DNA Viruses: Structure, Classification, and Human Diseases

DNA Viruses: Classification and General Features

Overview of DNA Virus Families

DNA viruses causing human disease are classified into seven families based on their DNA type, envelope presence, size, and host specificity.

• dsDNA Viruses: Poxviridae, Herpesviridae, Papillomaviridae, Polyomaviridae, Adenoviridae

• ssDNA Viruses: Parvoviridae

• Mixed dsDNA/ssDNA: Hepadnaviridae

• Envelope: Some families possess an envelope, affecting transmission and immune evasion.

• Host Range: Viruses may be species-specific or capable of zoonotic transmission.

Poxviridae

Structure and Transmission

• Genome: Double-stranded DNA

• Capsid: Complex structure with envelope

• Size: Second largest viruses infecting mammals

• Transmission: Primarily via inhalation; close contact required

Diseases Caused by Poxviridae

• Smallpox (Variola): Two forms: Variola major and minor. Virus spreads via blood to skin, causing pox and scarring, especially on the face. First human disease eradicated.

• Molluscum Contagiosum: Caused by Molluscipoxvirus. Characterized by smooth, waxy papules on face, trunk, and genitalia. Heals without treatment in immunocompetent individuals.

• Animal Poxviruses: Usually species-specific; zoonotic transmission is rare and typically mild.

Stages of Poxvirus Lesions

The progression of skin lesions in poxvirus infections follows distinct stages:

• Macule: Flat, reddened area

• Papule: Raised, solid lesion

• Vesicle: Fluid-filled blister

• Pustule: Pus-filled lesion

• Crust: Dried exudate

• Scar: Permanent mark after healing

Smallpox Lesions

Molluscum Contagiosum Lesions

Herpesviridae

Structure and Latency

• Genome: Linear dsDNA

• Capsid: Enveloped polyhedral

• Entry: Envelope fusion with host cell membrane

• Latency: Virus remains inactive in cells; reactivation causes recurrent disease

• Nomenclature: Human herpesviruses (HHV) numbered by discovery order

HHV-1 and HHV-2 (Herpes Simplex Viruses)

• Manifestations: Oral herpes, genital herpes, ocular herpes, whitlow, neonatal herpes

• Transmission: Close contact, entry via mucous membrane breaks

• Pathogenesis: Virus spreads cell-to-cell via syncytia formation

• Epidemiology: HHV-1: casual contact in children; HHV-2: sexual activity (ages 15–29)

Oral Herpes Lesions

Ocular Herpes and Whitlow

Diagnosis, Treatment, and Prevention

• Diagnosis: Characteristic lesions

• Treatment: Chemotherapeutic agents limit lesion duration and viral shedding; do not cure or eliminate latent virus

• Prevention: Gloves for healthcare workers; sexual abstinence or monogamy

HHV-3 (Varicella-Zoster Virus)

• Diseases: Varicella (chickenpox) in children; herpes zoster (shingles) in adults

• Transmission: Highly infectious; entry via respiratory tract or eyes

• Pathogenesis: Virus travels via blood; skin lesions appear 2–3 weeks post-infection

• Latency: Virus can reactivate as shingles

Latency and Reactivation of VZV

Shingles Lesions

Dermatomes

Diagnosis, Treatment, and Prevention

• Diagnosis: Chickenpox diagnosed by lesions; shingles more difficult

• Treatment: Chickenpox is self-limiting; shingles managed symptomatically

• Prevention: Difficult due to viral shedding before symptoms

HHV-4 (Epstein-Barr Virus)

• Diseases: Infectious mononucleosis, Burkitt’s lymphoma, nasopharyngeal cancer, oral hairy leukoplakia

• Transmission: Saliva

• Pathogenesis: Infects pharynx, parotid glands, then B lymphocytes; latent infection suppresses apoptosis

• Immune Response: Cytotoxic T cells kill infected B cells; symptoms depend on immune status

Diagnosis, Treatment, and Prevention

• Diagnosis: Based on characteristic signs

• Treatment: Chemotherapy for Burkitt’s lymphoma; symptomatic relief for mono

• Prevention: Difficult due to widespread transmission

HHV-5 (Cytomegalovirus)

• Pathology: Infected cells become enlarged (cytomegaly)

• Transmission: Bodily secretions; requires close contact and large exchange

• Complications: Severe in fetuses, newborns, immunodeficient patients

Diagnosis, Treatment, and Prevention

• Diagnosis: Detection of enlarged cells and cellular inclusions

• Treatment: Difficult in fetuses/newborns; Fomiversen for CMV eye infections

• Prevention: Abstinence and safe sex

Other Herpesvirus Infections

• HHV-6 (Roseolovirus): Causes roseola; pink rash, possible link to multiple sclerosis

• HHV-8 (Rhadinovirus): Associated with Kaposi’s sarcoma

Papillomaviridae and Polyomaviridae

Papillomavirus Infections

• Diseases: Papillomas (warts) on skin/mucous membranes; genital warts linked to cancer

• Transmission: Direct contact, fomites, autoinoculation

• Diagnosis: Observation of papillomas; PAP smear for genital cancers

• Treatment: Removal of warts; prevention difficult except for genital warts (abstinence/monogamy)

Polyomavirus Infections

• Diseases: Tumors, urinary tract infections (BK virus), progressive multifocal leukoencephalopathy (JC virus)

• Pathogenesis: Latent infections in kidneys; reactivation depends on immune status

Adenoviridae

Structure and Diseases

• Genome: Single, linear dsDNA

• Diseases: Common cold, mild diarrhea, conjunctivitis (pinkeye)

• Transmission: Respiratory droplets, endocytosis in respiratory tract

Hepadnaviridae

Structure and Replication

• Genome: Both single- and double-stranded DNA

• Replication: Via RNA intermediary (unique among DNA viruses)

• Includes: Hepatitis B virus (HBV)

Hepatitis B Infections

• Disease: Hepatitis (liver inflammation), jaundice, liver enlargement, abdominal distress, bleeding

• Transmission: Saliva, semen, vaginal secretions; needles, sexual intercourse, childbirth

• Complications: Coinfection with hepatitis D increases risk of permanent liver damage; associated with liver cancer

Viral Particles and Diagnosis

• Body Fluids: Contain three types of virus particles

• Diagnosis: Detection of viral antigens

• Treatment: No universally effective treatment; prevention via vaccination and safer sexual practices

Parvoviridae

Structure and Disease

• Genome: ssDNA (only human pathogen with this genome)

• Size: Smallest DNA virus

• Disease: B19 virus causes erythema infectiosum (fifth disease); characterized by skin reddening, aggravated by sunlight