Herpes

Question 1

Herpes Viruses

Answer 1

• Herpes viruses are large enveloped viruses.
• The icosahedral nucleocapsid containing a single linear double-stranded DNA molecule is assembled in the nucleus. This results in a nuclear inclusion body

Question 2

Classification of Herpes

Answer 2

• Alphaherpesviruses eg. HSV-1
Wide host range; latency in neurons
• Betaherpesviruses eg. CMV
Narrow host range; latency in monocytes
• Gammaherpesviruses eg. EBV
Narrow host range; latency in lymphocytes associated with cell transformations

Question 3

Replication of Herpes Virus

Answer 3

• An extracellular virion adsorbs to a susceptible cell.
• The virion envelope fuses with the plasma membrane of the host cell which releases the nucleocapsid into the cytoplasm.
• The nucleocapsid goes to the nucleus and releases its DNA into the nucleus.
• The DNA becomes circular.
• Synthesis of mRNA, viral proteins, and viral DNA
takes place.
• Progeny nucleocapsids are assembled in the nucleus. Large arrays of nucleocapsids result in characteristic nuclear inclusion bodies.
• Virus-encoded glycoproteins are inserted into the nuclear membrane and into the plasma membrane.
• Enveloped infectious virions are produced by nucleocapsid budding through the nuclear membrane.
• The virus-encoded glycoproteins in the plasma membrane can result in cell fusion to produce multinucleate giant cells (compare measles).

Question 4

Latency

Answer 4

Latency is the single most important biological characteristic of herpes viruses: In contrast to most other viruses, herpes viruses are not eliminated on recovery from primary infection but remain in a clinically “latent” state that can subsequently result in recurrent disease.
Thus for each herpes virus you must consider both the primary and the recurrent disease. The nature of the “latent” state is not completely understood.

Question 5

Latent state-recurrent infections

Answer 5

Patients whose infection is clinically latent may sporadically produce infectious virions while remaining asymptomatic.
Note that the latent state-recurrent infection allows
parent child transmission and maintenance
of the virus in very small populations (contrast
measles).

Question 6

Characteristics of Latency in Herpes

Answer 6

• Virus is neither destroyed or transformed
• Its largely an in vivo phenomenon
• It’s lifelong
• It allows persistence of viral genome in non-infectious state
• Genome is quiescent with potential to become active
• Recrudescence of virus leads to infection of new individuals

Question 7

Types of Herpes Viruses

Answer 7

There are eight herpes viruses that affect humans though we would only learn about five:
• Herpes simplex virus type-1,
• herpes simplex virus type 2, 
• varicella-zoster virus, 
• cytomegalovirus
• Epstein Barr (EB) virus

Question 8

Characteristics of Herpes Virus Types

Antigenically Relatedness

Answer 8

Herpes simplex type-1 and type-2 viruses are closely related antigenically. They are distinguished by typespecific monoclonal antibodies or PCR.

Question 9

Characteristics of Herpes Virus Types

Latency

Answer 9

Herpes simplex type-1, herpes simplex type-2, and
varicella-zoster viruses remain latent in sensory ganglion cells. When the infection is reactivated the virus makes its way to the skin by a neural pathway and thus the skin lesions are unilateral (primary lesions are often bilateral). With all 3 viruses, recurrent disease is seen in immunologically normal persons, however the pathology is more severe in the immunosuppressed.

Question 10

Characteristics of Herpes Virus Types

EB virus

Answer 10

EB virus is latent within B lymphocytes; the latently infected cell type in cytomegalovirus infection is uncertain.
Recurrent disease is seen almost entirely in immunosuppressed patients with both viruses.