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unpublished...the slide manufacturer says is a type of false positive).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 deaths.

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 viruses).

References:

1. CAP Today, August 2003, page 1.

   posted 30 May 2003; latest update 8 September 2003)