Prevalence of human pathogenic Yersinia enterocolitica in Swedish pig farms

Abstract

Background: Pigs are the most important reservoir for human pathogenic Yersinia enterocolitica. We investigated the herd prevalence of human pathogenic Y. enterocolitica in Swedish pig farms by analysing pen faecal samples using a cold enrichment of 1 week and thereafter subsequent plating onto chromogenic selective media (CAY agar).

Results: Pathogenic Y. enterocolitica was found in 32 (30.5%) of the 105 sampled farms with finisher pigs. Bioserotype 4/O:3 was identified at all but one farm, where 2/O:9 was identified. Pen-prevalence within the positive herds varied from 1/4 to 4/4 pens. The calculated intra-class correlation coefficient ICC (0.89) from a model with a random effect for grouping within herd indicated a very high degree of clustering by herd. None of the explored risk factors, including herd size, herd type, pig flow, feed type, access to outdoors, evidence of birds and rodents in the herd, usage of straw, number of pigs in sampled pen and age of pigs in pen were significantly associated with Y. enterocolitica status of the pen. The use of high pressure washing with cold water was significantly associated with Y. enterocolitica in the pen (OR = 84.77, 4.05-1772). Two culture methods were assessed for detection of Y. enterocolitica, one of which included the use of a chromogenic agar (CAY agar) intended for detection of human pathogenic Y. enterocolitica. The chromogenic media was found equal or superior to traditional methods and was used in this study. The isolates obtained were characterised by biotyping, serotyping, mass spectrometry (MALDI-TOF) and PCR. Characterisation by MALDI-TOF gave identical results to that of conventional bioserotyping. All porcine isolates were positive for the ail and inv genes by PCR, indicating that the isolates were most likely pathogenic to humans.

Conclusions: Human pathogenic Y. enterocolitica was found in nearly one-third of the Swedish pig farms with finisher pigs. The use of high pressure washing with cold water was associated with the presence of Y. enterocolitica in the pen. A modified culturing method using a chromogenic agar was efficient for detection of pathogenic Y. enterocolitica in pig faeces. The use of masspectrometry for identification and subtyping was in agreement with conventional biotyping and serotyping methods.

Keywords: Biotyping; Farm; Mass spectrometry; Pig; Prevalence; Risk factors; Serotyping; Yersinia enterocolitica; Zoonosis.

Fig. 1

a Yersinia enterocolitica bioserotype 2/O:9 isolate, displaying pink colour with a mauve coloured “bull´s eye”, on CHROMagar™ Y. enterocolitica (CAY) plate incubated at 25 °C for 24–48 h. b Swarming colonies of a Yersinia enterocolitica bioserotype 1B/O:8 isolate on CHROMagar™ Y. enterocolitica (CAY) plate incubated at 25 °C for 24–48 h

Fig. 2

a Yersinia enterocolitica bioserotype 4/O:3 isolate on CHROMagar™ Y. enterocolitica (CAY) plate incubated at 25 °C for 24–48 h. White colony formations with a transparent “bull’s eye”. b Yersinia enterocolitica bioserotype 1A/O:8 isolate on CHROMagar™ Y. enterocolitica (CAY) plate incubated at 25 °C for 24–48 h. Blue swarming colonies with dark metallic blue “bull’s eye”

Fig. 3

Number of pig herds sampled per month. Black cells in the diagram represent pen samples where Y. enterocolitica was isolated; grey cells represent negative samples