Will we ever eradicate animal tuberculosis?

A one health-focused literature review on bovine and zoonotic tuberculosis in Pakistan from the past two decades: challenges and way forward for control

Bovine tuberculosis (bTB), caused by Mycobacterium bovis (M. bovis), is a globally prevalent zoonotic infectious disease. World Organization for Animal Health (WOAH) estimates indicate that up to 10% of the total human TB cases in developing countries are attributed to M. bovis. Pakistan ranks 4th in global milk production with a livestock population of over 212 million animals. Over 8 million families are involved in raising these animals as a means of livelihood. To date, there is an absence of national-level data on the prevalence of bTB and an effective control program is still lacking. The multifaceted impacts and substantial economic losses render addressing bTB a daunting, but highly important challenge. In this review, we summarise all the freely available literature on M. bovis infection from Pakistan using Google scholar and PubMed databases. A total of 40 animal studies were identified using search terms: "bovine tuberculosis in Pakistan, bTB, Pakistan, Mycobacterium bovis in Pakistan, M. bovis in Pakistan"; while seven human studies were identified using the terms: zoonotic tuberculosis in Pakistan', 'M. bovis in humans Pakistan', 'zTB in TB patients in Pakistan". We have summarized all these studies to identify critical risk factors involved in transmission of bTB among animals and humans. Despite lack of comprehensive and geographically representative studies, the literature suggests a varying prevalence of bTB in animals, ranging from as low as 2% to as high as 19%. Regarding zTB prevalence in humans, estimates range from 1.5% to 13% in high-risk group of farm and abattoir workers, with notably higher percentages in extra-pulmonary TB cases. The review also addresses the challenges that Pakistan faces in formulating an effective policy for the control and eradication of bTB. We conclude with one-health based recommendations as a way forward for controlling TB caused by M. bovis in cattle and humans.

Identification and molecular characterization of Mycobacterium bovis DNA in GeneXpert® MTB/RIF ultra-positive, culture-negative sputum from a rural community in South Africa

This study investigated the presence of Mycobacterium bovis (M. bovis) DNA in archived human sputum samples previously collected from residents who reside adjacent to the M. bovis-endemic Hluhluwe-iMfolozi wildlife park, South Africa (SA). Sixty-eight sputum samples were GeneXpert MTB/RIF Ultra-positive for M. tuberculosis complex (MTBC) DNA but culture negative for M. tuberculosis. Amplification and Sanger sequencing of hsp65 and rpoB genes from DNA extracted from stored heat-inactivated sputum samples confirmed the presence of detectable amounts of MTBC from 20 out of the 68 sputum samples. Region of difference PCR, spoligotyping and gyrB long-read amplicon deep sequencing identified M. bovis (n = 10) and M. tuberculosis (n = 7). Notably, M. bovis spoligotypes SB0130 and SB1474 were identified in 4 samples, with SB0130 previously identified in local cattle and wildlife and SB1474 exclusively in African buffaloes in the adjacent park. M. bovis DNA in sputum, from people living near the park, underscores zoonotic transmission potential in SA. Identification of spoligotypes specifically associated with wildlife only and spoligotypes found in livestock as well as wildlife, highlights the complexity of TB epidemiology at wildlife-livestock-human interfaces. These findings support the need for integrated surveillance and control strategies to curb potential spillover and for the consideration of human M. bovis infection in SA patients with positive Ultra results.

Unveiling insights into bovine tuberculosis: A comprehensive review

The frequent zoonotic disease known as "bovine tuberculosis" is brought on by the Mycobacterium bovis bacteria, which can infect both people and animals. The aim of this review article is to provide an explanation of the etiology, history, epidemiology, pathogenesis, clinical symptoms, diagnosis, transmission, risk factors, public health importance, economic impact, treatment, and control of bovine tuberculosis. Primarily, bovine tuberculosis affects cattle, but other animals may also be affected. Bovine tuberculosis is present throughout the world, with the exception of Antarctica. Cattle that contract bovine tuberculosis might suffer from a persistent, crippling illness. In the early stages of the disease, there are no symptoms. The tuberculin test is the primary method for detecting bovine tuberculosis in cows. Depending on its localized site in the infected animal, M. bovis can be found in respiratory secretions, milk, urine, feces, vaginal secretions, semen, feces, and exudates from lesions (such as lymph node drainage and some skin lesions). This illness generally lowers cattle productivity and could have a negative financial impact on the livestock business, particularly the dairy industry. The most effective first-line anti-tuberculosis chemotherapy consists of isoniazid, ethambutol, rifampin, and streptomycin. Second-line drugs used against bovine tuberculosis include ethionamide, capreomycin, thioacetazone, and cycloserine. To successfully control and eradicate bovine tuberculosis, developed nations have implemented routine testing and culling of infected animals under national mandatory programs.

The Establishment of a Novel γ-Interferon In Vitro Release Assay for the Differentiation of Mycobacterial Bovis-Infected and BCG-Vaccinated Cattle

BCG vaccination is increasingly reconsidered in the effective prevention of bovine tuberculosis (bTB). However, the primary challenge in BCG vaccination for cattle is the lack of a technique for differentiating between infected and vaccinated animals (DIVA). This study aimed to establish a novel DIVA diagnostic test based on an interferon-gamma in vitro release assay (IGRA). The plasmid encoding three differential antigens (Rv3872, CFP-10, and ESAT-6) absent in BCG genes but present in virulent M. bovis was previously constructed. Thus, a recombinant protein called RCE (Rv3872, CFP-10, and ESAT-6) was expressed, and an RCE-based DIVA IGRA (RCE-IGRA) was established. The RCE concentration was optimized at 4 μg/mL by evaluating 97 cattle (74 of which were bTB-positive, and 23 were negative) using a commercial IGRA bTB diagnostic kit. Further, 84 cattle were tested in parallel with the RCE-IGRA and commercial PPD-based IGRA (PPD-IGRA), and the results showed a high correlation with a kappa value of 0.83. The study included BCG-vaccinated calves (n = 6), bTB-positive cattle (n = 6), and bTB-negative non-vaccinated calves (n = 6). After 3 months post-vaccination, PPD-IGRA generated positive results in both vaccinated and infected calves. However, RCE-IGRA developed positive results in infected calves but negative results in vaccinated calves. In conclusion, this DIVA method has broad prospects in differentiating BCG vaccination from natural infection to prevent bTB.

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

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.