Losses of adult sheep due to ovine Johne's disease in 12 infected flocks over a 3‐year period

Whittington

et al. 2007

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 (<2%), medium (2-10%) and high (>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.

Section: Introductionmentioning

confidence: 99%

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

Whittington

et al. 2007

“…paratuberculosis (MAP), results in substantial economic losses due to reduced production, increased mortalities and the costs associated with the disease control programmes (Bush et al, 2006). The infection usually enters a flock by introduction of infected sheep and then spreads within the flock mainly by the faecal-oral route.…”

Section: Introductionmentioning

confidence: 99%

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

Johnson

et al. 2013

This study was conducted to evaluate the effectiveness of Gudair TM 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 Gudair TM 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 prevaccination 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 postvaccination 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 90 th percentile of the prevalence 2 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.

“…Losses to the American dairy industry alone were estimated to be US$ 200 to 250 million annually (Ott et al, 1999). Similarly in Australia, paratuberculosis caused an average reduction of income of AU$ 13,715 per farm per year on 12 infected Merino sheep flocks (Bush et al, 2006). Control programmes initiated in many countries to combat the disease have met with limited success, partly due to incomplete understanding of disease epidemiology, particularly the factors that affect disease prevalence and mycobacterial survival.…”

Section: Introductionmentioning

confidence: 99%

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

Whittington

et al. 2009

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 <2%, medium 2-10% and high >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 2 consequent retention of the bacteria in the topsoil, thus making them available in higher numbers to grazing sheep.