Equine Herpesvirus (EHV): The Essential Guide to Vaccination for Respiratory & Abortion Prevention

Equine Herpesvirus (EHV) vaccination is crucial for horse health. Vaccines effectively reduce the severity of EHV-1 and EHV-4 respiratory disease symptoms and are highly effective in preventing EHV-1 induced abortions in pregnant mares. Regular vaccination, combined with strong biosecurity, protects individual horses and herd health from this widespread and often devastating virus.

Understanding Equine Herpesvirus (EHV) can feel like navigating a maze, especially when considering how to protect your beloved horses. This common virus is a major concern for horse owners, breeders, and trainers worldwide, often causing respiratory illness, devastating abortions, and sometimes even severe neurological problems. The good news is that practical, effective solutions exist to minimize its impact. This article will walk you through everything you need to know about EHV, focusing on the vital role of vaccination for respiratory health and preventing abortions, providing clear, actionable insights to keep your horses safe and sound.

What is Equine Herpesvirus (EHV)?

Equine Herpesvirus (EHV) refers to a group of viruses that commonly infect horses. While there are nine known types, EHV-1 and EHV-4 are the most significant for horse health. These viruses are widespread globally and are a leading cause of infectious disease in equine populations.

EHV-1 vs. EHV-4: Understanding the Differences

• EHV-1 (Equine Herpesvirus Type 1): This is the most concerning type due to its ability to cause three distinct and severe forms of disease:
  • Respiratory Disease: Fever, nasal discharge, cough, and lethargy.
  • Abortion: Late-term abortions in pregnant mares (typically in the last trimester). This is often the most economically devastating form.
  • Equine Herpesvirus Myeloencephalopathy (EHM): A neurological form characterized by ataxia (incoordination), weakness, and paralysis. While less common, it can be fatal and is not reliably prevented by current vaccines.
• EHV-4 (Equine Herpesvirus Type 4): Primarily associated with respiratory disease in young horses, similar to EHV-1. While EHV-4 can, in rare cases, cause abortion, it is far less common than EHV-1-induced abortions and is not associated with the neurological form.

How Does EHV Spread?

EHV is highly contagious and spreads readily among horses. Understanding its transmission routes is key to implementing effective prevention strategies.

• Direct Contact: Nose-to-nose contact between infected and susceptible horses is a primary mode of transmission.
• Aerosol Droplets: Coughing and sneezing by an infected horse can release virus particles into the air, which can then be inhaled by other horses.
• Indirect Contact (Fomites): The virus can survive on surfaces like feed buckets, water troughs, stalls, grooming tools, tack, and even human hands and clothing. These contaminated objects, known as fomites, can then transfer the virus to other horses.
• Aborted Fetuses and Placenta: Tissues and fluids from an EHV-1 abortion are highly contaminated with the virus and pose a significant risk of spread.
• Latent Infection and Reactivation: A critical aspect of EHV is its ability to establish a latent (dormant) infection within a horse’s body, often in nerve cells. Horses that have been infected may carry the virus without showing symptoms. Under stress (e.g., travel, training, illness), the virus can reactivate and be shed, potentially infecting other horses, even if the carrier horse shows no clinical signs.

Clinical Signs of EHV Infection

The symptoms of EHV vary depending on the strain of the virus and the horse’s age and immune status. Recognizing these signs early is crucial for isolation and veterinary intervention.

Respiratory Form (EHV-1 & EHV-4)

This form is most common in foals, weanlings, and yearlings, but can affect horses of any age. Symptoms typically appear 2-10 days after exposure:

• Fever (often the first sign, sometimes biphasic)
• Nasal discharge (clear, then becoming cloudy or pus-like)
• Cough
• Lethargy and depression
• Loss of appetite
• Enlarged lymph nodes under the jaw

Abortion Form (EHV-1)

EHV-1 is a leading cause of infectious abortion in mares. Abortions usually occur late in gestation (typically 7-11 months), often without any prior signs of illness in the mare. The aborted fetus may appear fresh or show signs of decomposition. Rarely, foals born alive to EHV-1 infected mares may be weak, jaundiced, or have neurological deficits and typically die within a few days.

Neurologic Form (EHV-1 Myeloencephalopathy – EHM)

While less common, EHM is the most severe manifestation of EHV-1 infection and can be life-threatening. Symptoms can develop rapidly and vary in severity:

• Ataxia (incoordination), especially in the hind limbs
• Weakness
• Urine dribbling or inability to urinate
• Decreased tail tone
• Recumbency (inability to stand)
• Paralysis
• Head pressing or circling (less common)

It’s important to note that EHM can occur in vaccinated or unvaccinated horses, and the neurological form is not reliably prevented by current EHV vaccines.

Diagnosing EHV

Accurate and timely diagnosis of EHV is essential for implementing control measures and preventing further spread. A veterinarian will typically use a combination of clinical signs and laboratory tests.

• Nasal Swabs: Used to detect the virus in respiratory secretions, particularly in horses showing respiratory signs.
• Blood Samples: Can be used to detect the virus or measure antibody levels, indicating exposure or active infection.
• PCR (Polymerase Chain Reaction) Testing: A highly sensitive and specific test used on nasal swabs, blood, or tissue samples (e.g., from aborted fetuses) to detect the viral DNA. This is often the preferred method for rapid diagnosis.
• Tissue Samples: In cases of abortion, tissues from the aborted fetus and placenta are critical for confirming EHV-1 as the cause.

Why Vaccinate Against EHV?

Vaccination is a cornerstone of EHV prevention and control, offering significant benefits to individual horses and the broader equine community.

• Reduces Severity of Respiratory Disease: While vaccines may not always prevent infection, they significantly lessen the severity and duration of respiratory symptoms caused by EHV-1 and EHV-4. This means less discomfort for the horse and a faster recovery.
• Prevents EHV-1 Abortion Storms: For pregnant mares, EHV-1 abortion prevention vaccines are highly effective in protecting against the devastating impact of late-term abortions. This is critical for breeding operations.
• Limits Viral Shedding: Vaccinated horses, if they do become infected, typically shed less virus for a shorter period. This reduces the overall viral load in the environment and helps to protect other horses.
• Protects the Herd: A well-vaccinated herd creates a level of herd immunity, making it more difficult for the virus to spread widely. This is especially important in environments with frequent horse movement, such as boarding stables, show grounds, or training facilities.
• Economic Benefits: Preventing illness, abortions, and potential neurological disease saves significant veterinary costs, lost training time, and the emotional and financial impact of losing a foal.

Types of EHV Vaccines

It’s important to understand that not all EHV vaccines are created equal, particularly when it comes to preventing abortion. There are generally two categories of EHV vaccines available:

• Vaccines for Respiratory Disease: These vaccines typically contain both EHV-1 and EHV-4 components. They are designed to reduce the incidence and severity of respiratory signs. Examples include Calvenza-03 EHV, Rhinomune, and Equi-Shield EHV.
• Vaccines for Abortion Prevention: These are specifically formulated and licensed to prevent EHV-1 induced abortions in pregnant mares. They contain an inactivated (killed) EHV-1 virus. The most well-known example is Pneumabort-K. These vaccines are given at specific stages of pregnancy to ensure protection.

It is crucial to use the correct vaccine for the intended purpose. A vaccine labeled for respiratory disease may not provide adequate protection against abortion, and vice versa. Always consult your veterinarian to determine the most appropriate vaccine for your horse’s specific needs and risk factors.

EHV Vaccination Protocols

Vaccination protocols for EHV vary depending on the horse’s age, use, pregnancy status, and risk of exposure. A veterinarian will develop a tailored vaccination program for your horses.

General Guidelines for EHV Vaccination

The following table provides a general overview of common EHV vaccination schedules. Always consult your veterinarian for a precise schedule tailored to your horse’s individual circumstances and regional disease prevalence.

Horse Category	Vaccine Type	Primary Series	Booster Frequency	Key Considerations
Foals	EHV-1/EHV-4 Respiratory	Start at 4-6 months, 2 doses, 3-4 weeks apart	Every 3-6 months until 2 years old	High risk for respiratory disease; maternal antibodies can interfere.
Weanlings/Yearlings	EHV-1/EHV-4 Respiratory	2 doses, 3-4 weeks apart (if not previously vaccinated)	Every 3-6 months	Frequent exposure at sales, shows, training facilities.
Adult Horses (Non-Pregnant)	EHV-1/EHV-4 Respiratory	Annual booster (if previously vaccinated)	Every 3-6 months for high-risk horses (e.g., show, race, frequent travel)	Reduces respiratory signs and viral shedding.
Pregnant Mares	EHV-1 Abortion Prevention (Killed)	3 doses during gestation	5th, 7th, and 9th months of gestation	Crucial for preventing EHV-1 induced abortion; use only abortion-specific vaccine.
Stallions/Broodmares (Non-Pregnant)	EHV-1/EHV-4 Respiratory	Annual booster	Every 3-6 months if on breeding farms with high turnover.	Reduces risk of shedding to pregnant mares.

Specific Considerations for Pregnant Mares

Vaccination against EHV-1 for abortion prevention is one of the most critical aspects of a broodmare’s health program. The specific timing of these vaccinations is vital:

• First Dose: Typically given at the 5th month of gestation.
• Second Dose: Given at the 7th month of gestation.
• Third Dose: Given at the 9th month of gestation.

This schedule ensures continuous high levels of protective antibodies during the critical last trimester, when EHV-1 abortions are most likely to occur. Only inactivated (killed) EHV-1 vaccines specifically licensed for abortion prevention should be used in pregnant mares.

Efficacy and Limitations of EHV Vaccines

While EHV vaccines are powerful tools, it’s important to understand what they can and cannot do.

• Reduced Disease Severity: EHV vaccines are highly effective at reducing the severity and duration of respiratory illness caused by EHV-1 and EHV-4.
• Abortion Prevention: EHV-1 abortion prevention vaccines are very effective at preventing EHV-1 induced abortions when administered correctly and on schedule.
• Reduced Viral Shedding: Vaccinated horses that become infected typically shed less virus for a shorter period, which helps reduce environmental contamination and transmission to other horses.
• Not 100% Protective Against Infection: No EHV vaccine guarantees complete prevention of infection. Vaccinated horses can still be exposed to and carry the virus, but they are less likely to develop severe disease.
• No Reliable Protection Against EHM: Current EHV vaccines do not reliably prevent the neurological form (EHM). While vaccination may reduce the overall viral load in a population, outbreaks of EHM can still occur in vaccinated herds. This is a critical limitation to understand.
• Biosecurity Remains Key: Due to the limitations of vaccines, especially regarding EHM and complete prevention of infection, strong biosecurity measures are always necessary to complement vaccination.

Beyond Vaccination: Essential Biosecurity Measures

Vaccination is a critical component of EHV control, but it is not a standalone solution. Robust biosecurity practices are essential to minimize the risk of EHV transmission, especially given the virus’s ability to cause latent infections and the limitations of current vaccines against the neurological form.

Biosecurity Practice	Description	Why It’s Important for EHV
Quarantine New Arrivals	Isolate all new horses for 21-30 days upon arrival. Monitor for signs of illness.	Prevents introduction of EHV (and other diseases) from potentially infected or shedding horses into your existing herd.
Isolate Sick Horses	Immediately separate any horse showing signs of illness (fever, nasal discharge, neurological signs) from the rest of the herd.	Minimizes direct and indirect contact transmission from actively shedding horses.
Dedicated Equipment	Use separate buckets, brushes, and tack for isolated or sick horses. If shared, disinfect thoroughly between uses.	Reduces transmission via fomites. EHV can survive on surfaces.
Hand Hygiene	Wash hands thoroughly with soap and water or use hand sanitizer after handling each horse, especially sick ones.	Prevents mechanical transmission of the virus from horse to horse via human hands.
Disinfection Protocols	Regularly clean and disinfect stalls, trailers, and common areas. Use disinfectants effective against enveloped viruses like EHV.	Eliminates virus particles from the environment, reducing the risk of indirect transmission.
Minimize Horse Movement	Limit unnecessary movement of horses, especially during outbreaks or when risk is high.	Reduces opportunities for horses to encounter infected animals or contaminated environments.
Monitor Temperatures	Take and record daily temperatures of all horses, especially during high-risk periods or potential exposure.	Fever is often the first sign of EHV infection, allowing for early detection and isolation.
Stress Reduction	Minimize stressors like overcrowding, poor nutrition, and excessive training.	Stress can trigger latent EHV infections to reactivate and cause viral shedding.
Control Pests	Implement rodent and insect control programs.	While not primary vectors, pests can potentially carry contaminated material.

When to Consult Your Veterinarian

Your veterinarian is your primary partner in managing EHV and maintaining your horse’s health. Always consult them for:

• Tailored Vaccination Protocols: They can assess your horse’s individual risk factors, age, use, and geographical location to recommend the most appropriate EHV vaccination schedule.
• Signs of Illness: If your horse shows any signs of EHV (fever, nasal discharge, cough, abortion, neurological symptoms), contact your vet immediately. Early diagnosis and intervention are crucial.
• Outbreak Management: In the event of an EHV outbreak on your farm or in your area, your veterinarian will guide you on isolation, testing, and additional biosecurity measures.
• Pregnant Mare Care: Proper EHV-1 abortion prevention requires precise timing and the correct vaccine, which your vet will oversee.
• New Horse Arrivals: Discuss quarantine and testing protocols for new horses joining your herd.
• General Health Concerns: For any questions about your horse’s overall health, preventative care, or other vaccine needs, your vet is the best resource. For example, understanding how often your dog needs shots is equally important for your canine companions; you can find more information on Dog Vaccines & Boosters: How Often Does Your Dog Need Shots? to ensure all your animals are well-protected.

Frequently Asked Questions (FAQs) About EHV Vaccination

1. What is the difference between EHV-1 and EHV-4?

EHV-1 is the more concerning type, capable of causing respiratory disease, abortions in pregnant mares, and the severe neurological form (EHM). EHV-4 primarily causes respiratory disease, especially in young horses, and rarely leads to abortion or neurological issues.

2. Can EHV vaccines prevent the neurologic form (EHM)?

No, current EHV vaccines do not reliably prevent the neurological form (EHM) caused by EHV-1. While vaccination reduces respiratory disease and viral shedding, horses can still develop EHM even if vaccinated. Biosecurity is especially critical for EHM prevention.

3. How often should my horse be vaccinated for EHV?

The frequency depends on your horse’s age, use, and risk factors. High-risk horses (e.g., show horses, performance horses, those in large herds) may need respiratory EHV vaccines every 3-6 months. Pregnant mares require specific EHV-1 abortion prevention vaccines at the 5th, 7th, and 9th months of gestation. Always consult your veterinarian for a personalized schedule.

4. Are EHV vaccines safe for pregnant mares?

Yes, EHV-1 vaccines specifically licensed for abortion prevention (which are inactivated or “killed” vaccines) are considered safe and highly recommended for pregnant mares when administered according to veterinary guidelines. They are crucial for protecting against EHV-1 induced abortions.

5. What happens if my mare aborts despite vaccination?

While EHV-1 abortion vaccines are highly effective, no vaccine is 100% protective. If a mare aborts, it’s crucial to contact your veterinarian immediately. They will collect samples from the mare and the aborted fetus to diagnose the cause, rule out other pathogens, and implement control measures to protect the rest of the herd.

6. Can EHV affect humans?

No, Equine Herpesvirus (EHV) is species-specific and does not cause disease in humans. It poses no direct threat to human health.

7. Is there a cure for EHV?

There is no specific cure for EHV. Treatment focuses on supportive care to manage symptoms, prevent secondary infections, and help the horse recover. This may include anti-inflammatory drugs, fluid therapy, and antibiotics for bacterial complications. In cases of EHM, intensive nursing care is often required.

Conclusion

Equine Herpesvirus poses a significant threat to horse health, with its potential to cause respiratory illness, devastating abortions, and severe neurological disease. While the virus is widespread, a proactive approach combining strategic vaccination and stringent biosecurity measures offers the best defense. Regular vaccination helps reduce the severity of respiratory symptoms and, crucially, prevents EHV-1 induced abortions in pregnant mares, safeguarding both individual horses and the stability of breeding programs. Remember, your veterinarian is an invaluable partner in developing a comprehensive EHV prevention plan tailored to your horse’s specific needs, ensuring their well-being for years to come.