What is the evolutionary history and global geographic distribution of varicella (chickenpox) caused by varicella‑zoster virus?

Evolution and Geographic Distribution of Varicella-Zoster Virus

Varicella-zoster virus (VZV) most likely originated in Europe rather than Africa, with current evidence suggesting that extant VZV strains dispersed from Europe to other continents, contradicting the earlier "Out-of-Africa" hypothesis 1.

Evolutionary History

Origin and Ancient Evolution

• VZV belongs to the Herpesviridae family, with phylogenetic models estimating the base of this viral family at approximately 400 million years ago, coinciding with the formation of the supercontinent Pangaea 2, 3
• The virus has co-evolved with humans throughout history and is present in all nationalities globally 3

Recent Phylogenetic Evidence

The most recent high-quality phylogeographic analysis (2020) challenges the traditional "Out-of-Africa" model:

• European origin is now considered most likely for circulating VZV strains 1
• Europe was identified as the ancestral location for most internal nodes of the VZV phylogeny, including the ancestor of clade 5 strains 1
• This contradicts earlier hypotheses that VZV migrated out of Africa with human populations 60,000 years ago 2

Genetic Structure and Clades

Established Clades

VZV can be divided into five major confirmed clades (clades 1-5) and two provisional clades requiring further confirmation 4:

Clades 1,3, and 4: Predominantly European strains Clade 2: Asian distribution (most common in East Asia) Clade 5: Previously thought to be African, but recent evidence suggests European ancestry with subsequent spread 1

Genetic Characteristics

• VZV is remarkably genetically stable compared to other herpesviruses 5
• The prototype VZV genome contains nearly 125,000 base pairs divided into 70 open reading frames 2
• Isolates within a clade display >99.9% identity to one another 2
• Members of different clades show 99.8% identity to each other 2
• Only 0.25% nucleotide variation (58 segregating sites out of 23,266 examined) in UK samples 5
• Nucleotide diversity is an order of magnitude smaller than herpes simplex virus type 1 5

Recombination Events

• Recombination has been rare due to extensive genetic correlation between widely spaced sites 5
• However, recombinant strains have been identified in regions where multiple clades co-circulate, particularly in Brazil 5
• Recent studies from China (Urumqi) identified strains that could not be attributed to established clades, suggesting ongoing recombination events 6

Global Geographic Distribution

Regional Clade Dominance

Europe and North America:

• Predominantly clades 1,3, and 4 4, 3
• United Kingdom shows presence of multiple strains 5

Asia:

• Clade 2 dominates in East Asia 4, 3
• China (Shanghai) shows 81.8% clade 2 predominance with high homogeneity 6
• China (Urumqi) demonstrates more genetic diversity, including clade 5 and unclassified strains due to geographical location 6

Africa and Indian Subcontinent:

• Minimal genetic differentiation between samples from Guinea Bissau, Zambia, Bangladesh, and South India 5
• Only one or fewer substitutions in 1,895 bases surveyed, suggesting recent rapid spread and/or low mutation rates 5
• Clade 5 previously attributed to Africa, but recent evidence suggests European origin with subsequent introduction 1

South America:

• Brazil contains mixtures of genotypes where two or more strains co-occur 5
• At least one probable recombinant between major strains identified 5

Factors Influencing Distribution

• Regional dominance of specific VZV clades depends on environmental factors, evolutionary conditions, and host-virus interactions 4
• Geographical separation contributes significantly to VZV complexity and genetic recombination 6
• Immigration and travel alter genotype distribution patterns 4

Clinical Epidemiology Context

Transmission Characteristics

VZV is highly contagious, with 80-90% of susceptible people exposed in household settings developing clinical infection 7:

• Transmission occurs via direct contact, airborne droplets, or infected respiratory secretions
• Virus enters through upper respiratory tract or conjunctivae
• Infected hosts are contagious from 1-2 days before rash onset until all lesions are crusted

Climate-Related Distribution Patterns

Critical caveat: VZV spreads less readily in tropical climates than in temperate climates 7:

• A higher proportion of adults from tropical countries remain serosusceptible to VZV compared to adults from cooler climates
• This has significant implications for disease severity when tropical-origin adults are exposed in adulthood

Age Distribution (Pre-vaccine Era)

In the 1980s-1990s United States 7:

• 90% of cases occurred in children <15 years
• 33% in 1-4 year-olds, 44% in 5-9 year-olds
• By immediate pre-vaccine era: 95.5% of 20-29 year-olds, 98.9% of 30-39 year-olds, and 99.6% of those ≥40 years had VZV immunity

Emerging Diversity

Four different VZV genotypes that do not segregate into the previously defined five clades have recently been identified, indicating wider than anticipated diversity among newly collected VZV strains worldwide 2. This finding suggests that VZV evolution and geographic distribution are more complex than previously understood, with ongoing surveillance needed to track emerging variants.