Herpesviruses

Herpesviruses

Introduction

The herpesviruses are large, enveloped DNA viruses. They exhibit many common features, such as similar morphology of virions, basic mode of replication in the host cells, and capability to establish latent and recurrent infections. All herpesviruses are structurally similar. The herpesviruses measure 120–200 nm in diameter. The virus has a characteristic morphology and consists of four distinct structural elements: (a) DNA core, (b) capsid, (c) tegument, and (d) envelope.

The DNA core consists of a linear double-stranded DNA (dsDNA) molecule with molecular weight varying from 125 to 229 kilobase pairs (kbp). The core is surrounded by an icosahedral capsid containing 162 capsomeres. This is enclosed by a glycoprotein-containing envelope. Between the envelope and capsid is an amorphous structure called tegument, which contains viral proteins and enzymes that initiate replication.

Envelope is the outermost component and is composed of lipids. It is derived from the modified host cell nuclear mem-brane through which the naked virions project during replica-tion. It carries surface spikes about 8 mm long. As enveloped viruses, the herpesviruses are sensitive to fat solvents, such as alcohol, chloroform, ether, and bile salts.

Herpesviruses replicate in the host cell nucleus, and both replication and assembly occur in the nucleus. The herpes-virus encodes for several glycoproteins that facilitate viral attachment, fusion, and immune evasion. The virus buds from nuclear membrane and is released by exocytosis and cell lysis.

Classification

Herpesviruses are ubiquitous. All human herpesviruses are included in the family Herpesviridae, which is divided into three subfamilies based on viral characteristics, pathogen-esis of the disease, and clinical manifestation of the disease (Table 57-1). At least eight important herpesviruses are known to cause infection in humans (Table 57-2). They are officially designated as human herpesvirus types 1–8, but the common names are still used.