Equine Herpesvirus: How a Virus Triggers Abortion in Horses
"Understanding EHV-1's strategy for evading the immune system could lead to better prevention."
Equine herpesvirus-1 (EHV-1) is a major threat to horses. It can cause everything from mild respiratory issues to late-term abortions and neurological problems. What makes EHV-1 so complex is that different strains behave differently. Some are more likely to cause abortions (abortigenic), while others lead to neurological issues (neurovirulent). Both types replicate in the lining of the upper respiratory tract, but abortigenic strains tend to form larger, more noticeable patches.
Scientists have been trying to figure out why these strains differ in their effects. One idea is that abortigenic strains might be better at dodging the horse's immune defenses, specifically the type I interferon response. Interferons are crucial proteins that help the body fight off viruses. If a virus can suppress or avoid this response, it could replicate more effectively and cause more severe disease.
This article delves into new research that explores how abortigenic and neurovirulent EHV-1 strains interact with the interferon response. By understanding these interactions, we can potentially develop better strategies to protect horses from the devastating effects of EHV-1.
EHV-1 and EHM Impact
Of the nine known equine herpesviruses, EHV-1, EHV-3, and EHV-4 pose the highest disease risk to U.S. horses, and these viruses do not pose a risk to people. EHV-1 is associated with serious disease, including equine herpesvirus myeloencephalopathy (EHM). In one reported outbreak, 90 horses were infected with EHV-1 or EHM, while another 72 were suspected cases; a separate 2025 study reported an EHM case-fatality rate of 12% among its cases.
Biosecurity and Vaccination
Accepted EHV-1 control relies on vaccination alongside a comprehensive biosecurity program. Guidance includes keeping returning horses at least 30 feet from others, quarantining groups that traveled together as a cluster, and preventing nose-to-nose contact. These measures can limit signs and infection spread, but vaccination is not presented as a standalone solution.
A Widespread, Persistent Virus
EHV-1 has long been recognized as one of the most important and prevalent viral pathogens affecting horses and as a major threat to the equine industry. Oregon guidance reports that almost all horses have been infected by age two, with initial exposure generally occurring in foals through contact with their dams. The virus therefore represents both an established historical presence in horse populations and an ongoing disease concern.
Decoding EHV-1's Immune Evasion Tactics: What the Study Reveals
To investigate the interferon response, researchers used two models: primary respiratory epithelial cells (EREC) and respiratory mucosa explants (tissue samples) from horses. They infected these with both abortigenic and neurovirulent EHV-1 strains and then measured interferon alpha (IFNα) levels.
- Interferon Production: Both types of EHV-1 triggered IFNα production in the respiratory tissues. There was no significant difference in the amount of interferon produced between the abortigenic and neurovirulent strains.
- IFN Susceptibility: Pretreating cells with recombinant equine IFNα (rEqIFNα) suppressed the replication of both types of EHV-1. Conversely, blocking interferon signaling with Ruxolitinib (Rux) increased viral replication, indicating that both strain types are susceptible to interferon's antiviral effects.
- Abortigenic Strain Anomaly: In a surprising twist, a moderate concentration of rEqIFNα (100 U/ml) actually increased the replication of abortigenic EHV-1 in some horses, compared to a lower concentration (10 U/ml). This effect wasn't seen with the neurovirulent strains.
- Plaque Size Differences: When interferon signaling was blocked, the typically smaller viral plaques caused by neurovirulent strains became larger. However, the plaque size of abortigenic strains remained relatively unchanged.
Research on EHV-1 Infection
Current research describes EHV-1 as a highly contagious respiratory pathogen transmitted through horse-to-horse contact. The Wagner Laboratory identifies fever, nasal discharge, and lethargy among the associated clinical signs. A 2025 review examines equine herpesvirus infections in horses and donkeys, including their viral causes and clinical presentations.
Neurologic Disease Is Serious but Variable
EHV-1 neurologic disease can progress quickly, causing incoordination, weakness, and difficulty standing. However, EHM is not uniformly fatal: the Equine Disease Communication Center reports that 60-70% of affected horses recover with treatment in most outbreaks. These outcomes show why severe neurologic signs should not be equated with inevitable death, even though the disease remains serious.
EHV-1 Versus Other EHV Types
EHV-1 most commonly causes respiratory illness but can also cause pregnancy loss or neurologic disease. By contrast, EHV-4 primarily causes respiratory disease and rarely leads to abortion or neurologic signs. This comparison places EHV-1 as the more important concern when reproductive or neurologic complications are involved.
What Does This Mean for Horse Owners?
This research highlights the crucial role of the interferon response in controlling EHV-1. It also suggests that abortigenic EHV-1 strains might have evolved ways to lessen the impact of interferon, potentially explaining their ability to cause more severe respiratory disease and abortions.
Exposure, Latency, and Outbreak Risk
EHV-4 primarily causes respiratory disease and rarely produces abortion or neurologic signs, while most horses are exposed early in life and may carry the virus in a latent state. Recent cases have been linked to a rodeo event, with horses from Texas and Oklahoma diagnosed with equine herpesvirus. Together, these reports highlight the contrast between widespread early exposure and the outbreak risks associated with movement and congregation of horses.
A Growing Equine Healthcare Market
The equine healthcare market is projected to surpass US$1.3 billion by 2035. Transparency Market Research identifies rising equine diseases as a driver and specifically includes equine influenza and equine herpesvirus among the relevant indications. This projection suggests that EHV will remain part of the veterinary healthcare market's future priorities.
Protecting the Equestrian Community
Equine herpesvirus affects more than individual horses because transmission can threaten the broader equestrian community. Washington State guidance emphasizes that timely intervention can help prevent spread and mitigate the virus's impact on equine health and that community. The systemic challenge is therefore coordinating prompt recognition and response across connected horse populations.
Consequences for Horses and Owners
EHV-1 primarily causes upper respiratory disease and abortions or stillbirths in horses. In some cases, it causes a neurologic disease called equine herpesvirus myeloencephalopathy. For horse owners, the same infection can therefore involve respiratory illness, reproductive loss, or potentially disabling neurologic disease.
While more research is needed to fully understand these mechanisms, these findings open new doors for developing more effective prevention and treatment strategies. Potential avenues include:
<ul> <li><b>Targeted Vaccines:</b> Designing vaccines that specifically boost the interferon response against abortigenic strains.</li> <li><b>Immunomodulatory Therapies:</b> Developing therapies that fine-tune the interferon response to prevent the virus from exploiting it.</li> <li><b>Early Detection:</b> Improving diagnostic tools for early detection of EHV-1, allowing for timely intervention and minimizing the risk of outbreaks.</li> </ul>