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 determine the prevalence and effect of Neospora caninum infection and persistent infection (PI) with bovine viral diarrhea virus (BVDV) on weight gain, morbidity, and mortality rate in dairy-breed steer calves located on a feedlot in California.
Design—Prospective cohort observational study.
Animals—900 dairy-breed steer calves in 2 pens.
Procedures—The 3- to 4-month-old calves were evaluated for serum antibodies against N caninum and infection with BVDV at entry to the feedlot. Five months later, sera were again analyzed for anti–N caninum antibodies; calves that were determined to have BVDV infection initially were retested to evaluate PI status. Average daily gain, morbidity, and deaths were recorded for all calves.
Results—Among 900 calves, prevalence of N caninum infection was 16.7% (95% confidence interval, 14.3% to 19.3%); prevalence of BVDV-associated PI was 0.2% (95% confidence interval, 0.03% to 0.9%). Morbidity rate and time to first illness were not significantly different between calves that were seropositive or seronegative for N caninum.Atthe second sample collection, weight and average daily gain of calves that were seropositive for N caninum was less than that of seronegative steers in 1 pen, whereas these measures did not differ between groups in the other pen. Statistical power was insufficient to evaluate the effect of BVDV PI on any outcome measurement.
Conclusions and Clinical Relevance—Although N caninum serostatus had no significant effect on morbidity rate, some seropositive calves had reduced growth, compared with seronegative calves, 5 months after entry to the feedlot.
Objective—To estimate transmission of bovine viral
diarrhea virus (BVDV) and crude morbidity and mortality
ratios in BVDV-vaccinated and unvaccinated dairy
heifer calves managed under typical dairy drylot conditions.
Design—Randomized clinical trial.
Animals—106 female Holstein calves.
Procedure—Seroconversion rates for BVDV types I
and II and proportional morbidity and mortality ratios
were compared between calves given a killed BVDV
type-I vaccine at 15 days of age and a modified-live
BVDV type-I vaccine at 40 to 45 days of age (n = 53)
and calves given no BVDV vaccines (53). Sera were
collected at 45-day intervals as calves moved from
individual hutches to corrals holding increasingly larger
numbers of calves. Seroconversion was used as
evidence of exposure to BVDV.
Results—Crude proportional morbidity (0.16) and
mortality (0.17) ratios for control calves did not differ
significantly from those of vaccinated calves (0.28
and 0.12, respectively). The proportion of control
calves that seroconverted to BVDV type I through 9
months of age (0.629) was significantly higher than
that of vaccinated calves that seroconverted, unrelated
to vaccination, during the same period (0.536).
Estimated overall protective effect of vaccination
against BVDV type I through 4 to 9 months of age
was 48%. The proportion of control calves that seroconverted
to BVDV type II (0.356) was not different
from that of vaccinated calves (0.470).
Conclusions and Clinical Relevance—Findings suggest
that calfhood vaccination may be an appropriate
strategy to help reduce short-term transmission of
some but not necessarily all strains of BVDV. (J Am
Vet Med Assoc 2001;219:968–975)
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 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 evaluate risk of bovine viral diarrhea
virus (BVDV) infection between birth and 9 months of
age for dairy replacement heifers raised under typical
dry-lot management conditions.
Design—Longitudinal observational study.
Procedure—Calves were randomly selected from
2 dairies that used killed and modified-live BVDV
vaccines. Repeated serologic and BVDV polymerase
chain reaction assays were used to estimate
risk of BVDV infection in calves of various
ages (1 to 60 days; 61 to 100 days; 101 days to 9
months) and to estimate overall infection rate by 9
months of age.
Results—Risk of BVDV infection increased with age
(maximum risk, 150 to 260 days). Proportion of calves
infected with BVDV by 9 months of age was higher for
dairy A (0.665), compared with dairy B (0.357).
Percentage infected with BVDV type I did not differ
between dairy A (18.2%) and dairy B (15.2%), whereas
percentage infected with BVDV type II for dairy A
(50%) was twice that for dairy B (21%). Between 210
and 220 days of age, infection with BVDV regardless of
type was > 1.3%/d on dairy A and 0.5%/d on dairy B.
Conclusions and Clinical Relevance—Under drylot
conditions, a considerable amount of BVDV
infection may occur before 9 months of age. Risk
of infection increases with age. Although dairies
may appear to have similar management practices,
there can be considerably different risks of BVDV
infection among dairies. (J Am Vet Med Assoc
Objective—To compare seroprevalence of antibodies
against equine arteritis virus (EAV) in horses residing
in the United States with that of imported horses.
Sample Population—Serum samples from 364 horses
on 44 equine operations in California and 226 horses
imported from various countries.
Procedure—Serum samples were collected from
each imported horse and from up to 20 horses on
each operation. For resident horses, the number of
sampled horses on each operation was determined
on the basis of the number of horses on the operation.
Samples were tested for antibodies against EAV
by use of a serum neutralization test.
Results—1.9% of resident horses and 18.6% of
imported horses were seropositive to EAV, including
16.1% of imported stallions.
Conclusions and Clinical Relevance—Results indicate
that the EAV seroprevalence of horses residing
in California is considerably lower than that of imported
horses, including imported stallions. (J Am Vet
Med Assoc 2001;219:946–949)
Objective—To determine whether time until culling
or risk of culling was associated with Neospora caninum serostatus
among Holstein cattle in dairy herds
Design—Retrospective cohort study.
Animals—3,412 cows in 56 herds.
Procedure—Blood samples were collected, and
serum was tested for antibodies against N caninum.
Information on cows that were culled was collected
during the 1- to 2-year period that producers were
unaware of serostatus of individual cows in their
Results—Herd prevalence of N caninum-seropositive
cows ranged from 0 to 68.3% (median, 7.0%). During
the time of the study, 184 of 359 (51.3%) N caninum-seropositive
cows were culled, compared with 1,388
of 3,053 (45.5%) seronegative cows. Mean time from
blood sample collection to culling for seronegative
cows (289 days; 95% confidence interval, 280 to 299
days) was not significantly different from mean time
for seropositive cows (296 days; 95% confidence
interval, 269 to 323 days). Survival analysis indicated
that N caninum serostatus was not associated with
time to culling or risk of culling.
Conclusions and Clinical Relevance—Results suggest
that N caninum serostatus of Holstein cows in
Ontario was not significantly associated with either
time to culling or risk of culling. Thus, N caninum
serostatus alone should not be used to determine
whether cows should be culled. (J Am Vet Med Assoc 2002;221:1165–1168)
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)