Rhinopneumonitis virus (rhino) can cause respiratory disease, abortion, or neurological signs. Rhinopneumonitis is caused by an equine herpes virus, EHV-1. The equine herpes virus labeled EHV-2 does not cause disease, and EHV-3 causes a venereal disease). One thing that all herpes viruses have in common is their ability to live within their host indefinitely. Once a horse is infected with rhinopneumonitis, it seems that infection can be reactivated later when the horse is stressed.
Young horses are most often affected when they are brought together in groups. Rhinopneumonitis is spread between coughing horses or on the hands of unknowing people. An upper respiratory infection is a common result of virus invasion. Uncomplicated cases will resolve on their own in a week or two. Occasionally the illness will leave the horse susceptible to bacterial pneumonia or a guttural-pouch infection.
Another common result of rhino infection is abortion in pregnant mares. Some mares will carry their fetus to term, only to deliver a weak sickly foal with slim chances of survival.
The neurological form of rhinopneumonitis is less common. A high fever precedes the development of signs ranging from incoordination and weakness to paralysis. These horses may recover with nursing care, but it can take anywhere from a few days to several months.
Two different subtypes of the EHV-1 virus exist in nature. Subtype one usually causes abortion or nervous system disease (myeloencephalopathy), while subtype two (sometimes called EHV-4) is most often the cause of respiratory infections.
There are two kinds of vaccine available now to help protect your horse from rhinopneumonitis. One is a modified live-virus vaccine, while the other contains a killed product. Whether killed or live, many rhino vaccines contain only subtype one, the major cause of abortions. Recent studies show that a protection induced by the modified-live virus vaccine is superior to that induced by the inactivated combination vaccine (1).
Many vaccines that are currently available contain only EHV-1 subtype one. It's hoped that the two strains of virus have enough in common that vaccination offers some protection against subtype two, the cause of respiratory disease. Vaccines have recently been produced that contain subtype 2, of EHV-4.
While respiratory infection is not prevented, its severity is reduced and the length of illness shortened when any vaccine is used. For the competitive show or race horse, any advantage is better than none. Rhino vaccinations are not necessary for the adult pleasure horse. Watch for new developments, since research continues in the search for a better vaccine.