Chicken Pox - Varicella Zoster Virus
VARICELLA-ZOSTER VIRUS
· VZV causes chickenpox (varicella) upon recurrence, and causes herpes zoster, or shingles.
· As an alphaherpesvirus,
VZV shares many characteristics with HSV.
· Causes Clinical chicken pox
(primary infection).
· 90% of cases before age 10, peak
incidence 2-8 years.
· Recurrent infection - shingles
(herpes zoster) in adults.
·
VZV is seen worldwide and is highly infectious,
·
Human is the only natural host of the virus.
1. Virus transmission and
Structure
· Virus entry through inhalation –
airborne transmission.
· Direct contact.
· Vesicle fluids.
·
Viral particle size: 180-200nm in diameter.
·
Lipid envelope encloses the icosahedral form of the nucleocapsid.
·
DNA is linear, Smallest genome.
·
Replicates slower.
(Image
source: https://www.creative-diagnostics.com/tag-varicella-zoster-virus-antigens-39.htm)
2. Replication
· Replicates in the respiratory tract (mucosal
epithelial) and invades lymph nodes.
· Local replication is followed by
spread to tonsils and other regional lymphoid tissues, where VZV gains access
to T cells.
· Virus envelope glycoprotein
binds to mannose-6-phosphate receptor8 or myelin-associated
glycoprotein9.
·
Infected
T cells then deliver the virus to cutaneous sites of replication.
·
Viremia: spreads the virus to target organs.
·
Incubation
period 14-18 days.
·
Belong to the alpha herpesvirus subfamily
of herpesviruses.
·
Double stranded DNA enveloped virus.
·
Genome size 125 kbp, long icosahedral capsid (shaped like
spheres),
·
Transmission
mainly occurs via droplets or particles in the air.
4. Pathogenesis
· VZV is generally acquired by
inhalation, and primary infection begins in the tonsils and mucosa of the
respiratory tract.
· The virus then progresses via the
bloodstream and lymphatic system to the cells of the reticuloendothelial
system.
· Reticuloendothelial system –Phagocytic
Cells in fixed tissues (Liver, Heart gall bladder).
· A secondary viremia occurs after 11
to 13 days and spreads the virus throughout the body and to the skin.
· The virus infects T cells, and these
cells can home to the skin and transfer virus to skin epithelial cells.
· The virus overcomes inhibition by
interferon-α, and vesicles are produced in the
skin and cause lesions in successive crops.
· Fever and systemic symptoms occur
with the rash.
· The lesions are superficial and all
the stages
-Macule,
Papule, Vesicle and scab.
· The virus becomes latent in the
dorsal root or cranial nerve ganglia after the primary infection.
· The virus can be reactivated in
older adults when immunity wanes or in patients with impaired cellular
immunity.
· On reactivation, the virus
replicates and is released along the entire neural pathway to infect the skin,
causing a vesicular rash along the entire dermatome, known as herpes zoster,
or shingles.
· This damages the neuron and may
result in post herpetic neuralgia.
Mechanism of spread of varicella-zoster virus (VZV) within the
body
5.
Clinical Symptoms of Chickenpox
· Rash appears first on the head, neck, and trunk.
· Vesicles contain clear fluid (itch).
· New vesicles appear during the first
week.
· Mild fever, malaise, headache.
· Recovery in 2 weeks.
· Adult infections are more severe
(pneumonia).
·
Neonatal
infection (encephalitis).
6. Laboratory
diagnosis
o Specimen collection - skin lesions,
respiratory specimens, or organ biopsy specimens.
o Immunofluorescent antibody staining
o Polymerase Chain Reaction (PCR) - vesicles,
scabs, maculopapular lesions.
o ELISA.
o In-vitro cultivation - Human
Diploid Fibroblasts (MRC-5 cells), melanoma (MeWo) cells, or retinal pigmented
epithelial (ARPE-19) cells.
o These cells are also used to
isolate VZV, research, and vaccine preparation.
7. Treatment
· ACV, famciclovir, and valacyclovir
· Live attenuated vaccine Oka (VARIVAX)
– 2 doses, Intramuscular or Subcutaneous administration, VARIVAX approximately
0.5 mL.
· First dose is administered at 12 to
15 months of age.
Second dose is administered at 4 to 6 years of age.
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