Herpes Virus Vaccination in Horses

Herpes Virus Vaccination in Horses: What It Does and What It Doesn’t

By Dr Duncan Houston

Equine herpesvirus is one of the most important infectious diseases in horses, not because it is rare, but because it is common, unpredictable, and capable of causing serious outbreaks. Most horses are exposed at some point in their lives. Many recover uneventfully. Some do not.

The neurologic form, known as Equine Herpes Myeloencephalopathy, is where concern rises sharply. These cases can lead to incoordination, paralysis, and, in severe situations, death. The challenge is that vaccination does not fully prevent this form of the disease. That is where confusion and frustration often start.

The goal of this article is clarity. Vaccination still matters, but it needs to be understood properly and combined with good biosecurity to actually reduce risk.

Quick Answer

Equine herpesvirus vaccines help reduce respiratory disease and abortion risk and may reduce viral shedding, but they do not reliably prevent the neurologic form of EHV-1. Vaccination remains important as part of overall disease control, but it must be combined with strict biosecurity and early detection to reduce the risk of serious outbreaks.

What Is Equine Herpesvirus?

Equine herpesvirus is a group of viruses that infect horses, most commonly EHV-1 and EHV-4.

EHV-1 can cause:

• respiratory disease

• abortion in pregnant mares

• neurologic disease

EHV-4 mainly causes respiratory disease.

The neurologic form of EHV-1 is less common but far more serious. It affects the spinal cord and nervous system, leading to coordination problems, weakness, and in severe cases inability to stand.

Why the Neurologic Form Is So Concerning

Equine Herpes Myeloencephalopathy can develop quickly and unpredictably.

Common signs include:

• hind limb weakness

• incoordination

• difficulty urinating

• tail weakness

• inability to stand in severe cases

Not every infected horse develops neurologic disease. That is part of what makes outbreaks difficult to manage. You cannot always predict which horses will be severely affected.

What Do Current Vaccines Actually Do?

This is where expectations need to be realistic.

Most currently available vaccines are designed to:

• reduce respiratory disease

• reduce viral shedding

• reduce abortion risk in pregnant mares

They are not specifically labeled to prevent neurologic disease.

That means:

• vaccinated horses can still become infected

• vaccinated horses can still develop neurologic signs

• outbreaks can still occur in vaccinated populations

This is often misunderstood. Vaccination is not a guarantee against EHM.

Why Vaccination Still Matters

If vaccines do not fully prevent neurologic disease, it is fair to ask why they are still used.

They remain important because they can:

• reduce the amount of virus a horse sheds

• reduce severity of respiratory disease

• lower overall viral load within a group

• potentially reduce the impact of an outbreak

In practical terms, vaccination may not stop every case, but it can help reduce spread and severity across a population.

What Research Suggests About Disease Severity

Some studies have suggested that certain vaccination protocols may reduce the severity of disease in exposed horses, even if they do not prevent infection completely.

In controlled conditions:

• unvaccinated horses were more likely to develop severe neurologic signs

• vaccinated horses sometimes showed milder clinical signs

This is not absolute protection, but it is clinically relevant. A reduction in severity can still make a significant difference in outcomes during an outbreak.

Does Vaccination Increase the Risk of EHM?

This question comes up regularly.

There is no clear, consistent evidence that vaccination increases the risk of neurologic disease. Some discussions have explored possible immune-related mechanisms, but current evidence does not support avoiding vaccination because of increased EHM risk.

The more practical concern is not that vaccination causes the problem, but that it may give a false sense of security if biosecurity is ignored.

How Worried Should You Be?

Low risk

• closed herd

• minimal movement between properties

• low exposure to outside horses

Action: Routine vaccination and good management are usually sufficient.

Moderate risk

• occasional travel

• new horses introduced periodically

• shared equipment or facilities

Action: Regular vaccination and basic biosecurity are important.

High risk

• frequent travel

• show horses

• boarding or mixed facilities

• high turnover of horses

Action: Vaccination plus strict biosecurity and monitoring are essential.

Critical risk

• known outbreak nearby

• horses showing fever or neurologic signs

• confirmed EHV exposure

Action: Immediate veterinary involvement and strict isolation protocols are required.

What Is a Sensible Vaccination Strategy?

While protocols vary, a practical approach often includes:

• regular boosters for horses that travel or compete

• targeted vaccination for pregnant mares to reduce abortion risk

• use of appropriate vaccines based on the horse’s risk profile

• timing vaccinations to reduce gaps in protection

The exact schedule should be tailored to the horse’s lifestyle, not applied blindly.

Why Biosecurity Matters More Than People Think

Because vaccines do not fully prevent neurologic disease, biosecurity becomes critical.

Key measures include:

• avoiding direct contact between unfamiliar horses

• not sharing equipment without cleaning

• monitoring temperatures during travel or after exposure

• isolating new arrivals

• separating horses showing signs of illness

Decision checkpoint

If you rely on vaccination alone without biosecurity, you are leaving a major gap in disease control.

Early Detection Is Critical

One of the most useful tools during risk periods is regular temperature monitoring.

Fever is often one of the earliest signs of EHV infection.

Practical steps:

• check temperature daily during high-risk periods

• increase to twice daily if there is known exposure risk

• act quickly if a fever is detected

Catching a case early can help limit spread.

When Is This an Emergency?

Call your veterinarian immediately if a horse shows:

• incoordination

• weakness, especially in the hind limbs

• difficulty standing

• inability to urinate

• sudden neurologic changes

• multiple horses developing fever or illness

These signs may indicate neurologic disease and require urgent attention.

Common Mistakes Owners Make

Assuming vaccination prevents all forms of EHV

It does not prevent neurologic disease reliably.

Ignoring biosecurity in vaccinated horses

This increases outbreak risk.

Delaying action when fever appears

Early response is critical.

Mixing new horses without quarantine

This is one of the fastest ways to introduce disease.

Relying on appearance instead of monitoring

Some horses look normal early in infection.

Practical EHV Risk Management

FAQs

Does vaccination prevent EHM completely?

No. It reduces some risks but does not reliably prevent neurologic disease.

Should I still vaccinate my horse?

Yes. Vaccination remains an important part of overall disease control.

What is the earliest sign of EHV infection?

Fever is often one of the earliest detectable signs.

How does EHV spread?

Primarily through respiratory secretions and close contact between horses.

Is biosecurity really necessary if horses are vaccinated?

Yes. It is essential because vaccines are not a complete barrier.

Final Thoughts

Equine herpesvirus is not a problem that can be solved with one tool. Vaccination helps, but it is only part of the strategy. The real protection comes from combining vaccination with good management, early detection, and strict biosecurity when risk increases.

If you understand what vaccines can and cannot do, you are in a much stronger position to protect your horses. That clarity matters more than assuming protection that is not there.

If you are unsure how to build a vaccination plan or manage EHV risk in your specific setup, ASK A VET™ can help you tailor a practical approach based on your horse’s lifestyle and exposure risk.