Laboratory diagnosis of Varicella-Zoster Virus

• Collection of specimen, viral isolation and cell culture
Because cutaneous VZV infection involves the release of virus into the vesicle, viral isolation is enhanced by obtaining vesicular fluid and infected cells from the base. VZV is most often recovered from early, clear vesicles. Infectious VZV is usually recoverable from varicella lesions for 3 days, and for 7 days or longer from herpes zoster lesions. VZV can be cultured on human embryonic lung or Vero cells. CPE may not be evident for 3-7 days (Nauschuetz, 2000).
The shell vial culture method may improve the sensitivity of VZV isolation and permit earlier identification of positive specimens, within 1 to 3 days (Brinker and Doern, 1998).
• Serology
The humoral immune response to VZV limits serologic approaches to the diagnosis. Serologic diagnosis by IgG titers is retrospective, and methods to detect IgM Abs lack sensitivity and specificity; false-positive results are common. Because VZV reactivation also induces IgM antibodies to VZV in many patients, their presence in serum does not differentiate primary from recurrent infection. Defining VZV immune status is important to assess the risk for primary VZV infection in exposed individuals, the risk for reactivation in patients who are receiving immunosuppressive therapy. The most sensitive methods detect binding of Abs in sera to membranes of VZV infected cells, referred to as the FA membrane Ag assay or to purified VZV protein preparations by EIA, neutralization and RIA are sensitive (Gershon et al.,1998).
• Molecular detection
Detection of VZV DNA by PCR is now widely used, particularly for diagnosis of severe disease where rapid differential diagnosis is required (Minson, 2005).