DNA Viruses Infecting Humans: Structure, Pathogenesis, and Clinical Manifestations

DNA Viruses Infecting Humans

Overview of Human DNA Viruses

DNA viruses are a diverse group of pathogens that infect humans, causing a wide range of diseases. They are classified based on their structure (naked or enveloped), genome type (single or double-stranded DNA), and replication strategies. Major families include Papillomaviridae, Polyomaviridae, Adenoviridae, Parvoviridae, Herpesviridae, Poxviridae, and Hepadnaviridae.

Papillomaviridae (Human Papillomavirus, HPV)

Structure and General Features

• Small, naked, icosahedral capsids

• Circular double-stranded DNA (dsDNA)

• Highly tissue-specific; most infections are transient

• Persistent infections may rarely lead to tumors (e.g., squamous cell carcinoma)

Clinical Manifestations of HPV

• Papilloma = wart; 90% of infections are asymptomatic

• Common warts: Painless, elevated, solid, rough growths

• Plantar warts: Deep warts on the soles of the feet

• Genital warts: Most common sexually transmitted infection (STI) in the U.S.; can range from tiny, flat, painless bumps to extensive, branching solid masses (condylomata acuminata)

• Transmission: Direct contact or contaminated fomites; incubation period ranges from 2 weeks to over a year

HPV Pathogenesis and Replication

HPV infects basal keratinocytes in the skin or mucosa, leading to cell proliferation and wart formation. The virus requires differentiation of host cells for its life cycle, making it difficult to study in vitro.

HPV Genotypes and Cancer Risk

• Over 230 genotypes identified; classified as genital-mucosal (~40 types) or non-genital cutaneous

• 15 high-risk types (e.g., HPV 16, 18) are associated with increased risk of cancer (cervical, head, and neck)

• HPV causes more head and neck cancers than cervical cancer in the U.S.

HPV Detection, Treatment, and Prevention

• Early detection: Visual inspection, Pap smear, HPV DNA tests

• Most warts regress spontaneously; treatments include podophyllin, cauterization, freezing, or laser surgery

• Vaccines: Three effective recombinant protein vaccines (e.g., Gardasil 9) targeting high-risk and common wart-causing strains; most effective if given before sexual activity

HPV and Cancer Progression

High-risk HPV types can integrate their DNA into host cells, disrupting regulatory genes and promoting oncogenesis. Viral proteins E6 and E7 inactivate tumor suppressors p53 and Rb, leading to uncontrolled cell proliferation.

Polyomaviridae

Structure and General Features

• Small, naked, icosahedral capsids

• Circular dsDNA genome

• Most infections are transient; persistent infections rarely lead to tumors

Human Polyomaviruses and Disease

• Common in humans; >13 types identified

• Diseases mainly in immunosuppressed individuals (e.g., BK and JC viruses)

• JC virus: Causes progressive multifocal leukoencephalopathy (PML), a fatal brain infection in immunocompromised patients

• BK virus: Causes nephropathy in renal transplant patients

Polyomaviruses of the Skin

• Merkel cell polyomavirus (MCV): Associated with Merkel cell carcinoma, a rare aggressive skin cancer

• Other skin polyomaviruses: May cause dermatoses in immunocompromised individuals

Adenoviridae

Structure and Clinical Features

• Over 50 types; naked, icosahedral capsids

• Infect lymphoid tissue, respiratory or intestinal epithelia, or conjunctiva

• Spread by respiratory, ocular secretions, fecal-oral route, or waterborne transmission

• Diseases: Colds, pharyngitis, conjunctivitis, pneumonia, acute hemorrhagic cystitis, gastroenteritis

• Oncogenic in animals, not in humans

Parvoviridae

Structure and Clinical Features

• Very small, naked, icosahedral capsids

• Linear single-stranded DNA (ssDNA) genome

• Human parvovirus B19: Causes erythema infectiosum (fifth disease) in children, characterized by a biphasic illness with fever and a lacy rash

• Can cause severe anemia in fetuses and individuals with hemolytic disorders

Herpesviridae

Structure and General Features

• Large, enveloped, icosahedral capsids with dsDNA

• Replicate in the nucleus; establish latency and can reactivate

• Clinical complications are more severe in immunocompromised individuals

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

• HSV-1: Usually causes oral lesions (cold sores), but can also cause genital infections

• HSV-2: Primarily causes genital lesions, but can also infect the oral region

• Both establish latency in sensory ganglia and can reactivate

HSV Epidemiology and Pathogenesis

• Transmitted by close contact with secretions, lesions, or asymptomatic shedding

• Virus multiplies in sensory neurons, migrates to ganglia (latency), and reactivates under stress, fever, UV light, or hormonal changes

Clinical Manifestations of HSV

• Herpes labialis: Fever blisters/cold sores

• Herpetic gingivostomatitis: Generalized oral infection, especially in children

• Herpetic keratitis: Eye infection, can lead to blindness

• Herpes genitalis: Painful genital ulcers, recurrent episodes

• Herpetic whitlow: Painful infection of fingers, common in healthcare workers

• HSV encephalitis: Most common sporadic viral encephalitis in the U.S.

Diagnosis, Treatment, and Control of HSV

• Diagnosis: Clinical appearance, Tzanck smear (multinucleated giant cells), immunofluorescence, PCR, viral culture

• Treatment: Nucleoside analogues (e.g., acyclovir), topical or systemic therapy

Other Herpesviruses

• Varicella-zoster virus (VZV): Causes chickenpox (primary infection) and shingles (reactivation)

• Epstein-Barr virus (EBV): Infectious mononucleosis, associated with Burkitt lymphoma, nasopharyngeal carcinoma

• Cytomegalovirus (CMV): Congenital infections, mononucleosis-like syndrome, severe disease in immunocompromised

• HHV-6, HHV-7: Roseola, pityriasis rosea, possible links to other diseases

• HHV-8 (KSHV): Kaposi's sarcoma, especially in AIDS patients

Poxviridae

Structure and Clinical Features

• Largest and most complex animal viruses; lack typical nucleocapsid

• Replicate in the cytoplasm of epidermal and subcutaneous cells

• Cause skin pustules (pox) that leave scars

Major Poxvirus Diseases

• Smallpox (variola): First disease eradicated by vaccination; caused severe systemic illness and characteristic rash

• Molluscum contagiosum: Benign skin lesions, usually self-limited

• Mpox (monkeypox): Similar to smallpox but milder; zoonotic

Hepadnaviridae (Hepatitis B Virus, HBV)

Structure and Replication

• Enveloped, icosahedral capsid; partially double-stranded DNA genome

• Uses reverse transcriptase in replication

Hepatitis B Virus and Disease

• Transmission: Blood, sexual contact, perinatal

• Infects hepatocytes; can cause acute or chronic hepatitis

• Chronic infection increases risk of cirrhosis and hepatocellular carcinoma

• Prevention: Subunit vaccine, first dose at birth

Additional info: This guide covers the structure, replication, pathogenesis, clinical manifestations, and prevention of major DNA viruses infecting humans, with emphasis on those most relevant to medical microbiology and infectious disease.