Mo. Rev. Stat. § 57.111
Whenever any sheriff or deputy sheriff of any county in this state is expressly requested, in each instance, by a sheriff of an adjoining county of this state to render assistance, such sheriff or deputy shall have the same powers of arrest in such county as he has in his* own jurisdiction.
(L. 1979 S.B. 259 § 1)
*Words "they have in their" appear in original rolls.