Mo. Rev. Stat. § 287.420
No proceedings for compensation for any accident under this chapter shall be maintained unless written notice of the time, place and nature of the injury, and the name and address of the person injured, has been given to the employer no later than thirty days after the accident, unless the employer was not prejudiced by failure to receive the notice. No proceedings for compensation for any occupational disease or repetitive trauma under this chapter shall be maintained unless written notice of the time, place, and nature of the injury, and the name and address of the person injured, has been given to the employer no later than thirty days after the diagnosis of the condition unless the employee can prove the employer was not prejudiced by failure to receive the notice.
§ 287.420, RSMo