The distribution of pathogenic Y. enterocolitica
strains has a close correlation with geographic and climate factors but with evident regional disparity. The distribution of pathogenic Y. enterocolitica
strains in China (25
) is similar to that in other countries (8
). This was confirmed by using epidemiology and laboratory isolation, which found that typical pathogenic Y. enterocolitica
strains carrying the ail
, and virF
virulence genes are found in the cold northern city of Xuzhou in Jiangsu Province and are only of serotypes O:3 and O:9, whereas no pathogenic Y. enterocolitica
strains were found in the warmer southern coastal city of Nantong in Jiangsu Province. No pathogenic strains were isolated by the research team of Enshu Yu in Nantong in the 1980s, and the distribution of serotypes was similar to our previous findings (E. Yu et al., unpublished data). It is interesting that Y. enterocolitica
O:8, a common and important pathogenic serotype in the United States, is nonpathogenic in China, even though many O:8 strains were isolated from humans and animals (24
The Y. enterocolitica ystB
gene encodes a heat-resistant enterotoxin similar to YstA (17
). Strains carrying ystB
are considered nonpathogenic and lack the pathogenic marker genes ail
, and virF
encoded on the chromosome and plasmid pYV. Some researchers believe that these strains are pathogenic for humans and can cause local outbreaks (19
). A total of 157 strains carrying the ystB
gene were isolated during this study, and a high proportion of the isolates from diarrhea patients in Xuzhou belonged to biotype 1A. Our results show that it is difficult to detect pathogenic strains with these characteristics compared to the ease of detection of typical pathogenic strains. We consider strains without these virulence genes to be unable to infect hosts, as they cannot break through the immune barrier; therefore, the enterotoxin encoded by ystB
cannot induce pathogenesis. This requires further study.
It is well-known that swine are the primary source of pathogenic Y. enterocolitica strains throughout the world, and this is also the case in most provinces of China. However, the investigation in Xuzhou, Jiangsu Province, during 2004 and 2008 showed that dogs belonging to farmers may be a source of pathogenic Y. enterocolitica infections in these areas (Table ). Y. enterocolitica is extensively distributed among animals in the two areas investigated, where most strains are nonpathogenic. Some pathogenic serotype O:3 strains were isolated from dogs and swine belonging to farmers.
The primary PFGE patterns of the serotype O:3 strains in the Chinese Y. enterocolitica
isolate bank (with more than 10 strains) were K6GN11C30012 (54 strains), K6GN11C30021 (31 strains), K6GN11C30015 (23 strains), and K6GN11C30016 (11 strains). Strains isolated from dogs belonging to farmers shared the same PFGE pattern (pattern K6GN11C30021), showing that they may have come from the same clone. Strains isolated from humans and livestock in Henan Province and the city of Tianjin also had the same PFGE pattern (pattern K6GN11C30021) (Fig. ). Strains isolated from dogs in recent years have the same PFGE patterns as strains isolated from diarrhea patients in 1994, indicating that the strains have persisted for more than 10 years in humans and dogs. Another restriction enzyme instead of NotI, FseI, was chosen for use in PFGE analysis of these strains, and the results confirmed that the six strains isolated from dogs had the same PFGE patterns as strains isolated from diarrhea patients in 1994 (Fig. ). This suggests that Y. enterocolitica
strains from humans and dogs belonging to farmers are closely correlated and provides molecular evidence that the same pathogen transmitted between humans and dogs came from the same source, e.g., pigs. Domestic dogs (Canis familiaris
) belonging to farmers may be infected through pigs or other infected livestock, and the source of the organism for the farmers is thus the dogs belonging to those farmers because of the close contact of dogs with people. As the PFGE patterns of isolates from both sources were indistinguishable, we considered the transmission of the pathogen to be from animals (domestic dogs belonging to farmers) to humans, but it also may be possible that the reverse route of transmission, that is, from humans to animals, occurs. Fredriksson-Ahomaa et al. reported that raw pork can be an important source of Yersinia enterocolitica
4/O:3 infections in dogs and cats, and these pets may be one source of human infection, especially in young children (6
The primary PFGE patterns of serotype O:9 strains in the Chinese Y. enterocolitica isolate bank were K6GN11C90008 (58 strains), K6GN11C90010 (54 strains), K6GN11C90018 (47 strains), and K6GN11C90004 (25 strains). The pattern of the O:9 strain from a diarrhea patient in 1999 from Xuzhou was K6GN11C90007. At the same time, one O:3 strain isolated from a pig had a different PFGE pattern. These results show the polymorphism of the pathogenic strain distribution and that more transmission strands exist in Jiangsu.
Unfortunately, Y. enterocolitica pathogenic strains have not been isolated from specimens from outpatient clinics. We speculate that the reason is that the symptoms of diarrhea caused by Y. enterocolitica are often mild and most cases are self-limiting. Farmers with mild diarrhea seldom visit outpatient clinics due to poverty, and it is too late for strain isolation when the patients show complications and visit the hospital. Additionally, it is rare for humans to be infected through food due to Chinese cooking and eating habits, even though many domesticated animals in China carry pathogenic Y. enterocolitica isolates. Chinese seldom consume raw food, and when they cook food, they completely kill any Y. enterocolitica isolates that may be present. Our surveillance program for Y. enterocolitica was adjusted in 2009. The investigation includes households in the community and countryside, and fecal samples from patients with mild diarrhea are used to isolate Y. enterocolitica to obtain a more complete understanding of Y. enterocolitica infections among the local population.
In conclusion, our data show that pathogenic Y. enterocolitica
strains are of serotype O:3, and the distribution of pathogenic strains showed a notable regional disparity, with such strains primarily occurring in the northern cold city of Xuzhou, similar to the case in other countries (8
). Although it is difficult to determine the pathway of transmission of the pathogen between human and domestic dogs (Canis familiaris
) belonging to farmers, resulting in the zoonotic disease of yersiniosis, some strains isolated from human and dogs in Xuzhou had the same subtype. Therefore, we believe that dogs belonging to farmers may be another primary carrier of pathogenic Y. enterocolitica
in local communities and may be a potential source for human infections.