Sunday, August 15, 2004

"In the Absence of a wildlife reservoir......." Gamma Interferon

The use of the Gamma interferon 'Bovigam' test is another 'Holy Grail' expounded by the ISG and enthusiastically endorsed by cohort, Elaine King's NFBG.
It'll kill a whole heap more cows, but will it clear Tb more quickly from a herd?

Pilot schemes are running in Wales, Staffordshire, Cheshire, Hereford & Worcs., and Shropshire.
Defra say they would like to do more.
The ISG say they should do more.

The gamma interferon test was trialled over 5 years in Northern Ireland - Yup that's our bit, part of the UK - and abandoned!
Irish vets described it as follows in the Veterinary Record: "An optimist may say it was 'hopeful', a pragmatist would describe it as 'equivocal'".
They were puzzled at Defra's enthusiasm for a repeat 'trial' on a much smaller scale, particularly as their results were available. (Aren't we all?)
The 'trial' in N.I ran for 5 years and involved 6,500 cattle - but of course not the ISG and John Bourne who are so keen to replicate it.
By the end of February 2004, 88 herds had been recruited for Defra's (repeat) performance in GB.

Our man in NZ tells us that over there it is 'relatively successful' in overcoming the 'latency' of the intradermal skin test, which will not show exposure to m. bovis 30 - 50 days before testing, and may not show up until 7 months afterwards. (7 years in Australia!). In NZ it may shorten the time spent under restriction.
Defra are co operating with NZ on trials, but in the UK there is a 'But', and it's a very big one.

Gamma interferon is a laboratory based blood test which measures immune response to - tuberculosis - and that means any tuberculosis, not just m.bovis.

M. bovis is only one strain of Tb, but the current 'Bovigam' test will also pick up avian Tb, para Tb (Johnnes) and skin Tb amongst others. And therein lies the problem. In NZ they are able to use a single skin jab - they have no avian strains, and an onfarm questionaire weeds out any Johnnes candidates before slaughter.
Over here we kill the lot - only to find that nearly 35 percent - YES 35 percent - are not infected with m.bovis at all. Gamma interferon is only 65 percent specific to m.bovis.

As usual, we are indebted to Mr. Benjamin Bradshaw, the Honourable Member for Exeter for his answers to Owen Paterson's Parliamentary Questions.
What a pity he doesn't seem to read his answers more frequently...

Parliamentry Question . 8th. dec 2003 Col 218W
Mr. Bradshaw: "The test ..... results in a higher probability of false positives"

Under EU rules it is also secondary to the approved intradermal skin test, used everywhere else with no problems.

VLA (Veterinary Laboratories Agency) confirm that in the case of a big herd breakdown, (sometimes found where testing is on a 4/5 year programme rather than annual) and where confirmed cases may be developing ahead of skin tests, then the gamma-inteferon 'bovigam' test may shorten the time under restriction.
But only, the man said:
"in the absence of a wildlife reservoir to keep reinfecting........."

We are not in NZ, we do have many strains of TB and we do have that maintenance reservoir..

Keep focused readers..


Anonymous said...

Is it really so bad if the gamma interferon-based test picks up M avium as well as M tb? M avium is pretty nasty, we don't want that in our dairy cattle, it survives pasteurisation and there is evidence for a link between M avium and human intestinal illness.

Matthew said...

Thanks for opening this one up. The skin test in the UK, has been developed to differentiate between Avian tb and bovine. At the moment gamma interferon does not. As to whether it would be a good thing to deliberately slaughter for Avian tb, I leave that to vets (and Richard) to comment on. Para Tb or 'Johnnes' (which may be linked to Chrones) is the one I would agree should be eradicated from our dairy cattle. Gamma interferon as used at present is only 65percent sensitive to m.bovis, so is very much a blunt instrument. I suppose it comes down to how many cattle you want to kill. And if the wildlife maintenance reservoir is still infecting those that are left, what good will it do?