The highly infectious bacterial disease spreads through direct contact, making biosecurity crucial in preventing outbreaks.
32 horses are potentially exposed, but no new cases have been identified.
This is the eighth facility in the state to quarantine due to the virus in two months.
Vaccination and biosecurity measures can help protect horses who travel as well as those they may infect when they return home.
Forty potentially infected horses may have moved back to other parts of Multnomah or Clackamas county.