Toxoplasma gondii oocyst-specific antibodies and source of infection.

Infection source can determine cost-effective public health interventions. To quantify risk of acquiring Toxoplasma gondii from environmental sources versus from meat, we examined serum from pregnant women in Chile. Because 43% had oocyst-specific antibodies, we conclude that contaminated meat remains the primary source of infection but that environmental sources also pose substantial risk.

Simulation model for evaluation of testing strategies for detection of paratuberculosis in midwestern US dairy herds.

We developed a stochastic simulation model to compare the herd sensitivity (HSe) of five testing strategies for detection of Mycobacterium avium subsp. paratuberculosis (Map) in Midwestern US dairies. Testing strategies were ELISA serologic testing by two commercial assays (EA and EB), ELISA testing with follow-up of positive samples with individual fecal culture (EAIFC and EBIFC), individual fecal culture (IFC), pooled fecal culture (PFC), and culture of fecal slurry samples from the environment (ENV). We assumed that these dairies had no prior paratuberculosis-related testing and culling. We used cost-effectiveness (CE) analysis to compare the cost to HSe of testing strategies for different within-herd prevalences. HSe was strongly associated with within-herd prevalence, number of Map organisms shed in feces by infected cows, and number of samples tested. Among evaluated testing methods with 100% herd specificity (HSp), ENV was the most cost-effective method for herds with a low (5%), moderate (16%) or high (35%) Map prevalence. The PFC, IFC, EAIFC and EBIFC were increasingly more costly detection methods. Culture of six environmental samples per herd yielded >or=99% HSe in herds with >or=16% within-herd prevalence, but was not sufficient to achieve 95% HSe in low-prevalence herds (5%). Testing all cows using EAIFC or EBIFC, as is commonly done in paratuberculosis-screening programs, was less likely to achieve a HSe of 95% in low than in high prevalence herds. ELISA alone was a sensitive and low-cost testing method; however, without confirmatory fecal culture, testing 30 cows in non-infected herds yielded HSp of 21% and 91% for EA and EB, respectively.

Evaluation of five different antigens in enzyme-linked immunosorbent assay for the detection of avian pneumovirus antibodies.

Five different antigens were evaluated in enzyme-linked immunosorbent assay (ELISA) tests for the detection of avian pneumovirus (APV) antibodies. Two of the 5 antigens were prepared from recent APV isolates from Minnesota. The 2 older isolates were passage 63 of a strain currently used as a live, attenuated vaccine and a Colorado strain isolated for the first time in the United States and currently used in an ELISA test. The fifth antigen is based on an APV recombinant N-protein. Basic parameters and positive-negative threshold of the assays were established for all 5 antigens on the basis of data obtained by testing 46 known negative and 46 known positive serum samples. Subsequently, 449 field samples were tested by all 5 ELISAs. The optical density difference (ODD) was calculated by subtracting optical density of the sample in the negative antigen well from that in the positive antigen well. In the current ELISA test based on the Colorado strain, an ODD of 0.2 is considered to be the cutoff value to classify samples as negative or positive. In this study, however, use of different cutoffs, based on ODD of negative control plus 3 SD or values estimated from Receiver operating characteristic analysis, was considered to be more appropriate for the various antigens used. Overall person-to-person and day-to-day variability was found to be large for all tests using either ODD or sample to positive ratio to report results. In addition, results suggest that antigenicity of the APV isolates in the United States has not changed between 1997 and 2000.

The effect of anatomic site and age on detection of Staphylococcus aureus in pigs.

Despite active research into methicillin-resistant Staphylococcus aureus (MRSA) in pigs since 2004, the ecology of the susceptible ancestral organism has been neglected. A longitudinal study of pigs in 2 intensive production systems was conducted to investigate the effects of age and anatomical site on detection of S. aureus. Sampling was replicated in 2 cohorts per farm, with swabs collected from the nares, tonsils, skin (axilla), and rectum in lactating sows, suckling, weaned, and market-age pigs, plus the vagina of sows. No MRSA were isolated, but S. aureus was detected in a least 1 site in 175 (91.1%) out of 192 pigs. Pig-level prevalence did not differ among the age groups, but the proportion of positive samples (all sites) was higher in market-age pigs (75.2%) and nursery-age pigs (63.2%) than in sows (40.7%) and suckling piglets (38%). Prevalence did not differ among nasal (67.9%), skin (62.3%), and tonsil (61.7%) swabs, but was lower in rectal (42%) and vaginal swabs (39.6%). Multiple multilocus sequence typing (MLST) and spa types were found in both production systems, but all isolates were of ST398, ST9, or ST5. These MLST lineages have been variably predominant among reports of MRSA in pigs on 3 continents, and the presence of methicillin-sensitive variants in several countries raises the likelihood that MRSA in pigs has likely resulted from independent acquisition of the mecA gene by multiple S. aureus lineages that have been adapted to swine over the long term, rather than recent introduction of novel clones into swine populations.

Breed-specific reference intervals for assessing thyroid function in seven dog breeds.

Thyroxine (T4), free T4 (FT4), and thyrotropin (TSH) concentrations were measured in serum from 693 healthy representatives from 7 dog breeds (Alaskan Malamute, Collie, English Setter, Golden Retriever, Keeshond, Samoyed, or Siberian Husky) to determine whether breed-specific reference intervals (RIs) are warranted. Veterinarians reviewed the health history, performed a physical examination, and approved laboratory data for the enrolled dogs. Many purebred dogs had T4 and FT4 concentrations that were at, or below, the lower limits previously determined for non-breed-specific RIs. Mean concentrations of T4, FT4, and TSH varied significantly among breeds. The range of mean concentration of T4 (19.7 nmol/L [1.53 µg/dL] in English Setters to 29.0 nmol/L [2.25 µg/dL] in Keeshonds) and FT4 (12.6 pmol/L [0.98 ng/dL] in English Setters to 20.2 pmol/L [1.57 ng/dL] in Samoyeds) was considerable. Median TSH values ranged from 6.10 mIU/L (0.07 ng/mL; Alaskan Malamute and Golden Retriever) to 17.6 mIU/L (0.26 ng/mL; Collie). Mean T4 and FT4 concentrations were higher in females. Increasing age was associated with decreasing T4 and FT4, and increasing TSH concentration. The substantial ranges across breeds of measures of central tendency (mean, median) for all hormones indicate that breed-specific RIs are warranted. RIs encompassing the central 95% of reference values for all breeds combined, and for individual breeds, were calculated using nonparametric (TSH) and robust (T4, FT4) methods. Use of breed-specific RIs in combination with careful attention to the potential for pre-analytical and analytical variability in test results will improve thyroid function assessment in these breeds.

Evaluation of result variability with a commercial Johne's disease enzyme-linked immunosorbent assay kit and repeat testing of samples.

Two hundred one serum samples from individual dairy cows with a range of results on initial testing with a commercial Johne's disease enzyme-linked immunosorbent assay (ELISA) kit were repeat tested 5 times in each of 2 laboratories with kits produced by the same manufacturer. The results for the samples with all 10 replicates showed that the values for individual samples often had a coefficient of variation greater than 20%. As expected, the standard deviation for the results increased as the average value increased and the coefficient of variation was greater in samples with low mean values. The different lots of the commercial ELISA kit used in this study had a significant effect on both the optical density and the calculated sample to positive (S/P) ratio for test replicates. Based on the variability detected in S/P ratios of replicate samples, application of a single cutoff point to interpret individual test results as positive or negative for antibodies to Mycobacterium avium subspecies paratuberculosis could result in inconsistent classification of animals as positive or negative for Johne's disease. Such inconsistency in test interpretation leads to frustration in large animal veterinarians or producers trying to make management decisions based on individual test results. Instead of dichotomizing the test results as positive or negative based on a single cutoff value, reporting numerical values and supplying a classification scheme that includes a suspect category reflecting the uncertainty inherent in the test is recommended to provide more reliable result interpretation.

The effect of pooling sera on the detection of avian pneumovirus antibodies using an enzyme-linked immunosorbent assay test.

Pooling of samples is a cost-effective approach to estimate disease prevalence and to identify infected individuals. The objective of this study was to evaluate the use of serum pools for the detection of avian pneumovirus infection in turkey flocks by enzyme-linked immunosorbent assay, so that a minimum number of tests can be performed without compromising the sensitivity and specificity of the test. A total of 900 field samples were tested; 20 samples from each of 45 flocks. All samples were tested individually followed by pool testing in groups of 3, 4, 5, and 7 samples each. The number of positive pools for a given pool size was positively associated with the number of positive samples. In a separate experiment, the effect of dilution was examined by pooling 1 positive sample with different numbers of negative samples to form pools of sizes 2-7. These laboratory results were analyzed and integrated into a simulation model aimed at evaluating cost-efficient testing procedures. The probability of detecting an infected flock depended on prevalence of infection, size of serum pool, and the cutoff value used for optical density difference. At a theoretical prevalence of 20%, the probability of detecting an infected flock was 0.93 and 0.86 for a pool of 2 and 7, respectively. The probability of detecting positive flocks increased with increased prevalence and decreased cutoff. Pooling of samples represented a significant reduction in the cost of testing, suggesting that pooling is more advantageous and cost effective than testing individual samples.

Effect of bovine viral diarrhea virus infection on fertility of dairy heifers.

A prospective field study in heifers from birth to first breeding was undertaken on two commercial dairies to assess the effect of bovine viral diarrhea virus (BVDV) congenital and post-natal infection (PNI) on fertility. A high BVDV Type 2 antibody titer (1:4096) at 10 months of age was associated with 32 more days to conceive, compared with a low titer (1:128). Conversely, infection with BVDV by 5-6 months of age and high BVDV Type 2 titers 1 month before conception or breeding was associated with improved fertility. Heifers with evidence of congenital BVDV infection had lower fertility than non-infected heifers (15-42 days longer time-to-first AI), which depended on BVDV Type 2 titers at 10 months of age. Neospora caninum infection was associated with additional services per conception (SPC) and Leptospira interrogans infection was associated with a delay in the time-to-first breeding. It appears that under field conditions, the effect of subclinical BVDV infection on subsequent heifer fertility may be due to a complex of interrelationships among multiple BVDV infections that depend on the type and timing of infection relative to reproductive development and events.

A method of probability diagnostic assignment that applies bayes theorem for use in serologic diagnostics, using an example of Neospora caninum infection in cattle.

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.

Quantification, risk factors, and health impact of natural congenital infection with bovine viral diarrhea virus in dairy calves.

OBJECTIVES: 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.

Predicted ages of dairy calves when colostrum-derived bovine viral diarrhea virus antibodies would no longer offer protection against disease or interfere with vaccination.

OBJECTIVE: To develop models that could be used to predict, for dairy calves, the age at which colostrum-derived 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 < or = 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.

Estimated within-herd prevalence (WHP) of Mycobacterium avium subsp. paratuberculosis in a sample of Minnesota dairy herds using bacterial culture of pooled fecal samples.

The objective of this study was to describe the estimated within-herd prevalence (WHP) of Mycobacterium avium subsp. paratuberculosis (Map) in a sample of infected dairy herds in Minnesota (N = 66) using test results from bacterial culture of pooled fecal samples. Fecal samples were collected from up to 100 cows in each herd and were tested using bacterial culture in pools of 5 cows based on age order. The mean herd size was 222 (44 to 1500) milking cows; the cows were predominantly Holstein. Using a frequentist approach, the within-herd mean individual fecal prevalence was 10% [95% confidence interval (CI) = 4% to 16%] assuming 70% test sensitivity and 99.5% test specificity. Using Bayesian methods, the estimated true within-herd individual cow prevalence was 14% (95% CI = 7% to 27%). Within-herd prevalence was higher in larger dairy herds than in herds with fewer cows. As Map is the causative agent of Johne's disease (JD), the results of this study could contribute to the success of a nationwide control program for this disease.