Mo. Rev. Stat. § 334.685

Any person licensed to practice as a physical therapist assistant in this state who retires from the practice shall file with the board an affidavit, on a form furnished by the board, which states the date on which the person retired from practice and such other information required by the board to verify such retirement. If a person reengages in practice as a physical therapy assistant after a person submits an affidavit of retirement required by this section, the person shall reapply for licensure as required by sections 334.650 to 334.685.