"Thus, if for example the true sensitivity of the test is 75 per cent, infection will remain undetected in one in four herds with a single infected animal. Given that only one confirmed reactor is detected at the disclosure test in about 30 per cent of breakdown herds, this represents a large number of additional infected herds that may remain undetected."And from that hypothesis, the ISG's electronic abacus expands the risk to thousands of cattle in hundreds of herds. This was repeated more recently by Cambridge University models - [link] with Dr. Andrew Conlan, ignoring the obvious, and stating unequivocally that:
Around 38 per cent of herds that are cleared experience a recurrent incident within 24 months, suggesting that infection may be persisting within herds.Not that those tested herds may be experiencing an insidious and constant reintroduction of infection from a non-bovine source? The cynical amongst us would suggest that would stop the funding stream generated by badger TB, stone dead. and that would never do.
Scientists also, in the time honoured fashion of Not Made Here, ignore work done on actual transmission opportunities from reactor cattle, both in Ireland and GB.
In 1978 and 1988 Eamon Costello and Louis O'Reilly tried in Ireland with such pairings. Six months -[link] of shared feed, water, and air failed to transmit anything at all, and twelve months - link showed early lesions in just 4 out of 10 pairings. Conclusion: transmission in the field from cattle was very difficult.
The 'Pathman' project - [link] reporting in 2007, spent £2.8m trying to do the same and salami sliced reactor cattle into very tiny bits. After taking 1600 samples from that project's candidates and 1000 from a parallel study, they report that all failed to transmit. - [link]
A further overview, also written a decade ago by practising veterinary surgeon, the late John Daykin and Dr. Lewis Thomas also squashes flat, the elusive reservoir - [link] of zoonotic Tuberculosis in cattle.
But still, right up to date, we hear the same sing-song lament from this 2016 Defra tome - [link] Page 9.
Because of the limitations of the test and the nature of the response to the bovine TB bacterium we may miss 20 to 25 per cent of TB-infected cattle using the standard interpretation of the test (these animals are known as false negatives).Defra personnel tend to ignore research and data which doesn't fit their particular bill, and true to form, they have ignored project SE4500, which examines slaughterhouse cases of TB.
Think about it: if the skin test was missing shed loads of cattle, then that abattoir surveillance - [link] designed for exactly the purpose of finding TB lesions, would be finding the 25 per cent of the annual kill that the skin test missed, would it not? So some 600,000 animals? That 'reservoir' which they seek?
Wind up your calculators dear readers, because there are too many noughts for us in that Defra paper.
But briefly it tells us, that out of 11.1 million animals from TB free herds, passing under the MHS officer's TB inspection microscope 2009 - 2013, just 5,366 samples proved positive for m.bovis. And that is nothing like 25 per cent of cattle, it is barely 0.05 per cent. We cannot find any more details as to whether these samples were from old, walled up lesions, or open active disease. But nevertheless, the figures and evidence from around the world do not support the ISG's and Defra's mischievous assumption that 'If for example....' the skin test (as used in the UK or parts of it) is rubbish.
So as cattle farmers, we are grateful to veterinary surgeon Den Leonard, for permission to quote his letter of explanation of the 'skin test' when used regularly as a whole herd test. The letter was featured in Farmers Guardian January 13th.2017.
"Many people ask for a ‘better’ test for TB, quoting the low sensitivity of the single intradermal comparative cervical test (SICCT) or ‘skin test’ as we all call it. Because it has a low sensitivity it misses some infected animals. However, because so many tests are done on a farm and an area basis, infected areas are soon discovered.
When this happens many thousands of tests are performed in that area, as well as repeatedly on infected premises, which overcomes this issue of sensitivity from an eradication point of view, as the test is given many opportunities to identify the presence of TB.
This is how the test is used across the world very successfully and indeed was how our country virtually eradicated TB when we were managing the badger population density at the same time (before the 1992 Protection of Badgers Act).
However, what is needed more than sensitivity is specificity, and this is where the skin test is the best test. It has a specificity of 99.98%, which means that only 1 in 5000 of the positive test results are NOT positive. If we chose a more sensitive test, then we would reduce the specificity. If we reduced the specificity to 99.5%, which still sounds really good, then 1 in 200 positive results would be incorrect.
An area with perfectly clean cattle in it, or even a farm, would never test clear because of the increased false positive rate.
The skin test can be interpreted in a more sensitive (as in misses less positives), and consequently less specific way, by altering the skin thickness thresholds – what is known as ‘severe interpretation’. Also the gamma interferon blood test is more sensitive and less specific. This alteration in testing is appropriate when you know that infection resides in a herd as you want to ensure you find more truly positive cattle quickly, so you ‘accept’ more false positives during that phase. Using the combination of the skin test, gamma interferon blood testing, and severe interpretation, areas without wildlife infection are rapidly cleared of infected cattle throughout the world.
Farmers get frustrated when positively testing cattle do not show up with lesions. This is because nobody is explaining the test result to them properly, or because myths get perpetuated by people unwilling to understand. This aspect has been covered well in David Denny’s letter.
There is no need for any new cattle tests; we just need better education of stakeholders by the government and by practicing vets.
Den Leonard. Lambert, Leonard and MayFrom what we've read, perhaps we should start that education process with Defra.
We finished the previous posting with Damien Hirst's cow safe in her hermetically sealed formaldehyde tank. We'll end this one with a coughing badger similarly incarcerated.
A much better idea for all our cattle, we think.