Infection: Chicken pox; primary varicella (VZV) infection; primary herpes zoster (HZ) infection
- Common childhood illness with characteristic vesicular lesions, occurring in successive crops, which are itchy. Feeling feverish and unwell usually precedes the rash.
- Can cause serious illness in pregnant women and in those with immune compromise.
Did you know?
- Characteristic feature is lesions at different stages of development (initially red spots, then fluid-filled vesicles, followed by crusting) on different parts of the body.
- Highly transmissible. People are considered infectious until new lesion formation has ceased and all lesions are crusted.
Who should I test?
- Laboratory testing is not usually required. The diagnosis is made clinically in the vast majority of people due to the characteristic nature of the rash.
- Laboratory testing is reserved for atypical cases or where diagnostic certainty is important e.g. possible exposures involving pregnant women, immune compromised.
Test of choice:
Request VZV PCR
Viral swab of vesicle fluid for VZV PCR (usually requires de-roofing of vesicle to obtain fluid)
- Good sensitivity – a negative result on an adequately collected sample makes the diagnosis very unlikely
- Excellent specificity – a positive result confirms the diagnosis
Note: swabs run for VZV PCR in our laboratory are run for HSV PCR in parallel, as these viruses can mimic each other.
Tests to avoid/specialist tests:
- IgM and IgG serology are not recommended for the diagnosis of chicken pox, as IgM is prone to false positives, and IgG seroconversion requires paired sampling, resulting in delayed diagnosis.
In the case of possible exposure to chicken pox in risk groups, rapid assessment of immunity to the varicella virus is required:
- Pregnant women – a reliable history of chicken pox is sufficient. Failing this, urgent IgG serology should be requested.
- Immune compromised – urgent IgG serology should be requested.
The above situations should be discussed urgently with clinical microbiology, infectious diseases, or obstetrics, as appropriate, as prophylaxis may be required.