Estudio epidemiológico de la Linfadenitis Caseosa en pequeños rumiantes en la región de Alto Alentejo (Portugal) y valoración de técnicas moleculares y serológicas para el diagnóstico de la enfermedad

Caseous lymphadenitis (CL) or pseudotuberculosis is a pyogenic disease, caused by Corynebacterium pseudotuberculosis, affecting mainly small ruminants, although it has also been described in deer, cows, horses, pigs, and humans. Despite its worldwide distribution and its economic impact on sheep and goat production, its real prevalence and distribution are underestimated in countries where CL it is not considered a notifiable disease. The present work is the first seroprevalence study in CL carried out in Alto Alentejo, the main livestock region of Portugal, with the aim of (1) assessing the validity and usefulness of various serological and molecular diagnostic techniques, (2) conducting a cross-sectional survey of the seroprevalence of C. pseudotuberculosis in small ruminants, (3) conducting a preliminary study of the risk factors associated with seropositivity and (4) describing the clinical cases found. To meet this aim, serum samples were taken from 756 animals in 82 farms, as well as abscess and pus samples from 58 animals with skin and visceral pyogranulomatous lesions. Prior to the study and to improve the sensitivity of multiplex PCR for the identification of C. pseudotuberculosis from purulent samples, the validity of this test was determined after including a dilution (1:10) of the DNA samples. The results obtained showed no improvement in their diagnostic validity, so we consider that the dilution should only be applied in highly purulent lesions that are negative to the classical protocol. The comparison of the direct ELISA test (ELITEST CLA # CK105A^®) with the multiplex PCR (reference test) showed a diagnostic sensitivity and specificity of 100% and 76.1%, respectively, with a Probability Ratio of 4.78 for positive results and greater than 100 for negative ones. On this basis, the ELISA would offer 100% certainty in discarding CL, regardless of the pretest probability, but the credibility of positive results would only be ≥ 80% when the real prevalence in the group was ≥ 46%. The epidemiological study of C. pseudotuberculosis infection reflected a seroprevalence at the farm level (dispersion) of 75.4% (95% CI [66.4% -84.5%]), with 16.7% of affected small ruminants (95% CI [14.1% -19.4%]), finding significantly greater susceptibility in the caprine species (PR 4.0 95% CI [3.31-4.84]), in dairy animals (PR 2.91 95% CI [2.49 -3.42]) and in those raised in a non-extensive production regime (PR 4.19 95% CI [3.67- 4.79]) and / or in medium and large farms. Finally, the pathological study confirmed the importance of the visceral form of the disease, especially in sheep species. The results found in this thesis confirm an actual problem and point out the importance of CL in small ruminants in Portugal, as also the usefulness of the direct ELISA technique in the detection of subclinical cases and as a screening test in disease control programs.