Estimation of sensitivity, specificity and predictive values of two serologic tests for the detection of antibodies against Actinobacillus pleuropneumoniae serotype 2 in the absence of a reference test (gold standard)

Latent-class models were used to determine the sensitivity, specificity and predictive values of a polyclonal blocking enzyme-linked immunosorbent assay (ELISA) and a modified complement-fixation test (CFT) when there was no reference test. The tests were used for detection of antibodies against Actinobacillus pleuropneumoniae serotype 2 in a survey of respiratory diseases in Danish finishing pigs. The estimates were obtained by maximum-likelihood and also by a Bayesian method (implemented with Gibbs sampling). Possible dependence of diagnostic errors was investigated by comparing models where independence was assumed to models allowing for conditional dependence, given the true disease status.No strong evidence of conditional dependence in either test sensitivity or specificity was found. Assuming independence, maximum-likelihood estimates and 95% confidence intervals of the sensitivity and specificity of the ELISA were 100% and 92.8% (90.1–95.5%) and the corresponding values of the CFT were 90.6% (85.8–95.4%) and 98.6% (98.0–99.3%), respectively. Bayesian estimates and posterior 95% credible intervals of the sensitivity and specificity of the ELISA were 99.7% (98.7–100%) and 92.7% (89.9–95.3%) and of the CFT were 90.6% (86.0–95.3%) and 98.7% (98.0–99.3%). The sensitivity and specificity of a combined test, where the CFT is subsequently applied to the pig sera that test positive in the ELISA, were estimated at 90.2% (85.6–95.0%) and 99.9% (99.8–100%), respectively. The cost of the combined test was less than the cost of the use of the CFT alone, at prevalences <54%. Prevalences and predictive values and their 95% limits were estimated in six sub-samples of data. The estimates of sensitivity and specificity obtained in the present investigation generally validate those reported from other sources.     

Comparison of ELISA and CFT assays for Chlamydophila abortus antibodies in ovine sera

OBJECTIVE: To compare the sensitivity and specificity of Chlamydophila abortus antibody assays, to find a suitable serological assay for testing sheep for export. DESIGN: Comparison of results from known positive and negative sheep populations. PROCEDURE: Fifty-five positive and fifty negative sera were analysed by four enzyme linked immunosorbent assays (ELISA), three using recombinant antigens based on the chlamydial polymorphic outer membrane proteins (POMP90-3, POMP90-4, POMP80-90) and one using a synthetic peptide based on chlamydial major outer membrane proteins (MOMP-P). They were also analysed by complement fixation tests (CFT) using crude antigens from chlamydia isolated from an Australian sheep, a Californian parakeet and a Texan turkey. Assay sensitivity and specificity were expressed as point estimates and 95% confidence intervals. Results were compared using McNemar's test for paired samples. RESULTS: ELISA sensitivity ranged from 70 to 98% and complement fixation test sensitivity from 60 to 96%; with POMP90-3 > POMP90-4 > CFT (parakeet) > CFT (turkey) > POMP80-90 > MOMP-P > CFT (sheep). There was no significant difference from POMP90-3 to POMP80-90 (P > 0.05). ELISA specificity ranged from 88 to 100% and CFT specificity was 100% for all three antigens; with CFT and POMP90-4 > MOMP-P > POMP80-90 > POMP90-3. There was no significant difference from CFT to POMP80-90 (P > 0.05). Changing the CFT cut-off from 1:32 to 1:4 substantially reduced the specificity with little improvement in sensitivity. CONCLUSION: Assays using POMP90-4, POMP80-90, CFT (parakeet) and CFT (turkey) had equivalent sensitivity and specificity; none of the ELISAs were more specific than any CFT. The POMP80-90 ELISA is recommended as an alternative to CFT (parakeet) but as its specificity is not ideal the search for a more specific assay should continue.     

Sensitivity and specificity of the agar-gel-immunodiffusion test, ELISA and the skin test for detection of paratuberculosis in United States Midwest sheep populations

Our objective was to estimate the sensitivity and specificity of the agar-gel-immunodiffusion test (AGID), the ELISA, and the skin test for the detection of Mycobacterium avium subspecies paratuberculosis (MAP) in sheep using Bayesian methods without a gold standard. Fourteen flocks (2 465 sheep) were used. Five flocks (450 sheep) were considered MAP non-infected and 9 flocks (2 015 sheep) had sheep infected with MAP. Sheep were skin tested and blood was collected for AGID and ELISA testing. Results were analyzed using a Bayesian 3-test in 1-population model fitted in WinBUGS. The model allowed for dependence (correlation) between the two serologic tests, but these two tests were assumed to be conditionally independent of the skin test. The estimated specificity was 99.5% (95% PI of 98.9-99.9%) for the AGID; 99.3% (98.4-99.8%) for the ELISA using an optical density measured cutoff of 0.20; 99.2% (98.1-99.8%) using a cutoff of 0.15; 97.5% (95.8-98.7%) using a cutoff of 0.10; and 98.7% (97.3-99.5%) for the skin test. The estimated sensitivities were 8.3% (6.2-10.7%) for the AGID; 8.0% (6.0-10.4%), 10.6% (8.3-13.1%), and 16.3% (13.5-19.4%) for the ELISA using the cutoffs 0.20, 0.15, and 0.10 respectively; and 73.3% (62.3-85.8%) for the skin test. The skin test was specific in non-infected populations and sensitive in infected populations, although in some cases a positive skin test might represent MAP exposure rather than infection. The AGID and ELISA were specific but lacked sensitivity. The AGID and ELISA consistently identified two different populations of infected sheep with only moderate overlap between positive test results.     

Evaluation of an enzyme-linked immunosorbent assay kit to detect Babesia bovis antibodies in cattle

The sensitivity and specificity of an enzyme-linked immunosorbent assay (ELISA) to detect antibodies; to Babesia bovis was evaluated in 1000 sera from Holstein heifers. Five hundred of them were from cattle naturally or experimentally infected with B. bovis and 500 from uninfected heifers born and raised in a region free of the vector of cattle babesiosis. Additionally, the ELISA was evaluated and compared with an indirect immunofluorescent antibody (IFA) test in 374 heifers inoculated with different kinds of B. bovis antigens in four trials. The cross-reaction was also evaluated in 50 heifers infected with Babesia bigemina and 50 heifers infected with Anaplasma marginale. The mean percentage positivity of negative sera in relation to the ELISA strong positive sera was 8%. The seropositive/seronegative cutoff point was set as twice the mean percentage positivity of negative cattle sera ( = 16%). The sensitivity of the ELISA was 98% with a 95% confidence interval (CI) of 96–99%. The specificity was 95% (CI 93–97%). The agreement was 97% and the kappa value was 0.93. The predictive values of positive and negative results were 95% and 98% respectively. ELISA showed a similar sensitivity to that of the IFA test to detect antibodies to different B. bovis antigens. Its sensitivity ranged from 97.1% to 100% (CI 89–100%), while the sensitivity of the IFA test ranged from 92.8% to 100% (CI 83–100%). ELISA cross-reacted in 8% and 6% of the sera carrying B. bigemina and A. marginale antibodies, respectively, while the IFA showed 4% cross-reaction in each situation. The ELISA evaluated has the advantages of a proper sensitivity, objectivity and capacity to be adapted to test large number of samples in a short period of time. The results indicate that the ELISA is a suitable replacement for the IFA test to detect B. bovis antibodies in cattle sera, especially in epidemiological studies.     

Evaluation of a new antibody-based enzyme-linked immunosorbent assay for the detection of bovine leukemia virus infection in dairy cattle.

The objective of this study was to validate a new blocking enzyme-linked immunosorbent assay (ELISA) (designated M108 for milk and S108 for serum samples) for detecting bovine leukemia virus (BLV) infection in dairy cattle. Milk, serum, and ethylenediaminetetraacetic acid-blood samples were collected from 524 adult Holstein cows originating from 6 dairy herds in Central Argentina. The M108 and S108 were compared with agar gel immunodiffusion (AGID), polymerase chain reaction and a commercial ELISA. Because there is currently no reference test capable of serving as a gold standard, the test sensitivity (SE) and specificity (SP) were evaluated by the use of a latent class model. Statistical inference was performed by classical maximum likelihood and by Bayesian techniques. The maximum-likelihood analysis was performed assuming conditional independence of tests, whereas the Bayesian approach allowed for conditional dependence. No clear conclusion could be drawn about conditional dependence of tests. Results with maximum likelihood (under conditional independence) and posterior Bayes (under conditional dependence) were practically the same. Conservative estimates of SE and SP (with 95% confidence intervals) for M108 were 98.6 (96.7; 99.6) and 96.7 (92.9; 98.8) and for S108 99.5 (98.2; 99.9) and 95.4 (90.9; 98.1), respectively. The ELISA 108 using either milk or serum to detect BLV-infected animals had comparable SE and SP with the official AGID and a commercial ELISA test, which are currently the most widely accepted tests for the serological diagnosis of BLV infection. Therefore, ELISA 108 can be used as an alternative test in monitoring and control programs.     

Evaluation of the diagnostic accuracy of the modified agglutination test (MAT) and an indirect ELISA for the detection of serum antibodies against Toxoplasma gondii in sheep through Bayesian approaches

The diagnostic accuracies of the modified agglutination test (MAT) and indirect ELISA test for the detection of serum antibodies against Toxoplasma gondii in sheep were evaluated through Bayesian approaches on two populations of sheep created from three different groups of animals (T. gondii-aborted ewes, colostrums-deprived newborn lambs, and ewe-lambs and adult ewes with unknown T. gondii infection status). Tests showed a high degree of agreement (kappa statistic = 0.93; 95% confidence interval = 0.87, 0.98) and a significant specificity (Sp) correlation (gamma(Sp) = 0.26; 95% credibility interval = 0.017, 0.61). When prior information was used for all unknown parameters the posterior medians for the sensitivity (Se) and Sp of the MAT and ELISA were, respectively, 92.6% (95% credibility interval = 85.2, 96.9), 95.5% (89.9, 98.7), 90.5% (83.4, 95.6), and 97.8% (94.2, 99.5). These estimates remained similar when uninformative priors were included. The Se estimates of the MAT and ELISA were higher than those obtained on pigs in other study using the same approach (Se = 80.6% and Sp = 89.5% for the MAT, and Se = 71.5% and Sp = 85.5% for the ELISA [Georgiadis, M.P., Wesley, O.J., Gardner, I.A., Singh, R., 2003. Correlation-adjusted estimation of sensitivity and specificity of two diagnostic tests. Appl. Stat. 52, 63-78]. This finding supported the believe that test performances may vary when applied on different animal species. Thus, if these tests are planned to be used on animal species other than sheep or pigs, their diagnostic accuracy should be re-assessed to prevent biased inferences from their results.     

Evaluating the accuracy of canine pregnancy diagnosis and litter size using real-time ultrasound

A total of 135 bitches suspected of being pregnant were examined with a real-time ultrasound equipment. 58 bitches were not pregnant. In pregnancy diagnosis ultrasound produced an overall accuracy of 99.3%, sensitivity 98.7%, and specificity 100.0%. Estimation of the litter size was fairly reliable in small litters of 1–3 puppies.     

Use of an enzyme-linked immunosorbent assay in a bovine brucellosis eradication program

An enzyme-linked immunosorbent assay (ELISA) was developed and was compared with the complement fixation test (CFT) in a bovine brucellosis eradication program. The ELISA detected significantly more reactors than the CFT in both strain 19 vaccinated infected herds (1.79% versus 1.14%) and non-vaccinated infected herds (4.2% versus 3.59%) but not in either vaccinated or non-vaccinated brucella-free herds. The specificity for both tests in brucella-free herds was greater than 0.998. The specificity and sensitivity of the ELISA were compared with those of 3 other tests (the Rose Bengal test; the indirect haemolysis test [IHLT] and the CFT) on serum from 151 animals cultured at slaughter. The calculated specificity of the ELISA in this infected group was lower than for both the CFT and the IHLT (0.58 versus 0.67 versus 0.75). The sensitivity however was much greater (1.0 versus 0.73 versus 0.71). The value of the ELISA when used in an eradication program is discussed.     

A comparison of three serological tests for the diagnosis of B. ovis infection in rams

The complement fixation test (CFT), the enzyme labelled immunosorbent assay (ELISA) and the gel diffusion precipitin test (GD) were compared, for the diagnosis of Brucella ovis infection in rams. The sensitivities of the tests in 109 rams which were shedding B. ovis in their semen were: CFT 96.3%; ELISA 97.2%; GD 91.7%. The specificities of the tests in 141 rams from non-infected flocks were: CFI 99.3%; ELISA 98.6%; GD 100%. Predictive values of the three tests were measured in 285 rams from infected flocks. Thirty-eight percent of these rams were shedding B. ovis in their semen. Predictive values of positive tests were: CFT 75.5%; ELISA 66.7%; GD 72.5%. Predictive values of negative tests were: CFI 97.1%; ELISA 97.6%; GD 93.8%.     

A comparison of standard serological tests for the diagnosis of bovine brucellosis in Canada.

Six agglutination and two complement fixation tests were compared with respect to specificity, sensitivity and relative sensitivity for the serodiagnosis of bovine brucellosis. Based on 1051 sera from brucellosis free herds, the specificity of the tests was 98.9% for the buffered plate antigen test (BPAT), 99.2% and 99.3% for the standard tube and plate agglutination tests (STAT and SPAT), respectively, and 99.8% for the 2-mercaptoethanol test (2MET). On this small sample, the rose bengal plate test (RBPT), card test (CARD) and the complement fixation test (CFT) correctly classed all sera as negative. On a sample of 167 culture positive cattle, the sensitivities of the tests were CFT: 79.0%, BPAT: 75.4, RBPT: 74.9%, CARD: 74.3%, SPAT: 73.1%, STAT: 68.9%, and 2MET: 59.9%. All tests combined detected only 82% of these infected cattle. Analysis of the relative sensitivity of the six agglutination tests gave the following ranking: BPAT greater than RBPT greater than CARD greater than SPAT greater than STAT. The 2MET ranked between the BPAT and RBPT or between the RBPT and CARD depending on the analysis used. The use of the BPAT as a screening test is recommended provided that a test of high specificity and sensitivity such as the CFT is used to confirm screening test reactions.     

Simulation-model evaluation of bovine tuberculosis-eradication strategies in Argentine dairy herds

We used stochastic modification of the Reed–Frost model to assess the impact of 14 different eradication strategies on bovine tuberculosis, under three scenarios of disease introduction, in Argentine dairy herds. All strategies investigated were based on a test-and-cull approach using either the caudal-fold test (CFT), the single cervical test (SCT), the gamma-interferon (γ-IFN) test or a combination of these tests. The maximum, minimum and most-likely sensitivity and specificity were investigated in three scenarios simulating different epidemiological conditions.

Simulation results were highly variable; therefore, it is difficult to predict the effect of disease-control strategies within individual herds. On average, the use of the SCT was less efficient in eradicating tuberculosis from the simulated herd than the CFT. Eradication would be achieved most efficiently by strategies in which the CFT was used assuming maximum possible sensitivity and specificity (difficult to achieve in the field) and/or the γ-IFN test—which has both economical and logistical limitations to its widespread application in Argentina. When disease-control was simulated in situations in which herd tuberculosis prevalence is ≥22%, all strategies we simulated were less efficient than herd depopulation. Considering that Argentine dairy producers are not compensated financially for cattle culled because of tuberculosis, eradication strategies currently used in the Argentine national tuberculosis eradication might not succeed.     

Evaluation of indirect fluorescent antibody test (IFAT) for the diagnosis and screening of lumpy skin disease using Bayesian method

The performance of indirect fluorescence antibody test (IFAT) for serological diagnosis and screening of lumpy skin disease (LSD) was evaluated using methods without gold standard. Virus neutralization test (VNT) was used as the second test and the study sites were selected from two different geographical places in Ethiopia to get different disease prevalence. The analysis of conditional dependent Bayesian model for the accuracy of IFAT showed that sensitivity, specificity, prevalence of the population Pi₁ and the population Pi₂ were 0.92 (0.89–0.95), 0.88 (0.85–0.91), 0.28 (0.25–0.32) and 0.06 (0.048–0.075), respectively. The posterior inferences obtained for VNT sensitivity, specificity and conditional correlation between the tests for sensitivity (rhoD) and specificity (rhoDc) were 0.78 (0.74–0.83), 0.97 (0.95–0.99), 0.052 (−0.03–0.15) and 0.019 (−0.01–0.06), respectively. The interval estimation of conditional correlation for both sensitivity and specificity clusters around zero and thus conditional dependence between the two tests was not significant. Although accuracy measure would not be the only basis for test selection, the result of our study demonstrated that IFAT has a reasonable high accuracy to be used for the diagnosis and sero-surveillance analysis of LSD in the target population.     

Validation of the Fluorescence Polarisation Assay (FPA) for the serological diagnosis of brucellosis

In order to evaluate suitability of Fluorescence Polarisation Assay (FPA) for serological Brucella diagnostic, 1739 samples of sera from cattle, pigs, sheep and goats (65 Brucella-positive, 960-negative and 714 false-positive sera) were investigated at a dilution of 1:10. The cut-off was adjusted by means of ROC analysis. Furthermore, the serum samples were examined for Brucella antibodies using SAT, CFT and ELISA and the results were evaluated regarding sensitivity and specificity. FPA, SAT, CFT and ELISA attained a sensitivity of 92.3, 98.5, 84.6 and 86.2%. In comparison, specificity varied with 87.8, 72.6, 92.5 and 85.8%, respectively. Accordingly, FPA is a suitable test for serodiagnosis of brucellosis.     

Comparison of serologic tests for detection of Brucella infections in cattle and water buffalo (Bubalus bubalis)

OBJECTIVE: To estimate sensitivity and specificity of 4 commonly used brucellosis screening tests in cattle and domestic water buffalo of Trinidad, and to compare test parameter estimates between cattle and water buffalo. ANIMALS: 391 cattle and 381 water buffalo. PROCEDURE: 4 Brucella-infected herds (2 cattle and 2 water buffalo) and 4 herds (2 of each species) considered to be brucellosis-free were selected. A minimum of 100 animals, or all animals > 1 year of age, were tested from each herd. Serum samples were evaluated for Brucella-specific antibodies by use of standard plate agglutination test (SPAT), card test (CT), buffered plate agglutination test (BPAT), and standard tube agglutination test (STAT). A Bayesian approach was used to estimate sensitivity and specificity of diagnostic tests without the use of a gold standard, assuming conditional independence of tests. RESULTS: Sensitivity and specificity estimates in cattle, respectively, were SPAT, 66.7 and 98.9; CT, 72.7 and 99.6; BPAT, 88.1 and 98.1; and STAT, 80.2 and 99.3. Corresponding test estimates in water buffalo, respectively, were SPAT, 51.4 and 99.3; CT, 90.4 and 99.4; BPAT, 96.3 and 90.7; and STAT, 75.0 and 98.8. Sensitivity of the CT and specificity of the BPAT were different between cattle and water buffalo with at least 95% probability. CONCLUSIONS AND CLINICAL RELEVANCE: Brucellosis serologic test performance varied by species tested, but BPAT had the highest sensitivity for screening cattle and water buffalo. Sensitivity and specificity of more than 2 screening tests can be estimated simultaneously without a gold standard by use of Bayesian techniques.     

An enzyme-linked immunosorbent assay (ELISA) for the detection of anti-chlamydial antibodies in pig sera

The evaluation of an enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies against chlamydiae in pig sera is described. The most widely used serological test is the complement fixation test (CFT). The CFT has a lack of sensitivity and specificity because of low antibody titers and unspecific reactions. Eight conventionally raised pigs were exposed to a pathogenic strain of Chlamydia suis, four controls were mock infected. The immune responses was monitored by CFT and indirect ELISA. There was no agreement between CFT and ELISA data. These results were confirmed by a study with 191 sera from nine pig farms. As shown by ELISA and PCR chlamydiae are widespread in swine.     

补体结合酶联免疫吸附试验方法的建立

为改进免疫学诊断技术的准确性,研究了一种基于补体结合的免疫学检测新技术———补体结合酶联免疫吸附试验（CF-ELISA）。CF-ELISA技术采用酶标记抗菊糖纯化豚鼠补体C3抗体及其酶显色系统作为补体参与反应的指示系统,用ELISA方法进行补体结合试验。经对布氏菌病抗体检测的初步试验结果显示,CF-ELISA技术可检测到0.01 IU的布氏菌病抗体,灵敏度与间接酶联免疫吸附试验（iELISA）相当,是虎红平板凝集试验（RBPT）试管凝集试验（SAT）的5 000倍、补体结合试验（CFT）的10 000倍。对349份确诊布氏菌病感染群牛、羊血清的检测结果显示,CF-ELISAi、ELISA、CFT、SAT、RBPT的阳性率分别为35.82%3、6.39%、31.81%、30.09%、36.1%,CF-ELISA与iELISA、CFT、SAT、RBPT的阳性符合率分别为：98.4%、88.8%、80.0%、90.6%。CF-ELISAi、ELISA、CFT、SAT、RBPT对490份布氏菌病阴性群牛、羊血清的阴性率分别为100%、99.6%、100%、99.4%、99.8%,CF-ELISA与iELISA、CFT、SAT、RBPT的阴性符合率分别为：99.6%1、00%、99.4%、99.8%。研究表明,CF-ELISA是具有高特异性和高敏感性的布氏菌病免疫学检测技术。     

Bayesian estimation of diagnostic accuracy of a new bead-based antibody detection test to reveal Toxoplasma gondii infections in pig populations

The success of a Toxoplasma gondii surveillance program in European pig production systems depends partly on the quality of the test to detect infection in the population. The test accuracy of a recently developed serological bead-based assay (BBA) was investigated earlier using sera from experimentally infected animals. In this study, the accuracy of the BBA was determined by the use of sera from animals from two field subpopulations. As no T. gondii infection information of these animals was available, test accuracy was determined through a Bayesian approach allowing for conditional dependency between BBA and an ELISA test. The priors for prevalence were based on available information from literature, whereas for specificity vague non-informative priors were used. Priors for sensitivity were based either on available information or specified as non-informative. Posterior estimates for BBA sensitivity and specificity were (mode) 0.855 (Bayesian 95% credibility interval (bCI) 0.702–0.960) and 0.913 (bCI 0.893–0.931), respectively. Comparing the results of BBA and ELISA, sensitivity was higher for the BBA while specificity was higher for ELISA. Alternative priors for the sensitivity affected posterior estimates for sensitivity of both BBA and ELISA, but not for specificity. Because the difference in prevalence between the two subpopulations is small, and the number of infected animals is small as well, the precision of the posterior estimates for sensitivity may be less accurate in comparison to the estimates for specificity. The estimated value for specificity of BBA is at least optimally defined for testing pigs from conventional and organic Dutch farms.     

Evaluation of a novel enzyme-linked immunosorbent assay for serological diagnosis of porcine proliferative enteropathy

A specific enzyme-linked immunosorbent assay (ELISA) for detection of antibodies to the porcine pathogen Lawsonia intracellularis was developed and evaluated using sera from na?ve, naturally infected as well as experimentally infected pigs. On the basis of 37 serum samples collected from experimentally infected pigs and 62 serum samples from naturally infected pigs the sensitivity of the ELISA was calculated to 98.0%. The specificity of the test was 99.3%, calculated on the basis of 273 serum samples collected in six herds free of L. intracellularis after medicated eradication. The novel ELISA was a specific and sensitive method for detecting specific antibodies, and may be a good alternative to the existing serological tests for L. intracellularis. It may be usable for diagnosis of proliferative enteropathy and for determination of a herd's epidemiologic status.     

Performance evaluation of two serological tests for contagious bovine pleuropneumonia (CBPP) detection in an enzootic area using a Bayesian framework

A Bayesian approach, allowing for conditional dependence between two tests was used to estimate without gold standard the sensitivities of complement fixation test (CFT) and competitive enzyme-linked immunosorbent assay test (cELISA) and the serological prevalence of CBPP in a cattle population of the Central Delta of the Niger River in Mali, where CBPP is enzootic and the true prevalence and animals serological state were unknown. A significant difference (P = 0.99) was observed between the sensitivities of the two tests, estimated at 73.7% (95% probability interval [PI], 63.4-82.7) for cELISA and 42.3% (95% PI, 33.3-53.7) for CFT. Individual-level serological prevalence in the study population was estimated at 14.1% (95% PI, 10.8-16.9). Our results indicate that in enzootic areas, cELISA performs better in terms of sensitivity than CFT. However, negative conditional sensitivity dependence between the two tests was detected, implying that to achieve maximum sensitivity, the two tests should be applied in parallel.     

An estimate of specificity for a Johne's disease absorbed ELISA in northern Australian cattle

Objective To estimate the specificity of an absorbed enzyme-linked immuno-sorbent assay kit^d for Johne's disease (JD) when used in mature cattle populations resident in northern Australia.
Design Blood samples were collected from beef cattle in northern Queensland, the Northern Territory and northern Western Australia, and from dairy cattle in northern Queensland. The specificity of a serological test for JD was estimated by testing the blood samples with an absorbed ELISA kit. Further samples were collected from cattle with positive ELISA results to determine the presence or absence of infection with Mycobacterium avium subsp paratuberculosis .
Procedure During 1995 and 1996, blood, tissue and gut contents were collected from beef cattle at abattoirs in Queensland and the Northern Territory; and blood and faecal samples were collected from dairy cattle in herds assessed to be most at risk for JD in northern Queensland. The blood samples were tested using an absorbed ELISA kit. Tissues and gut contents from beef cattle that had positive ELISA results were cultured for M avium subsp paratuberculosis , and tissues were examined histo-logically. Faecal samples from dairy cattle with positive ELISA results were cultured for M avium subsp paratuberculosis .
Results Estimates of specificity for this absorbed ELISA in mature northern Australian cattle were 98.0% (97.0 to 98.8%, 95% CI) in beef cattle, and 98.3% (96.7 to 99.3%, 95% CI) in dairy cattle.
Conclusion Estimates of specificity in this study were lower for beef cattle from the Northern Territory and northern Western Australia and for dairy cattle from northern Queensland than those quoted from studies on cattle in southern Western Australia. This should be considered when serological testing using the JD ELISA is carried out on northern Australian cattle.