Rhinopneumonitis (or rhino) in horses is an inflammation of the mucosa of the nasal cavities and the lungs caused by the equine herpes virus. This virus lives only in horses and has several distinct types, which can cause a whole range of illnesses (respiratory, neurologic, reproductive, neonatal). Respiratory disease is the most common by far.
Almost all adult horses are infected with the equine herpes virus as a consequence of natural exposure during their first months of their life. The respiratory form of rhinopneumonitis rarely causes severe disease in adult horses. Following infection, the virus lives in hibernation in the lymph nodes of the respiratory system, and the horse is infected for the remainder of his life. The immune system keeps these herpes viruses in check, so the immune system plays a very important role in determining whether infected horses exhibit symptoms of disease or not.
If the horse's immune system is compromised in the future by some other disease or problem or by the use of immunosuppressive medications (such as heavy doses of steroids), the virus may escape confinement by the immune system, enter the bloodstream, return to the respiratory lining, and cause further signs of respiratory infection or fever. This ability of the rhinopneumonitis virus to cause recurrent signs of disease is referred to as "viral recrudescence" (i.e., essentially 'rebirth' of the symptoms). No other virus in the horse is capable of latency and relapse in such a manner. In this regard, it is similar to the human herpes simplex virus that causes "cold sore" lesions on the lips of infected people. It just hangs out until your horse is stressed and then comes out.
The equine rhinopneumonitis virus is also an important cause of abortion in brood mares. The abortion hardly causes any residual stress or symptoms in the mare, and the foal that is aborted ("slips") looks like a well-developed healthy fetus. Brood mares are routinely vaccinated against rhinopneumonitis throughout pregnancy to minimize the risk of abortion attributed to infection by this virus. Large broodmare farms are plagued with this problem, because new strains ("mutants") arise and take the horse's immune system by surprise.
Sometimes, rhinopneumonitis viral attack on the developing foal during pregnancy fails to kill the foal and cause an abortion. In those cases, the foal may be born alive after an appropriate length of gestation (pregnancy) but has been compromised by the virus. These foals are unthrifty at birth as a result of viral damage to the liver and lungs, and they usually die.
Less commonly, the rhinopneumonitis virus causes neurological symptoms in affected horses. In these cases, the virus typically attacks the spinal cord and causes weakness, incoordination (ataxia) and collapse. Often, the resulting inflammation in the spinal cord also leads to paralysis of the bladder and cystitis in affected horses.
What to Watch For
The most common symptom is a mild respiratory disease that lasts for several days.
Veterinarians in the United States commonly undertake vaccination of horses against the equine herpes viruses (Subtypes EHV-1 and EHV-4). As a rule, vaccines against any of the herpes viruses are not very effective because herpes viruses live in the horse and befuddle the immune system.
Vaccines are probably most important to protect pregnant mares against viral abortion. For best protection against abortion, it is necessary that the pregnant mare is vaccinated at least three times during the gestation (typically at months 5, 7 and 9). Using the rhinopneumonitis vaccine to prevent respiratory disease in young horses requires that it be administered every 2 to 3 months.
The use of rhinopneumonitis vaccine once yearly, as a part of an annual vaccination program, is probably offering minimal protection from a serious challenge. It might be all you need if your horse lives in relative isolation, but if you're moving around, you ought to get him vaccinated 3-4 times per year. Unfortunately, vaccines that are currently available against rhinopneumonitis do not offer any protection against the neurological form of this disease.