10 Things You Might Not Know About Equine Herpesvirus-1 (EHV) and Equine Herpes Myeloencephalopathy (EHM)

10 Things You Might Not Know About Equine Herpesvirus-1 (EHV) and Equine Herpes Myeloencephalopathy (EHM)

1. EHV-1 is a common virus in the horse population. Horses are typically exposed to EHV-1 early in life, with 80-90% of horses reportedly infected by two years of age. The virus may not make the horse sick at the time, but it can enter a period of latency and become reactivated later in response to stress or other factors. Subclinical carriers can shed the infectious organism into the environment, presenting potential sources of infection for other horses. It is important to note that the bulk of the disease is rhinopneumonitis, a respiratory infection. Abortions and neurologic cases are rare.
2. The incubation period of EHV-1 infection is highly variable. The time from infection to observable clinical signs (incubation period) depends on the individual animal, the virulence (severity) of the virus, and environmental factors such as stress. The average incubation period is 4 to 7 days, with some taking up to 14 days. When neurological disease occurs, it is typically 8 to 12 days after the primary infection involving fever.
3. EHV-1 is commonly diagnosed from nasal swabs or blood samples. Polymerase chain reaction (PCR) is used to amplify the DNA of the virus so that it can be detected. In California, the neurologic form, EHM, is a reportable disease and therefore the California Animal Health and Food Safety Laboratory (CAHFS) must perform EHV-1 testing in suspected cases. Positive cases tested at a non-state diagnostic laboratory need to be confirmed by CAHFS. The laboratory recommends submitting both a nasal swab sample and a blood sample if possible.
4. There are three different primary genotypes of EHV-1: neuropathogenic, non-neuropathogenic and a new variant. The neuropathogenic genotype is designated as EHV-1:D-752. The non-neuropathic genotype is EHV-1:N-752 and the new variant is EHV-1:H-752. All three genotypes have been associated with neurologic signs. These three genotypes are genetically distinct, enabling testing to perform EHV-1 subtyping on nasal swabs and blood samples.
5. Random EHV-1 testing of horses outside of quarantine areas or in unexposed stables is not currently recommended. The detection of virus through PCR analysis does not provide a diagnosis in the absence of clinical signs and/or other corroborating information because low levels of non-replicating virus may be the source of the viral DNA detected. The relationship between individuals who test positive but never develop clinical signs and those that test positive and actually develop clinically significant disease is unknown. Research in this area is ongoing.
6. Fever is the most consistent initial clinical sign of EHV-1 infection. Elevated temperatures (≥101.5 °F in horses not receiving any anti-inflammatory drugs) that otherwise appear healthy can be early indicators of EHV-1 (and other) infections. Treating horses in the early stages of infection, or even before other clinical signs appear, often results in better outcomes and can significantly limit the spread of disease. Take and record horses’ temperatures regularly and talk to your veterinarian if you notice elevated temperatures in otherwise healthy horses. Enforce proper biosecurity measures to keep your horse healthy at home and away from home. Some microchips have temperature sensors (thermochips) and pair with a smartphone app to simplify regular body temperature monitoring.
7. There is no vaccine for EHM. Vaccines are available that are licensed for protection against the respiratory disease and abortion related forms of EHV. None of these vaccines prevents the neurologic form of EHV-1. However, they may decrease nasal shedding of EHV-1, thereby limiting the spread of infection and outbreaks. The most up-to-date guidelines on these vaccines is available through the American Association of Equine Practitioners (AAEP). The intent of vaccination requirements for licensed competitions is to promote proper vaccination protocols in accordance with AAEP recommendations.
8. The EHV-1 virus can travel through the air, but can only spread for a maximum distance of 30 feet. It is primarily spread directly through horse-to-horse contact and indirectly on contaminated objects (fomites) such as hands, clothing, tack, buckets, etc. (where it can remain viable for four to six hours). Infected horses primarily shed the virus in nasal secretions.
9. EHV-1 can survive in the environment for 7 - 30 days, but it is susceptible to disinfectants. Bleach and alcohol disinfectants are effective against EHV-1. However, it is important to remember that these types of disinfectants are inactivated by organic matter (soil, manure, etc.). Therefore, it is important to use soaps or detergents to remove any organic matter prior to applying a disinfectant. It is also important to follow manufacturer’s instructions as specific contact times are needed to inactivate disease organisms and vary by product. Accelerated hydrogen peroxide products, such as Accel/Rescue® and Intervention™, hold up better to organic matter and have a comparatively short contact time.
10. Treatment of EHM usually focuses on reducing inflammation. A combination of anti-inflammatories such as flunixin meglumine and prednisolone, free-radical scavengers such as dimethyl sulphoxide and vitamin E, and even aspirin have been used to treat clinical signs. Antivirals such as valacyclovir have been utilized to decrease viral load and prevent severe neurologic signs. However, to date, controlled studies to evaluate the efficacy of various approaches to EHM prevention are lacking. Additional supportive treatments can be implemented on a case-by-case basis.

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.