September 21 2007 ~ "a lateral flow device - similar to that of a home pregnancy test"
- Following their Statement 9 on September 16 IAH Pirbright brought out another statement, Statement 10 yesterday to explain the technology of the 'Rapid diagnosis' that they are using. The "lateral flow device" illustrated is not the rapid real time RT-PCR test that we have, for six years, been writing about but it can be used to detect virus antigen (they call this "virus particles") which is the virus protein which naturally coats the virus RNA. Depending on the format (but the basis of the test is similar) it can detect antigens or antibodies and as such, is usefully used at the penside. It does not amplify any that are there. It is therefore not as sensitive as PCR since it can only detect virus particles when there are enormous numbers - in the blister fluid of an acute lesion, for example. Once antibody has appeared in the animal and coated the virus protein and made the virus non - infectious, it will not be detectable by this virus protein (antigen) assay.
RT-PCR will detect infection when there are no visible lesions, (as will tissue culture). RT- PCR will still find the virus RNA for probably another few days at least after the antigen test as it will see past all the protein - as the RNA, if present, is extracted for the test. Pirbright uses the real time RT-PCR test to detect virus in the blood during the incubation period before there are clinical signs but when there is a viraemia (this can be associated with non-specific signs such as a raised temperature). This is because their first two tests - the lateral flow device and capture ELISA - are not sensitive enough; they do not amplify any virus present to bring it up to the level of detection like RT-PCR.
But Pirbright does not use RT-PCR as rapid on-site diagnosis, only (as far as we are aware from Statement 10) in the laboratory.
Real time RT-PCR used in kit-form on site, was the system offered to the UK in 2001 from Plum Island, rejected on the grounds that it was "unvalidated" (see below) but the kit, already used in other countries, can be a first line of defence test in the sort of acute situation that we had in early August. It can be done on nose swabs of animals not showing clinical signs and can detect active virus.
Long time readers of this website will no doubt be somewhat bemused to learn that Pirbright's lateral flow device, used to detect the antibodies of recovered animals is not validated.
But if Pirbright's arsenal of diagnostic equipment is not as impressive as it should be blame for this does not lie with IAH.
(Newbury Today reports that
Newbury MP Richard Benyon has urged it to pump more money into Compton, the sister IAH lab of Pirbright near Newbury. He said,
“I am fed up with the Institute's scientists and employees being the whipping boys for Government under-investment.”
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