Mo. Rev. Stat. § 278.090
Wherever in this law a referendum is provided for and is used, it shall be conducted by secret ballot in such a manner that any person connected with, or any official of such referendum or election cannot know how any land representative votes. Ballot boxes similar to those used in any general election in this state shall be used and such boxes shall only be opened and the ballots counted in the presence of at least three persons who are residents of the soil and water district and who shall sign a statement they witnessed the opening of the ballot boxes and the counting of the ballots therein, but such ballot boxes shall not be opened or any ballots counted until immediately after the time expires for casting a ballot. Any person designated to conduct such a referendum or assist in such a referendum, and in that manner gains knowledge as to how any land representative voted and reveals such knowledge to any other person shall be guilty of a misdemeanor.
§ 278.090, RSMo