A horse on the backside at Parx Racing has tested positive for equine herpesvirus, according to the track’s Twitter account.

New Parx Quarantine: A horse @parxracing tested positive for eq. herpes. No horses may leave the backstretch until further notice.

— Parx Racing (@parxracing) February 15, 2014

The track previously was under a quarantine for the entire month of December, as well as parts of November and January, for the same reason.  That quarantine was lifted on January 10.  However, a series of weather-related cancellations in December left the track with a sparse schedule.

It is standard in such cases for horses to be prohibited from shipping in or out of the racetrack in order to prevent the spread of the disease.  Animal control authorities typically seek a 21-day period in which all animals are free of symptoms of the disease before lifting such a quarantine.