A large-scale fatal outbreak of Pasteurella multocida among wild rodents in southwestern China

Background: Pasteurella multocida is an important and old zoonotic pathogen worldwide which has an impressive host spectrum including numerous domestic and wild animals as well as birds, causing specific diseases or outbreak with great economic impact. It has never been reported that P. multocida can cause an epidemic in wild rodents. In June 5–17, 2016, more than 1000 rodent deaths of an unknown cause quickly spread in the PuEr City, Yunnan province, southwestern China. Methods: The rodents in affected areas and outside of the epidemic areas were collected and screened for possible known pathogens including Yersinia pestis, rabies virus and hantavirus as well as other bacteria. The possible bacterial pathogens were isolated both by culture medium and by mouse inoculation in parallel. The isolates were identified by the Vitek GNI card and PCR assays for 16S rRNA genes. The pathogen strains were selected for whole genome sequencing analysis. Results: A total of 123 rodents were collected from 25 sample sites at affected area, among of which, all 119 dead rodents were negative for the pathogen under consideration except P. multocida, and all four live rodents were negative for P. multocida. In addition, 480 rodents collected from other 23 counties outside of the epidemic area in Yunnan were negative for with P. multocida. A total of 14 strains of P. multocida (six directly isolated from the field rodents and eight from the experimental mice that were injected with the organ substrates from the dead rodents) belonged to serogroup A and serogroup F represented by 9 N and 20 N were identified in these epidemic areas. Whole genome sequencing revealed that the serogroup F strain shared 99% similarity to P. multocida Pm70 from chicken, but contained a 50 k bp insertion sequence. The serogroup A strain shared 95% similarity to P. multocida FDAARGOS_385 from a human patient, but contained four large structural differences. Histological abnormalities were identified in the livers, lungs, hearts and brains of the inoculated mice. Conclusions: The simultaneous occurrence of both serotypes of P. multocida may have caused this sudden onset of mortality across the local rodent population in Yunnan Province, China. Further attention should be paid to this old bacterium in the world.