Wednesday, July 23, 2014

Flying under a flag of convenience?

This disease - zoonotic Tuberculosis - appears to attract the very worst in selective marketing.

From the notorious '74 per cent efficacy' - [link] attributed to BCG badger vaccine lobbed into an indiscriminately infected population, to the deliberate dumbing down - [ link] of statistics for 'other species' deaths from 'bovine' TB, we've seen many examples.

And it is to one of those 'other species' to which we return, with a very upbeat but totally misleading blurb about a a new blood test -[link] designed to seek out zTuberculosis in alpacas. And find it with 97 per cent accuaracy'.

Farmers Guardian - [ link] described the Enferplex test as needing:
"[] a single blood sample to be taken for testing from Camelids. The scheme also uses a statistical assessment to aid determination of herd infection status, pioneered by SureFarm’s Alastair Hayton. This has been approved by Defra.

The company said the scheme would enable herd level testing to confirm freedom from infection, testing of individual stock before movement or purchase and pre-export testing."
Sounds good? The National Beef Association magazine also covered the launch, which was attended by the NFU. The NBA splurge is expanded as follows:
"A £100,000K project, financed entirely by llama and alpaca owners in the UK has successfully developed a test, approved by government, which can prove at 97 per cent accuracy if an animal carries the disease [zTB] or not."
"It will give the owners of the UK's 30 - 40,000 llamas and alpacas peace of mind when exporting because it can demonstrate they are free of TB".
Well hallelujah to that. So what's wrong?

We would suggest a complete misunderstanding of the difference this test has exhibited in the trials between 'Sensitivity' - and 'Specificity'. The accuracy of any diagnostic test depends on a trade off between the two, and unfortunately, in the trials, Enferplex didn't fare too well as one of four assays trialled in terms of its ability to detect disease.

 The full results are in charts prepared by AHVLA  on this link -[link] and although in one chart, the Speciticity of Enferplex is approaching 97 per cent, the Sensitivity or ability to detect to disease is only 66 percent. Meaning that 34 per cent of candidate animals, carrying zTB may be missed.

In the third chart, the sensitivity of Enferplex is very low at 55 per cent, meaning it may miss 45 per cent of infected animals.

From the Alpaca tb website:
"The tables below show the sensitivity and specificity of the tests. The sensitivity of a test is the proportion of truly infected animals that are detected with a test - if a test has 55% Sensitivity it is missing 45% of infected animals.

The Specificity of a test is the proportion of truly uninfected animals that are correctly classified as test-negative. If a test has 99% specificity then 1% will be a false positive.

You can see that when used on their own, the Stat-Pak, IDEXX and Enferplex (2 antigen) have similar sensitivity and could miss around one third of infected camelids.

DPP and Enferplex 4 antigen could miss approaching half (45%) of infected animals."
More comment on this link - [link] and a button to direct readers to the actual paper.

Having looked at the tables, and in particular the Sensitivity or 'the ability to detect disease' in camelids, it would appear that IDEXX and STAT-PAK as a combined screen (after using a skin test primer)  give the best chance of not missing too many infected animals.

Conversely Enferplex, despite it's high profile launch, appears from these figures to be capable of missing anything from 33 - 45 percent of infected animals. As do the other blood assays as stand alone tests.

So one wonders - and not for the first time - what 'government' has to gain from its approval?

 UPDATE: The following information is from AHVLA:
The sensitivity of all the above tests is dependent on a prior skin test which boosts specific antibody responses. Without such a skin test the sensitivity of all the tests could drop by around 20% to 30%, which in the case of the high specificity options could reduce sensitivity to a level where the test is more likely to miss an infected animal than identify it.

This ability of the skin test to boost specific antibody in camelids has been published in 3 separate research papers to date:

Stevens et al., 1998, Canadian Veterinary Journal, 62:102-109 Dean et al., 2009, Veterinary Record, 12, 165(11):323-324 Bezos et al., 2013, Preventative Veterinary Medicine, 111(3-4): 304-313
So sadly it would appear that Enferplex, particularly as promoted, i.e without mention of a priming tuberculin antigen skin test, follows several other blood assay screens for zTuberculosis into a black box of hope, rather than a useful tool for any zTB eradication programme.

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