Whenever all the handlers of not less than 75 per centum of the volume of anti-hog-cholera serum and hog-cholera virus which is handled in the current of interstate or foreign commerce, or so as directly to burden, obstruct, or affect interstate or foreign commerce, have signed a marketing agreement entered into with the Secretary of Agriculture pursuant to section 852 of this title, the Secretary of Agriculture shall issue an order which shall regulate only such handling in the same manner as, and contain only such terms and conditions as are contained in such marketing agreement, and shall from time to time amend such order in conformance with amendments to such marketing agreement. Such order shall terminate upon termination of such marketing agreement as provided in such marketing agreement.