Mo. Rev. Stat. § 57.117

Hereafter no sheriff in this state shall appoint any under sheriff or deputy sheriff unless the person so appointed shall be, at the time of his or her appointment, a bona fide resident of this state or of an adjoining state. The provisions of this section authorizing the appointment of a person as an under sheriff or deputy sheriff who is a bona fide resident of an adjoining state shall not apply to a sheriff of any county with a charter form of government and with more than six hundred thousand but fewer than seven hundred thousand inhabitants or any city not within a county.