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Pseudotuberculosis, how to find the enemy hidden inside doors
Costa, L.; Huerta, B.; Galán-Relaño...
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Objectives: The actual prevalence of CLA (caseous lymphadenitis) in small ruminant flocks is underestimated in many countries and continues to spread without data and information about its real economic impact. The difficulty in the accurate identification of the causative agent in internal subclinical cases allows the disease to spread within and between flocks. This research intends to assess the utility of an ELISA (enzyme-linked immunosorbent assay) test in the detection of internal subclinical cases of CLA in farms and to simultaneously add data on the seroprevalence of the disease in Portugal.
Materials & Methods: In this research, we selected a field population of 82 flocks of small ruminants (55 sheep and 27 goat flocks).The procedure was carried out on the farm and in a regional slaughterhouse in the province of Alentejo in southern Portugal. Sera randomly collected from 756 small ruminants (approximately 10 samples per holding), 70% sheep (528/756) and 30% goats (228/756) were screened for anti-bodies against Corynebacterium pseudotuberculosis using the ELISA technique based on a recombinant phospholipase D (ELITEST CLA # CK105A®). The animals showing internal lesions (n = 58) were sampled and a conventional PCR assay was made for the identification of C. pseudotuberculosis in pyogranulomatous lesions and for the cross-sectional study to assess the performance of the ELISA test. The ELISA assay results were subjected to frequency analysis (prevalence of the disease). The same method was applied to the data regarding the species, productive aptitude, production regime, and dimension (size) of the herd. The performance of the ELISA test was evaluated by association tests, namely, χ2 (Chi- square – Fischer’s Exact Test) with a 95% confidence interval. The statistical analysis was performed with SPSS (Statistical Package for Social Sciences) 22.
Results: In this investigation, we found a prevalence of CLA of 34% (258/756), with the ELISA test showing a low specificity, 78% (35/45) and high sensitivity, 100% (13/13). The descriptive analysis of the results suggests that the proportion of disease higher in goats (χ2 (1, N = 756) = 30.773,p < 0.01). Regarding productive aptitude, dairy farms appear to be more susceptible to CLA (χ2 (1, N = 756) = 17.458, p < 0.01); the same propensity appears to exist regarding the production regime, with the farms in the non-extensive regime (intensive and semi-extensive, approximately 6% of the total sample) showing an increased propensity for the disease (χ2 (2, N = 756) = 22.561, p < 0.01). When the results of the ELISA are analyzed regarding the size of the farm, the data are also statistically significant. This data suggests that farms without the reported disease have an average of 85 animals, whereas farms with confirmed disease have an average of 171 animals (p < 0.001). This observation appears to mean that CLA is more prevalent in medium to large farms (50 - 200 animals and > 200 animals).
By performing a herd-level analysis (considering an infected farm with at least one animal positive for the ELISA), of the 82 farms in the study, 66 had at least one positive animal, leading to a value of 80% (66/82) of affected holdings. The performance of the ELISA test technique (ELITEST CLA # CK105A®) was evaluated by determining the positive predictive value (PPV) and negative predictive value (NPV). In this analysis, the ELISA test allowed the detection of 57% of subclinical animals, which was confirmed by the post- mortem examination and conventional PCR assay performed in pyogranulomatous lesions. In the group of animals in which both tests were performed (PCR and ELISA) (n = 58), 57% of the animals positive for the ELISA test were positive for the PCR assay (13/23), and all the negatives, 100%, in the ELISA test were negative for the PCR assay (35/35).
Conclusion: This is the first prevalence study of CL in small ruminants carried out in Portugal. This research clarifies an actual problem and pointed out the importance of CLA in small ruminant herds in Portugal showing that CLA is a prevalent disease in small ruminants, with 34% seroprevalence. Finally seems to demonstrate that the ELISA test is a good diagnostic tool for use in CLA eradication programmes, as can detect a valid proportion of subclinically infected animals.
Keywords: Caseous lymphadenitis, ELISA, Portugal, seroprevalence, small ruminants.
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About
Affiliation of the authors at the time of publication
Polytechnique Institute of Portalegre, Portalegre, Portugal;
Department of Animal Health - Faculty of Veterinary Medicine - University of Córdoba, Córdoba, Spain;
Department of Animal Health - Faculty of Veterinary Medicine - University of Córdoba, Córdoba, Spain;
Vetdiagnos - Veterinary diagnostic laboratory, Cantanhede, Portugal;
ICAAM - Institute of Mediterranean Agricultural and Environmental Sciences, Institute of Advanced Research and Training, University of Évora, Évora, Portugal.
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