Varicella zoster virus

Varicella zoster virus (VZV), also called human herpesvirus 3, is one of nine herpes viruses that can infect people. It causes chickenpox, which mostly affects children and young adults, and later can cause shingles in adults (rarely in children). A rare late complication is Ramsay Hunt syndrome type 2. The virus only infects humans and can survive outside the body for a few hours.

VZV multiplies in the tonsils and produces a range of symptoms. After the initial chickenpox infection, the virus goes dormant in nerve cells, including those in the cranial nerve ganglia, dorsal root ganglia, and autonomic ganglia. Years later, it can reactivate and cause shingles.

Chickenpox appears after an incubation period of 10 to 21 days (average 14 days). The virus enters through the respiratory system and targets the skin and peripheral nerves. Illness lasts about three to four days, and people are most contagious one to two days before the rash appears. Symptoms include pus-filled blisters that burst, scab, and heal, often on the face, throat, lower back, chest, and shoulders. Complications can include encephalitis, pneumonia (viral or bacterial), and bronchitis (viral or bacterial). Even after symptoms clear, the virus stays dormant in the trigeminal and dorsal root ganglia.

Shingles occurs when VZV reactivates, which happens in about one-third of cases. The lifetime risk of developing shingles is 20 to 30 percent (roughly one in four people), but for those aged 85 and older, the risk rises to one in two. A 2015 Swedish study estimated 315 cases per 100,000 people per year overall, and 577 per 100,000 for those 50 and older. VZV can also infect the central nervous system; a 2013 Swiss study reported 1.02 such infections per 100,000 people between 2003 and 2010. Shingles lesions and burning nerve pain usually appear on one side of the body, along one or two adjacent sensory nerves. Skin lesions heal over weeks, but pain often lasts longer. In 10 to 15 percent of cases, pain persists beyond three months, a condition called postherpetic neuralgia. Other serious complications include Mollaret's meningitis, zoster multiplex, brain artery inflammation leading to stroke, myelitis, herpes zoster ophthalmicus, and zoster sine herpete. In Ramsay Hunt syndrome type II, the virus affects the geniculate ganglion, causing painful blisters on the tongue and ear, along with one-sided facial weakness and hearing loss. Infection early in pregnancy can severely harm the fetus. Reye's syndrome, which causes continuous vomiting and brain dysfunction (drowsiness or aggression), can occur after initial infection, mostly in children and teenagers; using aspirin during infection raises this risk, and it can lead to coma or death.

The virus is closely related to herpes simplex viruses (HSV) and shares much of its genome. Its envelope glycoproteins (gB, gC, gE, gH, gI, gK, gL) match those in HSV, but VZV lacks an equivalent of HSV's gD protein and does not produce latency-associated transcripts (LAT). VZV virions are spherical, 180 to 200 nanometers in diameter, with a lipid envelope enclosing a 100-nanometer nucleocapsid made of 162 capsomeres arranged in an icosahedral shape. Its DNA is a single, linear, double-stranded molecule about 125,000 nucleotides long. The capsid is surrounded by a tegument of loosely associated proteins that help start virus reproduction, and the tegument is covered by a lipid envelope studded with glycoproteins about 8 nanometers long.

The genome was first sequenced in 1986. It is a linear duplex DNA molecule; a laboratory strain has 124,884 base pairs. The genome has two main isomers (P and IS) depending on the orientation of the S segment, each equally common, together making up 90 to 95 percent of molecules. The L segment can also invert, producing four linear isomers total. This distribution differs from HSV's equal probabilities, and the reason is unknown. A small number of molecules are circular, though little is known about them (HSV circularizes upon infection). There are at least 70 open reading frames.

VZV shares a common ancestor with HSV1 and HSV2; five of its roughly 70 genes have no HSV counterpart. It is less closely related to other human herpes viruses, but many homologous genes and conserved gene blocks exist. At least five clades of the virus have been identified.