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Tests based on tuberculin purified protein derivative (PPD) cannot distinguish between tuberculosis infection, Mycobacterium bovis BCG vaccination, or exposure to environmental mycobacteria. The present study investigated the diagnostic potential of two Mycobacterium tuberculosis-specific antigens (ESAT-6 and CFP10) in experimental animals as well as during natural infection in humans and cattle. Both antigens were frequently recognized in vivo and in vitro based on the induction of delayed-type hypersensitivity responses and the ability to induce gamma interferon production by lymphocytes, respectively. The combination of ESAT-6 and CFP10 was found to be highly sensitive and specific for both in vivo and in vitro diagnosis. In humans, the combination had a high sensitivity (73%) and a much higher specificity (93%) than PPD (7%).Tuberculosis (TB) remains a major global health problem. Human TB is the most frequent cause of death from a single infectious agent, being responsible for eight million new cases and approximately two million deaths annually (39). The AIDS epidemic and the appearance of multidrug-resistant strains of Mycobacterium tuberculosis have contributed to the resurgence of TB in humans. TB is also a significant problem in cattle and bovine TB represents a significant zoonotic infection, particularly in human immunodeficiency virus-infected humans in developing countries (8). Early diagnosis of infection is crucial to prevent the spread of both of these diseases, and improved methods are urgently required.Purified protein derivative (PPD), a crude, poorly defined mixture of mycobacterial antigens containing both secreted and somatic proteins, has been used for TB diagnosis and epidemiological studies for more than half a century. It has been used for many years as an in vivo skin test reagent in both humans and cattle. Alternatives to skin testing have been investigated. In 1990, an in vitro diagnostic test for Mycobacterium bovis infection in cattle was developed, based on the detection of gamma interferon (IFN-␥) liberated in whole blood cultures incubated in vitro with PPD (38). In 1994, an adaptation of this test was developed for the diagnosis of M. tuberculosis and Mycobacterium avium infection in humans, again using PPD-type antigens (33, 34). PPD contains many mycobacterial antigens, some of which are shared among pathogenic mycobacteria belonging to the M. tuberculosis complex (M. tuberculosis, M. bovis, and Mycobacterium africanum), environmental nontuberculous mycobacteria (NTM), and the vaccine substrain M. bovis bacille Calmette-Guerin (BCG) (13). Thus, although responsiveness to PPD is an important aid in the diagnosis of TB and can give an indication of exposure to mycobacteria, it is often impossible to distinguish BCG vaccination and exposure to NTM from M. tuberculosis infection (12). It has been apparent, therefore, that a new diagnostic reagent with specificity for M. tuberculosis and M. bovis is needed to overcome the limitations of PPD.The recent identification of regions of...

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Therefore, the development and maintenance of a Th1 IFN-c response is associated with a greater control of M. bovis infection. Animals progressing from a Th1-biased to a Th0-biased immune response developed more extensive pathology and performed less well in CMI-based diagnostic tests but developed strong IgG1 humoral responses.

Tuberculosis (TB) remains a global health problem in humans and animals, and improved diagnostic methods are needed urgently. This study examined the potential of an interferon-gamma blood test based on a recently identified low-molecular-mass secreted protein antigen, ESAT-6, for early detection of bovine TB. It was found that field cases of bovine TB and experimentally infected cattle exhibited strong in vitro interferon-y responses directed toward this antigen. Of importance, ESAT-6 reactivity was found to discriminate between cattle infected with TB and cattle sensitized by environmental mycobacteria, and the gene encoding this molecule was demonstrated to be absent from >90% of the nontuberculous mycobacterial strains isolated from healthy sensitized cattle. These results demonstrate the feasibility of using single defined antigens for the highly specific diagnosis of TB.