Mo. Rev. Stat. § 61.200

The county commission may, in their discretion, appoint the county surveyor of their respective counties to the office of county highway engineer, provided he be thoroughly qualified and competent, as required by sections 61.160 to 61.310; and when so appointed, he shall receive the compensation fixed by the county commission, and such fees as are allowed by law for his services as county surveyor; provided, the county surveyor may refuse to act or serve as such county highway engineer, unless otherwise provided by law. In the event that the county highway engineer cannot properly perform all the duties of his office, he shall, with the approval of the county commission, appoint one or more assistants, who shall receive such compensation as may be fixed by the county commission.