There has been a lot written about Japanese encephalitis and its current existence in Victoria and so I thought it would be topical to discuss the effect JE can have on horses.
Japanese Encephalitis (JE) virus is a flavivirus that can also be described as an arbovirus. An arbovirus, or an arthropod-borne virus, is one that is transmitted by hematophagous (meaning blood-eating) insects such as mosquitoes, sandflies, biting midges and ticks.
Horses and pigs are the two most common domestic animals that are susceptible to JE, however, horses are what are termed as “dead end hosts” for JE. This means that, although the horse is infected, the virus does not multiply in the horse’s blood or spread to other animals, as the amount of virus replicated in the blood is too small for a mosquito to ingest enough virus to reinfect another animal.
Pigs and waterbirds, however, are natural hosts and infection in these animals allows the virus to multiply and grow inside their body. Then, when bitten by a mosquito, the insect takes in enough virus to infect a horse or human, thus perpetuating the disease cycle in the environment. The virus cannot spread from one pig directly to another pig, or from one bird to another bird, and as with the horse, relies on the mosquito to transmit the infection and spread the disease.
Humans are also “dead end hosts” and can catch the virus when bitten by a mosquito carrying the disease, but cannot catch the virus from their horse or transmit the virus to their horse.
The incubation period, or time it takes to display symptoms once the horse has been bitten and infected with the virus, is usually 5-14 days. The disease in horses can be quite mild or subclinical and the horse may go through the active phase undiagnosed as the owner is unaware of any issues that alert them to the infection. Other cases show mild symptoms such as fever, anorexia or reduced feed intake, lethargy, pinpoint haemorrhages, and jaundice (yellow discoloration of the mucous membranes).
In a small percentage of horses, the predominant symptoms are neurological, with horses becoming uncoordinated, visually impaired, and unable to eat properly. Other clinical signs include teeth grinding, neck stiffness and paralysis of the limbs. In a small percentage of horses, JE produces a syndrome of hyperexcitability, where the horses profusely sweat and twitch due to muscle spasms, progressing to ataxia (uncoordinated movement) and recumbency (down on the ground), with death the outcome. The mortality rate for horses suffering JE is reported to be low (1-1.5%).
The course of the disease, or the duration that clinical symptoms are displayed, is typically around 2-3 days, with complete recovery achieved within 4-5 days of the initial signs, although there have been reports of some residual neurological signs persisting.
A provisional diagnosis of JE can be derived from the clinical symptoms seen in areas where JE is known to be endemic, however, this is not the case in Australia and more scientific data is required to make this diagnosis. Confirming the diagnosis is usually based on the detection of rising antibody titres to the virus, taken on blood samples 2-4 weeks apart, indicating the horse has been infected and the body has mounted an immunological response to fight the virus.
“There is no specific treatment for JE.”
Sometimes part of the virus can be detected in the fluid around the brain; however, collecting this fluid is far more difficult than taking a blood sample, so the latter is performed more frequently. In cases where the horse has died, tissue sampling is used to identify the viral components. If diagnosed here in Australia, the state government bodies are required to be informed as JE is classified as a “Notifiable Disease”.
There is no specific treatment for JE and any infection is treated symptomatically if required. By this I mean a horse may require nonspecific treatment such as phenylbutazone or flunixon if it spikes a high temperature, or a muscle relaxation or mild sedation if there are muscle spasms. Any horse showing signs of incoordination or impaired vision should be confined to a small “safe” area to prevent or minimise the risk of it injuring itself in fences or falling over objects in a paddock.
Prevention is based around controlling access of the mosquito to the horse using topical insecticide repellents, cotton rugs and fly veils. Horses can be stabled from dusk to dawn when mosquito activity is at its highest level. Stables can also be insect-proofed with netting, and the walls sprayed with an appropriate residual insecticide to further limit mosquito activity around horses. The lighting in stables should be turned off during the night or light sources changed to fluorescent types to discourage flying insects from invading the stable.
Owners should eliminate or reduce mosquito breeding grounds around their properties where possible, by removing stagnant water sources or fixing water leaks that can be havens for mosquitoes. Similarly, clearing debris and rubbish from around buildings and yards to prevent large numbers of mosquitoes propagating can assist.
In countries where JE is endemic, a vaccine is available and this is given as a two-dose injection protocol, four weeks apart. The vaccine is not available in Australia for routine use but is given to horses here when they are booked to travel to certain Asian countries where the virus is endemic. EQ
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