Preliminary study on the potential cost at slaughter of OJD vaccination site lesions to the Australian sheep industry.

This report presents the results of a pilot project to predict the risk of discounts being applied in Australia due to OJD vaccination site lesions as vaccine use expands. This includes a review of discounting in New Zealand as well as a preliminary survey of the prevalence of lesions and actual discounts applied to slaughtered vaccinates in Australia. Information from New Zealand suggests that the greatest discounts are applied to trimmed high value lamb carcases that are destined for export in whole carcase form as a result of downgrading to a lower quality grade. In Australia the greatest risk of discounting is likely to be in purebred merino lambs sold into the prime lamb market, but the low proportion exported as carcases would limit the discount applied. The prevalence of lesions observed was 18% for mutton and 65% for lamb carcases. The value of the trim removed was insignificant, the labour cost of its removal was nil and no carcase was downgraded to a lower value grade. However this study was conducted at a time of low sheep supply in carcases vaccinated at the recommended site and caution should be exercised in extrapolating these results to oversupplied market situations or in sheep vaccinated at alternate sites

Risk factors for ovine Johne's disease in infected sheep flocks in Australia

We conducted a cross-sectional study in 2004-05 to investigate risk factors for ovine Johne’s disease (OJD) involving 92 infected Merino sheep flocks in Australia. In each enrolled flock we collected pooled faecal-samples from 3- to 5-year-old sheep and cultured them for Mycobacterium avium subsp. paratuberculosis (MAP) to determine their OJD status. Based on pooled faecal-culture (PFC) results, three outcome variables representing different facets of disease biology were derived: pool OJD status (binomial: positive or negative), log pool MAP number (continuous) and cohort OJD prevalence level (ordinal: low (10%) prevalence). We used these outcomes in three separate multivariable analyses to identify risk factors, which were based on a questionnaire administered during a face-to-face interview with the farmer. We found higher OJD infection in sheep whose dams had been in poor condition and kept at a high stocking rate during lambing and in sheep which had experienced a longer period of growth retardation during their lifetime. Flocks that had vaccinated for >2 years (rather than only 1 to 2 years) with a killed MAP vaccine had significantly lower OJD infection. In addition, practices including culling low body-weight sheep or selling sub-flocks experiencing high losses, sharing of roads between neighbouring farms, and greater frequency of application of super-phosphate fertilizers were associated with higher OJD. Of the confounders investigated, infection was higher in flocks experiencing high mortalities; in wethers compared to ewes; and in 3-year-old sheep compared to 4-year-old sheep. Keywords: risk factors, paratuberculosis, ovine Johne’s disease, Australia, Mycobacterium, epidemiology, cross-sectional studyMeat and Livestock Australi

Association of farm soil characteristics with ovine Johne's disease in Australia

Speculation about the association of soil characteristics with the expression of ovine Johne’s disease (OJD) prompted this cross-sectional study. We enrolled 92 sheep flocks in Australia during 2004-05 and in each enrolled flock collected pooled faecal samples from an identified cohort (group of same age and sex) of sheep and soil samples from the paddocks grazed by this cohort of sheep. Faecal pools were cultured to create three outcome variables: positive or negative status of faecal pools (pool OJD status, binary); the log number of viable Mycobacterium avium subsp. paratuberculosis (MAP) organisms per gram of faeces (log pool MAP number, continuous); and the prevalence of faecal shedders (cohort OJD prevalence level, ordinal: low 10%). Separate statistical models were then developed to investigate the association between soil characteristics and each outcome variable. Sheep raised on soils with a higher percentage of organic carbon and clay had a higher OJD prevalence whereas, sheep grazing on soils with a higher content of sand and nitrogen had a lower OJD prevalence. Iron content of the soil was positively associated with OJD infection but the association between soil pH and OJD was inconclusive. Parent soil type, the only farm level factor, was not significant in any of the final models. Study results indicate a higher risk of OJD in sheep raised on soils with greater organic matter and clay content. We hypothesise that this is due to adsorption of MAP to clay and the consequent retention of the bacteria in the topsoil, thus making them available in higher numbers to grazing sheep. Keywords: risk factors, paratuberculosis, mycobacterium, cross-sectional study, organic carbon, clay, sand, pH, iron.Meat and Livestock Australi

Comparison of pre- and post-vaccination ovine Johne's disease prevalence using a Bayesian approach

postprintThis study was conducted to evaluate the effectiveness of GudairTM vaccine in decreasing the prevalence of shedding of Mycobacterium avium subsp. paratuberculosis (MAP) in flocks of varying initial prevalence. Thirty seven self-replacing Merino flocks from New South Wales and Victoria (Australia) that had been vaccinating lambs with GudairTM for at least five years were enrolled in the study. These flocks had been tested prior to or at commencement of vaccination using pooled faecal culture, agar gel immunodiffusion or both tests. These pre-vaccination test results were used to estimate pre-vaccination prevalence. Post-vaccination prevalence was estimated from culture of usually 7 pools of 50 sheep collected from the enrolled flocks in 2008-2009, approximately five or more years after commencement of vaccination. A Bayesian model was developed to estimate and compare the pre- and post-vaccination prevalences for the enrolled flocks. Apparent pre- and post-vaccination prevalences for flocks were modelled as functions of the true pre- and post-vaccination prevalences, respectively, and the sensitivities and specificities of the respective diagnostic tests. Logit-normal models were specified on pre- and post-vaccination true prevalences and were then used to make inferences about the median and 90th percentile of the prevalence distributions and their differences. Priors were mostly specified based on published literature or analysis of abattoir surveillance data for this population of flocks. The analysis found a significant decline in ovine Johne’s disease prevalence from a pre-vaccination median prevalence of 2.72% [95% probability interval (PI): 1.40; 6.86%] to a post-vaccination median prevalence of 0.72% (0.39; 1.27%). However 30 of the 37 flocks still contained sheep that were shedding MAP in their faeces. The results suggest that vaccination with Gudair™ is usually effective in reducing the prevalence of faecal shedding but the response to vaccination is variable among flocks. This approach could be implemented in similar situations to compare prevalences where information from multiple diagnostic tests with varied sensitivities and specificities is available. Keywords: Ovine Johne’s disease; Gudair; Vaccination; Abattoir surveillance; Faecal culture; Agar gel immune-diffusion test

Comparison of pre- and post-vaccination ovine Johne's disease prevalence using a Bayesian approach

This study was conducted to evaluate the effectiveness of GudairTM vaccine in decreasing the prevalence of shedding of Mycobacterium avium subsp. paratuberculosis (MAP) in flocks of varying initial prevalence. Thirty seven self-replacing Merino flocks from New South Wales and Victoria (Australia) that had been vaccinating lambs with GudairTM for at least five years were enrolled in the study. These flocks had been tested prior to or at commencement of vaccination using pooled faecal culture, agar gel immunodiffusion or both tests. These pre-vaccination test results were used to estimate pre-vaccination prevalence. Post-vaccination prevalence was estimated from culture of usually 7 pools of 50 sheep collected from the enrolled flocks in 2008-2009, approximately five or more years after commencement of vaccination. A Bayesian model was developed to estimate and compare the pre- and post-vaccination prevalences for the enrolled flocks. Apparent pre- and post-vaccination prevalences for flocks were modelled as functions of the true pre- and post-vaccination prevalences, respectively, and the sensitivities and specificities of the respective diagnostic tests. Logit-normal models were specified on pre- and post-vaccination true prevalences and were then used to make inferences about the median and 90th percentile of the prevalence distributions and their differences. Priors were mostly specified based on published literature or analysis of abattoir surveillance data for this population of flocks. The analysis found a significant decline in ovine Johne’s disease prevalence from a pre-vaccination median prevalence of 2.72% [95% probability interval (PI): 1.40; 6.86%] to a post-vaccination median prevalence of 0.72% (0.39; 1.27%). However 30 of the 37 flocks still contained sheep that were shedding MAP in their faeces. The results suggest that vaccination with Gudair™ is usually effective in reducing the prevalence of faecal shedding but the response to vaccination is variable among flocks. This approach could be implemented in similar situations to compare prevalences where information from multiple diagnostic tests with varied sensitivities and specificities is available. Keywords: Ovine Johne’s disease; Gudair; Vaccination; Abattoir surveillance; Faecal culture; Agar gel immune-diffusion test