Diagnosis of Bovine Tuberculosis
Sudhir Kumar Prajapati1, Deepti Narang2, Pallavi Slathia2, Sonu S. Nair1, Athira V1, Bablu Kumar3, Prasad Thomas1, V K Chaturvedi1, P. Dandapat1 and Abhishek1
1Division of Bacteriology and Mycology, ICAR-Indian Veterinary Research Institute, Izatnagar, Bareilly-243122, Uttar Pradesh, India
2Department of Veterinary Microbiology, College of Veterinary Science, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana-141004, Punjab, India
3Division of Biological Products, ICAR-Indian Veterinary Research Institute, Izatnagar, Bareilly-243122, Uttar Pradesh, India
Abstract
Bovine tuberculosis (bTB) is a chronic disease that affects animals, particularly cattle, leading to a loss in productivity and signifies a crucial public health risk. Exact estimates of bTB prevalence are lacking in many nations, including India, despite the zoonotic threat and considerable economic expenses associated with the disease. Early TB diagnosis is crucial for the effective prevention and management of the disease. Conventional diagnostic methods like culture and microscopy, though considered the gold standard. For the early detection of the disease, molecular diagnosis and antemortem testing of cellular immune response (Tuberculin test and Interferon gamma Assay) are necessary. Additionally, serological tests like indirect ELISA can be performed to screen for anti-TB antibodies in a herd.
Introduction
Mycobacterium bovis is primarily responsible for the chronic granulomatous inflammatory disease known as bovine tuberculosis (bTB). Although mostly harming cattle, the disease has a wide range of hosts, including people. According to estimates, M. bovis accounts for 10% or less of all human TB cases in underdeveloped nations, which puts the world’s health at risk (Olea-Popelka et al., 2014; Gagneux et al., 2018). Worldwide, cattle, buffaloes, and many other wild species are susceptible to the infectious disease known as bovine tuberculosis (BTB), which is brought on by the bacterium Mycobacterium bovis (Ameni et al., 2007; Le Roex et al., 2013). Due to the disease’s chronic and progressive character, dairy cattle suffer a 10 to 25% reduction in productivity, which has a substantial economic impact and frequently leads to high animal morbidity.
The identification and removal of the diseased animal from the herd are necessary for the most efficient bTB control plan. Using a variety of tuberculin tests, including the single intradermal test (SIT), comparative intradermal, brief thermal, and Stormont tests, BTB is typically identified based on delayed hypersensitivity reactions. However, because of the sensitivity and specificity limitations of these tests, BTB cannot be diagnosed at all phases of infection by a single test (Bezos et al., 2014).
Epidemiology
A wide variety of mammals can become infected with mycobacteria, which has complicated efforts to eradicate the disease by creating infection reservoirs in wildlife and spreading the virus to cattle (De Lisle et al., 2002). According to Zanella et al. (2008), ingestion of tainted milk is a frequent and significant method of zoonotic tuberculosis transmission between humans and cattle. Numerous researchers in India have examined animal TB (Thakur et al., 2010). Even though the disease is present in India, it is more common in Africa, some parts of Asia, and America. In India, it is a serious health issue.
Transmission
Primarily, tuberculosis is a respiratory disease and is transmitted through air born routes within and between species during close contact. According to Ramos et al. (2015), Broughan et al. (2016), and Bapat et al. (2017), infected cattle are thought to be a potential source of infection since they release a considerable amount of mycobacterial organisms through droplet nuclei into the environment and may serve as a source of intra-herd transmission. In addition, the World Health Organisation has listed BTB as one of the seven neglected zoonotic diseases with the potential to spread to humans through direct contact with infected animals or consumption of raw milk, meat, and their products (Malama et al., 2013).
Microscopic examination
The diagnosis is confirmed by direct microscopic demonstration of very small red beaded rods (acid fast bacilli) in clinical specimens by Ziehl-Neelsen (ZN) staining technique. The Ziehl-Neelsen acid-fast stain, a conventional approach, and the pigmentation, growth rate, and gross and microscopic colony morphologies of cultures of the isolated causal organism are used to identify the mycobacteria. The distinct species of mycobacteria are identified by biochemical assays, such as niacin, catalase, nitrate reduction, and urease tests (Niemann et al., 2000).
Diagnosis
Traditional laboratory techniques like culture and smear microscopy can be used to diagnose TB (Parsons et al., 2011). Although these techniques are inexpensive, they have little sensitivity. The culture method, which is the gold standard, takes 6 to 8 weeks to demonstrate organism growth (Ameni et al., 2010). In vivo, detection of the dominant, pre-clinical cell-mediated immune response using the intradermal skin test is the main method used to diagnose bTB infection in animals (Humphrey et al., 2010). With the help of different tuberculin tests, including the single intradermal test (SIT), comparative intradermal test (CIT), and gamma interferon (-IFN) assay, this test is based on delayed hypersensitivity reactions. According to Inwald et al. (2003), the OIE (2009), and Gormley et al., (2006), the tuberculin used for diagnostic purposes in cattle is a combination of dominant mycobacterial proteins obtained from particular strains of M. bovis. However, a sizable portion of these antigens are also found in non-pathogenic mycobacterial species that are ubiquitous in the environment, and this cross-reactivity to common antigens might result in non-specific reactors, or false positives, which reduce the specificity of the test. As a result, the comparative intradermal test includes Mycobacterium avium (M. avium) tuberculin (Monaghan et al., 1994).
In India, the diagnosis and speciation of bTB are frequently done using polymerase chain reaction (PCR) technology. Recently, molecular techniques like PCR that perform better and are more inexpensive have been introduced. According to Sharma et al. (2012), this nucleic acid-based amplification (NAA) technique reliably amplifies the targeted region for the diagnosis of non-pulmonary TB. The tuberculin skin test (TST) has long been a helpful diagnostic and epidemiological tool for the management of bovine tuberculosis. Bovine tuberculosis can be diagnosed using the officially recognized -IFN assay. In many nations, eradication programs for bovine tuberculosis can combine the use of the -IFN assay with the intradermal tuberculin test (Palmer et al., 2006).
The identification and removal of the diseased animal from the herd is necessary for the most efficient BTB control plan. Using a variety of tuberculin tests, including the single intradermal test (SIT), comparative intradermal, brief thermal, and Stormont tests, bTB is typically identified based on delayed hypersensitivity reactions. However, because of the sensitivity and specificity limitations of these tests, bTB cannot be diagnosed at all phases of infection by a single test (Bezos et al., 2014). Animals are individually tested utilizing the comparative cervical tuberculin test (CTT) and gamma interferon (-IFN) assay in addition to SIT to boost diagnostic sensitivity to get around this issue (Schiller et al., 2010).
Conclusion
Bovine TB can be diagnosed in its early stages in live animals using blood PCR and assays based on the cell-mediated immune response (CITT and IFN assay), particularly during the stage of bacteriaemia. Combining the use of CITT and IFN-assay increased the accuracy of TB screening, while IFN-assay was more focused than CITT. Early diagnosis of TB can lead to quick segregation of infected animals, restrict transmission, and help in eradication of bovine TB from the country.
Reference
Bezos J, Alvarez J, Romero B, De Juan L, Dominguez L. Bovine tuberculosis: Historical perspective. Res Vet Sci 2014;97:S3-4.
de Lisle, G. W., Bengis, R. G., Schmitt, S. M. and O’Brien, D. J. 2002. Tuberculosis in free- ranging wildlife: detection, diagnosis and management. Rev. Sci. Tech. 21: 317- 334.
Gagneux S. Ecology and evolution of Mycobacterium tuberculosis. Nat Rev Microbiol. (2018) 16:202–13. doi: 10.1038/nrmicro.2018.8.
Gormley, E., Doyle, M.B., Fitzsimons, T., McGill, K. and Collins, J.D. (2006) Diagnosis of Mycobacterium bovis Infection in Cattle by Use of the Gamma-Interferon (Bovigam1) Assay. Veterinary Microbiology, 112, 171-179. https://doi.org/10.1016/j.vetmic.2005.11.029.
Niemann, S., Harmsen, D., Rusch-Gerdes, S. and Richter, E. 2000. Differentiation of clinical Mycobacterium tuberculosis complex isolates by gyrB DNA sequence polymorphism analysis. J. Clin. Microbiol. 38: 1-4.
OIE (World Organisation for Animal Health) (2018) Chapter 8.11. Infection with Mycobacterium tuberculosis Complex. Terrestrial Animal Health Code.
Olea‐Popelka, F. , Muwonge, A. , Perera, A. , Dean, A. S. , Mumford, E. , Erlacher‐Vindel, E. , Fujiwara, P. I. (2014). Zoonotic tuberculosis in human beings caused by Mycobacterium bovis ‐ a call for action. The Lancet Infectious Diseases, 17(1), e21–e25. 10.1016/S1473-3099(16)30139-6
Ramos, D. F., Silva, P. E. A. and Dellagostin, O. A. (2015). Diagnosis of bovine tuberculosis: review of main techniques. Braz. J. Biol. 75: 830-837.
World Health Organization (2018) Global Tuberculosis Report.
Zanella, G., Durand, B. et al. (2008). Mycobacterium bovis in wildlife in France. J. Wildl. Dis. 44: 99-108.
