Sunday, October 28, 2012

BCG vaccination - cattle

And now to cattle BCG vaccines: heavily promoted by guitarist, pop idol and astrophysicist Dr. Brian May as 'only months away' with no obvious road blocks from the EU and obviously the answer to all our TB problems.

But not those of increasing numbers of alpacas, domestic cats (which we understand VLA use to fill in their M. bovis spoligotype maps in some areas), sheepdogs, tourist attraction bison, rare breed goats or even a TB-riddled sacred bullock.

Earlier this year a £7m cattle TB vaccination study in the UK was completed. It was carried out by VLA at Weybridge, Surrey (which makes a change from their frequent hops to Africa) and it started in 2005. (SE3227)

In this study 60 uninfected cattle were penned with 39 skin test reactor cattle. Of these 60 uninfected cattle, 20 were left unvaccinated, 20 were vaccinated and 20 were both vaccinated and boosted. After 12 months all the cattle were slaughtered and examined for signs of TB.

Eight of the 60 cattle were found to be infected (i.e. 13 percent) of which two were un-vaccinated, four were vaccinated and two were vaccinated and boosted. So not a great result then?

VLA's Glyn Hewinson reported his heroic efforts to torture the old BCG jab into working order, in this Farmers Weekly article. But for us, that 50-60 per cent efficacy, a repeat of his efforts in Ethiopia and Botswana, just doesn't cut it.

Combined with a DIVA test with less than perfect sensitivity or specificity, we could end up with more slaughtered cattle, a reservoir of TB in cattle and an OIE (Office of International Epizootics) Produce Ban.  It is not the European Union which sets the bar for eradication of tuberculosis, it is the OIE.

So from another contributor, a scenario to ponder, assuming of course we have OIE approval to use a cattle vaccine offering 50 - 60 efficacy, and a DIVA test of dubious sensitivety. (And ignoring the overspill into other mammals.)

Herd A/B of 100 cows is in TB hotspot area. These would be vaccinated annually at a cost of £8.25 per dose with a vaccine needing a 3 or 4 day withdrawal period for milk and meat. The vaccine is, in the words of VLA, only 50 - 60 per cent effective.
  • Around this herd, live both healthy and TB infected badgers and while every effort has been made to keep badgers out of cattle sheds in the winter. In the summer out in the fields cattle to badger (or badger excretion) contact cannot be avoided. 
  • Out of 100 cows in herd AB vaccinated, Group A cows (50-60 animals) would be protected but Group B (40-50 animals ) would have no protection. So after a skin test, (still compulsory) which showed reactors, we will not know which cows are protected and which are not from this herd. Group A may react to the test because of the vaccine, group B could react to the test either because of the vaccine inefficiency but also because they could have been exposed to the bacteria itself. The vaccine should have a 'marker', so any cattle who react to the skin test will have to have a DIVA test to find out if the skin test reaction is from the BCG vaccine or exposure to disease.
  • So lets say that 10 cows react to the test: we then have to DIVA test those 10 animals.
  • However the current DIVA test is not 100% effective, far less than that. But being generous, we'll say it's 90 %. Thus the result of a blood screen will be that either 1 infected cow will be missed as a false negative, or 1 cow will be culled as a false positive. On the other hand, we may get lucky and find the infected one.
  • Any infected cows missed and remaining in the herd, become a risk to the herd before possibly being found at the next test or on slaughter. But the worst case scenario is that they are sold on and possibly take infection to another herd. Which is why Defra helpfully point out that 'A negative DIVA test does not guarantee freedom from TB'.
  • So what does all this mean for the farm, its 100 vaccinated cows and the farmer?
  • As happens now, the farm will be placed under herd movements restriction because of the 10 cows which have reacted to the test; in fact just one reactor will kick start restriction of the whole herd. So for the farmer nothing will have changed in spite of vaccinating his cattle, except he may get less reactors than if he had not vaccinated. The status of any reactors is unknown. 
  • So what will not have changed should we go down the cattle vaccination road?
  • The farm will be under restriction and subject to 60 day testing, plus DIVA blood testing until clear, but with the added risk of more false positive or false negative results from the DIVA test.
  • Cattle movement off the farm will not be allowed (except for slaughter) until the farm has had at least one  'satisfactory' skin test. Strictly licensed live movements after that, have to be within 30 days of a preMT (down from 60 days)
  • Tuberculosis in badgers will still go unchecked.
  • Tuberculosis overspill into other species like alpacas, sheep, goats, pigs and cats and dogs from wildlife will continue.
  • What will have changed if farmers accept vaccination?
  • Some cows will have protection from bovine TB, but which ones will be unknown and some (maybe as many as 50% ) will have no protection at all.
  • Someone (most likely to be the farmer as Defra are broke) will have to pay for the vaccine (£8.25) and DIVA test (£24) [ Defra figures] Vaccination is annual.
  • BCG vaccine withdrawal period is 3 to 4 days, no milk sold and possible disposal problems. Meat sale delays.
  • Undetected DIVA false negatives will/could increase hidden bovine TB within the national herd.
  • Cows will / could be culled for false positives to the DIVA test.
  • But ongoing “jobs” for those in labs doing "DIVA" tests and research?

 It is difficult to see any benefit from vaccinating cattle unless the vaccine is at least 95 percent effective in protecting cattle and unless the DIVA test is 98 / 99% effective in detecting vaccinated cattle from those with exposure to TB.

And this scenario is likely to cause even more confusion about tests for TB and if they work or not. So where is the supposed benefit to the industry or the cattle? The promotion of vaccinating cattle as "the answer" to the TB problem in the face of a known wildlife reservoir of bovine TB which can still infect the 40 to 50 percent of cattle not protected and which will probably involve GB in another Beef Ban is no "answer" at all. The European Commission are less than happy with the spin  Dr. May put on this idea as well, see their statement.

And our information is that 'in the face of the enormous challenge cattle face from infectious badgers, BCG is unlikely to work at all'. Remember, all it takes is is 1 cfu, a single Colony Forming Unit to infect a calf.

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