form that now does cause disease.
Other cases of neurological disease in horses have shown high levels of Murray Valley Encephalitis (MVE) antibodies, mainly in areas where sporadic human cases or small outbreaks of MVE occur every few years. This is usually at the end of the wet season.
Anti-bodies to both Kunjin and MVE viruses have recently been detected in sentinel chicken flocks, indicating recent exposure to these viruses. The chicken flocks are placed strategically along the Murray Valley by the NSW and Victorian DPI from November to the end of March with blood samples tested weekly.
What is Kunjin Virus and MVE?
Both Kunjin Virus and MVE are a type of arbovirus (flavivirus) spread by mosquitoes.
Waterbirds, especially herons and ibis act as a natural reservoir for Kunjin virus, and do not become sick when infected. Mosquitoes feed on the birds and then transfer the infection to horses. There has been no evidence to date that Kunjin virus has caused illness in either birds or humans in Australia.
The primary hosts of MVE virus during years of high virus activity are also believed to be wild water birds.
There is no known risk to humans from direct contact with horses infected with arboviruses in Australia. Hendra Virus, which can be contracted by humans from horses, is not related. It is caused by a morbillivirus and horses appear to be infected by eating food contaminated with flying fox (fruit bat) urine or birthing products.
If exposure to bats is suspected in horses showing neurological signs, contact your veterinarian immediately and avoid direct contact with the horse.
What are the clinical signs?
Neurological signs to watch for include:
- Reluctance to move
- Incoordination or unsteadiness - dragging the toes of the front feet and high stepping behind, or even pitching forward onto the nose and buckling at the knees when attempting to graze.
- Muscle twitching, swelling of the face, drooping lower lip
- Difficulty eating and drinking
Temperature usually remains in the normal range, which is an important differentiation from horses affected by Hendra Virus, where horses may present with a temperature of 40 degrees or more.
Although the signs are distressing, the majority of horses have recovered uneventfully, over a period of a few days to weeks. Around 10% of horses developed more severe disease and died in the first outbreak of Kunjin Virus.
Is there any treatment?
There is no specific treatment for viral infections in affected horses but prompt supportive therapy, including anti-inflammatory drugs and fluids, can help limit the severity and duration of the illness.
How can infection be prevented?
While completely eliminating the risk of infection is not possible, reducing the exposure of horses to mosquito bites will minimise the risks. This is achieved by rugging horses, including hoods, using suitable insecticides and keeping horses stabled at night when mosquitoes are most active.