Mythbuster Monday: Vaccination Fully Protects Against Neuro EHV-1

Marcella Gruchalak, Comments Off on Mythbuster Monday: Vaccination Fully Protects Against Neuro EHV-1
On Mythbuster Monday, we tackle a variety of equestrian myths to either bust or confirm. Today’s discussion: Can a horse get the neurological form of EHV-1 if it’s vaccinated?

It’s Mythbuster Monday, where Horse Nation dives into different equestrian myths and provides research-based evidence to either bust or confirm those myths. Today’s topic: Can a horse get the neurological form of EHV-1 if it’s vaccinated? What does the vaccination help with? Read on to find out!

Myth: If a horse is vaccinated for EHV-1, it can’t get the neurological form

Myth or Fact: Myth

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Equine Herpesvirus‑1 (EHV‑1) is a highly contagious virus that affects horses worldwide. It belongs to the herpesvirus family and primarily causes respiratory illness, neurological disease, and reproductive issues. In young horses, EHV‑1 often presents as a mild fever, nasal discharge, and coughing, while pregnant mares may experience abortion in the later stages of gestation. One of the most serious manifestations is equine herpesvirus myeloencephalopathy (EHM), a neurological form that can lead to hind limb weakness, loss of coordination, and in severe cases, paralysis.

The virus spreads mainly through direct contact with infected horses via nasal secretions, but it can also be transmitted indirectly through contaminated equipment, clothing, or surfaces. After infection, horses can harbor the virus in a latent state, which can reactivate later, especially under stress, potentially causing new outbreaks. While vaccines exist to reduce respiratory disease and abortion risk, biosecurity, early detection, and careful management are crucial in controlling EHV‑1 in equine populations.

But, does the vaccine prevent the neurological symptoms in horses?

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The 2022 study, “Efficacy of vaccination against equine herpesvirus type 1 (EHV-1) infection”, systematically reviewed randomized controlled trials assessing commercial and experimental vaccines against EHV‑1. The researchers analyzed eight trials in which horses were experimentally challenged with the virus. They found that vaccination generally resulted in only slight improvements in clinical signs and viral shedding, but it did not provide strong or consistent protection against severe forms of disease, including neurological complications (EHM). The authors noted that study heterogeneity and limited reporting made it difficult to compare results across trials, but the overall conclusion reinforced that current vaccines cannot reliably prevent EHM.

In short, while vaccination may help reduce respiratory disease or viral shedding, it does not guarantee protection against the neurological form of EHV‑1, highlighting the need for strict biosecurity and careful management to prevent outbreaks.

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The 2023 review by Osterrieder systematically examined 35 studies (out of over 1,000 screened) that evaluated vaccine use against EHV‑1 in horses. These included both commercial vaccines (modified-live, inactivated, mixed) and experimental ones (DNA, recombinant, deletion mutants, etc.). The authors assessed key outcomes such as fever (pyrexia), abortion, neurologic disease (EHM), viremia, and nasal shedding.

Critically, when it came to neurologic disease (EHM), none of the vaccine trials reviewed showed a statistically significant reduction in the incidence of neurologic signs in vaccinated vs. unvaccinated horses. The pooled relative risk was about 0.96, meaning essentially no meaningful difference.

This review strongly supports the idea that current EHV‑1 vaccines do not reliably protect against the neurological (EHM) form. While vaccination may mildly reduce some clinical signs or viral shedding, it does not provide strong, consistent prevention of neurologic disease, so relying solely on vaccination is not sufficient for preventing EHM.

Another study, “Impact of the Host Immune Response on the Development of Equine Herpesvirus Myeloencephalopathy (EHM),” looked at how a horse’s immune system affects whether it develops the neurological form of EHV‑1, called EHM. Researchers found that horses that stayed healthy had a quick, strong immune response that helped control the virus early. Horses that got EHM had a weaker or delayed immune response, which let the virus spread in the blood and affect the nervous system.

The study showed current EHV‑1 vaccines don’t reliably prevent EHM because they don’t always trigger the kind of immune response that protects against the neurological form. So even vaccinated horses can still develop EHM, and managing the spread of the virus with biosecurity, monitoring, and good care is still essential.

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According to Colorado State University’s veterinary extension, the common EHV‑1 vaccines available today do not protect against the neurological form of the virus. These vaccines are designed to help with the respiratory form (like fever and nasal discharge) and to reduce the risk of abortion in pregnant mares.

While the vaccines don’t reliably prevent EHM, CSU notes that they can help reduce how much virus an infected horse sheds and how much virus is circulating in its system. Because of this, vaccinating before a horse is exposed may help make any infection less severe.

After reviewing the research, it’s evident that the notion that EHV‑1 vaccination fully protects horses from the neurological form, EHM, greatly oversimplifies a complex disease process. Each study reinforces that while current vaccines can help reduce symptoms, they do not reliably prevent EHM.

Be that as it may, vaccination is still a critical component in maintaining your horse’s health:

  • Helps reduce viral shedding: Vaccinated horses are less likely to blast germs into the barn aisle, lowering everyone’s overall exposure.
  • Limits the spread of outbreaks: EHV-1 vaccines don’t prevent the neuro form, but they do shrink the window of opportunity for the virus to circulate.
  • Protects vulnerable horses: Pregnant mares, young horses, and stressed travelers benefit when the herd around them is immunized.
  • Herd immunity = fewer outbreaks: The higher the percentage of vaccinated horses, the harder it is for EHV-1 to gain traction and move through a population.
  • Supports biosecurity efforts: Vaccination adds a crucial layer of defense on top of isolation, hygiene, and smart management.

Vaccination alone cannot be counted on as complete protection, and biosecurity, monitoring, and careful management remain essential. Also, the horse’s immune response, age, stress level, and viral strain all play critical roles in whether neurological disease develops. Collectively, the findings show that EHV‑1 prevention requires a multi-layered approach; vaccines are a helpful tool, but only when combined with vigilant husbandry and outbreak control measures.

Do you have an equine myth you’d like us to tackle? If so, send it our way! Email your suggestions to [email protected]. Put Mythbuster Monday in your subject line.