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Abstract
A Bayesian latent class model was developed to estimate the true prevalence of bovine digital dermatitis (BDD) in Taranaki, New Zealand. This model allowed farms to have zero prevalence as well as also accounting for between farm heterogeneity that was conditional on whether a farm was positive for bovine digital dermatitis. The estimated true farm level prevalence was 68.9% (95% credible interval [CrI]: 50.0%-85.7%), while on infected farms the overall cow level prevalence (number of infected cows/total number of cows on infected farms) was 2.9% (95%CrI: 2.1%–4.3%). The sensitivity analyses suggested that the prevalence estimates were reasonably robust when the variation of the priors fell within the biologically plausible range. These results indicated that visual inspection of standing animals during milking was sufficiently accurate to identify infected farms. However, for every 100 animals identified through visual inspection, 84 animals with lesions were missed. In the other words, 46% (calculated as 84/184) of true positives at the animal level could be missed by visual inspection. The high and robust specificity (99.9%, 95%CrI: 99.8%–99.9%) suggested that lesions reported as BDD were very unlikely to be false positives.
... where π j is the prevalence in the j-th herd and Se TUS , Se CRS, Sp TUS, Sp CRS are the sensitivities and specificities of TUS and CRS, respectively. We chose to model within herd-prevalence using the logitnormal approach as used by Yang et al. (44). A range of methods have been used to model within herd prevalence in these models. ...
... The herd-level prevalence was modeled using a Bernoulli distribution with mean µ representing the proportion of herds that were disease free. The conditional within-herd prevalence was the within-herd true prevalence in affected herds only and was modeled using an intercept only random effect logistic regression (44) where α was the intercept, and ε j was the farm level random effect. ...
Introduction
Bovine respiratory disease (BRD) has a significant impact on the health and welfare of dairy calves. It can result in increased antimicrobial usage, decreased growth rate and reduced future productivity. There is no gold standard antemortem diagnostic test for BRD in calves and no estimates of the prevalence of respiratory disease in seasonal calving dairy herds.
Methods
To estimate BRD prevalence in seasonal calving dairy herds in Ireland, 40 dairy farms were recruited and each farm was visited once during one of two calving seasons (spring 2020 & spring 2021). At that visit the prevalence of BRD in 20 calves between 4 and 6 weeks of age was determined using thoracic ultrasound score (≥3) and the Wisconsin respiratory scoring system (≥5). Hierarchical Bayesian latent class analysis was used to estimate the calf-level true prevalence of BRD, and the within-herd prevalence distribution, accounting for the imperfect nature of both diagnostic tests.
Results
In total, 787 calves were examined, of which 58 (7.4%) had BRD as defined by a Wisconsin respiratory score ≥5 only, 37 (4.7%) had BRD as defined by a thoracic ultrasound score of ≥3 only and 14 (1.8%) calves had BRD based on both thoracic ultrasound and clinical scoring. The primary model assumed both tests were independent and used informed priors for test characteristics. Using this model the true prevalence of BRD was estimated as 4%, 95% Bayesian credible interval (BCI) (1%, 8%). This prevalence estimate is lower or similar to those found in other dairy production systems. Median within herd prevalence varied from 0 to 22%. The prevalence estimate was not sensitive to whether the model was constructed with the tests considered conditionally dependent or independent. When the case definition for thoracic ultrasound was changed to a score ≥2, the prevalence estimate increased to 15% (95% BCI: 6%, 27%).
Discussion
The prevalence of calf respiratory disease, however defined, was low, but highly variable, in these seasonal calving dairy herds.
... Therefore, this exam is prone to being subjective and can be imperfect; consequently, it has been suggested to use a latent class model approach to estimate its accuracy for bovine lameness (Dutton-Regester et al., 2018). In fact, latent class models have already been used to estimate DD prevalence and accuracy of inspection in milking parlor to diagnose DD (Yang et al., 2017). Therefore, alternative methods to trimming chute observation have been investigated for easier diagnosis during daily herd operations; for instance, during milking time (Thomsen et al., 2008;Relun et al., 2011;Solano et al., 2017). ...
... Another possible explanation for the low sensitivity of the borescope is the difference in the angulation of the feet when observed in the milking parlor versus in the trimming chute. The latter allows for a complete visualization of the feet's surface, whereas when observing feet in the milking parlor with the borescope, visualization of this area is impaired by feet's conformation (e.g., heel height), presence of feces, and position of the cow in the milking parlor (Thomsen et al., 2008;Yang et al., 2017). ...
Digital dermatitis (DD) is a worldwide infectious disease of cattle with high prevalence in dairy herds. It is a painful disease with welfare issues causing economical losses. Identifying the affected animals is crucial to establish early treatment and evaluate the efficacy of a control strategy. The "gold standard" diagnosis of DD is the direct observation of DD lesions in a trimming chute. However, the use of a trimming chute for daily diagnosis of DD in all cows is not possible. To facilitate DD monitoring between trimming sessions, lesions could be identified in the parlor during milking. Therefore, we evaluated the use of a commercial borescope in a rotary milking parlor. Our hypothesis was that a borescope is an adequate alternative to evaluate DD lesions between trimming sessions. Our objective was to assess the sensitivity (Se), specificity (Sp), positive predictive value (PPV), and negative predictive value (NPV) of a borescope for the diagnosis of DD in the milking parlor as compared to direct observation in a trimming chute, and to quantify the agreement between both techniques.
... Therefore, this exam is prone to being subjective and can be imperfect; consequently, it has been suggested to use a latent class model approach to estimate its accuracy for bovine lameness (Dutton-Regester et al., 2018). In fact, latent class models have already been used to estimate DD prevalence and accuracy of inspection in milking parlor to diagnose DD (Yang et al., 2017). Therefore, alternative methods to trimming chute observation have been investigated for easier diagnosis during daily herd operations; for instance, during milking time (Thomsen et al., 2008;Relun et al., 2011;Solano et al., 2017). ...
... Another possible explanation for the low sensitivity of the borescope is the difference in the angulation of the feet when observed in the milking parlor versus in the trimming chute. The latter allows for a complete visualization of the feet's surface, whereas when observing feet in the milking parlor with the borescope, visualization of this area is impaired by feet's conformation (e.g., heel height), presence of feces, and position of the cow in the milking parlor (Thomsen et al., 2008;Yang et al., 2017). ...
Digital dermatitis (DD) is a worldwide infectious disease of cattle that causes lameness, discomfort, and economic losses. The reference standard test to diagnose DD is visual observation in a trimming chute, which cannot be practically performed daily on dairy farms. Moreover, some lesion misclassification may occur using this standard diagnostic method. The possibility of misclassification makes the use of a trimming chute debatable as a perfect reference standard test. The objective of this study was, therefore, to assess the diagnostic accuracy of a commercial borescope and trimming chute exam. The accuracy (sensitivity and specificity) of the tests and DD prevalence were investigated using Bayesian latent class analyses. Our hypothesis was that a commercial borescope can be routinely used to diagnose DD in a milking parlor without previous feet cleaning. A cross-sectional study was performed in a freestall facility. The lesions were scored (M0, M1, M2, M3, M4, M4.1) in the milking parlor with a borescope, followed by an examination in the trimming chute 48 to 72 h after the borescope exam. A total of 870 hind feet were scored during 2 sessions of trimming chute exams and borescope exams in the milking parlor. The data were analyzed in 2 ways. First, data were dichotomized into DD lesions (M1, M2, M3, M4, M4.1) and absence of DD lesions (M0). Second, data were dichotomized into active DD lesions (M1, M2, M4.1) and inactive lesions (M0, M3, M4). A Bayesian latent class model allowing for conditional dependence between tests was used to estimate tests' accuracy, likelihood ratio, and DD prevalence. When the data were dichotomized into DD lesions (M1-M4.1) versus absence of DD (M0) lesions, the sensitivity and specificity of the borescope was 55% [95% credible interval (CrI) 40-71%] and 81% (95% CrI 75-88%). The sensitivity of trimming chute exams was 79% (95% CrI 68-88%), and specificity was 80% (95% CrI 71-89%). When the data were dichotomized into active lesions (M1, M2, M4.1) versus inactive lesions or absence of lesions (M3, M4, M0), the sensitivity and specificity of the borescope were, respectively, 32% (95% CrI 13-58%) and 91% (95% CrI 88-95%). The sensitivity and specificity of trimming chute exams were 91% (95% CrI 81-97%) and 81% (95% CrI 75-89%), respectively. In conclusion, it is possible to use the borescope in the milking parlor without cleaning the feet to monitor prevalence of DD lesions. However, an isolated borescope examination, especially for diagnosing active DD lesions, has low sensitivity for use as a surveillance method. For such use, the sensitivity could be improved by repeating the borescope exam on a regular basis.
... The initial separation of the herds into BDD-lesion-free and BDD-lesion positive was based on whether BDD lesions were observed; i.e. a herd with ≥1 lesion was defined as being BDD-lesion positive, otherwise it was defined as being BDD-lesion free [14]. However, simply basing herd status on the presence/absence of visible lesions probably leads to loss of information regarding probability of a herd having BDD and may introduce misclassification bias at the herd level, as there is a chance that a herd where BDD lesions are truly present could be wrongly classified as being BDD-lesion-free due to a combination of limited diagnostic sensitivity and low cow-level prevalence [17]. ...
... One method for overcoming this limitation is by using a Bayesian latent class model, which estimates the mean probability of a herd being BDD-lesion positive conditional on the number of test positive animals, the total number of animals tested, and the test characteristics [17]. Thus, the mean probability contains more precise information than the simple dichotomised outcome and increases the power of the study to determine the impact of risk factors on the likelihood of a herd being BDD-lesion positive. ...
Background
Bovine digital dermatitis (BDD) is considered the most important infectious cause of lameness in dairy cattle worldwide, but has only recently been observed in New Zealand. Although many studies have investigated the risk factors for BDD in confined dairy systems, information on risk factors in pasture-based system is limited. Therefore a cross-sectional study including 59,849 animals from 127 dairy herds in four regions of New Zealand was conducted to identify the herd-level factors associated with the probability of a herd being BDD-lesion positive and with within-herd BDD prevalence.
Results
Purchasing heifers was associated with increased odds of a herd being BDD-lesion positive (odds ratio [OR]: 2.33, 95% probability interval [PI]: 1.26–4.42) and a cow being BDD affected (OR: 3.76, 95%PI: 1.73–8.38), respectively. Higher odds of a herd being BDD-lesion positive (OR: 2.06, 95%PI: 1.17–3.62) and a cow being BDD affected (OR: 2.87, 95%PI: 1.43–5.94) were also seen in herds where heifers co-grazed with cattle from other properties. In addition, using outside staff to treat lameness was associated with higher odds of a cow being BDD affected (OR: 2.18, 95%PI: 0.96–4.98).
Conclusion
This study highlighted that movements of heifers are significantly associated with the spread of BDD within and between dairy herds in New Zealand. To minimise the risk of disease introductions in herds where moving heifers cannot be avoided, it is best to purchase heifers only from herds where BDD-freedom has been confirmed and, if heifers have to graze-off a farm, they should be reared as a single biosecure management group, especially since animals may be BDD-infected without having clinically obvious lesions.
Electronic supplementary material
The online version of this article (10.1186/s12917-019-1871-3) contains supplementary material, which is available to authorized users.
... The difference in detection rates was then compared using a univariable mixed-effects logistic regression model to account for potential clustering effects within individuals. The regression coefficients were converted as per Yang et al. [16] to represent a "population average" interpretation. All parameters, including the three regression coefficients (i.e., the intercept [logit of the proportion of positive detected by the PUC] and the effects of OS and OF, respectively, and the variance of the random effect σ 2 A presenting the within-animal variability) were estimated using the Markov chain Monte Carlo (MCMC) method. ...
Porcine circovirus type 3 (PCV3) has been identified worldwide and is associated with reproductive and systemic diseases, yet the dynamics of PCV3 within pig farms remain unclear. Building upon our previous study, which initialised comparisons of different sample types for the detection of PCV3 in a sow farm, this study expanded both the range of sample types and the timeline of sampling in piglets and sows to better understand the PCV3 dynamics. This study collected two additional sample types—oropharyngeal swab (OS) and oral fluid (OF) along with placental umbilical cord (PUC) blood and processing fluid (PF) that were used in the previous study. Data were collected from July to August and October 2022; the aforementioned four sample types from 51 litters were collected, and additional OS samples were collected from two to three identified piglets per litter on days 1, 7, 14, and 21 post-farrowing. Besides, blood swabs were taken from 135 sows subject to both PCR test and oestrogen measurement. PF showed the highest detection rates (50/51), while OS and OF revealed 33/51 (95% confidence interval [CI]: 51.2%–76.8%) and 37/51 (95% CI: 59.5%–83.5%) detection rates; both were higher than that of PUC blood (22/51, 95% CI: 30.2%–56.8%). Despite the similarity between OS and OF samples, they did not identify the same population as infected, as the agreement between the samples was only fair at 90% level. The Bayesian generalised linear mixed model suggested PCV3 was more likely to be detected in both OS and OF compared to PUC blood, and PCV3 was present in the farrowing room throughout the pre-weaning period using an OS. Finally, we observed higher PCV3 detection rates in sows after farrowing; however, no evidence was found that such a pattern was associated with the decreased concentration of oestrogen.
... As there were multiple herds enrolled in the study, then p i = p j(i) , where j is the herd for animal i. This creates a random effect distribution for p j which can be modelled using a logit-normal distribution (Yang et al., 2017) such that Logit(p j(i) ) ∼ N( 0 , 2 herd ). This random effect treats the same herd tested at the different time points as two separate units, as the time gap between the two herd tests was sufficient for the prevalence of a herd to change. ...
Background
A novel ELISA test has been developed to detect antigen‐specific IgG in early and late lactation cows in New Zealand.
Objectives
This study was to evaluate the discriminatory ability of the ELISA based on the detection of S. aureus‐specific IgG as a screening test.
Methods
The ELISA was used for the composite milk samples taken during routine herd testing in 2018–2019 milking season in New Zealand. In the absence of a gold standard test, the diagnostic specificity and sensitivity was estimated using a Gaussian mixture model.
Results
The ELISA test had a high accuracy (AUC = 0.98) to detect antigen‐specific IgG in early and late lactation cows with high somatic cell count due to either subsequent to or contemporaneous with the S. aureus invasion. Using an S/P ratio = 0.3 as the cut‐off value, the ELISA test has sensitivity of 0.9 and specificity of 0.95, while the sensitivity increased to 0.94 at a cost of a decreased specificity of 0.9 at a lower cut‐off value 0.26.
Conclusions
The integration of the ELISA test as a screening tool into specific control programs may be useful to reduce the spread of S. aureus infections, to aid with treatment decisions, and to establish a correct milking order.
... If a disinformative prior is selected, in this case using values that are unrealistic to the leptospirosis situation in farmed swine the HKSAR, the posterior inferences would be meaningless. One might propose the use of a uniform prior such as Beta (1, 1) in our analysis, as such a prior is diffuse and considered to be "non-informative", which might be appropriate as no studies have ever been conducted to investigate the leptospirosis prevalence in farmed swine in the HKSAR [33]. However, using this uniform prior would mean we assume that the mean prevalence was 50%, and that it was equally likely that all or none of the swine were infected. ...
Leptospirosis is an important zoonotic disease with several maintenance host species including swine. A cross sectional survey was undertaken between January to October 2020 to investigate the prevalence of leptospirosis in farmed swine in the Hong Kong Special Administrative Region (HKSAR) of China. Serum samples were collected from swine on seven farms (15 swine per farm; ten multiparous sows and five twelve-week-old weaners), while kidney samples were collected from 64 swine submitted for routine post-mortem (26 farms; average 2.4 swine per farm, range 1–6). Microscopic agglutination tests (MAT) to a panel of 24 Leptospira antigens did not reveal any evidence of seroconversion at a titre of 1:100. Polymerase chain reaction (PCR) testing of the kidney samples for Leptospira DNA did not detect any evidence of infection. Bayesian methods were used to compute the probability that the leptospirosis prevalence in farmed swine in the HKSAR was <3%, given none of the 105 swine sampled were positive on the MAT. The results of this study demonstrate no serological or molecular evidence of leptospirosis in farmed swine in the HKSAR. Subsequent statistical analysis supports the conclusion that the prevalence of leptospirosis in farmed swine in the HKSAR is negligible at present.
... ICDs (Infectious Claw Disorders) are present in almost all herds, all over the world. Even in an area like New Zealand where dairy is pastured permanently the whole year, ICDs are found [9]. In a follow-up study, the influence of claw trimming in the introduction and transmission was confirmed [10]. ...
... Bovine digital dermatitis (DD) is an infectious foot disease that causes varying levels of pain, discomfort, and lameness in dairy cattle [1] and has been increasingly found in dairy production systems worldwide [2,3]. In countries where cows are housed indoors, DD can be a major infectious cause of cattle lameness as well as a significant problem for the dairy industry due to losses in milk production [4], increased treatment costs [5], and negative impacts on animal welfare [6]. ...
Abstract Bovine digital dermatitis (DD) is an important infectious cause of cattle lameness worldwide that has become increasingly prevalent in New Zealand pastoral dairy herds. In this study, a simplified DD scoring system after considering both M and Iowa DD scoring systems was applied to explore the transmission dynamics of DD in a typical spring-calving pastoral New Zealand dairy herd. The modified model only included three compartments: normal skin, early stage lesions and advanced lesions. Lesions regressing after treatment were excluded as DD lesions are rarely treated in New Zealand. Furthermore, sub-classes within each lesion class were not defined due to the lack of variability in DD lesion presentations within New Zealand. The model was validated based on longitudinal field data from three dairy herds in the Waikato region during one lactation season (2017–18). The model suggested that in infected dairy herds, although DD prevalence will tend to increase year-on-year it is likely to remain relatively low (
... For DD lesion classification systems other than the M-score, sensitivity (Se) ranges from 65% to 72% and specificity (Sp) from 84% to 99% (Rodriguez-Lainz et al., 1998;Thomsen et al., 2008). Yang et al. (2017a) estimated a Se of around 63% and Sp of nearly 100% for visual inspection of the rear feet for presence or absence of lesions of DD during milking. M-scoring in the milking parlor, whether assisted by a telescopic mirror or not, appears to be both sensitive (90-100%) and specific (80-99%) in identifying cattle with DD (Relun et al., 2011;Stokes et al., 2012;Solano et al., 2017), although some misclassification has been reported when compared with M-scoring in the trimming chute, especially for M3 (Relun et al., 2011) andM4.1 (Solano et al., 2017). ...
Digital dermatitis (DD) is the leading infectious cause of lameness in dairy cattle, and it affects their welfare and productivity worldwide. At the herd level, DD is often assessed while cows are standing in a milking parlor, and lesions are most commonly evaluated using the M-score. The objective of this study was to examine the interobserver agreement for M-scores of the feet of standing cattle, based on digital color photographs of dairy cattle hind feet. A total of 88 photographs and written descriptors of the M-score were sent to 11 scorers working at 10 different institutions in 5 countries. The scorers received no formal training immediately before scoring the photographs; however, all regularly used the M-score to score DD. The answers for 36 photographs were excluded from the analysis because the photograph either had more than 1 M-stage as mode or not all scorers assigned an M-score to it. The M-scores of the 11 scorers from 52 photographs were available for analysis. Interobserver agreement was tested using Gwet's agreement coefficient (AC1) and the mode was assumed correct. Overall, moderate agreement emerged for the M-score (AC1 = 0.48). For the individual M-stages, almost perfect agreement existed for M0 (AC1 = 0.99), M1 (AC1 = 0.92), and M3 (AC1 = 0.82), and substantial agreement for M2 (AC1 = 0.61), M4 (AC1 = 0.65), and M4.1 (AC1 = 0.71). This outcome indicates the degree of individual variation in M-scoring in this context by unstandardized, experienced European observers, particularly for the M2, M4, and M4.1 stages. Standardized training is likely to improve the consistency of M-scoring and thus the generalizability of future DD research results on this important endemic disease.
Bayesian finite mixture models, frequently referred to as Bayesian latent class models have become increasingly common for diagnostic test data in the absence of a gold standard test. Most Bayesian analyses in the veterinary literature have dealt with a dichotomised diagnostic outcome. The use of Bayesian finite mixture models for continuous test outcomes, such as sample to positive (S/P) ratios produced by an ELISA, is much less common, despite continuous models taking advantage of all of the information captured in the test outcome. This paper revisits the idea of the Bayesian finite mixture model and provides a practical guide for researchers who would like to use this approach for modelling continuous diagnostic outcomes as it preserves all information from the observed data. Synthetic datasets and a dataset from literature were analysed to illustrate that a mixture model with continuous diagnostic outcomes can be used to estimate true prevalence and to evaluate test sensitivity and specificity. In addition, directly modelling the continuous test outcomes rather than dichotomising them, means that optimal cut-offs can be defined based on the test purpose rather than being determined before testing. Moreover, as animals with higher scores are more likely to be infected, using continuous data allows test interpretation to be made at the individual animal level. In contrast, dichotomization treats all animals above a cut-off as having the same infection risk. This study demonstrates that dichotomisation is not a ‘must’ when using Bayesian latent class analysis for diagnostic test data, and suggests that latent class analysis using continuous test outcomes should be favoured when evaluating veterinary diagnostic tests producing continuous outcomes.
Visual assessment in the milking parlour is a commonly used method to determine the prevalence and severity of bovine digital dermatitis (BDD). It is generally suggested that cows’ feet are washed prior to examination to maximise the sensitivity of the assessment, but concern has been expressed that washing cows’ feet could contaminate the teats and lead to intramammary infection. Furthermore, the evidence for washing cows’ feet is equivocal, as some studies have reported similar sensitivities for detecting BDD without washing as that reported by studies which used washing. Furthermore most of these studies have used data from housed cattle. The findings from these studies may not be applicable to cattle at pasture where feet are often contaminated with mud rather than faeces and lesions may be smaller and less severe. The aim of this study was to compare, in cattle at pasture, the sensitivities of BDD examinations before and after washing.
Two herds known to have BDD were enrolled and approximately half of each herd was screened for BDD by examining the cows’ hind feet before and after washing. The sensitivities of these examinations were estimated using a Bayesian superpopulation approach, and were found to be 0.34 (95% credible interval [CrI]: 0.088–0.69) and 0.63 (95%CrI: 0.46–0.78) for pre- and post-washing, respectively. There was a 93.95% probability that the sensitivity of examination post-washing was greater than that pre-washing. These results suggest that in pasture-based herds, many BDD lesions will be missed if cattle are examined without their feet being washed.
A cross-sectional study of 127 dairy herds distributed across four regions of New Zealand (NZ) was conducted to estimate the regional herd-level prevalence of bovine digital dermatitis (BDD) and the prevalence of cows with BDD lesions within affected herds. Each herd was visited once during the 2016–2017 lactating season and the rear feet of all cows in the milking herd were examined to detect the presence of BDD lesions. Of the 127 herds examined, 63 had at least one cow with a detected BDD lesion. Of the 59 849 cows observed, 646 cows were observed with BDD lesions. All of the herds in which BBD was detected were located in three of the four regions (Waikato, Manawatu and South Canterbury). No convincing lesions were observed on the West Coast. The probability of BDD freedom on the West Coast was predicted to be 99.97% using a Bayesian latent class model. For the three regions where BDD lesions were observed, the true herd level and cow level prevalences were estimated using a Bayesian superpopulation approach which accounted for the imperfect diagnostic method. Based on priors obtained from previous research in another region of NZ (Taranaki), the true herd level prevalences in Waikato, Manawatu and South Canterbury were estimated to be 59.2% (95% probability interval [PI]: 44.3%–73.9%), 43.3% (95%PI: 29%–59%) and 65.9% (95%PI: 49.5%–79.9%), respectively, while the true median within-herd prevalences were estimated as 3.2% (95%PI: 2%–5%), 1.7% (95%PI: 0.9%–3.1%) and 3.7% (95%PI: 2.4%–5.5%), respectively. All of these estimates except for the true herd level prevalence in Manawatu were fairly robust to changes in the priors. For Manawatu region, changing from the prior obtained in Taranaki (the best estimate of the herd level prevalence = 60%, 95% sure > 40%) to one where the mode was 50% (95% sure < 80%) reduced the posterior from 43.3% to 35.2% (95%PI: 20.1%–53.5%). The marked variation in BDD prevalence between regions and between farms highlights the need for further exploration into risk factors for disease.
AIMS: To assess the inter-observer agreement for detecting bovine digital dermatitis (BDD) lesions in digital colour photographs of the hind feet of cows, which had been taken while the animals were standing to be milked, between two trained observers.
METHODS: Thirty-six photographs were selected from a total of 184 photographs held by the first author (R1), who had classified them as negative (n = 11) or positive (n = 25) for BDD. They were delivered to a technician (R2) who had previously visually inspected cattle for BDD lesions, and who then recorded the photographs as being either BDD-positive or BDD-negative. The percentage agreement between R1 and R2, and two other inter-observer agreement statistics, Cohen’s κ and Gwet’s first-order chance correction agreement coefficient (AC1), were calculated. The cumulative membership probabilities of Cohen’s κ and Gwet’s AC1 were then calculated for different benchmark ranges of κ.
RESULTS: The percentage agreement between R1 and R2 was 33/36 (92%), Cohen’s κ was 0.80 (95% CI = 0.57–1.0) and Gwet’s AC1 was 0.86 (95% CI = 0.69–1.0). Based on the cumulative membership probabilities for Gwet’s AC1, there was 75% probability that the two observers had almost perfect agreement (κ ≥0.81). For both Cohen’s κ and Gwet’s AC1, there was >95% probability that the two observers had at least substantial agreement (κ ≥0.61).
CONCLUSIONS: The two trained observers had at least substantial agreement in identifying from a digital photograph as to whether BDD lesions were present or absent. Therefore results from the two could be used interchangeably.
CLINICAL RELEVANCE: Visual assessment for BDD lesions in the milking parlour can be subjective. However a high agreement between these two trained BDD inspectors means BDD prevalence reported from different regions in New Zealand by these two can be directly compared.
A Bayesian latent class model was used to estimate the sensitivity and specificity of an immunoglobulin G1 serum enzyme-linked immunosorbent assay (Paralisa) and individual fecal culture to detect young deer infected with Mycobacterium avium subsp. paratuberculosis. Paired fecal and serum samples were collected, between July 2009 and April 2010, from 20 individual yearling (12-24-month-old) deer in each of 20 South Island and 18 North Island herds in New Zealand and subjected to culture and Paralisa, respectively. Two fecal samples and 16 serum samples from 356 North Island deer, and 55 fecal and 37 serum samples from 401 South Island deer, were positive. The estimate of individual fecal culture sensitivity was 77% (95% credible interval [CI] = 61-92%) with specificity of 99% (95% CI = 98-99.7%). The Paralisa sensitivity estimate was 19% (95% CI = 10-30%), with specificity of 94% (95% CI = 93-96%). All estimates were robust to variation of priors and assumptions tested in a sensitivity analysis. The data informs the use of the tests in determining infection status at the individual and herd level.
The objectives were (1) to estimate the genetic parameters and breeding values of hoof lesions, (2) to estimate the phenotypic effect of each feet and legs conformation traits on hoof lesions, and (3) to estimate genetic correlations between hoof lesions with feet and legs conformation traits. The presence or absence of specific hoof lesions was recorded for each hoof. Lesions were classified into infectious (digital and interdigital dermatitis, foot rot, and heel erosion), horn (sole and toe ulcer, sole hemorrhage, and white line disease), and other lesions (interdigital hyperplasia, fissures, thin soles, and corkscrew claw). A total of 34,905 hoof health records from 27,179 cows and 365 herds, collected by 18 different hoof-trimmers in Ontario, Alberta, and British Columbia, were analyzed using linear animal models. In addition, 5 feet and leg conformation traits (foot angle, heel depth, bone quality, rear leg side view, and rear leg rear view) and locomotion from primiparous cows were considered (n = 11,419 and 6,966 cows, for conformation traits and locomotion, respectively). At least one lesion was found in nearly 40% of the hoof trimming records. The heritability estimates for hoof lesions ranged from 0.01 for front horn lesions to 0.09 for rear infectious lesions. Despite the low heritability estimates, we observed large variability in sire estimated breeding value (EBV) for resistance to hoof lesions. Positive genetic correlations were found between the occurrence of front and rear infectious lesions (0.77) and between front and rear horn lesions (0.61), but not between infectious and horn lesions (0.08). For most of the conformation traits, low scores were phenotypically associated with higher incidence of horn lesions, whereas we found no evidence of a phenotypic effect of feet and leg traits on infectious lesions. The heritability of the conformation traits ranged from 0.04 for rear leg rear view to 0.22 for bone quality, whereas that for locomotion was 0.03. The genetic correlations between hoof lesions and conformation traits were low to moderate, yet most of the estimates were associated with high standard errors. In conclusion, although hoof lesions are lowly heritable traits, sufficient genetic variation exists (as evidenced by large variability in sire EBV) for genetic improvement through direct selection in the long term. Standardization of hoof health data collection is encouraged.
The purpose of this study was to concurrently estimate the effect of different digital dermatitis (DD) treatment regimens and herd management practices on the occurrence of a new DD lesion. A controlled clinical trial was conducted and involved 4678 dairy cows from 52 French dairy farms where DD was endemic. Farms were allocated by minimisation to one of 4 treatment regimens, varying through the mode (footbath or collective spraying) and the frequency of application (2 days every 4 weeks or fortnightly). They were visited 7 times every 4 weeks by 14 trained investigators. Frailty Cox proportional hazards models were used to estimate the relative effect of potential risk factors and treatment practices on the time until the first occurrence of a DD lesion. At herd level, high initial DD prevalence strongly increased the risk for DD occurrence (HR=1.93, CI 1.23-3.04), as well as absence of hoof-trimming (HR=1.75, CI 1.36-2.27) and poor leg cleanliness (HR=2.44, CI 1.80-3.31). At animal level, Holstein breed (HR=1.92, CI 1.35-3.57) and high-productive cows (HR=1.26, CI 1.01-1.56) were identified to be at higher risk for DD compared to Normande breed and low-productive cows, respectively. Compared to individual topical antibiotic treatments alone, collective treatments tended to decrease the risk of DD occurrence only when applied over 2 days at least every fortnight (HR range=0.64-0.73).
The diagnosis of digital dermatitis (DD) in cows is crucial for researchers to evaluate the effectiveness of control measures. The objective of this study was to evaluate intra- and inter-observer agreement and accuracy of a simple method to score DD lesions that is based on the inspection of cleaned feet in the milking parlor with a swiveling mirror and a powerful headlamp. The hind feet of 242 Holstein cows from 4 farms were inspected concurrently by 5 observers (to assess inter-observer agreement) during 2 consecutive milkings (to assess intra-observer agreement). This inspection was followed by an inspection of the feet in a trimming chute, considered to be the gold standard test, to test for accuracy. The reliability of the method was assessed using both the 5 M-stages scoring system (M0 to M4, M standing for Mortellaro) and a simplified M-stages scoring system in which the M3 and M4 stages are merged. As most disagreements concerned the M3 and M4 stages, performance was improved with the simplified M-stages scoring system. With this simplified system, the method had good intra-observer agreement [percent of overall raw agreement (PA)=80%, weighted kappa (κ(w))=0.71, and good inter-observer agreement (PA=77%, κ(w)=0.66)]. The agreement with the gold standard test was moderate (PA=69%, κ(w)=0.58). Considering absence versus presence of a DD lesion on a foot, the method had good sensitivity (0.90) and specificity (0.80). The time spent for inspection varied from 30 to 60s per cow. Inspection of cleaned feet with a swiveling mirror and a powerful headlamp in the milking parlor is a reliable, cost- and time-friendly method, which may be useful for both farmers and researchers, especially when the DD status of many dairy cows has to be evaluated concomitantly. The reliability of this method might be improved by using more precise descriptive criteria for the discrimination of each M-stage.
Digital dermatitis is a serious problem in dairy production in many countries. In many settings, it is important to evaluate the digital dermatitis status of individual cows or an entire dairy herd. Such an evaluation has traditionally been done in a hoof trimming chute. An evaluation in the milking parlor can take place without disturbing the cows to a large extent, it can be done using less labor compared with an evaluation in a hoof trimming chute, and is cheaper than using a chute. The objective was to evaluate the sensitivity and specificity of a rapid screening method for digital dermatitis in the milking parlor, without using any specialized tools and taking approximately 15 s/cow. All lactating cows in 3 commercial Danish dairy herds were included. Cows were first scored for the presence of digital dermatitis during milking and the next day all cows were scored during hoof trimming. A 6-point nominal scoring system based on a visual inspection of the digital dermatitis lesions was used. For the analysis, the scores were dichotomized (digital dermatitis positive or digital dermatitis negative). Additionally, lesions were classified as small (diameter <or= 2 cm) or large (diameter >2 cm). Sensitivity and specificity were calculated using observations from the hoof trimming chute as the "gold standard" and observations during milking as the diagnostic test. Relatively large variation was found between herds with an overall sensitivity of 0.65 (95% confidence interval: 0.59 to 0.72) and a specificity of 0.84 (0.81 to 0.87). The sensitivity increased to 0.69 (0.62 to 0.76), when only large lesions were assessed. The method has several advantages compared with evaluation in a chute and may be a useful tool in the daily hoof health management in dairy herds.
The objective of this cross-sectional study was to reevaluate the prevalence of digital dermatitis (DD) and associated risk factors in The Netherlands. Between May 2002 and December 2003, information about the presence or absence of DD lesions and other claw disorders on the hind claws of dairy cows in The Netherlands was collected by 20 hoof trimmers during trimming of all dairy cows in 383 herds. A questionnaire was used to acquire information regarding management and housing. Additional information, such as parity, breed, and stage of lactation of the cows, was provided by the Dutch Herd Book Organization. Digital dermatitis was present in 21.2% (SE = 0.3) of the study population (n = 22,454 cows). The herd prevalence varied from 0% (9.1% of the herds) to 83.0%. Most common was herd prevalence between 5 and 10%. The Holstein-Friesian breed was at higher risk for DD [odds ratio (OR) = 1.7] than was the Meuse Rhine IJssel breed (dual purpose breed). The risk for DD decreased with increasing parity. Cows at the peak of their lactation (30 to 60 d in milk) and in the third parity had higher odds for DD in comparison with cows that were later than 60 DIM. The presence of other claw disorders, such as interdigital dermatitis/heel horn erosion (IDHE), interdigital hyperplasia (HYP), and interdigital phlegmon, appeared to be predisposing for DD. Based on estimation of the population-attributable fraction, it was concluded that if IDHE, HYP, and interdigital phlegmon were not present among the study population, respectively, 32.2, 9.0, and 1.1% of the DD cases could have been prevented. The risk for DD slightly decreased when cows affected by IDHE had access to pasture. Because of the interrelation between infectious claw disorders, an effective intervention strategy against DD should focus on an integrated approach to the control of all infectious claw diseases. Cows trimmed >12 mo before the study (during regular trimming of the entire herd) were at lower risk for DD than were cows that were trimmed at shorter intervals. Animals that had >8 h of access to pasture were at higher risk for DD (OR = 1.6) compared with no access to pasture. Finally, cows in small-sized herds (<45 cows) affected with HYP were at lower risk (OR = 0.6) for DD than were cows affected with HYP in medium- and large-sized herds (60 to 85 cows per herd).
Lactating Holstein-Friesian dairy cows in four separately housed groups in a herd with endemic digital dermatitis (DD) were monitored weekly for 4 weeks in December 2004 for the presence of and transition between five stages (M0-M4) of DD. Cows were also monitored for the presence of heel horn erosion (HHE) and interdigital hyperplasia. Prior to the study, two groups had been housed indoors on a high or low energy ration, one group had been grazed and one was a dry cow group. All cows received the same ration during the period of investigation. 'Active infection' was defined as transition from M0, M1, M3 or M4 to M2 and 'resolving M2 lesions' were defined as transition from M2 to another stage. M2 lesions were diagnosed on 106 occasions in the hind claws of 49 (36%) of 138 dairy cows; both hind claws were affected in nine cases (18%). M2 lesions were more often painful on palpation than other stages; 94% of M2 lesions were located plantar-medially near the interdigital cleft and 71% had a diameter of 2-4 cm. More M1 lesions than other stages were found within the interdigital space. When interdigital hyperplasia was present, claws were always concurrently affected by DD. The baseline incidence for 'active infection' was 6% per week, increasing to 11% when HHE was present, 14% when cows were previously housed indoors and fed a high energy ration and 16% when cows were 60-120 days in lactation. Topical treatment with chlortetracycline resulted in resolution from M2 of 79% per week. There were no significant effects of group, stage of lactation, parity or HHE on resolution of M2 lesions.
AIMS: This aim of this cross-sectional study was to investigate the herd and cow-level prevalence of bovine digital dermatitis (BDD) in dairy farms in the northern Taranaki region of New Zealand, and to identify whether there was any spatial clustering of herds with the disease.
METHODS: A survey of 224 dairy farms in the northern Taranaki region of New Zealand was undertaken from September 2014 to February 2015. Following training in robust criteria to confirm BDD visually, a technician inspected the rear feet of every milking cow on the farms during milking. The identity of cows with lesions and the feet involved were recorded. The proportion of cows affected among the inspected population (cow-level prevalence), the proportion of a herd affected (farm-level prevalence), and proportion of farms with ≥1 cow with lesions, were calculated. A bivariate K function analysis was then used to assess whether farms with ≥1 cow with lesions were clustered, after accounting for the distribution of the farms involved in the study.
RESULTS: Bovine digital dermatitis lesions were observed on 143/224 (63.8 (95% CI=57.5–70.1)%) farms. Within-farm prevalence was 0% on 81 (36.2%) farms, between >0 and <3% on 120 (53.5%) farms, with a maximum prevalence of 12.7% on one farm. Overall, cow-level prevalence was 707/60,455 (1.2 (95% CI=0.9–3.0)%), and on affected farms was 707/41,116 (1.7 (95% CI=1.4–2.1)%). In affected cows, 268/707 (37.9%) had a lesion on left foot only, 262/707 (37.1%) on the right foot only and 177/707 (25.0%) on both feet. The K function analysis showed no evidence of clustering of farms with BDD.
CONCLUSIONS: Bovine digital dermatitis was widespread among the survey farms, but there was no evidence that there was any clustering of herds with BDD. The cow-level prevalence on affected farms was much lower than reported elsewhere.
CLINICAL RELEVANCE: Although the prevalence at the cow level was low, if these data are representative of other regions of New Zealand, BDD could easily become a major problem on dairy farms in New Zealand, as has been observed in other countries.
The objectives of this cross-sectional study were to determine the prevalence and distribution of foot lesions and associated cow- and herd-level risk factors in dairy cows in Alberta, Canada. Foot lesion data were recorded electronically by 7 hoof trimmers on 28,607 cows in 156 dairy farms from June 2009 to November 2012. Foot lesion prevalence estimates differed between farms that had the whole herd trimmed at once (≥80% of lactating cows were trimmed; n = 69 farms and 8,020 cows) and farms on which part of the herd was trimmed (selection of cows was determined by farmer and <80% of lactating cows were trimmed; n = 87 and 20,587 cows). Estimates were consistently higher for the latter likely because farmers presumably prioritized lame cows in partial herd trims. On farms with whole-herd trims, digital dermatitis was the most common lesion among all housing types, present in 15% of cows and 94% of herds. Sole ulcers and white line disease were detected in 6 and 4% of the cows and 92 and 93% of herds, respectively. Other infectious and claw horn lesions each affected 1 to 2% of cows and 62 to 78% of herds. Intraclass correlation coefficients for hoof trimmers ranged from 0.01 to 0.20 for all lesions, indicating some clustering of recorded lesions by trimmer. Multilevel mixed logistic regression models were constructed (including hoof trimmer as fixed and farm as random effects) for the 3 most frequently identified lesions. Prevalence of digital dermatitis decreased with increasing parity, but this effect interacted with days in milk (DIM); primiparous cows had higher odds of digital dermatitis in mid lactation (100–199 DIM) and late lactation (≥200 DIM) compared with cows at other stages of lactation. In contrast, prevalence of sole ulcers and white line disease increased with increasing parity; compared with cows in parity 1, those in parity 4 had 5 or 7 times higher odds of having these lesions, respectively. Cows in mid lactation and late lactation had higher odds of sole ulcers and white line disease than cows at other stages of lactation, regardless of parity. Digital dermatitis prevalence was 2 times higher in herds housed in barns with access to an exercise area. The odds of sole ulcers and white line disease were ≥2 times higher in cows housed in freestalls than those housed in deep-bedded packs. Therefore, preventive measures for control of digital dermatitis merit emphasis, especially in primiparous cows and on farms with exercise areas. In addition, improving housing environment by providing a deep-bedded area for older cows in mid lactation or late lactation could reduce prevalence of claw horn lesions. We inferred that foot lesion data recorded by hoof trimmers can provide useful information not only to develop effective foot health programs at herd level, but also for disease surveillance and ge- netic improvement at regional and national levels.
Bovine paratuberculosis is a disease characterised by chronic granulomatous enteritis which manifests clinically as a protein-losing enteropathy causing diarrhoea, hypoproteinaemia, emaciation and, eventually death. Some evidence exists to suggest a possible zoonotic link and a national voluntary Johne’s Disease Control Programme was initiated by Animal Health Ireland in 2013. The objective of this study was to estimate herd-level true prevalence (HTP) and animal-level true prevalence (ATP) of paratuberculosis in Irish herds enrolled in the national voluntary JD control programme during 2013–14. Two datasets were used in this study. The first dataset had been collected in Ireland during 2005 (5822 animals from 119 herds), and was used to construct model priors. Model priors were updated with a primary (2013–14) dataset which included test records from 99,101 animals in 1039 dairy herds and was generated as part of the national voluntary JD control programme. The posterior estimate of HTP from the final Bayesian model was 0.23–0.34 with a 95% probability. Across all herds, the median ATP was found to be 0.032 (0.009, 0.145). This study represents the first use of Bayesian methodology to estimate the prevalence of paratuberculosis in Irish dairy herds. The HTP estimate was higher than previous Irish estimates but still lower than estimates from other major dairy producing countries.
Prevalences of foot lesions and lameness were recorded in 1'449 Swiss dairy cows during routine claw-trimming on 78 farms from June 2010 until February 2011. Lameness was present in 14.8 % of cows and on 80.8 % of investigated farms. Highest prevalences were seen for widened white line (80.7 %/100 %), signalling foot lesion (65.6 %/98.7 %), heel-horn erosion (34.2 %/88.5 %), digital dermatitis complex (29.1 %/73.1 %), severe hemorrhages (27.9 %/87.2 %), and Rusterholz' sole ulcers (11.5 %/74.4 %) at cow and herd level, respectively. Lower prevalences were found for subclinical laminitis (5.4 %/47.4 %), chronic laminitis (3.3 %/25.6 %), white line disease (4.7 %/42.3 %), double soles (2.6 %/33.3 %), interdigital hyperplasia (3.1 %/33.3 %), sole ulcers (0.4 %/6.4 %), toe infections caused by faulty claw-trimming (3.9 %/39.7 %) and by injury (0.1 %/2.6 %), deep lacerations (0.4 %/6.4 %), and interdigital phlegmona (0.1 %/1.3 %). Lameness and foot lesions were shown to represent important health problems of dairy cows under the conditions of the typical grass-based production system in Switzerland. Digital dermatitis has developed to the most relevant foot disease with a high impact on welfare of Swiss dairy cows within the past 10 years.
The objective of this study was to observe the dynamics of clinical cure and recurrence of the lesions of bovine digital dermatitis for 11months after treatment with topical lincomycin HCl. The study was a clinical follow-up of 39 active bovine digital dermatitis lesions (from 29 cows). Cows with active, painful bovine digital dermatitis (BDD) lesions on the interdigital commissure of the rear feet were identified on day 0. On day 1, lesions in all cows were photographed and full-skin thickness 6mm punch biopsies were obtained for histological evaluation. All lesions on all cows were treated with topical lincomycin paste under a light bandage. On days 12 and 23, a subsample of 10 lesions was randomly selected, photographed, and biopsied. On day 37, all lesions on all cows were photographed and biopsied. After day 37, lesions were evaluated on a monthly basis. All lesions were photographed at each observation until day 341 (end of study) but only cows that had macroscopically active lesions were biopsied. Of the 39 lesions treated on day 1, 21 (54%) required re-treatment on at least one occasion before day 341. Macroscopic classification agreed well with histological classification when lesions were small, focal and active (M1 lesions) or large, ulcerative and active (M2), but agreement was variable for lesions that had healed macroscopically (M5) or that were chronic (M4). A transition model showed that M1 and M2 lesions were 27 times more likely to be an M2 lesion on the next observation than to be a healed (M5) lesion.
Bovine digital dermatitis (BDD) is an epidermitis which is a leading cause of infectious lameness. The only recognized diagnostic test is foot inspection, which is a labour-intensive procedure. There is no universally recognized, standardized lesion scoring system. As small lesions are easily missed, foot inspection has limited diagnostic sensitivity. Furthermore, interpretation is subjective, and prone to observer bias. Serology is more convenient to carry out and is potentially a more sensitive indicator of infection. By carrying out 20 serological assays using lesion-associated Treponema spp. isolates, three serogroups were identified. The reliability of the tests was established by assessing the level of agreement and the concordance correlation coefficient. Subsequently, an ELISA suitable for routine use was developed.
Several studies have shown that foot lesions and clinical lameness occur before first calving and develop further during the lactation period. Lameness may cause production losses, but the relationship between foot lesions, particularly in the claw horn, and lameness in heifers is unclear. The objectives of this study were to describe the development of and evaluate the relationship between lameness and foot lesions in Danish Holsteins before and after first calving. In a longitudinal study, 147 heifers were examined for lameness and foot lesions 2–5 times over an 18-month period. Lameness was assessed by means of a visual locomotion score and foot lesion severity was recorded.
Outbreaks of papillomatous digital dermatitis (PDD) have been reported from many countries, but there is little information on prevalence and geographic distribution of the disease. Our objectives were to describe prevalence of PDD on dairies belonging to the two main dairy-producer associations in Chile, and to evaluate a milking-parlor screening method for detection of PDD in dairy cows. First, a self-administered questionnaire was sent to all 214 association dairy managers. Seventy percent of the 119 respondents had observed PDD on their dairies. About 83% of the 63 responding managers had first seen PDD during the previous 10 years. Subsequently, a random sample of 43 dairies stratified by association was selected from the total 214 for PDD screening. During milking, we inspected cows' feet for PDD lesions; 91% of dairies had cows affected by PDD; median PDD prevalence for milking cows in dairies was 6.1%. Finally, in one dairy, PDD screening at the parlor was compared to PDD diagnosis by inspecting restrained cows in a chute. The screening method had a sensitivity of 0.72, and a specificity of 0.99. This study demonstrated that PDD was widespread on study dairies in Chile. The screening test presented can be used as a cost efficient method to estimate prevalence of PDD either on individual dairies or in large-scale surveys.
To compare efficacy of topical treatment with oxytetracycline solution or 1 of 4 nonantibiotic solutions among dairy cows with papillomatous digital dermatitis (PDD).
Randomized field trial.
66 cows in a single herd.
Cows were randomly assigned to be treated with oxytetracycline solution; a commercial formulation of soluble copper, peroxide compound, and a cationic agent; 5% copper sulfate solution; acidified ionized copper solution; hydrogen peroxide-peroxyacetic acid solution; or tap water. Cows were examined 14 and 30 days after initial treatment. During each examination, pain and lesion scores were recorded.
On the basis of pain and lesion scores, oxytetracycline and the commercial formulation appeared equally effective for treatment of PDD and significantly more effective than 5% copper sulfate solution, acidified ionized copper solution, hydrogen peroxide-peroxyacetic acid solution, and tap water. Proportions of cows with signs of pain or visible lesions after treatment were significantly lower for cows treated with oxytetracycline or the commercial formulation than for cows in the other groups.
Oxytetracycline and a commercial formulation of soluble copper, peroxide compound, and a cationic agent appeared to be effective for treatment of PDD in dairy cows.
One hundred and eleven dairy cows with digital dermatitis, on six commercial farms, were used to test the efficacy of a footbath containing erythromycin for treating the condition. Four days after 55 of the cows had walked through the footbath after two successive milkings their lesions were significantly less active and painful than those of the 56 untreated cows. These 56 cows were then treated in the same way and both groups were re-examined seven days later. There were no significant differences between the clinical signs observed in the two groups, and the benefits of the treatment had persisted for the 11 days of the trial.
The efficacy of three non-antibiotic products (copper sulphate, formalin and peracetic acid) was compared with the efficacy of erythromycin, when the four substances were applied in footbaths for the treatment of cows with digital dermatitis. The cows were divided into four groups on the basis of their current housing and randomly allocated to one of the four treatments. Cattle allocated to the non-antibiotic treatments were footbathed daily for seven days, but the cattle treated with erythromycin were footbathed for two days only. Complete records were obtained for 252 lesions from 169 cows. There were significant reductions in the lesion scores of all four groups, but there was no significant effect of treatment and no significant interaction between treatment and time.
We reviewed Bayesian approaches for animal-level and herd-level prevalence estimation based on cross-sectional sampling designs and demonstrated fitting of these models using the WinBUGS software. We considered estimation of infection prevalence based on use of a single diagnostic test applied to a single herd with binomial and hypergeometric sampling. We then considered multiple herds under binomial sampling with the primary goal of estimating the prevalence distribution and the proportion of infected herds. A new model is presented that can be used to estimate the herd-level prevalence in a region, including the posterior probability that all herds are non-infected. Using this model, inferences for the distribution of prevalences, mean prevalence in the region, and predicted prevalence of herds in the region (including the predicted probability of zero prevalence) are also available. In the models presented, both animal- and herd-level prevalences are modeled as mixture distributions to allow for zero infection prevalences. (If mixture models for the prevalences were not used, prevalence estimates might be artificially inflated, especially in herds and regions with low or zero prevalence.) Finally, we considered estimation of animal-level prevalence based on pooled samples.
Digital dermatitis is commonly reported to be most severe in first lactation heifers. It has been suggested that this initial infection is followed by the development of a limited immunity to the organisms which cause digital dermatitis. If this is the case then exposure to digital dermatitis prior to calving should reduce its severity after calving. A study was undertaken to examine whether such exposure significantly affected the development of digital dermatitis post-partum. Twelve weeks prior to calving, 60 Holstein heifers were blocked on the basis of their antibody titre to Borrelia burgdorferi and randomly allocated to one of three pre-calving environments: clean straw, used straw or cubicles. There was no significant effect of pre-calving environment on the development of digital dermatitis after calving indicating that "exposure" pre-calving did not reduce the development of digital dermatitis after calving. The most important factors determining the development of digital dermatitis after calving were presence of absence of visible lesions of digital dermatitis at Week-12 and at calving.