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Equine Herpesvirus (EHV-1 and EHV-4)

“EHV-1” and “EHV-4” are closely related equine herpesviruses that can cause respiratory disease, abortion and neurological disease, with a dormant lifelong infection which can suddenly re-activate and result in renewed viral shedding (“EHV-3” is a venereal form of herpes, and is mainly transmitted via sexual intercourse).

EHV-1 and EHV-4 are endemic, or present in most horse populations throughout the world. Like EHV-4, EHV-1 can cause respiratory symptoms, but EHV-1 is also noted for its greater ability to cause abortions and neurological disease. EHV-1 neurologic or paralytic disease, also known as “EHM” (equine herpes virus myeloencephalopathy) is unique from other equine neurologic diseases, in that it can be spread by aerosol transmission directly from horse to horse without a vector such as a mosquito, making it a disease of concern for race tracks and horse shows.

EHV-1 and EHV-4 can be directly transmitted between horses by nasal or ocular discharges, aborted foetal material, or simply by direct aerosol transmission of the viral particles from horse to horse. The virus has an uncanny ability to lie dormant within an animal, so that no symptoms may be seen until some type of stress triggers its reactivation. Once a horse is an EHV-carrier, it remains infected with the virus throughout its life, which means that the virus can suddenly reactivate and cause disease symptoms, or that the horse can shed the virus sporadically, infecting nearby horses. These “silent carriers” who shed the disease without displaying symptoms can be a danger to any unprotected horse.

Vaccination has been proven to reduce clinical signs, and to decrease viral shedding. The reduction in viral shedding may reduce the incidence of abortion. Although no vaccine has been proven effective against equine herpesvirus neurologic disease, it is important to take measures to prevent or reduce the transmission of EHV-1 as part of a comprehensive protection strategy.

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  • EHV respiratory disease

    • biphasic temperature (102° – 106 °C)
    • anorexia
    • lethargy
    • lymph node enlargement
    • nasal and ocular discharge (“red eye”)

    EHV abortion, or neonatal disease

    • Infection to abortion interval varies from 2 wks – several months
    • Mare with silent respiratory infection suddenly aborts in 7th –11th month of pregnancy
    • Neonatal disease: respiratory or liver problems – poor prognosis

    EHV-1 neurologic disease (“EHM”)

    • Rapid onset similar to a stroke, followed by– rapid deterioration within 48 hours
    • Need not have respiratory symptoms, but may have fever
    • Head tilts
    • Drags toes
    • Ataxia, particularly hind leg weakness
    • “Floppy tail”– incontinence and /or difficulty in defecation
  • Equine herpesvirus disease should be considered on the basis of clinical symptoms and a history of abortion storms (many mares aborting on one farm). Respiratory disease is attributed to EHV-1 or EHV-4 based upon the identifying the virus from nasal swabs, or through blood serology. EHM can also be diagnosed using these methods, as well as through examination of the cerebral spinal fluid. In cases of EHV-abortion, the foetus and placenta are examined for pathological lesions, and the virus may be isolated from the foetal organs. All aborted foal foetuses and placentas should be retained and examined for EHV presence regardless of the suspicion of another possible cause of abortion.

  • Treatment for horses with EHV diseases revolves largely around supportive care based upon the symptoms displayed. Thick bedding and comfortable conditions are prerequisite, and broad-spectrum antibiotics, anti-inflammatory drugs, or anti-herpesvirus drugs may help to alleviate clinical symptoms. Horses with EHM may need to be put into a sling if they are unable to support themselves.

  • Management is a very important aspect in controlling EHV disease, coupled with a total herd vaccination programme. The main aims of EHV control should be the reduction clinical respiratory signs and the reduction of virus shedding.

    All new horses entering farm premises should be quarantined for 14-21 days, and their temperatures taken daily. Hygiene is essential, as viral particles could be transmitted between horses if they are carried on equipment, or on handlers’ clothing. Following transportation, all horse boxes should disinfected. Any horses displaying respiratory symptoms should be isolated until diagnosis, as the virus is very labile, and cannot travel any great distance. Aborting mares should also be placed in isolation.

    As natural immunity is very short-lived, vaccination is an integral part of an EHV control strategy, as it has been proven to reduce viral shedding, and reduce the incidence of abortions.

  • Will vaccination protect against neurological disease?
    All EHV-1 strains can cause respiratory disease and abortion, but some are “non-neurologic, and thus do not result in neurologic disease. No vaccine has been proven to protect against the virulent mutation of EHV-1 which results in this serious neurologic disease. Therefore, it is prudent to take every precaution possible, including vaccination, to reduce the virus circulating in an equine population.

    Should horses be vaccinated in the event of an EHV-neurologic outbreak nearby?
    There is not much point in waiting until an outbreak to vaccinate. Horses which are ill should never be vaccinated, and indeed vaccinating exposed horses in the midst of a neurologic outbreak would have little effect other than limiting viral shedding. Horses which were previously vaccinated, however, are at no greater risk in the event of an outbreak.