Objective—To investigate herd characteristics and management practices associated with a high seroprevalence of Mycobacterium avium subsp paratuberculosis (MAP) in dairy herds in central California.
Sample Population—60 randomly selected cows from each of 21 dairy herds.
Procedures—Sera of selected cows were tested for antibodies against MAP by use of an ELISA test kit. Cows with a test sample-to-positive control sample (S:P) ratio of ≥ 0.25 were considered seropositive, and herds with ≥ 4% seropositive cows were considered high-seroprevalence herds. Data on herd characteristics and management practices were collected via interviews with owners. Bayesian logistic regression was used to model the predictive probability of a herd having a high seroprevalence on the basis of various herd characteristics and management practices.
Results—9 of 21 (43%) herds were classified as high-seroprevalence herds. Five variables (history of previous signs of paratuberculosis in the herd, herd size, exposing cattle to water from manure storage lagoons, feeding unsalable milk to calves, and exposing heifers ≤ 6 months old to manure of adult cows) were included in the predictive model on the basis of statistical and biological considerations. In large herds, the predictive probability of a high seroprevalence of MAP infection decreased from 0.74 to 0.39 when management changed from poor to good practices. In small herds, a similar decrease from 0.64 to 0.29 was predicted.
Conclusions and Clinical Relevance—The seroprevalence of MAP infection in California dairies may be reduced by improvements in herd management practices.
Objective—To investigate the epidemiologic and financial impacts of targeted sampling of subpopulations of cows, compared with random sampling of all cows, for classification of dairy herd infection status for paratuberculosis.
Animals—All cows from 4 infected herds with a low-to-moderate prevalence of paratuberculosis and from 1 noninfected herd in California.
Procedure—The infection status of each cow was classified on the basis of results of an ELISA or combined ELISA and fecal culture results. Thirteen sampling schemes designed to randomly sample cows on the basis of lactation number, stage of lactation, and milk production were evaluated. Sampling without replacement was used to obtain a probability of herd detection of paratuberculosis for each evaluated sampling method and for simulated sample sizes between 30 and 150 cows. Marginal cost-effectiveness analysis was used to determine the cost increase relative to the increase in detection probability.
Results—Sampling cows in the third or higher lactation and ≥200 days into lactation yielded the highest detection probability in most instances, resulting in a detection probability that was 1.4 to 2.5 times that obtained by sampling 30 cows in the second or higher lactation. Costs of testing via the alternative method with a 95% detection probability were approximately $300 lower in a high-prevalence herd (31%) and $800 lower in a low-prevalence herd (9%), compared with use of the reference method.
Conclusions and Clinical Relevance—Detection of herds with paratuberculosis could be improved, and costs of testing substantially reduced by sampling targeted groups of cows.
Objective—To develop a better system for classification of herd infection status for paratuberculosis (Johne's disease [JD]) in US cattle herds on the basis of the risk of potential transmission of Mycobacterium avium subsp paratubeculosis.
Sample—Simulated data for herd size and within-herd prevalence; sensitivity and specificity for test methods obtained from consensus-based estimates.
Procedures—Interrelationships among variables influencing interpretation and classification of herd infection status for JD were evaluated by use of simulated data for various herd sizes, true within-herd prevalences, and sampling and testing methods. The probability of finding ≥ 1 infected animal in herds was estimated for various testing methods and sample sizes by use of hypergeometric random sampling.
Results—2 main components were required for the new herd JD classification system: the probability of detection of infection determined on the basis of test results from a sample of animals and the maximum detected number of animals with positive test results. Tables were constructed of the estimated probability of detection of infection, and the maximum number of cattle with positive test results or fecal pools with positive culture results with 95% confidence for classification of herd JD infection status were plotted. Herd risk for JD was categorized on the basis of 95% confidence that the true within-herd prevalence was ≤ 15%, ≤ 10%, ≤ 5%, or ≤ 2%.
Conclusions and Clinical Relevance—Analysis of the findings indicated that a scientifically rigorous and transparent herd classification system for JD in cattle is feasible.
Objective—To evaluate sensitivity of microbial culture
of pooled fecal samples for detection of
Mycobacterium avium subsp paratuberculosis (MAP)
in large dairy herds and assess the use of the method
for estimation of MAP prevalence.
Animals—1,740 lactating cows from 29 dairy herds
Procedure—Serum from each cow was tested by
use of a commercial ELISA kit. Individual fecal samples
were cultured and used to create pooled fecal
samples (10 randomly selected fecal samples/pool; 6
pooled samples/herd). Sensitivity of MAP detection
was compared between Herrold's egg yolk (HEY) agar
and a new liquid culture method. Bayesian methods
were used to estimate true prevalence of MAP-infected
cows and herd sensitivity.
Results—Estimated sensitivity for pooled fecal
samples among all herds was 0.69 (25 culture-positive
pools/36 pools that were MAP positive).
Sensitivity increased as the number of culture-positive
samples in a pool increased. The HEY agar
method detected more infected cows than the liquid
culture method but had lower sensitivity for
pooled fecal samples. Prevalence of MAP-infected
cows was estimated to be 4% (95% probability
interval, 2% to 6%) on the basis of culture of
pooled fecal samples. Herd-level sensitivity estimate
ranged from 90% to 100% and was dependent
on prevalence in the population and the sensitivity
for culture of pooled fecal samples.
Conclusions and Clinical Relevance—Use of pooled
fecal samples from 10 cows was a cost-effective tool
for herd screening and may provide a good estimate
of the percentage of MAP-infected cows in dairy
herds with a low prevalence of MAP. (Am J Vet Res