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On Sunday, Dr Oura kindly sent the following commentary (in blue) to questions from

Here are my comments below in blue:
I am trying to keep abreast of bluetongue developments with my website and have been doing so since August 2006.  I am now increasingly baffled by the way reports of animals tested for bluetongue after import refer to "positive" results.  It seems that famers and vets - and particularly journalists - in the UK assume that any positive result means that the animal can then spread bluetongue - but is this really the case?  No this is not true. I have today posted a promed response which hopefully addresses this (see below).
Could you help me by telling me if my own understanding is correct. 
    a) That only animals with active virus still in the bloodstream could infect a midge that could then go on to infect another animal.  Only animals with active virus in their blood or possibly in their skin can transmit the virus to midges.
    b) That a vaccinated animal will not be a risk and a positive test for antibodies is a good sign.  As long as the vaccine is efficacious, vaccinated animals have been vaccinated correctly and vaccinated animals have been left for 60 days after the final vaccine shot before they are moved, vaccinated animals should not be a risk. Alternatively if vaccinated animals are tested negative by PCR at least 14 days after their final vaccine shot they should not be a risk. These are the EC rules that animals have to conform to if they are permitted to move out of a PZ (with only one serotype circulating) to a free area. 
Yes a positive test for antibodies is a good sign in that the animal is likely to have been vaccinated or the animal is immune following previous infection (also see promed posting as sheep vaccinated with a single shot of vaccine may not seroconvert but should still be protected). It is important to note that the situation becomes far more complicated in areas where more than one serotype are actively circulating.
    c) That what the DEFRA post import tests are picking up may well not be infectious animals but animals that have traces of Bluetongue from which they have recovered or antibodies from vaccine - and a clear distinction needs to be made between active disease and traces of  now inactive virus. Yes you are correct that a distinction needs to be made. See promed posting as this is hopefully explained - In the recent reports of PCR positive animals imported into the UK these animals have been found to be PCR positive in post-import testing and have, in the majority of cases, had a PCR result consistent with the animal being viraemic (early infection). In many of the samples from these imported animals we have gone on to successfully isolate virus from them.
I quoted the ProMed moderator, Arnon Shimshony, a few days ago who said that it seems likely that "no renewed activity of BTV-8 has been detected, so far, on British soil..." I went on to write :   
The moderator makes an important distinction between "positive for active bluetongue virus" and "detection of bluetongue" in otherwise healthy imports. The detection of antibodies or "non-viable virus components" by post-import testing procedures by DEFRA is not the same thing as finding active virus that could be spread if Culicoides midges were to bite an infected animal, get infected with the virus, and then go on to bite other ruminants. Midges that are not themselves infected can spread the disease only if they ingest blood from animals who have been in contact with Bluetongue and have active, infective virus in their blood." 
Yes this is true but I'm afraid many of the animals we have imported recently into the UK have been viraemic and in the early stages of infection.. (Sent to ProMed by Dr Oura)

Recent reports that not all animals vaccinated with inactivated BTV-8 vaccines seroconvert post-vaccination have been causing alarm. People have concluded from this that the vaccine may be failing and may not be protective. We have found at the bluetongue CRL here at Pirbright that not all sheep seroconvert after one shot of inactivated vaccine (when antibodies are detected using the commercial routinely-used competitive ELISAs). However, we have every reason to believe that these animals are protected as challenge experiments carried out by the vaccine companies have proved that animals that have not seroconverted are protected. Antibodies are only a component of the immune system and previous scientific research indicates that there are likely to be cell-mediated immune responses that are contributing to protection against BTV. Also low levels of antibodies may be present in vaccinated animals that are below the threshold of detection of the competitive ELISAs. This low level of antibodies may be sufficient to produce an effective memory response on challenge.

It seems clear that, for animals vaccinated with inactivated BTV-8 vaccines on a single occasion, the absence of antibodies detected by the cELISA tests does not correlate with a lack of protection. In other words seronegative vaccinated animals are still likely to be protected from BTV-8. Therefore reports of low levels of seroconversion post-vaccination should not put people off vaccinating their stock. However this does cause a problem in that, at the present time, we are unable to reliably confirm by testing that animals have been successfully vaccinated.

Also I would like to try to clear up another issue that is causing some confusion about PCR testing. PCR detects viral RNA and not infectious virus. Viral RNA detected by PCR persists in infected animals often for around 100-200 days post-infection. However infectious virus persists for a maximum of 60 days post-infection. Therefore animals will be PCR positive for a considerable time period after they have cleared infectious virus from their blood.

In the recent reports of PCR positive animals imported into the UK these animals have been found to be PCR positive in post-import testing and have, in the majority of cases, had a PCR result consistent with the animal being viraemic (early infection). In many of the samples from these imported animals we have gone on to successfully isolate virus from them.

Therefore, it is important to be cautious about interpreting PCR results as animals will in some cases be PCR positive however they may not be a risk to the local midge population. Laboratories however are able to go some way to interpret the PCR results and, depending on the CT levels, can predict if the animals are likely to be in the early (viraemic) or late (non-viraemic) stages of infection. The only test available to confirm that the animal is viraemic is virus isolation and the drawback of this is it takes at least a week to perform.

Dr Chris Oura Head of the CRL for Bluetongue, IAH, PIrbright, UK.