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What New Information on EHV-4 Should Veterinarians Keep in Mind When Diagnosing and Treating Respiratory Disease?

By Nicola Pusterla, DVM, PhD, DACVIM, DAVDC-Equine

Equine herpesvirus-4 (EHV-4) has long been recognized as a common cause of respiratory illness in young horses, often resulting in mild clinical signs like fever, nasal discharge and occasional lethargy. However, our newly published study sheds light on how this virus may occasionally behave more aggressively than previously thought. The study abstract can be viewed here.

In a study of 183 horses with acute fever and respiratory signs, 13.7% of those who tested positive for EHV-4 in nasal swabs also had the virus detected in their bloodstream (viremia).1 This is a stark contrast to equine herpesvirus-1 (EHV-1), where viremia is expected in nearly all infected horses. EHV-4 doesn’t have the same ability as EHV-1 to invade the bloodstream. This likely accounts for why we don’t see EHV-4 causing complications like abortion or myeloencephalopathy.

EHV-4 viremia was only found in horses already shedding the virus through nasal secretions. None of the control horses (febrile horses with no detectable EHV-1 in nasal secretions) tested positive for EHV-4 in their blood. This indicates that viremia is closely tied to active viral shedding, reinforcing that blood testing may only be relevant in confirmed shedding cases.

YOUNG HORSES ARE MORE LIKELY TO BE VIREMIC
Although rare, the detection of EHV-4 in the blood raises important clinical considerations. Viremia suggests the virus can move beyond the respiratory tract, potentially affecting other organ systems. Notably, younger horses were more likely to be viremic and showed a higher incidence of distal limb edema—a finding that surprised our research team. While the exact cause is speculative, it may be due to interaction of EHV-4 with blood vessels of the distal limbs.

The study also found that over a quarter (27.3%) of EHV-4-positive horses were co-infected with other respiratory pathogens, including equine influenza virus (11%), Streptococcus equi subspecies equi (10%), equine rhinitis B virus (9%), and even EHV-1 (2%). Stress and latent infections likely play a role in these co-infections—and when multiple pathogens are at work, the clinical signs can be more severe.

CLINICAL TAKEAWAYS
This research offers a few practical insights:

1. Monitor young horses closely: Younger horses with fever and respiratory signs should be watched carefully for signs beyond the respiratory tract, like limb edema.
2. Consider blood testing in severe cases: While routine blood testing for EHV-4 isn’t necessary, it could be informative in young horses showing more severe or unusual clinical signs, especially when nasal shedding is confirmed.
3. Watch for co-infections: Horses with EHV-4 may also be battling other respiratory pathogens, which could worsen clinical signs.
4. Biosecurity still matters: Even though EHV-4 complications are rare, biosecurity measures and vaccination protocols should remain a priority, especially in facilities housing young stock.

While EHV-4 viremia is uncommon, understanding its potential impact helps us provide more comprehensive care. Our findings reinforce that EHV-4 typically causes mild disease but, on occasion, it can cross into the bloodstream and contribute to more complex clinical presentations. Being aware of this can improve how we manage and support affected horses. This study offers a timely reminder that even familiar pathogens like EHV-4 can surprise us, underscoring the importance of ongoing research and vigilant clinical observation.

1. Pusterla N, Barnum S, Lawton K, Craig B, James K. Investigation of the frequency and selected prevalence factors of equid alphaherpesvirus 4 viremia in horses with acute onset of fever and respiratory signs. Journal of Veterinary Diagnostic Investigation. 2025;37(2):349-353. doi:10.1177/10406387241308080.

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