A letter about the very real concerns of farmers about serological testing - and the reply it received a month later from a DVMSubj: Procedures for blood testing.
Dear Mr Bennett,
I and my neighbouring farmers have heard a number of rumours about the blood testing of stock in Devon. I should be grateful if you would explain the situation.
1. I have been told by the MAFF Devon Helpline, that you are currently testing every sheep and every goat in the 3 km zones; by the NFU that you are testing a sample of the sheep in the 3 km zones and that if you test more widely it will be only on farms with both sheep and cattle; by a correspondent in Powys that all sheep flocks are to be tested; and I have seen a suggestion that the EU requires that all flocks in a 10 km radius round each Infected Premises are tested. Would you please clarify?
2. I have found the following description of the procedure for testing on an internet website. I should be grateful if you would confirm whether or not it is accurate and if not accurate, in what respect? It seems to differ from the procedure you hve been following at Knowstone, although it was apparently issued earlier.
(MAFF) 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.
- 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.)
- 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.
3. Peter Morris of the NFU has told me that this is an accurate description of your procedure; but a friend who asked about blood testing procedure over the weekend was told by the duty vet that a single positive test result would result in the slaughter of all animals on the farm involved. What are we to believe?
4. With regard to the procedure described, would you please clarify the description of procedure on finding two or more positive results? Is this two positive returned from the first screening test - or two positives when each individual animal is tested? I am not clear what triggers slaughter of the whole flock and the justification for this. If one historic case is considered non infective, why should two be considered so dangerous as to trigger a contiguous cull? Or is one positive considered likely to be a false result - if so, why kill the animal?
5. What is your attitude to the possibility described in item 2, that animals will be killed unnecessarily, to save on lab. time. I understand that these positive results are considered to be, at worst, animals which have been exposed to infection and have recovered - not actively infectious - so there does not seem to be a pressing need to kill them quickly (if at all).
6. Dr Donaldson of Pirbright has told me that the initial screening test can throw up as many as 5% false positives - 1 in 20. If the minimum sample is to be 30 sheep, it seems very likely that there will be at least one false positive and fairly likely that there will be two in the first test. Since most sheep flocks have at least 200 animals, it seems almost inevitable that there will be two false positives in the second test, where each animal is tested and identified with the result. If the animals testing positive are indeed taken to be historic recovered cases, why not leave them or kill them and leave the rest - as when one animal tests positive? If you are to follow the policy described without discretion, it seems inevitable that you will be killing rather a large number of animals unnecessarily. Do you have any comments and what is your reaction to this possibility?
7. The above comments arise from the accuracy of the lab. tests themselves. What additional inaccuracies are to be expected from the way the samples are collected and transported and the results transmitted? How does your procedure eliminate unnecessary killing arising from mistakes in sampling and transmission of results? What safeguards are there against deliberate tampering with samples and results?
8. What is your procedure for collecting samples; and who will collect samples? We have prevented anyone, let alone anyone who has recently been in contact with animals from visiting our farm. The person who collects the samples will have undoubtedly been in close contact with other animals and will be with ours. What guarantees are we to be given with regard to biosecurity?
9. What guarantees that samples from our farm are transmitted safely in good condition to the lab. (which will undoubtedly be working under pressure with large numbers of samples) and that the correct results returned?
10. What scope will there be for reassessing an apparently anomalous result - or for appealing against or challenging a result?
11. What qualifications and experience will the persons carrying out the tests and interpreting the results have? There must hardly be a flock in the country that does not have a selection of sheep with footrot, feet damaged by footrot in the past; orf and other skin damage. What distinguishes these from FMD lesions, old FMD lesions, etc. - and what assures us that one of these non FMD symptoms will not trigger the killing of ours and our neighbours' animals?
12. Is there any independent scrutiny of the procedure?
I apologise for taking up your time with these questions; but we have ourselves been given inaccurate information several times by the MAFF helplines and we have heard disturbing accounts of poor bioscurity by MAFF personnel from friends and neighbours, without the alarming stories reported in the press and the NFU bulletins and our confidence is very low. At the same time these matters are of the greatest importance to us and our friends and neighbours.
The reply to Lawrence's email of June 11th arrived by post on Monday July 16th - a month and five days later.Department of Environment, Food and Rural Affairs
State Veterinary Service
Clyst House, Winslade Park, Clyst St Mary, Exeter EX5 1DY
Telephone: Exeter (01392) 266373 Direct Dialling: (01392) 266376 GTN: 1378-6376
Fax: Exeter (01392) 266375 e-mail: firstname.lastname@example.org
Mr L Wright
Middle Campscott Farm
Date: 3 July 2001
Dear Mr Wright
FOOT AND MOUTH DISEASE: SEROLOGICAL SURVEILLANCE
I am writing in response to your e-mail in which you set out a number of queries concerning the blood testing program being carried out from this office in support of area clearance activity. I apologise for the delay in sending you this reply and I am grateful for your patience.
I hope to answer all the points you have raised and offer clarification on other points in the approximate order that they appear in your e-mail.
1. The serological surveillance is being carried out as a requirement of an EU Directive to enable lifting of restrictions on farms in the protection zones, the zones of 3km radius, around infected premises (this does not of course lift the restrictions on the infected premises as this requires a different course of action involving cleansing and disinfection and a restocking procedure using sentinel animals).
2. In these protection zones it is necessary to take samples from each sheep flock or goat herd following a period of 21 days after completion of preliminary cleansing and disinfection on the infected premises. Negative results and the removal of any sheep flocks in which positive samples are found will result in the restrictions being lifted which in turn may allow the reduction of the Infected Area.
3. At a later stage, again as part of the EU requirement, it will be necessary to sample a proportion of sheep flocks in the area defined between the protection zone and a circle of 10 km radius around the infected premises. This blood sampling is required as part of the measures to regain FMD free status for the country. Full details of this programme have not yet been produced but I understand that about 10% of flocks in this area will be sampled.
4. The number of samples taken in the protection zones depends on the number of separate epidemiological groups of sheep in the flock rather than on the total flock size, with a maximum of 59 samples being taken from each of these groups. Only if the flock is considered to be one epidemiological group would 59 be the maximum number of samples.
5. In Knowstone, the blood sampling was carried out as a part of diagnostic procedure not the serological surveillance measures described above. This sampling was designed to disclose any disease in sheep in this area of active infection and was only carried out on a limited number of farms around the infected premises. This explains why the procedure was different to the procedure for protection zone clearance as already described.
6. Blood samples are subjected to the ELISA test which as you state can produce false positives because it has a relatively low sensitivity threshold. Any such samples are re-tested using the Virus Neutralisation Test which has far greater sensitivity.
7. If samples test positive then a return visit is made to the farm premises and a number of sheep are blood sampled and also subjected to Probang sampling, which is carried out to ascertain whether virus is present in the gullet of the sheep. Culling of the flock or group then follows. If no further action is necessary. If only one sheep in the flock tests positive then only that sheep is culled from the flock following Probang sampling of that sheep. To date we have only had one flock with one positive, others having considerably more and none to date with only two positives. Currently of those results received back at this office only 8 out of 888 have been positive.
8. If virus is found in any of the Probang samples the farm is redefined as an Infected Premises and all susceptible livestock culled. Under the current policy, contiguous premises would also be considered for culling. Of course if during the various sampling visits, lesions of Foot and Mouth disease are found then the premises would be defined as an Infected Premises following confirmation of disease, either on clinical grounds or through laboratory testing of tissue and blood samples. In both cases a new protection zone would be established around the farm and the whole procedure repeated.
9. I have no grounds to believe that any policy allows the slaughter of animals merely to save on laboratory time. All testing methods used are internationally recognised for the diagnosis of FMD.
10. I am not sure what you might consider to be "inaccurate collection and transport". All samples are collected either by veterinary surgeons or trained lay blood samplers. Samples are labelled and the farm premises noted on the relevant forms. Samples and paperwork are carried by courier to the testing laboratories. Obviously we rely on the laboratories to have systems to enable accurate interpretation and reporting of results and they must have fulfilled DEFRA requirements in this regard before being awarded any contracts.
11. All blood samplers follow our strict guidelines on personal cleansing and disinfection and there is no evidence to suggest that any such staff have carried infection from one farm to another. If you wish to discuss arrangements if/when you become involved in the serosurveillance then this should be done when you are contacted to arrange an appointment.
12. Only veterinary surgeons can give a clinical opinion about suspect disease. That opinion of course depends upon what is seen during their inspection. The vets are aware of the differences between orf, footrot and other skin damage but you will also be aware that FMD in sheep can be very difficult to diagnose of symptoms are mild. In such circumstances it may be appropriate to take tissue and blood samples for diagnostic purposes.
13. I am not sure what you mean by "anomalous result". There is no appeal process.
14. Pirbright and Porton Down are independent laboratories that are carrying out the blood testing on behalf of DEFRA. I am not aware of any other independent scrutiny.
Finally I would apologise for any misleading information that you have received form this centre in the past. In many areas there has been an evolution of policy such that advice had to be changed as more became known about the outbreak. As my final point I would stress that the protection zone bleeding is a positive step towards removing the restrictions from farms and ultimately the Infected Area restrictions.
M J Bennett
Divisional Veterinary Manager