aq224 at FreeNet.Carleton.CA
Mon Oct 17 21:33:48 EST 1994
In a previous article, bhjelle at unm.edu () says:
>>There seems to be some question here w.r.t. the antibody assay used and
>>76-118 and also strain 79-90, whereas NEV was in lung sections of C.
>>glareolus. Over 2000 sera were tested and 29 (1.4%) were positive
>>with tires ranging from 1:16 to 1:128.
>>As a point of interest, there were only 2 patients with titres of 1:128.
>>One was a 57 year old female from Alberta and the other was a 72 year
>>female from Ontario. Antibody was found in all provinces. This
>>indicates contact with a Hantaan-like virus. The current strains with a
>>pulmonary syndrome are more related to the Prospect Hill virus.
>>obviously time to do some more testing for relevant antibody. The
>>specific virus in the Alberta case can be identified from patient
>>samples using a nested set RT-PCR with G2 consensus primers and
>>the amplicons for comparison to the 4 Corners strain, etc.
>I agree, PCR would be very helpful in confirming or refuting
>the possibility of false-positive ELISAs. In searching for
>unrecognized infection by Four Corners virus in humans
>by western blot, they appear to be exceedingly rare. We
>have run many hundreds (including hundreds with clear occupational
>risks) using FCV nucleocapsid and G1 antigens (see J Virol 1994;
>68:3000), and found *no* examples of unrecognized infection.
>This was in the endemic area, not Canada! ELISAs have
>a known problem with lack of specificity, so we are
>of course concerned that "positives" are truly positive.
>In fact, it would be exceedingly surprising if the
>elderly ladies you allude to had any reason whatsoever
>to have contracted a Hantaan-like virus (assuming they
>had no history of HFRS), much more likely they are
It is of interest to note that "these ladies" did not have antibodies (i.e.
<1:16) to the NEV strain in lung sections. If they were false positives,
this seems unlikely. There are a couple of points that are of interest.
One needs to know specifically which antigen and which antibody are
reacting in a specific assay. There is always a problem when using
"passaged" virus in other than the original host, because of selection and
propagation of non-pathogenic virions. This could easily apple to the
cell culture virus in the IFAT used, but is unlikely with the mouse lung.
The only fact is that it was infectious virus and not part of the virion.
>>BTW, the ELISA test appears to using nucleocapsid protein expressed in
>>E.coli as per Zoller in JMV 39:3, 1993. This is coded for by the S
>>segment and is highly conserved and probably useful for screening.
>>Zoller claims the Elisa to be superior to the indirect
>>assay. Not a bad compliment for the IFA also. It would be even more
>>strain specific though, if the G protein had been expressed in a
>>eucaryotic expression system, IMHO.
The problem with using glycoproteins expressed in eucaryotic systems
is that any slight modification to the conformation due to changes in
glycosylation result in antigens with different epitopes which are not
valid for specific assays such as implied here.
> >There is of course a large
between using >these assays to confirm infection in someone with recent
>clinical history of hanta-like disease, and using them
>to detect remote infection in someone with no clinical
>history. Like night and day. One is a very high prior
>probability setting, the other very low. That is why
>people invent things like the western blot, to add a
>substantial layer of specificity to the testing algorithm.
I think it is possible to assay for IgM and IgG to determine the current
versus past exposure/infection status.
The western blot does have some positive attributes. If the patients
serum reacts with a specific protein band of the virus it qualifies the
specificity of the antibody to some degree.
>It is just unfortunate to hear how we (apparently) have
>to reinvent the wheel in the field of hantaviruses,
>when these matters have already been largely resolved
>in the fields of HIV, HTLV, HCV, etc.
Its called progress :) or is it epidemiology!
NLVO, Ottawa, Can
Ottawa, Ontario, Canada
e-mail dmcleod at hpb.hwc.ca
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