Five of the six horses previously identified with equine herpesvirus type 1 at Golden Gate Fields tested negative in samples taken Jan. 5, the director of the large animal clinic at the University of California-Davis reported.
"This strongly suggests that they are no longer shedding virus and therefore no longer contagious," the director, Dr. David Wilson, said in an e-mail sent the morning of Jan. 6.
He said re-sampling of the horses, all of whom are segregated in the barn of trainer Lloyd Mason at the Albany, Calif., track, would be conducted Jan. 7.
The remaining horse to test positive was the last one to become infected, Wilson said.
"Our hope is that he will become negative next week," Wilson said in the message. "We will retest him every couple of days so that we can find out as soon as possible when he becomes negative."
He said a modification or lifting of quarantine restrictions would be based on continued clinical monitoring of horses at the track, including daily temperature taking.
The California Horse Racing Board imposed travel restrictions on about 2,100 horses stabled at Golden Gate, Bay Meadows, and the Alameda County Fairgrounds in Pleasanton after the first horse in Mason's barn tested positive for the highly contagious virus on Dec. 28.
An update on the quarantine status is expected to be issued by Dr. Rick Arthur, the CHRB's equine medical director.
"The fact that we have had no new cases since the first horse was diagnosed ... together with the follow-up results on the six horses we identified as shedding the virus during surveillance sampling, is very encouraging," Wilson said.