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Serological Testing

Information from a knowledgeable vet

" I have finally managed to get hold of/assemble some details of the MAFF official policy/procedures with regard to serological testing and surveying prior to lifting of restrictions - which I believe to be accurate.

They are using a new test for this type of testing - a competitive ELISA test (rather than the Blocking ELISA test used and still being used for diagnostic testing) - which is easier to set up and standardise reagents apparently so has higher throughput and should produce results more rapidly too.

They are calculating number of animals to be sampled based of being 95% confident of identifying FMDV virus infection at 5% prevalence. There is a table for use in the field to calculate the number of animals that should be sampled. A maximum of 60 samples is required regardless of the size of the flock.

First Sample

- The samples are NOT identified/tied-to individual animals on first round of sampling.

- If all samples are negative to the Competitive ELISA the flock is 'labelled' as negative. (No further action necessary.)

Second sample

- If there is a single positive sample - regardless of the number of samples taken - then it will be re-tested using a Virus Neutralisation Test (VNT)

- - If it's still positive then they will do a second round of sampling but this time sample EVERY animal and label

In second test samples identified to individual animals

- the samples will be identified/tied-to individual animals.

- If there is again only a single positive result then just the animal yielding the positive result will be culled and the flock then 'labelled' negative.

If two or more positive samples found in second round of tests

If there are two or more positive samples then there is no VNT carried out and no second round of sampling/testing - the whole flock will be 'labelled' as positive and will be culled as a Dangerous Contact (DC).

- The animals will be examined on slaughter for FMD lesions - if FMD lesions are found the premises will be 're-labelled' as an IP and Contiguous Cull (and possibly 3Km cull procedures) will come into effect.

This is Page Street policy and is to be followed throughout UK.

I understand that this policy was communicated from Page Street about 2 weeks ago to Control Centres and is to be a UK wide policy - ie. England, Scotland and Wales. There is no known 'scope' for regional variations.

Because they are using a new test - Competitive ELISA - there remains some debate about the issue of false positives, since this is a new test whose performance characteristics may not yet be fully understood. . Some of those who understand this 'stuff' believe that labelling a flock as positive solely on the basis of two positive samples using this test - without confirmation using VNT tests - risks 'over diagnosis'. Others - eg. the French - it seems believe that the Competitive ELISA is more accurate than VNT so no second-setp VNT tets are required. Overall it seems that MAFF has accepted this risk - of possible occasional 'over diagnosis ' - because doing confirmatory VNT tests is fiddling and time consuming of lab resources and may not increase precision when lab.resources are still a limiting factor in the rate at which flocks can be tested and hence would delay lifting restrictions. So it appears MAFF have opted to err on the 'safe side' rather than either delay things and/or risk leaving an infected flock alive longer.

Of course since we are still in the early stages of this serological testing/survey phase we should expect that this policy may be subject to some 'tuning' as results come in. I'm told that there is still 'great uncertainty about serology' with regard to this phase of the FMD control progamme, and that one should bear in mind that those people who 'administer' the policies at MAFF HQ are not themselves experts in this field.

However, I am left with the impression that this serological testing/survey policy has been drawn up with significantly more care and consultation with the real experts than the discredited 12/48 hour and 3Km cull policies."

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