Mycobacterial detection

Tuberculosis and Johne’s Disease in cattle

Bovine Tuberculosis (TB) is an infectious disease of cattle. It is one of a number of disorders caused by mycobacteria, which can also infect many other mammals including humans, badgers, deer, dogs and alpacas.

The mycobacterium, is a genus of bacteria that includes a number of pathogens such as:

Detection of mycobacterium is difficult

  • The presence of mycobacterium is difficult to detect. It can colonise hosts without them showing any adverse signs.
  • Diagnostic tools are poor in terms of sensitivity and specificity.
  • Mycobacterial infections are problematic to control. The cell wall has a unique structure, which is resistant to acids, alkalis and many antibiotics. So infections can be spread through bodily fluids and faeces and are not destroyed by disinfectants.
  • Identification is complicated. Mycobacterium is a slow growing organism so a culture test, although accurate, takes several weeks and is therefore not practical for detecting and understanding the disease. The long incubation time and use of chemicals to inhibit the growth of other bacteria also reduces the viability of the cells and impairs the sensitivity and reliability of the test.

Current test for bovine tuberculosis – a TB skin test

The primary screening test for tuberculosis in cattle is SICTT – a tuberculin skin test that detects the presence of a mycobacterial immune response in the animal.

For this test, a veterinarian injects a small amount of Avian and Bovine tuberculin into a fold of skin to see if there is an immunologic reaction. If the animal is infected a localised swelling can be detected three days later.

The test is thought to be only 50-60% effective, takes three days to produce a result, requires at least two visits by a vet and cannot distinguish between infected and vaccinated cattle, thus removing vaccination as a potential control mechanism.

A second test (gamma interferon) can now be used to confirm an infection but this test is laborious, expensive and has very low specificity so has limited utility

Current test for Johne’s Disease

A common test for Johne’s Disease is the serum antibody ELISA test, which measures the immune response. The limitations of the test are well known as the sensitivity is low and it cannot detect pre-clinical infection.

New research has revealed the potential for a faster, simpler blood test for both diseases

Bacterial dogma suggested that Mycobacterial infection could not be detected in blood (bacteraemia) until the disease was well progressed, however recent research has shown that bTB and MAP circulate in the blood of sub-clinically infected animals at much higher levels than expected.

This has created the opportunity for a new approach to testing.

Bacteriophage test

Bacteriophage amplification technology was originally developed as a test for TB in humans. This test uses a bacteriophage; a type of virus that rapidly infects a particular genus of bacteria and can only replicate in a viable cell. Its presence indicates live Mycobacteria in a sample. So instead of trying to culture the slow growing mycobacterium, a fast replicating phage is used as a proxy.

To conduct the test, mycobacterial cells are first isolated from a sample, incubated with the phage and the amplification of the phage either monitored by conventional microbiology (fully quantitative within 24 hours) or using molecular read-out technology (e.g.PCR) to produce a highly sensitive and specific result within 6-8 hours.

The advantage of the phage test is that it can identify the disease at a much earlier stage. It is also non-subjective, simple to perform and offers a fast result in 6 hours from sample receipt.

Towards eradicating the disease

The ability to detect viable MAP or bTB in the blood of sub clinically infected animals offers the opportunity for increased sensitivity of diagnosis in the early stages of the infection and hence much more effective disease control, improved monitoring and the identification of sources of infection

The phage test also allows the detection of mycobacteria in milk. This has significant implications for quality control of dairy products and potential transmission of Mycobacterial disease to humans.

Find out more about tests in this Actiphage® video below or by visiting PBD Biotech's YouTube channel.

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