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.

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

Symptoms :

• EHV-4 causes serious nasal discharge that turns mucousy, mild coughing, increased lung sounds, fever; Older horses can become infected but these are more likely to be carriers and not show any signs of the disease, can cause abortion in older mares. 
• EHV-1 Foals and yearlings develop the respiratory form during the autumn and winter months - can cause abortion and neurologic signs, paralysis
• Typically there are no clinical signs in the mare other than abortion. Normally there are premonitory signs of an impending abortion, such as udder enlargement, mammary secretions and relaxation of the vulva.

Incubation period : The virus can survive for 14 - 45 days in the environment and transmission is via the respiratory tract or from aborted fetuses, membranes and fluid. Foals can also pass the infection onto healthy. Pregnant mare can abort their fetus from 4 months onwards although it is more apparent between the 8th and 11th months. The abortion can occur anywhere from 14 days to several months after initial infection.

Transmission : Close contact with infected horses, exposure to aborted fetus, placenta and/or fluids.

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 - nasal swab taken early in course of disease or find virus in tissues (aborted fetus) Distinguishing between EHV-1 and EHV-4 may be difficult due to the similarity in clinical signs between the two virus subtypes. Blood samples and swabs, taken from the throat of suspect cases, should be sent to a lab for diagnosis.

Treatment : 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. Isolate animals exhibiting signs, disinfect stable equipment, bedding (remove and burn it).  Booster vaccinations during outbreak may help. 

Prevention of abortion

• Vaccination of mares against herpesvirus at 5, 7 and 9 months of gestation and against EAV when indicated will reduce the chance of abortion.
•  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. 
• A Caslick's suture should be placed if indicated by perineal conformation.
• Pregnant mares should be separated from other horses, especially from new stock, young stock, etc.
• Quarantine stable until at least 30 days after last case is identified.

SAFTEY - Foal your mares at home. Ensure any horse transport used is steam cleaned and disinfected with an approved product regularly. The virus can survive in dirty conditions for several weeks.

MY ADVICE - Artificial Insemination and quarantine any pregnant mares that come onto your property to foal.