Equine Viral Encephalomyelitis is an infectious mosquito-borne disease of horses, characterised clinically by paralysis and other signs of nervous derangement. The same virus strains can cause serious human disease as well as infecting poultry and other farmed birds including quail, ostriches and emus.
1 - Epidemic Venezuelan Equine Encephalomyelitis (VEE)
2 - Western Equine Encephalomyelitis (WEE)
3 - Eastern Equine Encephalomyelitis (EEE)
Birds, small mammals and possibly reptiles and amphibians are the natural reservoir of the virus. Transmission of infection to horses and man occurs following bites by mosquitoes or biting flies. The disease is not directly contagious between horses and man.
Equine Viral Encephalomyelitis is a notifiable disease. That means if you suspect it you must tell the Animal and Plant Health Agency (APHA) immediately. Failure to do so is an offence.
The virus strains which cause EVE are restricted to geographical areas of North, Central and South America as their names suggest. VEE has spread into Central America, causing devastating epidemics as far north as Texas. 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.
Clinical signs of disease
The incubation of the disease after infection with the virus is from 1 to 3 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 (EEE) 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 VEE does not cause as high a mortality as WEE/EEE, 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 VEE there may be damage to other organs such as the pancreas, liver and heart.
Blood samples and brain tissue samples can be used for diagnosis of the disease.