Validity of luminometry and bacteriological tests for diagnosing intramammary infection at dry-off in dairy cows.

The objective of this cross-sectional study was to estimate the validity of laboratory culture, Petrifilm and Tri-Plate on-farm culture systems, as well as luminometry to correctly identify IMI at dry-off in dairy cows, considering all tests to be imperfect. From September 2020 until December 2021, we collected composite milk samples from cows before dry-off and divided them into 4 aliquots for luminometry, Petrifilm (aerobic count), Tri-Plate, and laboratory culture tests. We assessed multiple thresholds of relative light units (RLU) for luminometry, and we used thresholds of ≥100 cfu/mL for the laboratory culture, ≥50 cfu/mL for Petrifilm, and ≥1 cfu for Tri-Plate tests. We fitted Bayesian latent class analysis models to estimate the sensitivity (Se) and specificity (Sp) for each test to identify IMI, with 95% credibility interval (BCI). Using different prevalence measures (0.30, 0.50, and 0.70), we calculated the predictive values (PV) and misclassification cost terms (MCT) at different false negative-to-false-positive ratios (FN:FP). A total of 333 cows were enrolled in the study from one commercial Holstein herd. The validity of the luminometry was poor for all thresholds, with an Se of 0.51 (95% BCI = 0.43-0.59) and Sp of 0.38 (95% BCI = 0.26-0.50) when using a threshold of ≥150 RLU. The laboratory culture had an Se of 0.93 (95% BCI = 0.85-0.98) and Sp of 0.69 (95% BCI = 0.49-0.89); the Petrifilm had an Se of 0.91 (95% BCI = 0.80-0.98) and Sp of 0.71 (95% BCI = 0.51-0.90); and the Tri-Plate had an Se of 0.65 (95% BCI = 0.53-0.82) and Sp of 0.85 (95% BCI = 0.66-0.97). Bacteriological tests had good PV, with comparable positive PV for all 3 tests, but lower negative PV for the Tri-Plate compared with the laboratory culture and the Petrifilm. For a prevalence of IMI of 0.30, all 3 tests had similar MCT, but for prevalence of 0.50 and 0.70, the Tri-Plate had higher MCT in scenarios where leaving a cow with IMI untreated is considered to have greater detrimental effects than treating a healthy cow (i.e., FN:FP of 3:1). Our results showed that the bacteriological tests have adequate validity to diagnose IMI at dry-off, but luminometry does not. We concluded that although luminometry is not useful to identify IMI at dry-off, the Petrifilm and Tri-Plate tests performed similarly to laboratory culture, depending on the prevalence and importance of the FP and FN results.

Assessment of L-lactatemia as a predictor of respiratory disease recognition and severity in feedlot steers.

The bovine respiratory disease complex (BRD) is a major health issue in feedlot cattle and one of the primary reasons for antimicrobial use in the North American feedlot industry. The purpose of the present study was to assess blood L-lactate levels of feedlot steers at high risk of developing BRD during the early feeding period. Blood samples were obtained at initial processing and again after BRD confirmation (using bronchial lavage or thoracic ultrasound exam). The study involved 232 recently weaned steers received at a single research feedlot that were processed without metaphylactic antimicrobial treatment. Blood samples were obtained for determination of L-lactatemia and temperament scores (very quiet or stoic [score 1], average [score 2] and very excited [score 3]) were systematically assigned at initial processing. A subsample of calves that were later confirmed as cases of BRD were sampled at first pull (day 0), and at subsequent observation points on days 3, 6, 9 and 15 following initial BRD diagnosis for blood lactate determination as a potential indicator of subsequent death. The clinical BRD cumulative incidence in the cohort was 38% (87/232). Temperament was associated with the probability of becoming a BRD case during the early feeding period. Stoic or very excited calves showed 2.2 times higher odds (95%CI: 1.3, 3.8) of becoming BRD cases compared to calves with average temperament. The impact of L-lactatemia differed by temperament strata. In calves with a temperament score of 2 (average temperament) every 1-log unit increase of lactatemia at processing resulted in 1.9 times higher odds (95% CI: 1.2, 3.1) of becoming a BRD case; this relationship was not significant in calves with a score of either 1 or 3. Twenty-nine confirmed BRD cases were studied for the dynamic lactate assessment analysis. L-lactate at first pull was not significantly different between survivors (median 3.3mmol/L; range 0.8-7.8mmol/L) and non-survivors (median 2.7mmol/L; range: 1.6-5.4mmol/L) steers. However, the dynamic assessment of L-lactatemia was associated with the hazard of death using Cox proportional hazard survival analysis. A 1-log increase of lactatemia increased the hazard of dying prior to the next observation by a factor of 36.5 (95% CI: 3.5-381.6). For calves showing a normal temperament score (i.e. temperament score of 2), a misclassification cost term analysis was conducted to identify potential L-lactate test thresholds for identifying future BRD steers. When planned test usage was for informing decision of administering or not a metaphylactic treatment at processing, experts agreed that false-negative (not treating a calf that would have benefit from treatment) to false-positive (wrongfully treating a calf that would have remained healthy) health costs ratio ranged from 8:1 to 20:1. In this situation, a threshold of 5mmol/L would have best informed treatment decision. When using L-lactate for informing the type of antimicrobial used at processing, false-negative to false-positive health costs ratio ranging from 1:1 to 3:1 could be expected and, again, a L-lactate threshold of 5.0mmol/L would have minimized the costs associated with calves' misclassification and could be used to identify calves that would benefit from a more efficient metaphylactic treatment. This study provides an interesting perspective on the potential application of chute-side markers or diagnostic tests to stratify the risk of future pull for BRD in cattle during processing in order to adapt antimicrobial treatments accordingly.