The tonsillar carriage of Yersinia species by pigs: a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Veterinary Science at Massey University

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Massey University
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The impetus for this study arose due to the increasing isolation of species of Yersinia from people, with pigs being suspected as major reservoirs of human pathogenic strains of the organism in New Zealand. The general aims of the study, conducted in two phases, among pigs from several herds sent for slaughter at an abattoir in Palmerston North were: (i) to determine the presence of human pathogenic strains of Yersinia in the tonsils of slaughtered pigs and their distribution among selected herds. (ii) to determine the seasonal effect on prevalence of isolation and type of organism isolated, and (iii) to determine the in vitro virulence characteristics of strains of the organism isolated from the tonsils of slaughter pigs, and their potential public health implications. The first phase involved a cross-sectional study, conducted between August and September, 1993. Tonsils were collected from 124 pigs from eight farms and were examined for the presence of species of Yersinia. A total of 77 (62.1%) strains of Yersinia were isolated, this consisted of 42 (33.9%), 27 (21.8%), 7 (5.6%) and 1 (0.8%) strains of Y. enterocolitica. Y. pseudotuberculosis, Y. frederiksenii and Y. kristensenii respectively. Yersinia enterocolitica serotypes 0:3, 0:5, 27 and Y. pseudotuberculosis comprised 26 (33.8%), 12(15.6%) and 27 (35.1%) of the total number of isolates respectively. Yersiniae were isolated from all eight farms with individual farm prevalences ranging from 20% to 100%, while the number of species per farm ranged from 1 to 3. The pyrazinamidase activity test correctly identified 48 of the isolates as pathogenic or non-pathogenic yersiniae, (a specificity of 96%). The second phase, a longitudinal study, was conducted over a period of twelve months (February 1993 – January 1994), among pigs from four farms, selected according to the particular strain of Yersinia prevailing in the herd. A total of 705 pigs were examined for the carriage of species of Yersinia in their tonsils. A total of 264 isolates were obtained, consisting of 198 (75%), 55 (20.8%), 5 (1.9%), and 1 (0.4%) strains of Y. enterocolitica, Y. pseudotuberculosis, Y. intermedia, Y. frederiksenii and Y. kristensenii respectively. Yersinia enterocolitica serotypes 0:5,27 and 0:3 comprised 105 (39.8%) and 78 (29.5%) of the total number of isolates respectively. Yersinia pseudotuberculosis comprised 55 (20.9%) with serotype III, 39 (14.8%) the most consistently isolated serotype. Yersiniae were isolated throughout the year particularly in the colder months. Yersinia enterocolitica serotypes 0:3 and 0:5,27 were found throughout the year with the lowest prevalence in the warmer months. However, a seasonal variation existed among serotypes of Y. pseudotuberculosis, with serotypes I and II found only in the winter and spring. Serotype III was found throughout the year, except for February. During phase two of the study, 150 isolates of Yersinia were tested for in vitro virulence-associated characteristics. The autoagglutination test. CR-MOX agar, and the pyrazinamidase assay, coupled with salicin and aesculin tests, were highly successful in separating pathogenic from non-pathogenic strains of Y. enterocolitica. Likewise, the three assays successfully identified virulence activity in the majority of strains of Y. pseudotuberculosis with specificity among the three assays ranging between 90-100% for both Y. pseudotuberculosis and Y. enterocolitica. The study also revealed marked variation in prevalence and type of Yersinia species isolated from pigs from different farms. The fact that particular serotypes predominate and persist on specific farms strongly suggest that there are factors such as source of pigs, management practices or contact with other animals which determine their status. Identification of these determinates could lead to control or eradication of important yersiniae from pig farms. The overall prevalence of 41.1% ranks New Zealand among countries with reported high isolation rates of the organism and further emphasises the fact that pigs constitute major reservoirs for human pathogenic strains of Yersinia worldwide. The infection among slaughter pigs in New Zealand may be of human health concern and thus warrants further investigation particularly to determine whether the strains isolated from pigs are identical to those involved in human disease.
Swine Diseases, Yersinia