We are now able to offer testing as of 1 June 2003
by way of an IFA serological procedure (at LML) to detect patient's
IgM & IgG antibodies in serum (made our "first in the
midlands diagnosis in Nov. 2003). The IFA slides last indefinitely
and allow same-day test results. Importantly, the true positive
test pattern is a "membrane" type; we have noted one
case with cytoplasmic positivity (which...unpublished...the slide
manufacturer says is a type of false positive). The WNV is said
to cause a very rapid antibody response; so, a negative response
in a person symptomatic for greater than 48 hours probably represents
solid evidence that the patient does not have a WNV infection.
A positive result should be viewed as presumptively diagnostic
of WNV (we will then forward sample to DHEC for confirmation at
their more relaxed pace). Testing will be in our private lab on
the LMC-main campus, day shift Monday-Saturday.
Our preliminary population background study using
a rapid EIA kit suggests that...by the EIA method...there
will be 6% false positives for "IgG evidence of previous infection" & 6% "IgM
evidence of active/acute infection"; but this $2000 kit's
antigen reagent expires 14 days after reagent reconstitution...way
too expensive for our lab volume.
The virus is carried by mosquitoes and infects birds,
horses, and humans. Symptoms are fever, severe headache, and muscle
pains. In 2002, 4156 severe cases were documented in the USA...284
As of August 20031, it is estimated
that of USA patients actually infected with the virus, 20% develop
only "West Nile Fever" and 1-2% the true CNS illness.
Serology can be done on serum and/or spinal fluid. IMPORTANTLY,
serum IgM has been found to stay elevated as long as a year after
contracting the first acute illness. So, serum IgM elevation is
presumptive evidence...in the correct context...of acute WNV infection,
especially with rising titer. IgM positive cerebrospinal fluid
is diagnostic of CNS involvement by WNV.
Cross reactions (false positives): at
least with serum and CNS ELISA types of testing, cross reactions
are known against some other flavoviruses, especially St. Louis
encephalitis virus (also yellow fever, dengue, or Japanese encephalitis
CAP Today, August 2003, page 1.
30 May 2003; latest update 8 September 2003)