They are Arizona, California, Colorado, Idaho, New Mexico, Nevada, Oregon, Utah and Washington.
Suspected and confirmed cases involving the potentially deadly virus are reported to be under voluntary or state quarantine, APHIS said.
Of the 75 confirmed cases, 58 are in horses that were at the Ogden, Utah event.
Eleven horses associated with the outbreak are dead or have been euthanized, APHIS said.
There are 15 newly identified premises with suspect or confirmed cases identified in the latest report.
"APHIS will continue to monitor the situation and work with the States to make decisions as needed to control the spread of the virus."
Of the 75 infected horses, 28 suffered from EHM (Equine Herpes Virus Myeloencephalopathy) - the neurological form of the disease.
Of the 11 deaths, nine were in horses exposed at the Ogden, Utah, cutting event, that appears to be at the centre of the outbreak. Two of the fatalities were in horses exposed secondarily.