A humoral diagnostic test outperforms cellular tests in a farm with a latent tuberculosis outbreak caused by a new Mycobacterium tuberculosis complex spoligotype that affected sheep but not goats

Ramon A Juste; Leire Fernández-Veiga; Miguel Fuertes; Ignacio Fernández-Ortiz de Murua; Guillermo Cardona; Maria V Geijo; Joseba M Garrido; Iker A Sevilla

doi:10.3389/fvets.2023.1310205

A humoral diagnostic test outperforms cellular tests in a farm with a latent tuberculosis outbreak caused by a new Mycobacterium tuberculosis complex spoligotype that affected sheep but not goats

Front Vet Sci. 2024 Jan 22:10:1310205. doi: 10.3389/fvets.2023.1310205. eCollection 2023.

Authors

Ramon A Juste¹, Leire Fernández-Veiga¹, Miguel Fuertes¹, Ignacio Fernández-Ortiz de Murua², Guillermo Cardona², Maria V Geijo¹, Joseba M Garrido¹, Iker A Sevilla¹

Affiliations

¹ Animal Health Department, NEIKER - Basque Institute for Agricultural Research and Development, Basque Research and Technology Alliance (BRTA), Derio, Spain.
² Servicio de Ganadería, Diputacion Foral de Alava, Vitoria-Gasteiz, Spain.

Abstract

Tuberculosis (TB) is a disease caused by members of the M. tuberculosis complex (MTC) that affects numerous species. M. caprae, a member of the complex which is close to M. bovis, is emerging and affects several different hosts that include goats, cattle, sheep, pigs, rabbits, wild boar, red deer, foxes and also humans. A new M. caprae spoligotype (SB2737) was isolated from an outbreak of sheep tuberculosis affecting a mixed sheep (323)-goat (29) farm in 2021. The index case was detected by the La Rioja slaughterhouse veterinary inspection. Tracing back to the farm of origin, both species were submitted to Comparative Intradermal Tuberculin Test (CITT) and M. bovis-specific antibody ELISA tests. A subsample was also examined by IFN-γ release assay (IGRA) and all positives were slaughtered and pathologically and microbiologically investigated. Only 1.2% of sheep and no goat were positive in the CITT, and 11.4% in the IGRA sheep subsample, while up to 36.8% were positive in two consecutive M. bovis-specific antibody ELISA tests. Goats had always tested negative in annual intradermal follow-up since 2013. Upon confirmation of the immunologically positive sheep at slaughter, all the remaining negative animals were killed and 29.2% of sheep were still found infected. This raised the final overall prevalence to 37.5%. Antibody ELISA was the most sensitive (81.4%) in vivo detection method still showing a 85.0% specificity relative to pathological and microbiological tuberculosis status. It was nearly 10 times more sensitive than skin test and had an 86.8% positive predictive value. Notwithstanding a possible singular pathogenesis of the new spoligotype, this outbreak adds up to previous reports suggesting that sheep tuberculosis could be huge reservoir of infection worldwide overlooked by skin test low sensitivity or simply lack of investigation. This makes it urgent to extend the use antibody tests to address the Trojan horse of hidden M. tuberculosis complex infections on bovine TB control programs.

Keywords: Mycobacterium caprae; SB2737; bovine tuberculosis; goat tuberculosis; sheep tuberculosis; skin test; tuberculosis antibody test.

Grants and funding

The author(s) declare financial support was received for the research, authorship, and/or publication of this article. This study has been carried out partly with funds from Grant PID2019-105155RB-C33 and Grant PID2022-142939OR-C21 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe.”