Resumo

Bovine Tuberculosis (BTB), caused by Mycobacterium bovis, remains an endemic infection in several countries. It causes particular problems on cattle dairy farms and poses a zoonotic health risk to the population exposed to products of animal origin. In many countries, disease control is based on a test-and-slaughter policy, using tuberculin skin tests (TST) as first diagnostic methods with effective results. Nevertheless, there are some animals that present inconclusive reactions and are kept in the herd until another intradermal test can be performed, usually only after a 60-90 days period. Therefore, during that period, those animals may disseminate the disease among the herd. The aim of this study was to evaluate the usage of an MPB70-ELISA in CCT(comparative cervical skin test)-inconclusive cows confirmed as infected by PCR and or bacteriological culture. A total of 77 adult crossbreed Holstein - Gir cows from a dairy herd from Rio de Janeiro, Brazil, with a recent history of bovine tuberculosis after the introduction of new cows in herd, were tested by the CCT (with both bovPPD and avPPD) in accordance to the regulations of the Brazilian Department of Agriculture. Serum samples obtained from cows with inconclusive-CCT were tested in an ELISA using the recombinant M. bovis protein MPB70 as capture antigens. Animals that were inconclusive to the intradermal tests were slaughtered. During necropsy, mediastinal and bronchial lymph nodes, as well as lung samples, were collected. Samples from each animal were cut into two parts and processed for bacterial culture (Lӧwenstein-Jensen with 0.5% pyruvate media) and for multiplex PCR that employed primers from the IS6110 sequence of M. tuberculosis complex and the primer RvD1Rv2031c from M. bovis. CCT revealed 19 (24.7%) positive cows, while eleven presented inconclusive results (14.3%). Ten (90.9%) out of the 11 inconclusive-CCT cows were positive to M. bovis; five of them only by PCR, one only by culture and four by both PCR and culture. MPB70-ELISA succeed to identify as reactive nine (90%) out of those ten cows. This study suggests that MPB70-ELISA could correctly identify as infected the animals that were previously classified as inconclusive by CCT.