Microbiology and Host Defense: DNA Viruses Overview

DNA-Containing Viruses Overview

• Major groups of clinically relevant DNA viruses include adenoviruses, parvoviruses, papillomaviruses, polyomaviruses, herpesviruses, hepadnaviruses, and poxviruses.
• Key symptoms and characteristics vary widely among these virus families.

Adenoviruses

• Medium-sized, nonenveloped viruses (70-90 nm) with icosahedral symmetry; genome is linear dsDNA (26-48 kb).
• Associated with acute respiratory diseases, conjunctivitis, and gastroenteritis.
• 67 known types infect humans, primarily targeting mucous membranes.

Parvoviruses

• Very small (18-26 nm), nonenveloped viruses with icosahedral symmetry, possessing ssDNA (~5 kb).
• Replicate in actively dividing cells; stable over pH range of 3-9.
• B19 virus causes Hydrops fetalis, aplastic anemia, and Fifth disease (erythema infectiosum).

Papillomaviruses

• Naked icosahedral viruses (55 nm) with double-stranded supercoiled DNA.
• Human Papillomavirus (HPV) can lead to genital cancers; associated with wart-like lesions.
• Animal-specific papillomaviruses do not infect humans.

Polyomaviruses

• Small (45 nm), nonenveloped, heat-stable viruses; structurally related to HPV.
• Notable human pathogens include JC virus linked to progressive multifocal leukoencephalopathy and BK virus causing nephropathy in transplant patients.

Herpesviruses

• Large viruses (150-200 nm) with a lipid-containing envelope and icosahedral nucleocapsid structure; linear dsDNA genome (120-240 kb).
• Cause lifelong latent infections; include HSV-1 (oral lesions), HSV-2 (genital herpes), and Varicella-Zoster virus (chickenpox and shingles).
• Epstein-Barr virus causes infectious mononucleosis, while HHV-8 is associated with Kaposi's sarcoma.

Hepadnaviruses

• Small (40-48 nm), enveloped viruses with partially dsDNA molecules (~3.2 kb).
• Known for causing acute and chronic hepatitis B infections; persistent infections increase liver cancer risks.
• Contains icosahedral nucleocapsid core within a lipid envelope.

Poxviruses

• Largest known viruses (220-450 nm) with complex structure; contain linear covalently closed dsDNA (130-375 kb).
• Pathogenic types include Smallpox, Molluscum contagiosum, and Monkeypox.
• Smallpox eradicated globally in 1980; stored in restricted laboratories.

Variola (Smallpox) Virus

• Two types: variola major (3-40% fatality) and variola minor (>1% fatality).
• Highly contagious, spread via respiratory droplets; no known animal reservoir.
• WHO-led eradication campaign in 1967 confirmed eradication by 1980; posing bioterrorism threat due to stability and high infectivity.

Smallpox Pathogenesis

• Entry through inhalation, initially infecting upper respiratory tract, followed by regional lymph nodes, leading to viremia and dissemination to organs.
• Incubation period lasts 12-14 days; clinical symptoms include fever and malaise before papulovesicular rash appears.
• Characteristic uniform vesicular rash forms, with lesions appearing simultaneously on face, arms, and extremities.

Diagnosis and Pathological Features

• Presence of eosinophilic inclusions (Guarnieri bodies) in infected cells.
• Viral proteins may impair host immune defenses, potentially leading to severe inflammatory responses.
• Diagnosis includes vesicular scrapings for analysis.

DNA-Containing Viruses Overview

• Major groups include adenoviruses, parvoviruses, papillomaviruses, herpesviruses, polyomaviruses, hepadnaviruses, and poxviruses.
• DNA viruses can be categorized based on envelope presence and capsid symmetry.

Clinically Important DNA Viruses

• Adenovirus: No envelope, icosahedral, ds linear DNA. Causes pharyngitis, conjunctivitis, and UTI.
• Parvovirus: No envelope, icosahedral, ss linear DNA. Associated with Fifth disease in children and anemia due to RBC destruction in fetuses.
• Papillomavirus: No envelope, icosahedral, ds circular DNA. Human Papilloma Virus (HPV) linked to warts and cervical/anal squamous cell carcinoma.
• Herpesvirus: Envelope present, icosahedral, ds linear DNA. Includes HSV-1 (oral lesions, cold sores), HSV-2 (genital herpes), Varicella-Zoster virus (chickenpox, shingles), Epstein-Barr virus (mononucleosis), and Cytomegalovirus (congenital abnormalities).
• Polyomavirus: No envelope, icosahedral, ds circular DNA. JC virus causes progressive multifocal leukoencephalopathy; BK virus affects transplant patients.
• Hepadnavirus: Envelope present, icosahedral, partially ds circular DNA. Hepatitis B virus leads to hepatitis.
• Poxvirus: Envelope present, complex structure, ds linear DNA. Includes smallpox (eradicated), molluscum contagiosum (umbilicated warts), and monkeypox (similar, less deadly).

Adenoviruses

• Medium-sized, nonenveloped, icosahedral viruses with fiber spikes enhancing host attachment.
• Pathogenic adenoviruses can cause vesicular skin lesions.

Poxviruses

• Genomes are linear, double-stranded DNA ranging from 130-375 kb.
• Poxvirus replication occurs in the cytoplasm, with numerous enzymatic proteins, including RNA polymerase.

Smallpox (Variola Virus)

• Consists of variola major (3-40% fatality) and variola minor (>1% fatality).
• Highly contagious via respiratory droplets; survives well in the environment.
• Global eradication confirmed by WHO in 1979; now stored in secure facilities.

Bioterrorism Concerns

• Smallpox is stable, high infectivity, and no longer widely vaccinated against, increasing population susceptibility.
• Response strategies are available through the CDC.

Smallpox Pathogenesis

• Virus enters via inhalation, replicates in the respiratory tract, and spreads through lymphatic dissemination.
• Incubation lasts 12 to 14 days, followed by fever and a characteristic papulovesicular rash developing into pustules.
• Guarnieri bodies are eosinophilic inclusions used for diagnosis.

Mpox (Monkeypox)

• Spreads through direct contact and has similar symptoms to smallpox, but lower fatality (1-10% depending on clade).
• Vaccine (JYNNEOS) developed for mpox and smallpox; requires 2 doses for effective protection.

Virus Replication Cycles of Major Virus Families

• Parvoviridae: No envelope, nucleus, 24 hours replication cycle.
• Polyomaviridae: No envelope, nucleus, 48 hours replication cycle.
• Adenoviridae: No envelope, nucleus, 25 hours replication cycle.
• Hepadnaviridae: Envelope present, nucleus/cytoplasm, 12-24 hours replication cycle.
• Herpesviridae: Envelope present, nucleus, 15-72 hours replication cycle.
• Poxviridae: Envelope present, cytoplasm, 20 hours replication cycle.