Equine Herpesvirus (EHV-1) and 1 (EHV-4)

(Equine Viral Rhinopneumonitis)

This is usually a disease of young animals, particularly foals, but can occur in naive older animals.  Herpesviruses require CLOSE CONTACT, and remember, ONCE INFECTED, ALWAYS INFECTED.  These viruses tend to spread rather slowly through a population of horses, unlike influenza. Based on the fact that this disease is a relatively low risk in the area where you practice and is included (however mininally) in the client’s vaccination schedule, put this one lower on the list as well.  She has had no problem with abortions in her mares.

Infectious agent: All Alphaherpesvirinae are enveloped, dsDNA viruses. There are 5 alpha herpesviruses that infect horses (EHV-1, 2, 3, 4, 5), but EHV-1 and EHV-4 are the ones that cause serious clinical disease in horses.  EHV-1 and 4 used to be considered subtypes of the same virus, but now are described as two different, but closely related ones.  EHV-4 is the respiratory agent, although it has been known to cause abortion and neurologic disease.  Likewise, EHV-1 is the reproductive disease that has been known to cause respiratory and neurologic disease. Confusing, yes? And to make things worse, the respiratory diseases caused by either EHV-1 or EHV-4 can look exactly alike.

History: Young animals under 2 years of age.  Occurs most frequently on breeding farms, sale or training facilities.  May occur more more frequently in the fall.

Clinical Signs: EHV-4 causes serous nasal discharge that is later mucoid or mucopurulent, possible mild coughing, increased lung sounds, fever; can cause abortion in older mares.  EHV-1 causes abortion and neurologic signs IN ADDITION to respiratory signs.

Incubation period:2-10 days

How is it transmitted?

IT REQUIRES CLOSE CONTACT, like moms and foals

Is LATENCY an issue?

Pathogenesis: Virus penetrates epithelium of upper and lower respiratory tracts within hours, replicates, then infects the local blood vessels and respiratory lymphoid tissues within 24 hours.  Immunosuppression occurs, which may predispose animal to secondary lower airway bacterial disease.

Diagnosis: Clinical signs of mild respiratory disease in young horses, viral isolation (rarely done) using nasopharyngeal swab taken early in course of disease OR find virus in tissues (aborted fetus) and apply flourescent or immunoperoxidase stain. Serology can be used, however, vaccination can cause high titers.   Distinguishing between EHV-1 and EHV-4 may be difficult due to the similarity in clinical signs between the two virus subtypes.  A type specific serologic test can distiguish between EHV-1 and 4.

Treatment and Prevention: In uncomplicated cases, complete recovery occurs in several weeks.  However, remember that herpes infection is for life, and reactivation of the virus can cause recurrent disease or continuous shedding!  Symptomatic treatment is best; avoid antibiotics unless complicated by secondary infection.  REST THE HORSE AND GRADUALLY RETURN IT TO WORK.  Isolate animals exhibiting signs, disinfect stable equipment, bedding (remove and burn it).  Booster vaccinations during outbreak may help.  Quarantine stable until 30 days after last new case is identified.

For prevention, EHV-1 and EHV-4 vaccines are available, but their efficacy at controlling respiratory disease is questionable.  Vaccination may at least reduce severity and duration of disease compared to unvaccinated horses.  Initially, 2 doses are given at 2-3 week intervals, then booster every 3 months to a year (each product is different).  For abortion protection, vaccinate mares at 5, 7, and 9 months gestation.

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