Pathogenic DNA Viruses: Structure, Taxonomy, and Clinical Significance

Pathogenic DNA Viruses

Overview of Pathogenic DNA Viruses

Pathogenic DNA viruses are a diverse group of viruses that cause a variety of diseases in humans. They are classified into several families based on their genome structure, capsid symmetry, presence or absence of an envelope, and the diseases they cause. Understanding their taxonomy and characteristics is essential for diagnosis, treatment, and prevention of viral diseases.

• DNA viruses can have single-stranded (ssDNA), double-stranded (dsDNA), or partially double-stranded genomes.

• They may be enveloped (surrounded by a lipid membrane) or naked (lacking an envelope).

• Capsid symmetry is typically icosahedral.

• Major families include Adenoviridae, Hepadnaviridae, and Parvoviridae.

Family	Strand Type	Enveloped/Naked	Capsid Symmetry	Size (nm)	Representative Genera (Disease)
Adenoviridae	Double	Naked	Icosahedral	60–90	Mastadenovirus (conjunctivitis, respiratory infections)
Hepadnaviridae	Partial single and partial double	Enveloped	Icosahedral	42	Orthohepadnavirus (hepatitis B)
Parvoviridae	Single	Naked	Icosahedral	18–26	Erythrovirus (fifth disease)

Additional info: Other DNA virus families include Poxviridae, Herpesviridae, Papillomaviridae, and Polyomaviridae, but only the three above are discussed in detail here.

Adenoviridae

Adenoviridae are non-enveloped DNA viruses with an icosahedral capsid and characteristic spikes. They are important human pathogens, causing a range of respiratory and ocular diseases.

• Genome: Single, linear dsDNA

• Capsid: Naked, polyhedral with spikes

• Transmission: Spread via respiratory droplets, fomites, and contaminated water

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

• Pathogenesis: Enter cells via endocytosis; cause symptoms such as sneezing, sore throat, cough, headache, malaise

• Special note: One strain (Adv36) may be associated with human obesity

• Treatment: Cidofovir for severe infections

• Prevention: Attenuated vaccine available for military personnel

Hepadnaviridae

Hepadnaviridae are enveloped DNA viruses with a unique genome structure. The most clinically significant member is the Hepatitis B virus (HBV), which is the only DNA virus known to cause hepatitis in humans.

• Genome: Both single- and double-stranded DNA (circular, partially double-stranded)

• Capsid: Enveloped, icosahedral

• Replication: Involves an RNA intermediate and reverse transcriptase

• Target: Invades and replicates in liver cells

Hepatitis B Infections

• Symptoms: Jaundice, liver enlargement, abdominal distress, bleeding into skin and organs

• Complications: Severe liver damage, increased risk with coinfection by hepatitis D virus

• Transmission: Blood, saliva, semen, vaginal secretions; via needles, sexual contact, childbirth

• Asymptomatic cases are common

• Prevention: Vaccination has significantly reduced incidence

Diagnosis, Treatment, and Prevention

• Diagnosis: Detection of viral antigens in body fluids; three types of particles (Dane, spherical, filamentous)

• Treatment: No universally effective treatment

• Prevention: Vaccination, abstinence, monogamy

Hepatitis B and Hepatic Cancer

• HBV is associated with hepatic (liver) cancer, especially in regions with high HBV prevalence

• HBV genome can integrate into hepatic cancer cells, which express HBV antigens

• Chronic HBV carriers are at higher risk for hepatic cancer

• Vaccination may help eliminate hepatic cancer related to HBV

Parvoviridae

Parvoviridae are the smallest DNA viruses and the only human pathogens with a single-stranded DNA genome. The primary human pathogen is B19 virus, which causes erythema infectiosum (fifth disease).

• Genome: Single-stranded DNA (ssDNA)

• Capsid: Naked, icosahedral

• Diseases: Erythema infectiosum (fifth disease)

• Symptoms: Reddening of the skin, especially on the cheeks; sunlight aggravates the rash

Review Questions and Key Facts

• Which DNA virus causes respiratory infections? Answer: Adenoviruses

• Which disease is characterized by a harmless rash aggravated by sunlight? Answer: Fifth disease (caused by Parvovirus B19)

• Are many people infected with Hepatitis B asymptomatic? Answer: True