The 2025 EHV-1/EHM outbreak produced half of last year’s cases in just one month. See why this event-linked spike is more serious and unlike a typical year. A Data-Driven Look at an Unusual and Concerning Trend
The equine community is no stranger to infectious disease. Equine Herpesvirus-1 (EHV-1) and its neurologic manifestation, Equine Herpesvirus Myeloencephalopathy (EHM), are reportable conditions that appear every year in the United States. But the scope, speed and concentration of the current outbreak linked to a major horse event has set it apart from historical norms—and the data makes that clear.
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One Month vs. One Year: A Statistical Red Flag
Between Nov. 1, 2025, and Dec. 4, 2025, EDCC reporting confirmed:
- 56 total cases
- 16 EHV-1
- 40 EHM
In comparison, during the entire calendar year of 2024 (Jan. 1–Dec. 31), there were:
- 111 total cases
- 8 EHV-1
- 103 EHM
Percentage Comparison: One Month of 2025 vs. All of 2024
Despite representing only about 34 days of activity, the current outbreak accounts for:
- 56 of 111 total cases last year → 50.5% of 2024’s annual total in just one month
- 40 of 103 EHM cases last year → 38.8% of last year’s EHM total in one month
- 16 EHV-1 cases vs. 8 last year → 200% (double) all EHV-1 cases in 2024
When a single month produces half the total case load of the previous year, epidemiologists consider it a meaningful deviation—not a seasonal fluctuation.
Why These Numbers Matter
EHM is not just another respiratory disease. It is a highly contagious neurologic illness with an estimated 60–80% survival rate, meaning up to one in three clinically affected horses may not survive even with aggressive treatment.
In typical years, EHM cases are sporadic, appearing in isolated clusters or individual farm outbreaks. But this year’s data shows:
- An unusually fast rise in case counts
- A higher proportion of EHV-1–positive horses converting to neurologic EHM
- A multi-state spread pattern consistent with large-scale horse movement
This pattern aligns with what officials have reported: the current outbreak is linked to a major equestrian event, dramatically altering exposure risk, viral load distribution and interstate transmission.
Why This Outbreak Is Different
1. Concentration of Horses in One Location
High-density stabling environments, especially multi-week events, provide exactly the conditions EHV-1 needs to spread: shared airspace, shared equipment, communal wash racks and movement between barns.
2. Increased Stress and Travel
Hauling, competition schedules, and environmental stress can dampen a horse’s immune system, making viral shedding more likely.
3. Accelerated Transmission Rate
The numbers show an outbreak that is not unfolding gradually; instead, cases are appearing in multiple states within days, highlighting exposure taking place from a large event where a mass amount of horses were located during a specific time.
4. Neurologic Cases Are Occurring Early and Often
In this outbreak, the EHM to EHV-1 is high:
- 40 EHM / 16 EHV-1 = 71% neurologic expression.
This elevated neurologic conversion rate is a primary reason veterinarians are on heightened alert.
A Reportable Disease, But Not a Predictable One
EHV-1 and EHM are always reportable, and every year brings confirmed cases. But reportable does not mean routine, especially when the numbers diverge this sharply from baseline.
In 2024, the spread was steady across 12 months. In 2025, the epicenter event created amplification conditions, resulting in a spike that compressed nearly half a year’s usual caseload into four weeks.
The difference is not simply timing; it is scale, concentration and velocity.
The Takeaway: This Outbreak Is Not “Normal”
The data from Nov. 1 to Dec. 4 confirms that:
- Case counts are far ahead of what would be expected based on historical patterns.
- The neurologic severity is high.
- The spread is rapid and geographically broad.
- The outbreak origin was linked to a major event which has created an exponential expansion that is not seen in typical seasonal activity.
As horses traveled to other events and back home secondary exposures climbed, ongoing reporting will remain essential and can be found on EDCC’s website: https://equinediseasecc.org/ .
This outbreak is a reminder of what the equine industry has long understood:
Biosecurity is not optional, and large-scale equestrian gatherings and horse owners who attend large events should be prepared for disease scenarios while preparing and returning from events.
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