Search Results

Abstract

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 calves.

Animals—466 calves.

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 analysis methods.

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)

Abstract

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.

Abstract

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)

Abstract

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)

Abstract

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 not abort.

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 caninum.

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; 63:318–325)

Abstract

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.

Animals—446 calves.

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 2001;219:1426–1431)

Abstract

Objective—To compare seroprevalence of antibodies against equine arteritis virus (EAV) in horses residing in the United States with that of imported horses.

Design—Serologic survey.

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)

Abstract

Objective—To determine whether time until culling or risk of culling was associated with Neospora caninum serostatus among Holstein cattle in dairy herds in Ontario.

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 herds.

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)

Abstract

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 water buffalo.

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)

Abstract

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 c-ELISA function.

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 curves.

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, respectively.

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)