12/20/2025
๐ ๐๐๐-๐: ๐๐ก๐ ๐๐ข๐ฌ๐ค ๐๐จ๐ฎ ๐๐จ๐งโ๐ญ ๐๐ฅ๐ฐ๐๐ฒ๐ฌ ๐๐๐
My last post clearly caused a kerfuffle when I posted the forms of EHV-1 that were reportABLE rather than the number of cases that have currently been reportED. So here I am to make amends! ๐
I want to start by sharing some foundational information about this virus and how outbreaks can begin.
๐๐ก๐ ๐๐๐ซ๐ญ ๐๐ ๐๐จ๐งโ๐ญ ๐๐๐
What many people donโt realize is that most horses are exposed to EHV-1, or its close relative EHV-4, very early in life. Research suggests that 80โ90% of horses are infected by the time they are two years old (Allen et al., 2002). Mare and foal populations play an important role in this cycle. Pregnancy and parturition can trigger reactivation of the virus in broodmares, leading to transmission to their foals (Paillot et al., 2008). This creates a quiet, ongoing epidemiological cycle that helps explain why exposure is so widespread across the horse population.
The result is a large number of horses that appear completely healthy but are latently infected and carrying EHV-1 throughout many equine populations.
Latency means the horseโs immune system doesnโt eliminate the virus after infection, it contains it. The virus isnโt gone, it just enters a dormant or โsleepingโ state, hiding in specific cells such as nerve or immune cells. Under certain stressors such as transport, illness, pregnancy, foaling, or management changes, the virus can reactivate, begin replicating again, and be shed. Importantly, this shedding can occur with little or no outward sign of illness.
This is one of the most important, and least understood, drivers of EHV-1 transmission. These horses can act like a Trojan horse, quietly spreading virus without anyone realizing it. And the hardest part? We still donโt know exactly what triggers reactivation and why it occurs in some horses and not others.
๐๐จ ๐ฐ๐ก๐๐ญ ๐๐จ ๐ฐ๐ ๐ค๐ง๐จ๐ฐ ๐๐๐จ๐ฎ๐ญ ๐ญ๐ก๐ข๐ฌ ๐๐ฎ๐ซ๐ซ๐๐ง๐ญ ๐จ๐ฎ๐ญ๐๐ซ๐๐๐ค?
The map shows the current (as of 12-17-25) reported counts of different forms of EHV-1 associated with the Womenโs Professional Rodeo Association (WPRA) World Finals and Elite Barrel Race event in Waco, TX. I have also included EHM cases reported in other states that have not been tied to this outbreak, which highlights that this virus is endemic - meaning it is consistently present within the horse population, even outside of recognized outbreaks.
The cases in Pennsylvania and Maryland were reported in November, and a case reported in Minnesota in December involved a different viral strain. Canada was not included, as there are no active neurologic cases reported there at this time.
For the WPRA-associated outbreak, current reports include:
โข 47 EHM cases
โข 9 EHV-1 cases without neurologic signs
โข 4 EHV-1 cases without clinical signs
Itโs important to remember that not all states require reporting of non-neurologic EHV-1 cases, which means these numbers likely do not capture the full scope of viral circulation. Reported cases represent only what has been detected and disclosed.
๐๐จ๐ง๐๐ฅ๐ฎ๐ฌ๐ข๐จ๐ง
This is exactly why biosecurity matters all the time, not just during outbreaks, and not just for horses that look sick.
Cheers,
Dr. DeBoer
Allen GP. Respiratory Infections by Equine Herpesvirus Types 1 and 4, 2002. Int. Vet. Inf. Serv. 2002.
Paillot R, Case R, Ross J, Newton R, Nugent J. Equine herpes virus-1: virus, immunity and vaccines. Open Vet. Sci. J. 2008;2(1):268-91.
