Each district health officer shall receive the salary prescribed for such position by the state personnel board. The salary shall constitute full authority for the district health officers and they shall receive no other salary payment or fees from any other public source. No district health officer shall engage in the private practice of medicine, maintain an office for the practice of medicine, nor accept nor receive any fee, gratuity or emolument of any form for rendering medical or surgical service to any citizen of this state, except that permission for such practice may be given by the secretary of the health and environment department [secretary of health] in any district, the board of which has declared an emergency to exist.