Assessment of the efficacy of an autogenous vaccine against Yersinia pseudotuberculosis in young Merino sheep

Abstract

AIMS

To assess the efficacy of an autogenous vaccine against Yersinia pseudotuberculosis III in preventing clinical disease and deaths due to yersiniosis in young Merino sheep, and to determine the effect of vaccination on the prevalence of faecal shedding of pathogenic Yersinia spp., daily liveweight gain, and development of antibodies to Yersinia spp. following vaccination and natural exposure.

METHODS

In six groups (three groups each from two farms) of young Merino sheep, 148–150 animals were systematically allocated to be vaccinated twice with an autogenous, formalin- killed bacterin vaccine containing Y. pseudotuberculosis serotype III or to remain non-vaccinated. All vaccinated and non-vaccinated sheep were run together in their original groups throughout the trial. Faecal and blood samples were collected, and liveweight measured, at the time of vaccination and subsequently over a 6-month period to determine faecal shedding of Y. enterocolitica and Y. pseudotuberculosis, seroprevalence of antibodies to Yersinia outer membrane proteins (YOP) and changes in liveweight.

RESULTS

None of the six trial groups experienced an outbreak of clinical yersiniosis during the study period. On Farm A, the prevalence of shedding of either or both Yersinia spp. was <40% on all but one sampling occasions. On Farm B the prevalence of shedding of both Yersinia spp. peaked at 98%, 96 days after vaccination. Mean liveweight and daily liveweight gain at the end of the study were similar in vaccinated and non-vaccinated groups on both farms (p>0.1), as was the prevalence of faecal shedding of Yersinia spp. (p>0.2), and the proportion of animals that became seropositive for antibodies to YOP following vaccination (p>0.1).

CONCLUSIONS AND CLINICAL RELEVANCE

This vaccine had, at most, limited effects on seroconversion and, under the conditions of this study, had no demonstrable impact on liveweight, mean daily liveweight gain or faecal shedding of Yersinia spp. Further studies are needed to determine the efficacy of this vaccine during outbreaks of yersiniosis or following experimental challenge with pathogenic Yersinia spp..

KEY WORDS:

• Sheep
• bacterial enteritis
• Yersinia pseudotuberculosis
• vaccine
• yersiniosis

Acknowledgements

This study was supported by Meat and Livestock Australia (grant B.STU.0247) and the Scobie and Claire Mackinnon Trust. The authors gratefully acknowledge D Rees and other staff from the Mackinnon Project for their technical assistance, as well as the farm owners and managers who generously donated their time and facilities for this study. We would also like to express our gratitude to R Bushell and R Batey for their assistance with microbiology and antibiotic sensitivity testing. Dr Kelly Stanger received an Australian Postgraduate Award to support her involvement in this study.