Horses offered for entry from Mexico shall be inspected as provided in §§ 93.306 and 93.323; shall be accompanied by a certificate and otherwise handled as provided in § 93.314; and shall be quarantined and tested as provided in § 93.324: Provided, That horses offered for importation from tick-infected areas of Mexico shall be chute inspected, unless in the judgment of the inspector a satisfactory inspection can be made otherwise. If upon inspection they are found to be apparently free from fever ticks, before entering the United States they shall be dipped once in a permitted arsenical solution or be otherwise treated in a manner approved by the Administrator.