EHV-1 and EHM Outbreaks: What Equine Veterinarians Need to Know About Transmission, Diagnosis, and Control

This article summarises key clinical and biosecurity lessons from a recent AAEP and Equine Disease Communication Center (EDCC) outbreak webinar, combined with current field reports and established EHV-1 guidance. It is written for practising equine veterinarians managing referral, ambulatory, performance, or breeding populations during active outbreaks.

Large equine events are wonderful for the sport, but they create the perfect environment for rapid viral transmission: shared airspace, shared touchpoints, high horse density, long travel distances, fatigued immune systems, and lots of unknown exposure histories.

This event was no exception.

More than 670 horses attended from 31 U.S. states and four Canadian provinces – a level of interstate mixing that means one infected horse can rapidly seed cases across the country. And we’re now seeing exactly that pattern.

Official cases have been confirmed in:

• Texas
• Oklahoma
• Louisiana
• South Dakota
• Colorado
• New Mexico
• Arizona
• Washington

Importantly, some recent EHM cases reported in Maryland and Pennsylvania were not associated with the Waco outbreak, reminding us that EHV-1 is always circulating at a low level in the population.

The index event associated with this outbreak occurred in Waco, Texas, underscoring how rapidly EHV-1 can disseminate following large, multi-state competitions.

Why This EHV-1 Outbreak Escalated So Quickly

Several converging factors made this outbreak particularly difficult to contain:

• High horse density at a single large event
• Extensive interstate and international travel immediately post-event
• Pre-clinical viral shedding prior to detection
• Delayed identification of index cases
• Shared infrastructure and human-mediated transmission

None of these factors are unique – but when they occur together, even robust biosecurity protocols can be overwhelmed.

EHV-1 vs. EHM – A Quick Refresher

EHV-1 refers to the virus; EHM (equine herpesvirus myeloencephalopathy) refers specifically to the neurologic disease caused by EHV-1.

EHV-1 is a common herpesvirus in the equine population. Most horses encounter it in their lifetime. But certain strains – and certain immune responses – cause far more severe disease.

There are three primary presentations:

1. Respiratory form

Fever, nasal discharge, cough. Common. Highly contagious.

2. Asymptomatic shedding

A horse may carry and shed the virus without looking sick. This is often the quiet driver of outbreaks.

3. Neurologic form (EHM)

This is the version we all fear – inflammation, micro-thrombosis, and spinal cord/brain injury leading to:

• Hindlimb weakness or ataxia
• Dog-sitting posture
• Toe-dragging
• Urine dribbling
• Tail paralysis
• Difficulty standing
• Cranial nerve deficits
• Rarely, seizures

In most outbreaks, 60–70% of affected horses recover with good nursing care. But the cases that deteriorate can do so quickly and severely, and those horses need you at your absolute best.

Transmission: Why Controlling This Virus Is So Challenging

The panel reinforced a critical point: EHV-1 doesn’t need direct contact to spread.

It moves via:

• Aerosolised respiratory particles
• Nose-to-nose contact
• Shared buckets, hoses, wash bays
• Human hands, clothing, phones, equipment
• Trailers
• Stalls
• In cool, moist conditions, EHV-1 can survive in the environment for up to three weeks.

In other words: this is a virus that exploits any lapse in biosecurity.

And horses rarely stay in one place long enough for an outbreak to remain contained without intervention.

Diagnostics: What Matters Most Right Now

If you remember nothing else from this section, remember this:

Fever is your earliest and most important warning sign.

Daily temperature monitoring is the fastest route to case detection, and it’s the tool that most reliably stops small clusters becoming large outbreaks.

Testing recommendations from the panel:

• Use PCR testing
• Collect nasal swabs + whole blood
• Prioritise clinically affected horses
• Repeat testing in 72 hours if signs worsen or remain suspicious

And a crucial myth-buster:

Both EHV-1 genotypes (“neuropathogenic” and “non-neuropathogenic”) can cause EHM.
Do not hang clinical decision-making on genotype alone.

Clinical Red Flags That Warrant Immediate Isolation

Veterinarians should isolate and investigate any horse showing:

• Fever ≥ 38.5°C (101.3°F)
• Sudden hindlimb weakness or ataxia
• Urinary incontinence or tail paresis
• Rapid deterioration within 24–48 hours
• Clustered febrile cases at a single facility

Early isolation based on fever alone remains the most effective outbreak-limiting intervention.

Treatment – What Actually Helps

The experts presented a clear, evidence-based approach:

1. Anti-inflammatory therapy

• NSAIDs for fever and initial signs
• Corticosteroids for moderate to severe neurologic cases (under appropriate supervision)

2. Antivirals

• Valacyclovir, ideally early in disease
• Best outcomes when used alongside supportive care and anti-inflammatory treatment

3. Anticoagulants

• Aspirin or heparin may reduce severity by addressing the micro-clotting that drives neurologic signs
• Several studies show significantly improved outcomes with combined antiviral + anticoagulant protocols

4. Supportive care

• IV fluids
• Nutritional support
• Catheterisation
• Frequent repositioning
• Sling support in recumbent horses

Your nursing care often determines whether a horse stabilises or deteriorates.

Biosecurity: The Hard Truth We All Know

If you work with performance horses, you already practice good biosecurity. But during an outbreak, “good” isn’t enough – it needs to be excellent, consistent, and non-negotiable.

The webinar highlighted the following essentials:

• Separate equipment for each horse
• No shared tack, buckets, hoses, or grooming tools
• Avoid communal wash racks
• Temperature checks twice daily
• Immediate isolation of any febrile horse
• Clear traffic flows for staff, farriers, chiropractors, dentists, and grooms
• Clean hands, clean clothing, clean boots — every time

And this line stuck with me:

“How you manage the first fever determines the size of your outbreak.”

They’re right.

Movement Restrictions & State-Level Updates

Texas’ State Veterinarian outlined how rapidly this outbreak has required:

• Immediate quarantine orders
• Over 1,500 trace investigations
• Coordination with more than 30 states and four Canadian provinces
• Event postponements
• Widespread communication with industry bodies, show organisers, and veterinarians

If your clients are travelling, you should monitor your state’s daily updates closely.

Why Every Vet Should Be Using the EDCC

The Equine Disease Communication Center (EDCC) remains the single most reliable source for outbreak reporting in the United States.
It ensures that:

• Only verified information is published
• Updates come directly from state officials and licensed veterinarians
• Rumours are filtered out
• Alerts are timely, accurate, and standardised

🟧 Report confirmed or suspected cases here:
https://equinediseasecc.org/forms/report-outbreak

🟧 EHV-1 clinical signs and information:
https://www.equinediseasecc.org/equine-herpesvirus

🟧 AAEP guidelines for EHV-1 and EHV-4:
https://aaep.org/resource/aaep-infectious-disease-guidelines-equine-herpesvirus-1-4/

Bookmark those links. Use them often.

In Summary

This outbreak is significant – but not unprecedented.
And with the right information, clear communication, and disciplined biosecurity, it is absolutely manageable.

As equine veterinarians, we are at our strongest when:

• We respond quickly
• We stay informed
• We support one another
• And we treat both horses and clients with steadiness and clarity

If you haven’t done so already, I strongly recommend watching the full AAEP/EDCC webinar. It’s practical, it’s thorough, and it’s an excellent briefing for anyone supporting horses in performance, breeding, or travel.

And as always – if you need a second opinion, updated guidance, or clinical support during an outbreak, early collaboration and clear communication remain essential. Outbreak control is most effective when veterinarians stay connected and informed.

Our horses deserve nothing less.

Key Takeaways for Equine Veterinarians

• EHV-1 spreads efficiently via aerosol, fomites, and human movement
• Fever detection remains the single most important early warning tool
• Genotype does not predict neurologic outcome
• Combined antiviral, anti-inflammatory, and supportive care improves prognosis
• Early isolation decisions determine outbreak scale