Objective—To develop models that could be used to
predict, for dairy calves, the age at which colostrumderived
bovine viral diarrhea virus (BVDV) antibodies
would no longer offer protection against infection or
interfere with vaccination.
Design—Prospective observational field study.
Animals—466 calves in 2 California dairy herds.
Procedure—Serum BVDV neutralizing antibody titers
were measured from birth through 300 days of age.
The age by which colostrum-derived BVDV antibodies
had decayed sufficiently that calves were considered
susceptible to BVDV infection (ie, titer ≤ 1:16) or
calves became seronegative was modeled with survival
analysis methods. Mixed-effects regression
analysis was used to model colostrum-derived BVDV
antibody titer for any given age.
Results—Half the calves in both herds became
seronegative for BVDV type I by 141 days of age and for
BVDV type II by 114 days of age. Rate of antibody decay
was significantly associated with antibody titer at 1 to 3
days of age and with whether calves were congenitally
infected with BVDV. Three-month-old calves were predicted
to have a mean BVDV type-I antibody titer of 1:32
and a mean BVDV type-II antibody titer of 1:16.
Conclusions and Clinical Relevance—Results provide
an improved understanding of the decay of
BVDV-specific colostrum-derived antibodies in dairy
calves raised under typical field conditions.
Knowledge of the age when the calf herd becomes
susceptible can be useful when designing vaccination
programs aimed at minimizing negative effects of
colostrum-derived antibodies on vaccine efficacy
while maximizing overall calf herd immunity. (J Am
Vet Med Assoc 2002;221:678–685)
Objective—To estimate risk and identify risk factors
for congenital infection with bovine viral diarrhea virus
(BVDV) not resulting in persistent infection and examine
effect of congenital infection on health of dairy
Procedures—Calves from 2 intensively managed drylot
dairies with different vaccination programs and
endemic BVDV infection were sampled before ingesting
colostrum and tested with their dams for BVDV
and BVDV serum-neutralizing antibodies. Records of
treatments and death up to 10 months of age were
obtained from calf ranch or dairy personnel. Risk factors
for congenital infection, including dam parity and
BVDV titer, were examined by use of logistic regression
analysis. Effect of congenital infection on morbidity
and mortality rates was examined by use of survival
Results—Fetal infection was identified in 10.1% of
calves, of which 0.5% had persistent infection and
9.6% had congenital infection. Although dependent
on herd, congenital infection was associated with
high BVDV type 2 titers in dams at calving and with
multiparous dams. Calves with congenital infection
had 2-fold higher risk of a severe illness, compared
with calves without congenital infection.
Conclusions and Clinical Relevance—The unexpectedly
high proportion of apparently healthy calves found
to be congenitally infected provided an estimate of the
amount of fetal infection via exposure of dams and
thus virus transmission in the herds. Findings indicate
that congenital infection with BVDV may have a negative
impact on calf health, with subsequent impact on
herd health. (Am J Vet Res 2003;64:358–365)
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 method of probability diagnostic
assignment (PDA) that uses continuous serologic
measures and infection prevalence to estimate
the probability of an animal being infected, using
Neospora caninum as an example.
Animals—196 N caninum-infected beef and dairy cattle
and 553 cattle not infected with N caninum; 50
dairy cows that aborted and 50 herdmates that did
Procedure—Probability density functions corresponding
to distributions of N caninum kinetic ELISA
results from infected and uninfected cattle were estimated
by maximum likelihood methods. Maximum
likelihood methods also were used to estimate N caninum
infection prevalence in a herd that had an excessive
number of abortions. Density functions and the
prevalence estimate were incorporated into Bayes
formula to calculate the conditional probability that a
cow with a particular ELISA value was infected with N
Results—Probability functions identified for infected
and uninfected cattle were Weibull and inverse
gamma functions, respectively. Herd prevalence was
estimated, and probabilities of N caninum infection
were determined for cows with various ELISA values.
Conclusions and Clinical Relevance—Use of PDA
offers an advantage to clinicians and diagnosticians
over traditional seronegative or seropositive classifications
used as a proxy for infection status by providing
an assessment of the actual probability of
infection. The PDA permits use of all diagnostic information
inherent in an assay, thereby eliminating a
need for estimates of sensitivity and specificity. The
PDA also would have general utility in interpreting
results of any diagnostic assay measured on a continuous
or discrete scale. Am J Vet Res (2002;
Objective—To determine management, fish, and
environmental risk factors for increased mortality and
an increased proportion of runts for white sturgeon
exposed to white sturgeon iridovirus (WSIV) and
white sturgeon herpesvirus-2 (WSHV-2).
Animals—White sturgeon in 57 tanks at 1 farm and
observations made for fish at another farm.
Procedure—A prospective cohort study was conducted.
Data on mortality, proportion of runts, and
potential risk factors were collected. Five fish from
each tank were examined for WSIV and WSHV-2 via
inoculation of susceptible cell lines and microscopic
examination of stained tissue sections. An ANCOVA
was used to evaluate effects of risk factors on mortality
and proportion of runts.
Results—Major determinants of number of dead fish
(natural logarithm [ln]-transformed) were spawn,
source (90% confidence interval [CI] for regression
coefficient, 0.62 to 2.21), and stocking density (90%
CI, 0.003 to 0.03). Main predictors of proportion of
runts (ln-transformed) were spawn, mortality incidence
density (90% CI, 0.004 to 0.03), age (90% CI,
–0.012 to –0.004), and the difference in weight
between the largest and smallest nonrunt fish (90%
CI, 0.0002 to 1.24). Additional observations indicated
a possible protective effect attributable to previous
exposure to the viruses.
Conclusions and Clinical Relevance—Mortality and
proportion of runts for white sturgeon after exposure
to WSIV and WSHV-2 may be reduced for a farm at
which the viruses are endemic by selection of specific
broodstock, stocking with fish that survived outbreaks
of viral disease, using all-in, all-out production,
and decreasing stocking densities. (Am J Vet Res
Objective—To estimate sensitivity and specificity of
4 commonly used brucellosis screening tests in cattle
and domestic water buffalo of Trinidad, and to compare
test parameter estimates between cattle and
Animals—391 cattle and 381 water buffalo.
Procedure—4 Brucella-infected herds (2 cattle and 2
water buffalo) and 4 herds (2 of each species) considered
to be brucellosis-free were selected. A minimum
of 100 animals, or all animals > 1 year of age, were
tested from each herd. Serum samples were evaluated
for Brucella-specific antibodies by use of standard
plate agglutination test (SPAT), card test (CT),
buffered plate agglutination test (BPAT), and standard
tube agglutination test (STAT). A Bayesian approach
was used to estimate sensitivity and specificity of
diagnostic tests without the use of a gold standard,
assuming conditional independence of tests.
Results—Sensitivity and specificity estimates in cattle,
respectively, were SPAT, 66.7 and 98.9; CT, 72.7
and 99.6; BPAT, 88.1 and 98.1; and STAT, 80.2 and
99.3. Corresponding test estimates in water buffalo,
respectively, were SPAT, 51.4 and 99.3; CT, 90.4 and
99.4; BPAT, 96.3 and 90.7; and STAT, 75.0 and 98.8.
Sensitivity of the CT and specificity of the BPAT were
different between cattle and water buffalo with at
least 95% probability.
Conclusions and Clinical Relevance—Brucellosis
serologic test performance varied by species tested,
but BPAT had the highest sensitivity for screening cattle
and water buffalo. Sensitivity and specificity of
more than 2 screening tests can be estimated simultaneously
without a gold standard by use of Bayesian
techniques. (Am J Vet Res 2002;63:1598–1605)
Objective—To estimate receiver-operating characteristic
(ROC) curves for a competitive ELISA (c-ELISA)
that is used in serodiagnosis of brucellosis in water
buffalo and cattle, to determine the most appropriate
positive cutoff value for the c-ELISA in confirmation of
infection, and to evaluate species differences in
Sample population—Sera from 4 herds of cattle
(n = 391) and 4 herds of water buffalo (381).
Procedure—Serum samples were evaluated for
Brucella-specific antibodies by use of a c-ELISA. On
the basis of previous serologic test results, iterative
simulation modeling was used to classify animals as
positive or negative for Brucella infection without the
use of a gold standard. Accuracy of c-ELISA for diagnosis
of infection was compared between cattle and
water buffalo by comparison of areas under ROC
Results—A positive cutoff value of 30% inhibition for
c-ELISA yielded sensitivity and specificity estimates,
respectively, of 83.9 and 92.6% for cattle and 91.4 and
95.4% for water buffalo. A positive cutoff value of
35% inhibition yielded sensitivity and specificity estimates,
respectively, of 83.9 and 96.2% for cattle and
88.0 and 97.4% for water buffalo. Areas under ROC
curves were 0.94 and 0.98 for cattle and water buffalo,
Conclusion and Clinical Relevance—ROC curves
can be estimated by use of iterative simulation methods
to determine optimal cutoff values for diagnostic
tests with quantitative outcomes. A cutoff value of
35% inhibition for the c-ELISA was found to be most
appropriate for confirmation of Brucella infection in cattle
and water buffalo. (Am J Vet Res 2003;64:57–64)