Yersinia enterocolitica O9 as a possible barrier against Y. pestis in natural plague foci in Ningxia, China

A survey of Yersinia spp, as related to plague control, was made in Haiyuan of Ganning loess plateau plague focus, Yanchi of Inner Mongolia plateau pl ague focus, and Yinchuan city, as a control area, in Ningxia, China. In Hai yuan, where the main plague reservoir was Mongolian ground squirrel (Citell us alaschanicus) living in the prairie, Y. enterocolitica O9 was frequently isolated from pigs, dogs, rodents living in and around houses, but only ra rely from hare and Mongolian ground squirrel. In Yanchi, where the main pla gue reservoir was Mongolian gerbil (Meriones unguiculatus) living in the pr airie and Y. pestis, which was isolated from rodents up to 1991, Y. enteroc olitica O9 was sometimes isolated from pigs and rodents. In all areas, some strains of Y. enterocolitica O3 and Y. pseudotuberculosis serotypes 3 and 4b were also isolated from pigs, dogs, and from rodents. We propose that an epidemiological link exists between the prevalence of Y. pestis and Y. ent erocolitica O9 in domestic and rodents living in these areas in China. The residential area in Haiyuan may be protected against Y. pestis by the domes tic animals and rodents which acquired cross-protection against Y. pestis b y infection with Y. enterocolitica O9, but this is not the case in the Yanc hi district.