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THE standard tests used to identify bovine TB in cattle rely on monitoring the immune response as an indicator of infection. While the skin test provides a simple and cost-effective assay, it cannot differentiate between infected animals and those that have been vaccinated against infection. In countries where the disease is endemic, vaccination – of both cattle and potential wildlife reservoirs – is considered to be the best long-term strategy to reduce the threat of bovine TB. However, introducing routine vaccination negates the value of the current standard diagnostic tests. There is therefore a real need for new tests that can differentiate between naturally infected and vaccinated animals (termed DIVA). One approach would be to directly detect Mycobacterium bovis, the main causative agent of bovine TB, in samples from infected animals. While this approach can be used for many bacterial infections, it is problematic when working with mycobacteria.
The slow growth of some pathogenic mycobacteria makes detection by traditional culture extremely difficult. For instance, culture results for M bovis can take up to eight weeks. Similarly, Mycobacterium avium subspecies paratuberculosis (MAP), which causes Johne's disease, can take up to 16 weeks to culture. Even rapid, automated culture methods for bovine TB take up to 15 days. The lengthy incubation times and poor levels of sensitivity achieved when culturing mycobacteria from blood limits the diagnostic power of this method and it has not been used for many years.
Molecular methods are often used to detect bacterial DNA as an alternative to culture. Blood assays based on PCR amplification of genomic signature sequences from Mycobacterium have …
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