TB Diagnosis in the Field
Bovine tuberculosis is a disease of economic and zoonotic importance caused by the bacterial pathogen Mycobacterium bovis.
Testing for Mycobacterium bovis
Natural infection of cattle in GB is seen, in over 90% of cases, as an infection of the respiratory tract. Infection is usually chronic, can remain sub-clinical for a long period and can be infectious long before clinical signs are visible. Research has shown that a high susceptibility of the lower respiratory tract with as few as five bacteria, cause progressive infection and pathology. This high susceptibility demands that diagnostic methods detect disease as early as possible.
In the UK, infected cattle are
identified using the single
intradermal comparative tuberculin
skin test. Field studies by VLA, have
confirmed that the parallel use of a
commercial test (Bovigam™), that
detects antigen-stimulated bovine
gamma interferon (IFN-
) in whole
blood cultures, can improve the
sensitivity for detecting infected
animals compared to the use of
either test in isolation.
The field trial demonstrated that in
problem herds approximately 12%
of animals, that did not test positive
to the tuberculin skin test, did test
positive to the IFN- blood test.
Of those gamma interferon positive
animals, approximately 18% were
shown to be visibly lesioned or
culture positive in almost half the
herds tested. Overall, a further 27%
of infected animals were diagnosed,
which were undetected by the
disclosing skin test.
The trial also showed that these
tests each identify a slightly
different cohort of infected animals
and the maximum value is gained
by the complementary use of the
IFN- test alongside the skin test.
A specificity value of around 97%
was obtained using a population of
approximately 1,000 TB- free cattle
when the gamma interferon test
was used in conjunction with the
tuberculins used for the skin test.
The IFN- blood test for cattle is
now a well established routine test
for specific disease situations and is
carried out at two of VLA's Regional
Laboratories, Sutton Bonington and
Luddington.
These findings were used to underpin new Defra policy to use the gamma interferon test in parallel with the skin test to improve the sensitivity of the testing regime and identify infected animals more quickly.
