Request form must state which investigation is required (either IgG - immune status, or IgG and IgM - query chickenpox infection).
NOTE: For diagnosis of acute chickenpox or zoster, send swab of vesicle fluid for VZV DNA PCR.
Immune status:
NOTE: This test is routinely available on-call for at-risk patients eg immunosuppressed, pregnant, neonatal, who have been exposed to chickenpox and for whom the immune status is unknown.
Give details of contact/exposure, also full clinical details eg pregnant, immunosuppressed etc.
Suspected acute chickenpox:
Give date of onset of rash.
Paired sera should be obtained wherever possible:
(a) acute sample - as early as possible in the illness, and
(b) convalescent sample 2 - 3 weeks after onset.
To ensure immediate processing, please contact the serology unit of the Department of Virology and Immunology directly :
Paul Austin , Section Leader ext 23006/22103
Helena Thompson , Technical Specialist ext 22103
IgG testing is performed 2 - 3 times a week. IgM testing is performed once a week.
Note: IgM response may be delayed or low level. Absence of detectable IgM need not exclude primary infection.
VZV is a member of the herpes virus family. Primary infection results in chickenpox . Complications include bacterial superinfection of skin lesions, pneumonia, central nervous system involvement, thrombocytopenia and more rarely, glomerulonephritis, arthritis and hepatitis. Chickenpox is most severe in the immunocompromised patient. VZV persists in latent form following primary infection and may reactivate dermatomally (zoster) as painful crops of vesicles. Infection in the first and/or early second trimester of pregnancy can result in fetal death or congenital varicella syndrome characterised by limb hypoplasia and skin scarring, eye abnormalities and CNS damage. Severe disease of the new-born can occur if the mother develops chickenpox 5 days before delivery and up to 2 days post-delivery.
In temperate climates, varicella is most common in late winter and early spring. The incubation period is 14-16 days. The infectious period is 2 days prior to rash until cessation of new vesicle formation and crusting of vesicles.
In primary infection, IgM class antibodies are usually detectable 2-5 days after rash onset, peak at 8-11 days and are usually undetectable within a few weeks of onset. IgG antibodies to VZV usually appear in the sera of patients within four days of symptom onset. Antibody levels peak at 4-8 weeks and remain high for at least 6-8 months. Thereafter, antibody levels may decline 2-3 fold but remain at low levels indefinitely.
Sensitive IgM assays may detect a transient IgM response in zoster, although this varies with different patients. The IgG antibody response in patients with zoster typically demonstrates a rise. Most patients with zoster have high levels of VZV IgG immediately after onset of lesions.
Administration of Varicella Zoster Immune Globulin (VZIG) if given within 96 hours after contact can prevent or modify the course of disease but it is ineffective once the disease has become established. Efficacy is increased if administration is early.
Candidates for VZIG where significant exposure has occurred are:
Immunocompromised children without a history of chickenpox
Susceptible (non-immune) pregnant women
New-born infant whose mother had onset of chickenpox within 5 days of delivery or within 48 hours after delivery
Hospitalised premature infants (> 28 weeks gestation) whose mother has no history of chickenpox or is seronegative
Hospitalised premature infants (< 28 weeks gestation or < 1000g) regardless of maternal history
Immediate testing (including after hours) is offered for immune studies in the clinical setting of pregnancy, immunodeficiency and chickenpox contact.
The VZV IgG assay is used to determine immune status. Patients who are IgG positive are considered immune. Patients giving equivocal or negative results should be considered non-immune.
A vaccine against VZV is available. Serology is not sufficiently sensitive for reliable detection of post-vaccination immune status.
Alternative laboratory techniques for diagnosis of primary infection and/or reactivation include:
1. PCR detection of VZV DNA in swabs of lesions. Detection of VZV DNA in peripheral WBCs is occasionally useful - please contact the virologist to discuss before requesting.
2. Meningitis/meningo-encephalitis. CSF for VZV DNA PCR.
3. Cerebral VZV vasculopathy. VZV DNA may not be detectable in CSF, may require VZV antibody on CSF. Please discuss with virologist. (See CSF below)
References
1 Trinity Biotech VZV IgM ELISA package insert.
2 2006 Red Book. Report of the Committee on Infectious Diseases 27th Edition. Published by American Academy of Paediatrics 2006.
For further information contact the laboratory (contact via Lablink: 22000 or (09) 307-8995 or 0800 522 7587) ,or:
the Virology team
virology@adhb.govt.nz