Estimate of the sensitivity of an ELISA used to detect Johne's disease in Victorian dairy cattle herds

A mechanistic model captures livestock trading, disease dynamics, and compensatory behaviour in response to control measures

Trade is a complex, multi-faceted process that can contribute to the spread and persistence of disease. We here develop novel mechanistic models of supply. Our model is framed within a livestock trading system, where farms form and end trade partnerships with rates dependent on current demand, with these trade partnerships facilitating trade between partners. With these time-varying, stock dependent partnership and trade dynamics, our trading model goes beyond current state of the art modelling approaches. By studying instantaneous shocks to farm-level supply and demand we show that behavioural responses of farms lead to trading systems that are highly resistant to shocks with only temporary disturbances to trade observed. Individual adaptation in response to permanent alterations to trading propensities, such that animal flows are maintained, illustrates the ability for farms to find new avenues of trade, minimising disruptions imposed by such alterations to trade that common modelling approaches cannot adequately capture. In the context of endemic disease control, we show that these adaptations hinder the potential beneficial reductions in prevalence suTrade is a complex, multi-faceted process that can contribute to the spread and persistence of disease. We here develop novel mechanistic models ofch changes to trading propensities have previously been shown to confer. Assessing the impact of a common disease control measure, post-movement batch testing, highlights the ability for our model to measure the stress on multiple components of trade imposed by such control measures and also highlights the temporary and, in some cases, the permanent disturbances to trade that post-movement testing has on the trading system.

Simulation modelling to estimate the herd-sensitivity of various pool sizes to test beef herds for Johne's disease in Australia

Johne's disease is a chronic intestinal disease affecting livestock. It leads to the shedding of Mycobacterium avium subspecies paratuberculosis (MAP) in the faeces, wasting and eventually death, with animal welfare, economic, and trade implications. The Johne's Beef Assurance Scheme, used in Australia to determine the risk of Johne's disease on beef properties and facilitate trade, is based on testing a subset of the herd with pooled faecal quantitative PCR. This study aimed to model the herd-sensitivity of pooled faecal testing under different Australian farming scenarios. Animals from simulated herds were randomly sampled and allocated into their respective pools. Each tested pool was provided a test outcome, with herd-sensitivity estimated as the probability of detecting a truly infected herd. The models simulated the test performance for the 'Sample' and 'Check' tests used in the assurance schemes (recommended sample sizes of 300 and 50, respectively) for a range of herd sizes, infection prevalence and MAP faecal shedding levels for the pool sizes of 5, 10, 15 and 20. Sensitivity and specificity input values of each pool size were obtained from a previous laboratory investigation. The herd-sensitivity estimate increased with herd size and infection prevalence levels, regardless of the pool size. Higher herd-sensitivity was also achieved for testing scenarios involving larger sample sizes. A pool size of 10 achieved similar herd-sensitivity to that of the current pool size for the majority of the Sample test and Check test scenarios. This was particularly evident when pool-specificity was assumed to be perfect. The overall herd-sensitivity of the Check test was very low for all infection prevalence levels and pool sizes, but it more than doubled, when the sample size increased from 50 to 100 animals (11% versus 26% for a herd size of 500 cattle with a 2% infection prevalence). The results show that the majority of beef producers participating in the assurance scheme can benefit from using a larger pool size for the pooled faecal quantitative PCR testing of their herd, in comparison to the pool size currently used.

Economic losses due to Johne's disease (paratuberculosis) in dairy cattle

Johne's disease (JD), or paratuberculosis, is an infectious inflammatory disorder of the intestines primarily associated with domestic and wild ruminants including dairy cattle. The disease, caused by an infection with Mycobacterium avium subspecies paratuberculosis (MAP) bacteria, burdens both animals and producers through reduced milk production, premature culling, and reduced salvage values among MAP-infected animals. The economic losses associated with these burdens have been measured before, but not across a comprehensive selection of major dairy-producing regions within a single methodological framework. This study uses a Markov chain Monte Carlo approach to estimate the annual losses per cow within MAP-infected herds and the total regional losses due to JD by simulating the spread and economic impact of the disease with region-specific economic variables. It was estimated that approximately 1% of gross milk revenue, equivalent to US$33 per cow, is lost annually in MAP-infected dairy herds, with those losses primarily driven by reduced production and being higher in regions characterized by above-average farm-gate milk prices and production per cow. An estimated US$198 million is lost due to JD in dairy cattle in the United States annually, US$75 million in Germany, US$56 million in France, US$54 million in New Zealand, and between US$17 million and US$28 million in Canada, one of the smallest dairy-producing regions modeled.

Estimation of the sensitivity and specificity of four serum ELISA and one fecal PCR for diagnosis of paratuberculosis in adult dairy cattle in New Zealand using Bayesian latent class analysis

In New Zealand, a new diagnostic approach for the control of paratuberculosis in mixed aged milking cows has been developed using a combination of ELISA and quantitative fecal PCR (f-qPCR). Our analysis was designed to evaluate performance of these individual tests in infected or infectious mixed aged cows across the prevalence of infection typically encountered on NZ dairy farms and calculate test accuracy when used as a screening test of serological ELISAs for four separate antigens read in parallel followed by a confirmatory quantitative f-qPCR test. Data from a cross-sectional study of 20 moderate prevalence herds was combined with existing data from 2 low and 20 high prevalence herds forming a dataset of 3845 paired serum and fecal samples. Incidence of clinical Johne's disease (JD) was used to classify herds into three prevalence categories. High (≥ 3% annual clinical JD for the last three years), moderate (<3 - 1%) and low (<1% incidence for at least the last five years). Positive tests were declared if> 50 ELISA units and f-qPCR at two cut-points (≥1 × 10⁴ genomes/mL or >1 × 10³ genomes/mL). Fixed Bayesian latent class models at both f-qPCR cut-points, accounted for conditional independence and paired conditional dependence. Mixed models at both f-qPCR cut-points, using a different mechanism to account for conditional dependencies between tests were also implemented. Models (24 in number) were constructed using OpenBUGS. The aim was to identify Mycobacterium avium subsp. paratuberculosis (MAP) infected cows that met at least one of two criteria: shedding sufficient MAP in feces to be detected by f-qPCR or mounting a detectable MAP antibody response. The best fit to the data was obtained by modelling pairwise dependencies between tests in a fixed model or by accounting for dependencies in a mixed model at a fecal cut-off of ≥1 × 10⁴ genomes/mL. Test performance differed with prevalence, but models were robust to prior assumptions. For the fixed model, at a prevalence of 0.29 (95 % probability interval (PI) = 0.25-0.33), as a screening plus confirmatory f-qPCR, post-test probability for disease in a positive animal was 0.84 (95 %PI = 0.80-0.88) and 0.16 (95 %PI = 0.15-0.18) for disease in a test negative animal. In low prevalence herds (0.01(95 %PI = 0.00-0.04)) the equivalent figures were 0.84 (95 %PI = 0.08-0.92) and 0.00 (95 %PI = 0.00-0.02). These results suggest this is a useful tool to control JD on dairy farms, particularly in herds with higher levels of infection, where the sampling and testing cost per animal is defrayed across more detected animals.

Is the transmission of Mycobacterium avium subspecies paratuberculosis (MAP) infection through milk intended to feed calves an overlooked item in paratuberculosis control programs?

As in many parts of the world, Chile shows a high Mycobacterium avium subsp. paratuberculosis (MAP) infection rate. Evidently, the control recommendations have been inefficient. In the author's opinion, the potential risk of MAP transmission to susceptible calves through milk consumption is largely overlooked. Two observational studies were performed, one to confirm MAP shedding in young stock in a high MAP-infected dairy herd and MAP load in milk intended to feed these calves was estimated. In parallel, in a second study, we estimated the relationship between MAP herd seroprevalence and the cow MAP shedding level as well as the presence of this pathogen in milk used to feed dairy calves. In the first study, 53.7% of cows and 22.5% of calves showed positive culture results. Among all cows tested, 9 (2.19%) animals had a positive serum ELISA, and MAP load in milk reached 10⁶ bacteria/mL. In the second study, three seroprevalence categories were established as follows: high (≥ 9%), medium (> 5% and ≤ 9%), and low (≤ 5%). Statistical significant differences among these categories were observed. Animals from the high seroprevalence category shed significantly more MAP than the others. However, in the low category, heavy shedder animals were also observed. Finally, in all study herds, MAP presence in milk intended to feed calves was reported, even from herds without ELISA-positive animals.

Modeling of alternative testing strategies to demonstrate freedom from Mycobacterium avium ssp. paratuberculosis infection in test-negative dairy herds in the Republic of Ireland

In light of the various adverse effects of Johne's disease on animal productivity and the debate on the role of its causative organism, Mycobacterium avium ssp. paratuberculosis, in the etiology of Crohn's disease, major dairy-producing countries around the world have implemented national control programs aimed at reducing the prevalence of this infection in cattle. A pilot control program was initiated in Ireland in 2013, with a key objective to provide farmers with test-negative dairy herds with tools and knowledge to increase their confidence of freedom over time. The aim of this study was to estimate the confidence of freedom obtained in test-negative Irish dairy herds over time with various sampling scenarios and to evaluate the cost-effectiveness of alternative scenarios for achieving an acceptable level of confidence of freedom in herds with no evidence of infection. A stochastic model was developed to simulate repeated annual testing of individual animals using ELISA and confirmatory assays over a period of 20 yr. Two scenarios modeled the current herd-screening options, whereas 14 alternative scenarios explored the effect of varying parameters from the current testing strategies, such as the frequency of testing, the eligibility criteria for selecting animals, the type of assay, the probability of introduction, and the assay sensitivity. Results showed that the current testing strategy with milk twice a year or serum once a year in all animals over 2 yr old provided the highest annual herd sensitivity, with a median value of 55%. Although the median confidence of freedom increased over time for all scenarios, the time required to reach 90 and 95% confidence of freedom was highly variable between scenarios. Under the testing scenario where serum tests were used once a year, the confidence of freedom reached 90% after 4 yr and 95% after 7 yr of testing. Some of the alternative scenarios achieved an acceptable level of confidence of freedom in a reasonable timeframe and at lesser cost than the current testing strategies. The results of this work are used to provide recommendations for the next phases of the program.

Case definition terminology for paratuberculosis (Johne's disease)

Paratuberculosis (Johne's disease) is an economically significant condition caused by Mycobacterium avium subsp. paratuberculosis. However, difficulties in diagnosis and classification of individual animals with the condition have hampered research and impeded efforts to halt its progressive spread in the global livestock industry. Descriptive terms applied to individual animals and herds such as exposed, infected, diseased, clinical, sub-clinical, infectious and resistant need to be defined so that they can be incorporated consistently into well-understood and reproducible case definitions. These allow for consistent classification of individuals in a population for the purposes of analysis based on accurate counts. The outputs might include the incidence of cases, frequency distributions of the number of cases by age class or more sophisticated analyses involving statistical comparisons of immune responses in vaccine development studies, or gene frequencies or expression data from cases and controls in genomic investigations. It is necessary to have agreed definitions in order to be able to make valid comparisons and meta-analyses of experiments conducted over time by a given researcher, in different laboratories, by different researchers, and in different countries. In this paper, terms are applied systematically in an hierarchical flow chart to enable classification of individual animals. We propose descriptive terms for different stages in the pathogenesis of paratuberculosis to enable their use in different types of studies and to enable an independent assessment of the extent to which accepted definitions for stages of disease have been applied consistently in any given study. This will assist in the general interpretation of data between studies, and will facilitate future meta-analyses.

Comparing ELISA test-positive prevalence, risk factors and management recommendations for Johne's disease prevention between organic and conventional dairy farms in Ontario, Canada

Johne's disease (JD) is a chronic, infectious disease in cattle. Between 2010 and 2013, a voluntary JD control program was successfully launched in Ontario, Canada, including a Risk Assessment and Management Plan (RAMP) and JD ELISA testing of the entire milking herd. Over the last decade, the organic dairy sector has been growing. However, organic farming regulations and philosophies may influence the risk for JD transmission on Ontario organic dairy farms. The aim of this cross-sectional study was to investigate differences in JD ELISA test positive prevalence, risk factors for JD and recommendations for JD prevention between organic and conventional dairy herds in Ontario. RAMP results (i.e. RAMP scores and recommendations) and ELISA results were available for 2103 dairy herds, including 42 organic herds. If available, additional data on milk production, milk quality, and herd characteristics were gathered. Organic and conventional herds had a similar herd-level JD ELISA test-positive prevalence (26.2% and 27.2%, respectively). Organic herds (4.2%) had a higher within-herd JD ELISA test-positive prevalence compared to conventional herds (2.3%) if they had at least one JD test-positive animal on the farm. Organic farms had lower risk scores for biosecurity (9 points lower), and higher scores in the calving (7 points higher) and the calf-rearing management areas (4 points higher). After accounting for RAMP score, organic farms received fewer recommendations for the calving management area (Odds Ratio=0.41) and more recommendations in the adult cow management area (Odds Ratio=2.70). A zero-inflated negative binomial model was built with purchase of animals and the herd size included in the logistic portion of the model. Herd type (organic or conventional), colostrum and milk feeding practices, average bulk tank somatic cell count, and presence of non-Holstein breeds were included in the negative binomial portion of the model. Organic farms had a higher number of test positive animals (Count Ratio=2.02). Further research is necessary to investigate the apparent disconnect between risk factors and recommendations on organic dairy farms.

Epidemiology of bovine Johne's disease (BJD) in beef cattle herds in Australia

OBJECTIVE

To describe the epidemiology of bovine Johne's disease (BJD) in beef herds in Australia.

DESIGN

Retrospective survey of beef herds detected with BJD between 1991 and 2006.

METHODS

Information about the management and physical characteristics of affected herds, index and clinical cases, testing and control programs, and attitudes to BJD were collated from existing data and personal interviews of herd owners or managers. Herds were excluded if they contained fewer than 30 breeding cows or operated as a dairy farm.

RESULTS

Records from 109 herds demonstrated the first detected ('index') case was 3.4-fold more likely to be a beef rather than dairy breed. However, further analysis revealed association with dairy cattle was an important risk factor for introducing BJD. Index cases were most likely detected by veterinarians investigating clinical cases of scouring or ill-thrifty animals during winter, particularly bulls or aged cows. Most herds with clinical BJD had only a single case, with only one high prevalence herd detected in the survey group. Over the period of observation, test and cull programs did not eradicate BJD unless combined with culling of known high-risk animals, but removal of high-risk cattle by partial or total destocking generally restored the trading status of affected herds.

CONCLUSION

Excluding cattle with dairy contact from beef herds, ensuring more effective farm biosecurity, promptly seeking veterinary advice regarding scouring cattle and sourcing replacement cattle from demonstrably low-risk herds, such as CattleMAP and 'Beef Only' herds, are simple strategies that should reduce the risk of introducing BJD infection into beef herds.

Epidemiology and economics of paratuberculosis

Johne’s disease is the clinical manifestation of Mycobacterium avium subsp. paratuberculosis (MAP) infection and has become widespread since it was first observed in the United States in the early 1900s. MAP is primarily spread through the fecal-oral route, and herds generally become infected by unknowingly purchasing infected animals. The economic losses from the disease are primarily due to decreased milk production, decreased weaning weights in nursing young stock, increased replacement costs, and decreased slaughter value.

The collection of lymphatic fluid from the bovine udder and its use for the detection of Mycobacterium avium subsp. paratuberculosis in the cow

The objective of the current study was to evaluate the feasibility of lymph collection from the bovine udder and to investigate if the lymphatic fluid might be of diagnostic value in cows infected with Mycobacterium avium subsp. paratuberculosis (MAP), the etiologic agent of paratuberculosis. Lymph fluid collection was attempted from 58 cows, and the reactions of the cows as well as the level of difficulty of the procedure were recorded in 56 animals. Lymph samples (51 in total) were tested for the presence of MAP by nested polymerase chain reaction. Collection of the lymphatic fluid caused no or mild signs of discomfort in 94.6% of the cows; in 51.8% of cows, lymphatic fluid was attained on the first attempt, while sample collection was unsuccessful in 12.1%. Mycobacterium avium subsp. paratuberculosis was detected in 43.1% of all lymph samples. The bacterium was present in 66.7% of cows with clinical Johne’s disease, in 42.8% of asymptomatic cows with a positive or suspicious enzyme-linked immunosorbent assay (ELISA) result in blood, and in 38.7% of cows with a negative ELISA result in blood. The present study shows that the procedure was well tolerated by most cows and can easily be performed on farm. The current report of the isolation of MAP from lymph fluid suggests that the present approach could be used for the early detection of Johne’s disease in cattle.

Endemic cattle diseases: comparative epidemiology and governance

Cattle are infected by a community of endemic pathogens with different epidemiological properties that invoke different managerial and governmental responses. We present characteristics of pathogens that influence their ability to persist in the UK, and describe a qualitative framework of factors that influence the political response to a livestock disease. We develop simple transmission models for three pathogens (bovine viral diarrhoea virus, bovine herpesvirus and Mycobacterium avium spp. paratuberculosis) using observed cattle movements, and compare the outcomes to an extensive dataset. The results demonstrate that the epidemiology of the three pathogens is determined by different aspects of within- and between-farm processes, which has economic, legal and political implications for control. We consider how these pathogens, and Mycobacterium bovis (the agent of bovine tuberculosis), may be classified by the process by which they persist and by their political profile. We further consider the dynamic interaction of these classifications with pathogen prevalence and with the action taken by the government.

Evaluation of Ziehl-Neelsen stained faecal smear and ELISA as tools for surveillance of clinical paratuberculosis in cattle in the Netherlands

Testing cattle suspected of clinical paratuberculosis is an important element of surveillance of paratuberculosis. The aim of this study was to evaluate the diagnostic-test characteristics of microscopic examination of Ziehl-Neelsen stained faecal smears for acid-fast Mycobacteria (ZN-test) and serum-ELISA in cattle suspected of clinical paratuberculosis in the Netherlands. Results of all samples submitted for ZN-test and serum-ELISA between April 2003 and April 2006 to our laboratory were retrieved. Results from cattle for which both tests were performed were analysed using two Bayesian latent-class models for evaluation of diagnostic tests in two populations without a gold standard, assuming (a) conditional independence of tests, or (b) conditional dependence of tests in both infected and non-infected cattle. Sampled cattle were divided into two populations in different ways using four known risk factors for clinical paratuberculosis: region, soil type, clinical signs, and age. For 892 cattle suspected of clinical paratuberculosis, both ZN-test and serum-ELISA results were retrieved: 250 ZN-positive and ELISA-positive, 12 ZN-positive and ELISA-negative, 260 ZN-negative and ELISA-positive, and 370 ZN-negative and ELISA-negative cattle. With priors based on the available literature, the posterior estimates of sensitivity, specificity, and positive and negative predictive values of the ELISA were always higher than those of the ZN-test. Furthermore, lower limits of the 95% credibility intervals of the posterior positive predictive values of the ELISA were >or=99.7%, and of the negative predictive values of the ELISA >or=56.4%. We conclude that the ELISA is preferred to the ZN-test to confirm the presumptive diagnosis of clinical paratuberculosis in the Netherlands. Little diagnostic information can be gained by performing the ZN-test in addition to the ELISA.

Prevalence and distribution of paratuberculosis (Johne's disease) in cattle herds in Ireland

A simple random survey was conducted in Ireland during 2005 to estimate the ELISA-prevalence of paratuberculosis, commonly called Johne's disease (JD), in the cattle population. Serum samples were collected from all 20,322 females/breeding bulls over 12 months-of-age in 639 herds. All samples were tested using a commercially available absorbed ELISA. The overall prevalence of infected herds, based on the presence of at least one ELISA-positive animal, was 21.4% (95% CI 18.4%-24.9%). Herd prevalence levels amongst dairy herds (mean 31.5%; 95% CI: 24.6%, 39.3%) was higher than among beef herds (mean 17.9%; 95% CI: 14.6%-21.8%). However, the animal level prevalence was similar. The true prevalence among all animals tested, was calculated to be 2.86% (95%CI: 2.76, 2.97) and for animals >= 2 yrs, it was 3.30% (95%CI: 3.17, 3.43). For animals in beef herds, true prevalence was 3.09% (95%CI: 2.93, 3.24), and for those in dairy herds, 2.74% (95%CI: 2.59, 2.90). The majority of herds had only one ELISA-positive infected animal. Only 6.4% (95% CI 4.7%-8.7%) of all herds had more than one ELISA-positive infected animal; 13.3% (CI 8.7%-19.7%) of dairy herds ranging from two to eight ELISA-positive infected animals; and, 3.9% beef herds (CI 2.4%-6.2%) ranging from two to five ELISA-positive infected animals. The true prevalence of herds infected and shedding Mycobacterium avium subspecies paratuberculosis is estimated to be 9.5% for all herd types; 20.6% for dairy herds; and 7.6% for beef herds. If ELISA positive animals <2-years-of-age are excluded, the true herd prevalene reduces to: 9.3% for all herd types; 19.6% for dairy herds; and 6.3% for beef herds based on a test specificity (Sp) of 99.8% and test sensitivity (Se) (i.e., ability to detect culture-positive, infected animals shedding at any level) of 27.8-28.9%.

Association among results of serum ELISA, faecal culture and nested PCR on milk, blood and faeces for the detection of paratuberculosis in dairy cows

Paratuberculosis is a chronic, infectious disease of ruminants that entails a serious concern for the cattle industry. One of the main issues relates to the efficiency of diagnosis of subclinically infected animals. The objective of this field study was to analyse the association among results of a serum enzyme-linked immunosorbent assays (ELISA), faecal culture and nested PCR tests on milk, blood and faeces for Mycobacterium avium subsp. paratuberculosis detection in dairy cows. Faeces, blood and milk samples were collected from 328 lactating dairy cows in four known infected herds. Results were analysed to determine associations and levels of agreement between pairs of tests. A total of 61 animals (18.6%) tested positive when all the tests were interpreted in parallel. The agreement between results in different pairs of tests was poor, slight and fair in two, five and three of the 10 possible combinations respectively. Faecal culture and faecal polymerase chain reaction (PCR) resulted in the highest kappa coefficient (0.39; fair agreement), with the lowest agreement being for ELISA and blood PCR (-0.036; poor agreement). Fisher's exact test resulted in statistically significant associations (P < or = 0.05) between the following test pairs: ELISA : faecal culture; ELISA : faecal PCR; milk PCR : faecal PCR, blood PCR : faecal PCR and faecal culture : faecal PCR. Enzyme-linked immunosorbent assays showed the highest complementary sensitivity values for all the possible two-test combinations, followed by faecal PCR. The combined use of ELISA and faecal PCR has the potential to increase the overall sensitivity for the diagnosis of paratuberculosis infection.

Evaluation of test-strategies for estimating probability of low prevalence of paratuberculosis in Danish dairy herds

Paratuberculosis is a chronic infection affecting cattle and other ruminants. In the dairy industry, losses due to paratuberculosis can be substantial in infected herds and several countries have implemented national programmes based on herd-classification to manage the disease. The aim of this study was to develop a method to estimate the probability of low within-herd prevalence of paratuberculosis for Danish dairy herds. A stochastic simulation model was developed using the R programming environment. Features of this model included: use of age-specific estimates of test-sensitivity and specificity; use of a distribution of observed values (rather than a fixed, low value) for design prevalence; and estimates of the probability of low prevalence (PrLow) based on a specific number of test-positive animals, rather than for a result less than or equal to a specified cut-point number of reactors. Using this model, five herd-testing strategies were evaluated: (1) milk-ELISA on all lactating cows; (2) milk-ELISA on lactating cows<or=4 years old; (3) milk-ELISA on lactating cows>4 years old; (4) faecal culture on all lactating cows; and (5) milk-ELISA plus faecal culture in series on all lactating cows. The five testing strategies were evaluated using observed milk-ELISA results from 19 Danish dairy herds as well as for simulated results from the same herds assuming that they were uninfected. Whole-herd milk-ELISA was the preferred strategy, and considered the most cost-effective strategy of the five alternatives. The five strategies were all efficient in detecting infection, i.e. estimating a low PrLow in infected herds, however, PrLow estimates for milk-ELISA on age-cohorts were too low in simulated uninfected herds and the strategies involving faecal culture were too expensive to be of practical interest. For simulated uninfected herds, whole-herd milk-ELISA resulted in median PrLow values>0.9 for most herds, depending on herd size and age-structure. None of the strategies provided enough power to establish a high PrLow in smaller herds, or herds with a younger age-structure. Despite this, it appears as if the method is a useful approach for herd-classification for most herds in the Danish dairy industry.

Current value of historical and ongoing surveillance for disease freedom: surveillance for bovine Johne's disease in Western Australia

'Confidence' in freedom from disease is generally derived from multiple sources of varied surveillance information, and typically this surveillance evidence has been accumulated over time. In the state of Western Australia (WA) the main surveillance evidence supporting Free Zone status in the national bovine Johne's disease (BJD) program comprises periodic surveys and the ongoing clinical diagnostic system. This paper illustrates a simple approach to current valuation of historical surveillance information, based on the calculated sensitivity of the surveillance processes, the time elapsed since the data were accumulated, and the probability of new introduction of disease into the population during that elapsed time. Surveillance system components (SSCs) contributing to the overall sensitivity of the surveillance system were the clinical diagnostic system and periodic targeted surveys. Sensitivity of each component was estimated using a stochastic scenario tree model of the surveillance process as implemented. Probability of introduction of BJD into WA during each time period was estimated retrospectively from a stochastic import risk analysis model applied to actual cattle importation data. The probability that the WA cattle population was free from infection (at design prevalences of 0.2% of herds and 2% of animals within an infected herd) was estimated following each of 11 years, giving a median probability that the State was free of BJD (at these design prevalences) at the end of 2005 of 0.89. The meaning of this result is discussed.

Pooled faecal culture for the detection of Mycobacterium avium subsp paratuberculosis in goats

OBJECTIVE

To evaluate pooled faecal culture for herd diagnosis of caprine Johne's disease and relate these findings to faecal shedding rates of Mycobacterium avium subsp paratuberculosis (Map).

DESIGN

Radiometric broth culture was applied to several pooling dilutions, and shedding rates were estimated from a regression equation based on bacterial growth rates and known processing losses during radiometric culture.

PROCEDURE

Sixteen faecal samples from goats naturally infected with sheep (n = 3) or cattle (n = 13) strains of Map, were diluted in normal goat faeces from 1 in 5 to 1 in 50. Cultures were confirmed by IS900 polymerase chain reaction and restriction endonuclease analysis, and mycobactin dependency. The numbers of viable Map in the culture inocula were determined by endpoint titration (most probable number) of nine samples and related to a cumulative growth index.

RESULTS

A pooling dilution of 1 in 25 with an incubation period of 10 weeks detected 13 of 16 culture positive goats, all shedding > or = 2 x 10(4) Map per gram of faeces. Two samples containing very low numbers of Map (< 2 x 10(3)/g) were only culture positive from undiluted faeces. Thirteen of 16 goats were considered to be shedding low to moderate concentrations of Map (< 2 x 10(5)/g faeces).

CONCLUSIONS

These data support a pooling dilution of 1 in 25 for application of pooled faecal culture as a diagnostic tool in caprine Johne's disease control. A test based on this dilution would reduce laboratory costs of whole herd testing in goats by approximately 40% relative to serology and 75 to 90% relative to individual faecal culture.

Peptide aMptD-mediated capture PCR for detection of Mycobacterium avium subsp. paratuberculosis in bulk milk samples

A peptide-mediated capture PCR for the detection of Mycobacterium avium subsp. paratuberculosis in bulk milk samples was developed and characterized. Capture of the organism was performed using peptide aMptD, which had been shown to bind to the M. avium subsp. paratuberculosis MptD protein (J. Stratmann, B. Strommenger, R. Goethe, K. Dohmann, G. F. Gerlach, K. Stevenson, L. L. Li, Q. Zhang, V. Kapur, and T. J. Bull, Infect. Immun. 72:1265-1274, 2004). Consistent expression of the MptD receptor protein and binding of the aMptD ligand were demonstrated by capturing different Mycobacterium avium subsp. paratuberculosis type I and type II strains and subsequent PCR analysis using ISMav2-based primers. The analytical sensitivity of the method was determined to be 5 x 10(2) CFU ml(-1) for artificially contaminated milk. The specificity of aMptD binding was confirmed by culture and competitive capture assays, showing selective enrichment of M. avium subsp. paratuberculosis (at a concentration of 5 x 10(2) CFU ml(-1)) from samples containing 100- and 1,000-fold excesses of other mycobacterial species, including M. avium subsp. avium and M. avium subsp. hominissuis. The aMptD-mediated capture of M. avium subsp. paratuberculosis using paramagnetic beads, followed by culture, demonstrated the ability of this approach to capture viable target cells present in artificially contaminated milk. Surface plasmon resonance experiments revealed that the aMptD peptide is a high-affinity ligand with a calculated association rate constant of 9.28 x 10(3) and an association constant of 1.33 x 10(9). The potential use of the method on untreated raw milk in the field was investigated by testing 423 bulk milk samples obtained from different dairy farms in Germany, 23 of which tested positive. Taken together, the results imply that the peptide-mediated capture PCR might present a suitable test for paratuberculosis screening of dairy herds, as it has an analytical sensitivity sufficient for detection of M. avium subsp. paratuberculosis in bulk milk samples under field conditions, relies on a defined and validated ligand-receptor interaction, and is adaptable to routine diagnostic laboratory automation.

Evaluation of three ELISAs for Mycobacterium avium subsp. paratuberculosis using tissue and fecal culture as comparison standards

Three serum ELISAs for detection of antibodies against Mycobacterium avium subsp. paratuberculosis (Mptb) were evaluated against culture of tissue and feces samples from 994 dairy cows collected at slaughter. Culture of ileum and associated lymph nodes for Mptb were positive for 160 (16.1%) of the 994 cows and 36 (3.6%) were fecal culture-positive for Mptb. Two of the ELISAs evaluated were absorbed indirect assays and the third was a non-absorbed indirect assay. Estimated sensitivities of the absorbed ELISAs when compared to tissue culture were 8.8% and 6.9%, while the unabsorbed ELISA had a sensitivity of 16.9%. Specificities were 97.6%, 96.0% and 90.8%, respectively. When compared to fecal culture, the sensitivities of the absorbed ELISAs were 16.6% and 13.9%, respectively, and the sensitivity of the unabsorbed ELISA was 27.8%. Specificities were 97.1%, 95.9% and 90.1%, respectively. Area under the curve (AUC) of receiver operator characteristic curves for the absorbed ELISAs when tissue culture was the standard were 0.553 and 0.547, while the unabsorbed ELISA had an AUC of 0.540. When fecal culture was the comparison standard, the AUC of the absorbed ELISAs was 0.575 and 0.574, while the unabsorbed ELISA was 0.529. Overall, the sensitivities of the ELISAs when compared to tissue culture were low. The apparent advantage of the unabsorbed ELISA with respect to sensitivity is at the cost of lowered specificity and test accuracy.