Herpesviruses are a diverse family of DNA viruses that cause a range of human diseases. They're split into three subfamilies: Alpha, Beta, and Gamma, each with unique characteristics and infection patterns.
These viruses are masters of persistence, establishing lifelong latent infections in specific cell types. They can reactivate under certain conditions, causing recurrent symptoms and posing challenges for treatment and prevention.
Herpesvirus subfamilies and diseases
Classification and characteristics
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Herpesviridae family divides into three subfamilies: , , and
Classification stems from biological properties including host range, replication cycle duration, cytopathology, and characteristics
Herpesviruses exhibit varying degrees of host specificity
Some infect a narrow range of hosts (HSV-1 primarily infects humans)
Others have a broader host range (EBV can infect various primate species)
Alphaherpesvirinae
Includes 1 and 2 (HSV-1, HSV-2) and (VZV)
Associated diseases:
HSV-1: oral herpes (cold sores)
HSV-2:
VZV: (primary infection) and (reactivation)
Characterized by rapid replication cycle and establishment of latency in sensory ganglia
Betaherpesvirinae
Comprises (CMV) and and 7 (HHV-6, HHV-7)
Associated diseases:
CMV: , in adults
HHV-6: (sixth disease)
HHV-7: roseola infantum, febrile illnesses in children
Features slower replication cycle and establishment of latency in secretory glands, lymphoreticular cells, and kidneys
Gammaherpesvirinae
Includes (EBV) and (KSHV)
Associated diseases:
EBV: , ,
KSHV: , ,
Characterized by tropism for lymphoid cells and establishment of latency in lymphoid tissues
Structure and replication of herpesviruses
Viral structure
Enveloped, viruses with distinctive icosahedral capsid
Tegument layer surrounds capsid, containing viral proteins important for initial stages of infection
Viral genome ranges from 120-230 kbp, encoding 70-200 genes
Variations exist among different herpesvirus species (HSV-1: ~152 kbp, EBV: ~172 kbp)
Envelope contains viral glycoproteins crucial for attachment and entry into host cells
Replication cycle
Attachment: viral glycoproteins bind to specific host cell receptors
HSV-1 glycoprotein D binds to nectin-1 or HVEM receptors
Entry: fusion of viral envelope with cell membrane or endocytosis
Uncoating: nucleocapsid transported to nuclear pore, viral DNA released into nucleus
Gene expression follows coordinated cascade:
Immediate-early genes: transcriptional regulators
Early genes: enzymes for DNA replication
Late genes: structural proteins
DNA replication occurs in nucleus using rolling circle mechanism
Produces concatemeric DNA cleaved and packaged into capsids
Assembly and egress:
Nucleocapsids acquire tegument proteins
Envelopment occurs by budding through nuclear membrane and cellular organelles
Latency and reactivation in herpesviruses
Latency mechanisms
Viral genome exists as circular episome in nucleus of specific cell types