Investigators say the fire also ignited 13 small grass fires along Highway 152 and Interstate 5 on Sunday. The Ford F-450 truck pulling the horse trailer was not damaged.
The driver told investigators he was unaware of the blowout until other drivers alerted him that his tire was smoking.
By the time firefighters arrived, the trailer was fully engulfed in flames. The blaze was contained within minutes, but the horses were found dead.
The driver had been on his way from a horse show in Southern California to his home in Oregon.
The Associated Press