The 10th state to join the list this week was Oklahoma, with one confirmed case of the neurological form of the disease.
The other states with confirmed cases are Arizona, California, Colorado, Idaho, New Mexico, Nevada, Oregon, Utah and Washington.
The death toll has risen by two since the last APHIS release a week ago, and now stands at 12. The two latest deaths were reported in California.
Of the 84 confirmed cases, 58 are horses that were at the Ogden, Utah, cutting horse event which appears to be at the centre of the outbreak.
In the last week, just two additional premises with suspected or confirmed cases have been reported.
Owners of horses known to have been exposed in this incident have been contacted by State Animal Health Officials, APHIS said, and suspect and confirmed cases are reported to be under voluntary or state quarantine.
All horse known exposed horses are reported to be under either voluntary or state quarantine.