Miss. Code § 41-55-55
After an air ambulance service district has been formed, any other county within the same highway patrol district or any county lying immediately adjacent to the district may join the district by the same procedure as if it were initiating the district, including the appointment of an additional member to the existing board of directors.
After a district has been in existence for at least fifteen (15) years, a county may join the district only with the approval of the board of directors of the district after the board of directors first finds that the services to the new member county will not result in an undue financial burden upon the district. The board of directors of the district may establish guidelines for the admission of new member counties but may not require a financial contribution in excess of that authorized by Section 41-55-47, Mississippi Code of 1972.
Miss. Code § 41-55-55