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
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)
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
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
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)