Pathogenic DNA Viruses: Structure, Diseases, and Clinical Features

Pathogenic DNA Viruses

Overview of DNA Viruses

DNA viruses that cause human diseases are classified into several families based on their genome type (double-stranded or single-stranded DNA), presence or absence of an envelope, size, and host range. These viruses are responsible for a variety of clinically significant diseases, ranging from skin lesions to systemic infections and cancers.

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

• ssDNA viruses: Parvoviridae

• Mixed genome: Hepadnaviridae (contains both dsDNA and ssDNA)

Poxviridae

General Characteristics

Poxviridae are large, complex, enveloped dsDNA viruses. They infect many mammals, but most animal poxviruses are species-specific and cannot infect humans. Human infection typically occurs through inhalation or close contact.

• Main human diseases: Smallpox (variola) and molluscum contagiosum

• Transmission: Close contact is necessary; some animal poxviruses can be transmitted to humans

Stages of Poxvirus Lesions

Poxvirus infections progress through distinct skin lesion stages, each with characteristic features:

• Macule: Flat, reddened area

• Papule: Raised, solid lesion

• Vesicle: Fluid-filled blister

• Pustule: Pus-filled lesion

• Crust: Dried exudate over the lesion

• Scar: Permanent mark after healing

Smallpox (Variola)

Smallpox, caused by the variola virus (genus Orthopoxvirus), was a highly contagious and deadly disease. It existed in two forms: variola major and variola minor. The virus spread via the bloodstream to the skin, producing characteristic pox and often resulting in facial scarring. Smallpox was the first human disease to be eradicated due to effective vaccination, lack of animal reservoirs, and the absence of asymptomatic cases.

Molluscum Contagiosum

This skin disease is caused by Molluscipoxvirus and is characterized by smooth, waxy papules, typically on the face, trunk, and external genitalia. It spreads by direct contact, especially among children, sexually active individuals, and immunocompromised patients. People with normal immunity usually recover without treatment.

Herpesviridae

General Characteristics

Herpesviruses are enveloped, polyhedral viruses with linear dsDNA. They are among the most prevalent DNA viruses and are known for their ability to establish latency in host cells, with potential for reactivation. Human herpesviruses are designated as HHV-1 through HHV-8.

Human Herpesviruses 1 and 2 (HHV-1, HHV-2)

These viruses, formerly known as herpes simplex viruses (HSV), cause slow-spreading skin lesions. HHV-1 is primarily associated with oral herpes, while HHV-2 is linked to genital herpes. Other manifestations include ocular herpes, whitlow (finger infection), and neonatal herpes.

• Transmission: Close bodily contact; virus enters through mucous membrane breaks

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

• Latency: Virus remains dormant in nerve ganglia, with potential for reactivation

Human Herpesvirus 3 (Varicella-Zoster Virus, VZV)

VZV causes two diseases: varicella (chickenpox) in children and herpes zoster (shingles) in adults. Chickenpox is highly infectious, with the virus entering via the respiratory tract or eyes and spreading through the bloodstream. After initial infection, the virus can remain latent and reactivate as shingles, typically in a single dermatome.

Human Herpesvirus 4 (Epstein-Barr Virus, EBV)

EBV is transmitted via saliva and initially infects the pharynx and salivary glands before invading B lymphocytes. It can cause infectious mononucleosis, Burkitt's lymphoma, and other diseases, depending on the host's immune status. The virus becomes latent in B cells and can suppress apoptosis.

Human Herpesvirus 5 (Cytomegalovirus, CMV)

CMV infection is common and usually asymptomatic, but can cause severe disease in fetuses, newborns, and immunodeficient individuals. Infected cells become enlarged ("owl's eye" appearance). Transmission requires close contact and exchange of bodily fluids.

Other Herpesviruses

• HHV-6 (Roseolovirus): Causes roseola, a pink rash, and may be linked to multiple sclerosis.

• HHV-8 (Rhadinovirus): Associated with Kaposi's sarcoma, especially in immunocompromised patients.

Papillomaviridae and Polyomaviridae

Papillomavirus Infections

Papillomaviruses cause papillomas (warts), which are benign epithelial growths. Warts can occur on various body surfaces and are sometimes associated with cancer risk, especially genital warts. Transmission occurs via direct contact or fomites, and autoinoculation can spread the virus on the same individual.

Polyomavirus Infections

Polyomaviruses can cause tumors and other diseases. BK and JC viruses are widespread; disease outcome depends on immune status. In immunocompromised individuals, BK virus can cause severe urinary tract infections, while JC virus may cause progressive multifocal leukoencephalopathy, a fatal CNS disease.

Adenoviridae

General Characteristics and Diseases

Adenoviruses are non-enveloped viruses with a linear dsDNA genome. They are a common cause of respiratory infections (common cold), conjunctivitis (pinkeye), and mild gastrointestinal disease. Transmission is primarily via respiratory droplets.

Hepadnaviridae

General Characteristics

Hepadnaviruses are enveloped DNA viruses with a unique genome containing both single- and double-stranded DNA. Hepatitis B virus (HBV) is the only DNA virus that causes hepatitis and replicates through an RNA intermediate.

Hepatitis B Infections

HBV causes hepatitis, an inflammation of the liver, which can lead to jaundice, liver enlargement, abdominal distress, and bleeding. The virus is transmitted through contact with infected body fluids, including blood, semen, and vaginal secretions. Chronic infection increases the risk of liver cancer.

Parvoviridae

General Characteristics and Disease

Parvoviridae are the smallest DNA viruses and the only human pathogens with a single-stranded DNA genome. The B19 virus causes erythema infectiosum (fifth disease), which presents as a reddening of the skin, often aggravated by sunlight.

Summary Table: Major DNA Virus Families and Associated Diseases

Family	Genome	Envelope	Main Diseases
Poxviridae	dsDNA	Yes	Smallpox, molluscum contagiosum
Herpesviridae	dsDNA	Yes	Herpes simplex, chickenpox, shingles, mononucleosis, CMV, roseola, Kaposi's sarcoma
Papillomaviridae	dsDNA	No	Warts, cervical cancer
Polyomaviridae	dsDNA	No	Urinary tract infections, leukoencephalopathy
Adenoviridae	dsDNA	No	Respiratory infections, conjunctivitis
Hepadnaviridae	dsDNA/ssDNA	Yes	Hepatitis B
Parvoviridae	ssDNA	No	Fifth disease (erythema infectiosum)