Mo. Rev. Stat. § 509.100: Failure to deny, effect.

Averments in a pleading to which a responsive pleading is required, other than those as to the amount of damage, are admitted when not denied in the responsive pleadings. When a reply is filed, whether required or not, all affirmative defenses of the answer which are not denied in the reply are deemed admitted. Averments in a pleading to which a responsive pleading is not filed and is not required shall be taken as denied or avoided.