Chickenpox never truly leaves: the virus hides in your nerves for decades
The virus behind childhood chickenpox does not depart when the spots heal. It retreats into nerve clusters near the spine and skull and can lie quiet for decades. In roughly a third of people it wakes again as shingles, and for those aged 85 and over the lifetime odds reach one in two.
Varicella zoster virus, also called human herpesvirus 3, is one of nine herpes viruses that infect people, and it infects only humans. It enters through the airways and multiplies in the tonsils, with an incubation period of 10 to 21 days, typically around 14. People are most contagious a day or two before the rash appears. The blisters fill with pus, burst and scab, most often on the face, chest, shoulders and lower back.
After recovery the virus settles into the trigeminal and dorsal root ganglia. When it reactivates, shingles follows the patch of skin served by one or two neighbouring sensory nerves, nearly always confined to a single side, with a burning pain. The rash fades over weeks, but in 10 to 15 percent of cases the pain lasts beyond three months, a condition called postherpetic neuralgia. A Swedish study published in 2015 estimated 315 shingles cases per 100,000 people a year overall, rising to 577 among those 50 and older. Rarely the virus attacks the facial nerve, causing Ramsay Hunt syndrome with facial weakness and hearing loss.
Under the microscope the virion is a sphere 180 to 200 nanometres across. Its genome, first sequenced in 1986, is double-stranded DNA of about 125,000 bases holding at least 70 genes, only five of which lack counterparts in the herpes simplex viruses, pointing to a shared ancestor. Strains fall into several clades with regional patterns, including European and North American lineages, an Asian group centred on Japan, and one apparently based in India.
Antivirals such as acyclovir work best when started early, although effective blood levels often need an intravenous drip. A live weakened vaccine grew from the Oka strain, isolated in the 1970s by Michiaki Takahashi and colleagues; Merck developed it as Varivax, and US approval came in 1995 after submission in 1990. It is now on childhood schedules in Australia, the United States and many other countries, though some worry that vaccine immunity may fade in adulthood.
Source: Varicella zoster virus
