DISCLOSURE
FUNDING:  Chao-feng Chen is supported by Hangzhou Science and Technology Sanitation Project (2016A02).
CONFLICTS OF INTEREST: The authors have no conflict of interest to declare.
ETHICS: There were not any ethics problems in our paper.
ACKNOWLEDG EMENTS: Miao-fu Li and Jing Wu: contributed to write the manuscript; Chao-feng Chen contributed to design of this work, statistical analysis and write article; Mei-jun Liu: contributed to evaluate quality and retrieved the required data, Chao-feng Chen: contributed to review the literature, performed the selection of the studies and helped gather references for the manuscript. Dr Yi-zhou Xu: contributed to design of this work, revised and approved the final version of the manuscript.