Update on EHV-1

Just a brief update on my last post, as the USDA released the latest stats on the EHV-1 outbreak last night.

     The incidence of equine herpes myeloencephalopathy (EHM) - i.e. actual neurologic disease caused by EHV-1 - remained very low, and only 11 deaths were reported for the more than 2,000 horses exposed.

     Here's a summary of the relevant stats:

• 415 horses were exposed at the event (primary exposure)
• another 1,635 horses were exposed via contact with horses returning from the event (secondary exposure) or via contact with horses exposed to the horses... (tertiary exposure)

     Of the 2,050 horses who were exposed, only 75 horses (3.7%) were confirmed as having EHV-1 infection or disease:

• 47 horses (2.3%) were confirmed to have EHV-1 infection but no neurologic signs
• 28 horses (1.4%) were confirmed to have EHM (i.e. EHV-1 + neurologic signs)

     Of those 75 horses confirmed as having EHV-1 infection or disease,

• 58 horses (77%) were primary infections
• 17 horses (23%) were secondary or tertiary infections

     So, although the exposure rate was high, the disease rate was low. Also, prompt quarantine measures seem to have been very effective at limiting secondary and tertiary infections.

     Assuming that all of the 11 horses who died had confirmed EHM (i.e. none of the horses merely positive for EHV-1 died),

• 39% of horses with EHM died
• 14.7% of horses with EHV-1 infection or disease died
• 0.5% of all exposed horses died

     So, while the mortality rate in horses with neurologic disease caused by EHV-1 was relatively high, the overall mortality rate in this outbreak was very low.

      And for my clients in Washington state:

• only 35 horses in our state were exposed, all of them primary exposures; there have been no reported secondary exposures
• only 4 horses were confirmed to have EHV-1 infection (but no neurologic signs)
• only 3 horses were confirmed to have EHM
• no horses have died

     Furthermore, most of the confirmed cases were in eastern Washington (Spokane, Chelan, Asotin, and Whitman counties). Thurston is the only county in western WA that has had a confirmed case of EHV-1.

     I think the worst of this outbreak is over now. Lessons learned will likely take a while to be distilled. Hopefully we'll have learned something useful before the next outbreak...

More soon,

-Dr. Chris King-

Nature's Apprentice

www.animavet.com