Equine viral encephalomyelitis: how to spot and report the disease

Advice on what to do if you suspect there is an outbreak of this infectious disease. 

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Equine viral encephalomyelitis is an infectious mosquito-borne disease of horses, characterised clinically by paralysis and other signs of nervous derangement.

The virus can cause serious human disease as well as infecting poultry and other farmed birds including quails, ostriches and emus.

There are a number of strains of the virus, including:

  • epidemic Venezuelan equine encephalomyelitis 
  • western equine encephalomyelitis 
  • eastern equine encephalomyelitis 

Latest situation: there has never been an outbreak of equine viral encephalomyelitis in the UK.


Clinical signs

The incubation of the disease after infection with the virus is from one to three weeks. In the initial stage there is fever, which may be accompanied by depression, and loss of appetite, but the reaction may be so mild it goes unnoticed. The virus causing eastern equine encephalomyelitis is the most virulent of the three types and the symptoms produced are the most severe, with a case fatality rate of up to 90%. The viraemia (level of virus in the blood) may be so high with this strain that horse to mosquito to horse cycling can occur.

The nervous signs, when they appear, are hypersensitivity to sound and touch with periods of excitement and restlessness with apparent blindness. Affected horses may walk blindly into objects or walls. Muscle twitchings may occur in the face and shoulder muscles. A period of severe depression follows. Affected horses stand with their heads hung low and may have a half-chewed mouthful of feed hanging from their lips. The animal appears to be asleep and is unable to hold up his head and often rests it on some solid object.

Although Venezuelan equine encephalomyelitis does not cause as high a mortality as other strains, the clinical signs are similar. However, a second generalised infection may be caused by the virus causing fever, depression, colic and diarrhoea.

Post-mortem changes and clinical signs correspond to damage of the brain tissue caused by the virus. In Venezuelan equine encephalomyelitis there may be damage to other organs such as the pancreas, liver and heart.

If you suspect signs of any notifiable diseases, you must immediately notify your Scotland: field service local office at the Animal and Plant Health Agency (APHA). Failure to do so is an offence. 

How equine viral encephalomyelitis is spread

The disease distribution is determined by climatic conditions as well as agricultural practices, such as irrigation, which favour the life cycle and spread of mosquitoes.

The virus strains which cause equine viral encephalomyelitis are restricted to geographical areas of North, Central and South America. Venezuelan equine encephalomyelitis has spread into Central America, causing devastating epidemics as far north as Texas. 

Human health implications

Humans can be infected by equine encephalitis viruses if they are bitten by an infected mosquito. Most people have no symptoms.

How to control the disease

If the disease is confirmed the outbreak will be controlled in line with the contingency plan for exotic notifiable diseases.


The main legislation covering the control of equine viral encephalomyelitis, is the Infectious Diseases of Horses Order 1987.


The policy on disease control is that prevention is better than cure. This approach works by reducing the chances of a disease entering the animal population, and if it does then it can be quickly spotted and dealt with through the preventative measures.

Biosecurity is about being aware of the ways disease can spread, and taking every practical measure to minimise the risk of disease spreading.


If you suspect signs of any notifiable diseases, you must immediately notify your Scotland: field service local office at the Animal and Plant Health Agency (APHA). Failure to do so is an offence. 

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