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28 February 2003
Description of Tuberculin Test for TB in Cattle
Tina Vernon BVSc MRCVS
Strathearn Farm & Equine Practice, Crieff, Perthshire
Tel: 07889 044 169
E-mail: tina.vernon@tesco.net
(Filed 28th Feb 2003)
© www.land-care.org.uk
The tuberculin test involves injecting a small
(0.1ml) dose of both avian and bovine tuberculin intradermally at
separate sites into the skin on the neck, and then measuring the
reaction to this at 72hours.
Cattle are commonly exposed to avian TB through
contact with birds/droppings at pasture or while housed. This is
of no consequence to the cattle, but there is marked cross reaction
between bovine and avian tuberculin on the intradermal test. A single
bovine test would be very unreliable and so a double comparative
test is used.
Generally speaking an animal tests clear if the
avian reaction is the same size or larger than the bovine reaction.
If the bovine reaction is larger than the avian, then the animal
is recorded as either an inconclusive reactor, or a reactor depending
on the size of the difference. Both cases are referred onto the
local Animal Health Office, together with all completed clear test
charts.
The fate of such animals depends on the herd history.
If the herd is closed (i.e. no animals brought in to the herd from
outwith the farm) and has had no history of TB and is in a “free
area”, usually only the affected animal will be placed under
restrictions and retested in 6 weeks.
In more risky cases, the animal may be slaughtered
and fully post mortemed to look for evidence of TB grossly and by
culture of the carcass. Movement restrictions may be applied to
the whole herd and any untested animals screened for TB.
In areas, or herds, with a known TB problem a
more severe interpretation of the comparative intradermal test is
sometimes used with reactions of equal size being regarded as positive.
There are a few problems with the intradermal
test, but it remains the most reliable test available at the moment.
Animals may be incubating the disease for some considerable time
before testing positive. Also there is always a subjective element,
and ideally both the injection and the reading of the test are carried
out by the same vet to minimise this.
Some animals test positive and then become clear
at 6 weeks. The most likely explanation is either contamination
of the injection site - e.g. by dirt introduced with the needle;
or accidental inversion of the two injections. Johnes disease/vaccination
can also interfere with the results.
Testing intervals are set by the local Animal
Health Office. Currently most of Scotland is on 4 yearly testing
with only animals that have calved, stock bulls and brought in animals
tested. In areas of the country at higher risk 6 monthly routine
testing is in place with all stock on the farm tested.
Extensive trials are on going to try and find
a more reliable test, particularly in the early stages of the disease.
A new blood test is looking promising.
© www.land-care.org.uk
Further Reading Recommended by Land-Care
Irvine, W. J. (2003). Just how bad is the TB problem in UK Cattle?
(Filed 25 February 2003, www.land-care.org.uk,
click here
to view).
Incidents of TB in Cattle in Scotland, 1995-2002.
(Filed 26 February 2003, www.land-care.org.uk,
click
here to view).
The Krebs Report (1997) and the Independent Scientific Review Group.
(Filed 27 February 2003, www.land-care.org.uk,
click here to view).
Independent Scientific Group (ISG) on Cattle TB (1998-current).
(Filed 27 February 2003, www.land-care.org.uk,
click here to view).
Badgers and Bovine TB Subcommittee (Westminster) meets with ISG
24th February 2003
(Filed 28 February 2003, www.land-care.org.uk,
click
here to view).
Badgers and TB in Cattle: the view of a dairy farmer.
(Filed 27 February 2003, www.land-care.org.uk,
click here
to view).
Cultybraggan Farm Diary (2002). Routine testing of Cultybraggan
cattle for Tuberculosis and Brucellosis, November 2002: All results
negative.
(Filed 26 November 2002, www.land-care.org.uk,
click
here to view).
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