Hal Schott, DVM, PhD, DACVIM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, answers your questions about equine herpesvirus....
What diseases do equine herpes viruses cause?
There are four herpes viruses that can cause disease in horses. Equine herpes virus 1 (EHV-1) causes respiratory disease, abortion, and occasionally neurological disease. Equine herpes virus 2 (EHV-2) is a relatively innocuous virus that has been implicated in outbreaks of mild rhinitis (nasal inflammation) and conjunctivitis (inflammation of the pink tissues around the eye). Equine herpes virus 3 (EHV-3) is the venereal form of the virus and causes lesions in breeding stallions and mares. Equine herpes virus 4 (EHV-4) also causes respiratory disease and, less commonly, abortion.
What is the difference between EHV-1 and EHV-4?
In the past, these two herpes viruses were classified as subtypes of EHV-1 but subsequent studies of their genetic makeup showed enough differences to classify them as different herpes viruses. Practically, the most important difference between EHV-1 and EHV-4 is the degree to which they invade the horse’s body beyond the respiratory tract. EHV-4 infections are mostly limited to the respiratory tract although they can occasionally extend further and, rarely, infect the placenta and developing fetus in pregnant mares and lead to abortion. EHV-1 also primarily affects the respiratory tract but is accompanied by a greater degree of viremia, circulation of virus particles through the bloodstream, than with infection with EHV-4. This greater viremia allows EHV-1 to be more invasive to other tissues, including the placenta (leading to abortion or birth of a weak foal) and the central nervous system (causing neurological disease in some affected horses).
How do herpes viruses differ from other viruses?
Herpes viruses are somewhat unique in that they are among a few types of viruses that result in lifelong infection of the host. The saying “herpes is forever” is often used to emphasize this point. However, despite persistence of the infection, clinical signs of rhinopneumonitis are only transient. As the affected patient recovers, herpes viruses become latent. In other words, they persist by hiding out in certain lymphoid tissues and ganglia of the nervous system. With stressful events or the normal immunosuppression that accompanies pregnancy, herpes viruses can recrudesce or become reactivated.
This can lead to another round of viremia and the potential for placental and fetal infection in the pregnant mare. Despite reactivation of the herpes virus, clinical signs of respiratory disease in adult mares are largely absent. This is because most horses are infected with EHV-1 and EHV-4 in the first year or two of life. These initial infections cause typical signs of viral respiratory disease, including fever, mild depression, nasal discharge, and coughing. However, once a horse has had two or three bouts of rhinopneumonitis, subsequent infection with a similar strain of EHV-1 or EHV-4 or recrudescence of a latent EHV-1 or EHV-4 rarely produces clinical signs. However, affected horses are actively shedding the virus during this time and can be a source of infection to other horses.
How common is infection with EHV-1 and EHV-4?
Infection with EHV-1 and EHV-4 is widespread in young horses, especially those that travel frequently and intermingle with other young horses. It has been estimated that 50-75% of horses that are 2-years-old or less have likely been infected at least once with EHV-1 and/or EHV-4. Thus, infection with these two viruses is one of the most common causes of respiratory disease in weanlings through young racehorses.
How are EHV-1 and EHV-4 spread between horses?
Both EHV-1 and EHV-4 are most commonly spread between horses by aerosol transmission from an infected horse to surrounding naïve horses. There are large numbers of infective viral particles in each drop of nasal secretions from an infected horse but, more important than direct contact between horses, is dissemination of virus in clouds of aerosol vapor from a coughing horse. The latter aerosol cloud can potentially travel 50 to 100 feet from the nose of an infected horse to its surrounding neighbors. Although of less importance in respiratory disease outbreaks with coughing horses, transfer of virus on animal caretakers’ hands and clothing, as well as on inanimate objects such as shared grooming and feeding utensils, can also be important in transmitting disease, especially between groups of horses housed a barn or more away from coughing horses. It cannot be overemphasized that simple preventive measures including hand washing and limiting traffic flow (of people and horses) and use of shared equipment can dramatically reduce the potential for spread of disease in potential outbreaks.
Hal Schott, DVM, PhD, DACVIM, Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University