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Abstract

Objective—To determine the proportion of adult cattle that change test status when an ELISA for antibodies against Mycobacterium avium subsp paratuberculosis (MAP) is used to assay samples collected twice at variable intervals and to determine whether cows with an initial strong positive result were more likely to maintain positive status, compared with all cows with an initial positive result.

Design—Cross-sectional observational study.

Animals—3,757 adult dairy cattle.

Procedure—Serum samples were obtained twice from cattle at intervals ranging from 77 to 600 days between collections. Samples were tested with an ELISA for detection of antibodies to MAP.

Results—Of 157 cattle with initial positive results (value for the sample divided by the value for positivecontrol serum [S/P] ≥ 0.25), 62 (39.5%) had negative results for the second sample. Of 71 cattle with an initial S/P value ≥ 0.40, 13 (18.3%) had a negative result (S/P < 0.25) for the second sample. Of 33 cattle with an initial S/P ≥ 0.70, 3 (9.1%) had a negative result (S/P value < 0.25) for the second sample. Interval between collection of samples did not affect results.

Conclusions and Clinical Relevance—Many cows changed ELISA status between samples collected at variable intervals. Cows with an initial high S/P value (≥ 0.70) were more likely to maintain positive status than cows classified as positive on the basis of cutoff values of ≥ 0.25 or ≥ 0.40. Veterinarians should expect variability in ELISA results when repeated testing of cattle is used as part of an MAP control program. (J Am Vet Med Assoc 2002;220:1685–1689)

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in Journal of the American Veterinary Medical Association

Abstract

Objective—To estimate seroprevalence of Mycobacterium avium subsp paratuberculosis (MAP) infection among adult dairy cows in Colorado and determine herd-level factors associated with the risk that individual cows would be seropositive.

Design—Cross-sectional observational study.

Animals—10,280 adult (≥ 2 years old) dairy cows in 15 herds in Colorado.

Procedure—Serum samples were tested with a commercial ELISA. A herd was considered to be infected with MAP if results of mycobacterial culture of ≥ 1 individual cow fecal sample were positive or if ≥ 1 culled cow had histologic evidence of MAP infection.

Results—424 of the 10,280 (4.12%) cows were seropositive. Within-herd prevalence of seropositive cows ranged from 0% to 7.82% (mean, 2.6%). Infection was confirmed in 11 dairies. Cows in herds that had imported ≥ 8% of their current herd size annually during the preceding 5 years were 3.28 times as likely to be seropositive as were cows in herds that imported < 8%. Cows in herds with ≥ 600 lactating cows were 3.12 times as likely to be seropositive as were cows in herds with < 600 lactating cows. Cows in herds with a history of clinical signs of MAP infection were 2.27 times as likely to be seropositive as were cows in herds without clinical signs.

Conclusions and Clinical Relevance—Annual importation rate, herd size, and whether cows in the herd had clinical signs typical of MAP infection were associated with the risk that individual cows would be seropositive for MAP infection. (J Am Vet Med Assoc 2004;225:97–101)

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in Journal of the American Veterinary Medical Association

Abstract

Objective—To evaluate sensitivities at the herd level of test strategies used in the Voluntary Johne's Disease Herd Status Program (VJDHSP) and alternative test strategies for detecting dairy cattle herds infected with Mycobacterium paratuberculosis.

Design—Nonrandom cross-sectional study.

Sample Population—64 dairy herds from Pennsylvania, Minnesota, Colorado, Ohio, and Wisconsin. Fifty-six herds had at least 1 cow shedding M paratuberculosis in feces; the other 8 herds were free from paratuberculosis.

Procedure—For all adult cows in each herd, serum samples were tested for antibodies to M paratuberculosis with an ELISA, and fecal samples were submitted for bacterial culture for M paratuberculosis. Sensitivities at the herd level (probability of detecting infected herd) of various testing strategies were then evaluated.

Results—Sensitivity at the herd level of the testing strategy used in level 1 of the VJDHSP (use of the ELISA to test samples from 30 cows followed by confirmatory bacterial culture of feces from cows with positive ELISA result) ranged from 33 to 84% for infected herds, depending on percentage of cows in the herd with positive bacterial culture results. If follow- up bacterial culture was not used to confirm positive ELISA results, sensitivity ranged from 70 to 93%, but probability of identifying uninfected herds as infected was 89%.

Conclusions and Clinical Relevance—Results suggest that the testing strategy used in the VJDHSP will fail to identify as infected most dairy herds with a low prevalence of paratuberculosis. A higher percentage of infected herds was detected if follow-up bacterial culture was not used, but this test strategy was associated with a high probability of misclassifying uninfected herds. (J Am Vet Med Assoc 2002;220: 1053–1057)

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in Journal of the American Veterinary Medical Association