The following information was obtained from UC Davis School of Veterinary Medicine Center for Equine Health

What is EHV-1?
EHV-1 (equine herpesvirus-1) is one of a large group of DNA viruses causing potentially serious disease in horses and other species. EHV-1 has two forms: one that causes abortion in mares and one that causes respiratory infection and neurological symptoms. The outbreak that has recently occurred in Orange County involved the EHV-1 respiratory/neurological form of the virus causing a condition known as Equine Herpes Myeloencephalopathy (EHM).
EHV-1 is contagious and is spread by direct horse-to-horse contact, by contaminated hands, equipment and tack, and, for a short time, through aerosolization of the virus within the environment of the stall and stable.

What are the Clinical Signs of EHV-1?
The initial clinical signs of the infection may be nonspecific and include fever of 102°F or greater. Fever may be the only abnormality observed. Other presenting signs may be combinations of fever and respiratory symptoms of nasal discharge and cough. Some horses have reddish mucous membranes.

Horses with neurological disease caused by EHV-1 infection can soon become uncoordinated and weak and have trouble standing. Difficulty urinating and defecating may also occur. Often the rear limbs are more severely affected than the front. Signs of brain dysfunction may occur as well, including extreme lethargy and a coma-like state.
The incubation period of EHV-1 infection is HIGHLY VARIABLE, depending on the host, on the virulence of the virus, and on environmental and other factors such as stress. The AVERAGE incubation period is 4 to 7 days, with the majority of cases being 3 to 8 days, but with some taking up to 14 days. When neurological disease occurs, it is typically 8 to 12 days after the primary infection involving fever. In most cases, horses exposed to EHV-1 will develop a fever and possibly nasal discharge and then go on to recover.

EHV-1 Control Measures
Horses exhibiting sudden and severe neurological symptoms consistent with a diagnosis of EHV-1 pose a definite risk to the surrounding population of horses. Consequently, early intervention to prevent the spread of infection is required. Disease control measures based on established medical practices and recent successful experiences to control the spread of EHV-1 indicate the following measures to be reasonable.

Isolation of Sick Horses
To prevent an infected horse from having any further contact with other horses in the stable environment, any individual horse with clinical signs consistent with neurological EHV-1 infection should be removed immediately from the area of other horses and placed in a separate enclosure designated for infectious disease isolation. This isolation stall or enclosure should be located well away from high traffic areas associated with other barns or training areas. Animal caregivers should take precautionary measures to ensure that they do not transmit disease to other horses through contaminated hands, clothing, equipment or tack. It is essential that the isolation facility have supervised oversight by an individual knowledgeable in disease control and quarantine procedures to avoid the possibility of contamination.

Segregation of Exposed Horses
Horses known to have had intimate contact with the diagnosed clinical case of EHV-1 should be maintained in their existing barns and segregated from other horses during exercise periods until the sick horse has been confirmed to have EHV-1 by PCR testing.

Quarantine Procedures
Once the confirmation of EHV-1 is made, appropriate focal quarantine measures to restrict the movement of all potentially exposed horses will be necessary to prevent the possible spread of disease to other locations. These procedures may begin with initial restrictions such as the quarantine of those individuals in the immediate area of exposure (i.e., a single barn or other unit of housing) within a horse facility. Horses in the immediate contact area of the clinically affected individual should be monitored closely but tested by PCR for EHV-1 only if they exhibit fever or clinical signs consistent with EHV-1 infection. Since stress may play a role in eliciting the onset of clinical signs, horses stabled in areas of known exposure should not be subjected to strenuous physical exercise or long-distance transport until their health status can be determined.

Infections other than EHV-1 can also spread by horse-to-horse contact, so keeping a horse with a fever isolated is a very good practice in any case.

If your horse develops fever, respiratory signs or neurological signs, immediately notify your veterinarian and do not move the horse or horses in the immediate area. Alert those who have horses in the adjacent area to cease all movement of horses in and out of the facility until a diagnosis is confirmed by testing. If horses are exposed and then travel to a new stable or show, the infection can spread to other horses at that new location.

EHV-1 does not persist in the environment for a long time, but disinfection of premises, stalls, trailers and so forth is indicated. If you handle a horse with EHV-1 and don’t wash hands or change clothing, you may spread the infection to other horses. A solution of 1 part chlorine bleach to 10 parts water is effective for decontaminating equipment and environment.

Vaccination
Commercially available vaccines for EHV-1 include two single-component inactivated vaccines (Pneumabort K and Prodigy) marketed for the prevention of abortion in pregnant mares; several multicomponent inactivated vaccines (Prestige, Calvenza, Innovator); and one MLV vaccine (Rhinomune) for the prevention of respiratory disease induced by EHV-1 and EHV-4. Each of these vaccines induce some, but not all, of the desired components of the immune response in the horse. Therefore, it is not surprising that NONE induces sterile immunity or complete protection from clinical disease. The best that can be hoped for is a reduction in the severity of clinical signs and in the amount of EHV-1 shed by vaccinated horses that do become infected.

There is evidence that viral shedding is reduced in horses with high circulating titers of virus-neutralizing (VN) antibody, as well as in those that have been vaccinated recently with the Rhinomune MLV vaccine. Of the available inactivated vaccines, Calvenza and both high antigenic mass vaccines marketed for prevention of abortion (Pneumabort K and Prodigy) stimulate the highest levels of VN antibody in experimental horses. One recent study to test the efficacy of Rhinomune against challenge with a “neuropathogenic” strain, and a challenge study performed almost 30 years ago to test the efficacy of Pneumabort K in preventing abortion, provided some evidence that these vaccines may have a place in control of outbreaks of EHM. The low number of horses enrolled in these studies justifies caution in interpretation of the results; however, a lower proportion of recently vaccinated horses developed EHM after challenge as compared to control unvaccinated horses in both studies.

On premises with confirmed clinical EHV-1 infection (any form), booster vaccination of horses that are likely to have been exposed already is not recommended. However, it seems rational to booster vaccinate nonexposed horses as well as horses that must enter the premises with one of the four vaccines listed above if they have not been vaccinated against EHV-1 within the past 60 days. This approach relies on the assumption that the immune system of most mature horses has already been “primed” by prior exposure to EHV-1 antigens through field infection or vaccination and can therefore be “boosted” within 7 to 10 days of administration of a single dose of vaccine.

While this approach does not guarantee protection of individual horses against the potentially fatal neurological form of EHV-1, the hope is that reduced nasal shedding of infectious EHV-1 by recently vaccinated horses will indirectly help protect other horses by reducing the dose of virus to which they are exposed.