Mo. Rev. Stat. § 509.210
In an action for libel or slander, it shall not be necessary to state in the petition any extrinsic facts, for the purpose of showing the application to the plaintiff of the defamatory matter out of which the claim arose, but it shall be sufficient to state, generally, that the same was published or spoken concerning the plaintiff; and if such allegation be not controverted in the answer, it shall not be necessary to prove it on the trial; in other cases it shall be necessary. The defendant may, in his answer, allege both the truth of the matter charged as defamatory and any mitigating circumstances admissible in evidence to reduce the amount of damages; and whether he prove the justification or not, he may give in evidence the mitigating circumstance.
§ 509.210, RSMo