FIVE regional meetings have been completed, involving 180 veterinary surgeons, to discuss issues around bovine TB. Hosted by the British Cattle Veterinary Association, the speakers explored some of the misconceptions and technical issues with testing for the disease and looked forward to future developments.
There was much to be reviewed and the role of testing was discussed and interrogated with the hope that veterinary surgeons and their clients will benefit from a clear understanding of what can, and cannot, be achieved from the current programme.
The limitations of TB diagnostics were presented by Andy Biggs (Vale Veterinary Group). The rising trend of infected cattle was demonstrated by a series of charts with a slaughter total of 40,116 in 2008. The spread of TB into new areas of the country accelerated after 2001 and the current duration of a breakdown is 12 months, increased from six months in 1986.
Recent insights indicate that infection can be established by inhalation of a single infectious particle containing five bacteria; that animals may be infectious before clinical signs or lesions are apparent; and cattle with confirmed bovine TB can be missed by the tuberculin skin test.
Confusion
The performance and interpretation of the skin test was covered in depth. The regional meetings were a month apart and in the intervening period discussion continued around the confusion of the current nomenclature of confirmed and unconfirmed.
The point that has been considered, long and hard, is the misunderstanding about the nomination of “non-visible lesions”. Detection of lesions post mortem is often difficult. Whether lesions are observed or not, the animal has responded to the skin test and is infected. The term “undetected” is more easily understood and this will mean a shift in understanding on the part of the farmer. Instead of assuming that the non-visible lesion finding means that the cow does not have TB, the test has shown that there is a problem but the tests cannot prove it.
Accuracy of any test for any disease is determined by the sensitivity and the specificity and for the skin test the sensitivity is the probability that the test is positive when used in a group of animals with the disease and the specificity is the probability that the test will be negative for cattle that do not have the disease.
Figures of 100% would be ideal but the sensitivity with bTB is recorded at 77%-95% and the specificity at 99% plus at a herd level, but sensitivity may be lower still in individual animals, particularly where there is a high prevalence of early infection, whatever the source.
The test is therefore effective at capturing the positive herd, with a consequent reduction in trading and a reduced risk of spread to other herds. However, there is more chance of missing an infected cow than slaughtering a cow incorrectly and the sensitivity accuracy can be improved by repeating the test so the chance of missing an infected cow reduces.
Low end
If the frequency of testing is reduced (e.g. to every four years) the sensitivity (accuracy) will be reduced and fewer infected cows will be identified. Because of latency with M. bovis, particularly in high incidence endemic areas, the sensitivity is at the low end of the range due to a higher new infection rate resulting in more cattle with early disease.
Quality control of the tuberculin was also discussed. If there is a variation in strengths of the avian and bovine tuberculin, then the performance parameters of the test will change. So, the outcome is that as a surveillance tool for the herd the Single Intradermal Comparative Cervical Tuberculin Test is effective in identifying infected herds, but as a control tool to identify and remove infected animals it is less effective.
Culture negative cows can still be infected and the test does not indicate whether the animal is infectious or diseased. The immune response can be detected within three weeks of infection but the incubation period of the disease can be from a few weeks to a lifetime.
In herds with a high incidence there will be more cattle becoming infected, more early infection, more animals in the latent period and more false negatives. With a low sensitivity, onethird of infected cows could be undisclosed in high incidence herds.
The point was made that “the very time you really need to rely on the bTB skin test the most, it lets you down the most”. With advanced clinical cases the immune system does not react, due to anergy, and this applies to both the skin test and gamma interferon. The way forward is not to keep testing herds next to herds where TB has been found, and effectively playing catch-up: “We need to get ahead of the game.”
There is a role for the gamma interferon test to increase the detection rate but there can be a sense of frustration that in endemic areas there is little point in using tests with increased sensitivity, and removing more cattle, when the infection is fed back by wildlife. An immediate reaction to new TB case breakdowns would be expected to reduce the duration of a breakdown, with testing based on risk.
Vaccines available
Glyn Hewison (Weybridge) outlined the vaccines available for cattle and badgers. The use of vaccine would reduce transmission and there is a need to address the disease in wildlife and cattle.
The efficacy of the BCG vaccine in man and animals varies from 0% to 80% but cattle use is benefiting from the £2 billion spent on human TB vaccines. There is evidence of improved protection from combining BCG with a genetic subunit or an improved prime BCG. The best protection in cattle has been obtained with BCG prime boosted with recombinant adenovirus.
For trade purposes the development of a test to differentiate naturally infected from vaccinated cattle (DIVA) is required. The natural infection with M. bovis has more antigens and there are bits missing from the genome that makes up the vaccine.
Development of a DIVA test is clearly feasible but the major issue is regulatory approval for its use. From the current policy point of view, a vaccine that is incompatible with routine tuberculin testing has a low priority. The most likely date for a vaccine to be developed and to pass the legal process is 2015.
The vaccination of badgers by injection is due to be rolled out in six pilot areas from next year, over a five year period, but the future is seen to involve oral baits. Any bait would not be meat-based due to the animal byproduct regulations but there is no prohibition against vaccinating badgers, although there would need to be a change in the law if badger vaccination was to become compulsory.
An oral vaccine delivery product (Liporale) is available in New Zealand and lipids protect BCG against stomach acids. This product is said not to show up on the skin test, which led one veterinary surgeon to look it up on the web during teatime. Efficacy in possums is said to be 95%.
There is a long-term strategy for the development and introduction of vaccines over the coming years and it is recognised that it will takeanumber of years after vaccine deployment to see a beneficial effect.
Practical politics
The UK bTB eradication plan is to be updated each year. Carl Padgett (Bay Vets Ltd) outlined the practical politics, which includes the national legislation, government intervention and the European dimension.
The current programme was laid down 20 years ago and there is a need to fundamentally rethink how to currently address TB.
The major issue is why is TB treated differently from other infectious diseases? Important reservoirs of infection need to be addressed and comparisons can be made to Johne’s Disease, with limitations of specificity and sensitivity. There are weaknesses in the testing structure but despite this herds are cleared of disease by a disciplined approach.
Carl highlighted an approach to identify the opportunities for spread with TB and to identify critical control points. Badger culling is moving ahead in Wales with a planned use of traps.
Speaking to Andy Biggs, he pointed out that Europe has accepted the updated plan from TBEG and that some €10 million will be made available to support the programme. Full details of the recommendations that have been accepted by the Minister are on the DEFRA website.
Any observations that readers wish to make to the group can be done by e-mail to tberadication.group @defra.gsi.gov.uk; comments to the BCVA TB working group by e-mail to office@cattlevet.co.uk.
Plans are afoot to run the TB myths roadshow in Wales and Northern Ireland.
