Mo. Rev. Stat. § 233.332
Notwithstanding any provisions of section 233.330 to the contrary, in any county of the third classification with a population according to the most recent federal decennial census in excess of four thousand nine hundred and not in excess of six thousand one hundred, if a special road district commissioner resigns, dies, is no longer eligible to serve as a commissioner or the commissioner's term of office expires, then the county commission may by ordinance appoint a person to fill the vacancy or in the alternative hold an election to fill such vacancy pursuant to section 233.330. No person shall be elected or appointed who is not an eligible voter in the special road district. Any person appointed pursuant to this section shall serve a term equal in duration to that of the previous special road district commissioner replaced by such person. The county commission may appoint a county commissioner to fill such vacancy.
§ 233.332, RSMo