Medical School · Year 2 · Microbiology · includes a discussion video

Lecture 11: DNA Viruses

Unit 2.8: Microbiology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the general characteristics and classification of DNA viruses
2. Explain Herpesviridae and associated clinical syndromes
3. Describe Papillomaviridae and Polyomaviridae
4. Explain Adenoviridae and Poxviridae
5. Describe Hepadnaviridae (Hepatitis B)
6. Explain Parvoviridae and associated diseases

Lecture Outline

I. DNA Virus Overview

DNA viruses comprise a diverse group of viral pathogens unified by their use of deoxyribonucleic acid as genetic material, conferring characteristics that distinguish them from RNA viruses in terms of replication fidelity, genomic stability, and evolutionary dynamics. The majority of human pathogenic DNA viruses possess double-stranded DNA genomes, providing the stability associated with complementary base pairing and proofreading during replication. Parvovirus B19 represents a notable exception, containing a single-stranded DNA genome that limits its coding capacity to only a few proteins. The genomic stability of DNA viruses means they exhibit lower mutation rates compared to RNA viruses, making them more amenable to vaccine development but also allowing for persistent infections where immune evasion depends on mechanisms other than antigenic variation.

Replication of most DNA viruses occurs within the host cell nucleus, exploiting cellular machinery for DNA replication and transcription while allowing viral genome access to splicing mechanisms and other nuclear processes. The poxviruses represent the major exception, replicating entirely within the cytoplasm using viral-encoded enzymes including their own DNA-dependent RNA polymerase, reflecting their evolutionary divergence and unusually large genomes capable of encoding this complex machinery. For nuclear-replicating DNA viruses, smaller viruses with limited coding capacity often depend heavily on host cell DNA polymerase and other replication proteins, while larger viruses may encode their own polymerases with distinct characteristics that can serve as antiviral drug targets. The dependence on nuclear replication influences viral assembly and the requirement for transport of viral components between cytoplasm and nucleus.

The classification of DNA viruses incorporates genomic structure, presence or absence of a lipid envelope, and morphological characteristics observable by electron microscopy. Enveloped DNA viruses include the Herpesviridae (herpes simplex, varicella-zoster, cytomegalovirus, Epstein-Barr virus), Poxviridae (smallpox, molluscum contagiosum), and Hepadnaviridae (hepatitis B virus). Non-enveloped DNA viruses include Papillomaviridae (human papillomaviruses), Polyomaviridae (JC and BK viruses), Adenoviridae, and Parvoviridae. The envelope status has practical implications for environmental stability and transmission: non-enveloped viruses generally survive longer outside the host and may transmit through fomites, while enveloped viruses require more intimate contact or large-volume body fluid exposure for transmission.

The clinical importance of DNA viruses extends across multiple domains including acute infection, latency and reactivation, congenital disease, and oncogenesis. Latency represents a hallmark of herpesvirus biology, with the ability to establish lifelong persistent infection in specific cell types followed by periodic reactivation under conditions of immune suppression or stress. Oncogenic potential is particularly notable among DNA viruses, with human papillomavirus causing cervical and oropharyngeal cancers, hepatitis B virus causing hepatocellular carcinoma, and Epstein-Barr virus associated with multiple malignancies. Congenital infection with cytomegalovirus represents the leading infectious cause of birth defects, while parvovirus B19 causes hydrops fetalis. Understanding these diverse pathogenic mechanisms informs prevention strategies, diagnostic approaches, and treatment modalities for this important group of human pathogens.

<image> Panel A: DNA virus genome comparison - horizontal bars representing different viral genomes showing size range (parvovirus smallest to poxvirus largest), with double-stranded versus single-stranded notation, circular versus linear structure indicated, and color coding for nuclear versus cytoplasmic replication site.

Panel B: Classification matrix by envelope status - two columns (enveloped and non-enveloped) listing viral families with representative icons showing morphology (spherical with spikes for enveloped, icosahedral for non-enveloped), transmission implications noted below each column.

Panel C: Replication strategy diagram - cell with nucleus showing nuclear replication pathway (most DNA viruses entering nucleus, using host/viral polymerase, transcription and splicing) versus cytoplasmic pathway (poxvirus with own polymerase remaining in cytoplasm), with assembly and release pathways.

Panel D: Clinical significance categories - four quadrants showing latency (herpesviruses with reactivation arrow), oncogenesis (HPV, HBV, EBV with cell transformation), congenital infection (CMV, parvovirus with fetus icon), and immunocompromised disease (reactivation syndromes in transplant/AIDS patients). </image>

II. Herpes Simplex Virus (HSV)

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) represent archetypal members of the Herpesviridae family, sharing the characteristic features of envelope, large double-stranded DNA genome, and the ability to establish lifelong latent infection. Both viruses possess complex envelopes containing multiple glycoproteins that mediate attachment and entry into host cells, making envelope-disrupting agents such as detergents and organic solvents effective for inactivation. Transmission requires direct contact with infected secretions or lesions, as the envelope precludes survival on fomites for extended periods. Following primary infection at mucosal or cutaneous sites, HSV establishes latency in sensory neurons, with HSV-1 classically residing in the trigeminal ganglia and HSV-2 in sacral ganglia, though overlap occurs reflecting changing epidemiology of infection sites.

The traditional distinction between HSV-1 and HSV-2 based on site of infection has become less absolute as patterns of transmission have evolved. HSV-1 historically caused primarily orolabial herpes ("cold sores") but now increasingly causes genital infections, particularly through oral-genital contact; it remains the predominant cause of herpes encephalitis and herpes keratitis. HSV-2 primarily causes genital herpes and is the major cause of neonatal herpes, transmitted during vaginal delivery when the mother has active genital lesions or asymptomatic shedding. Both viruses can cause severe disease in immunocompromised patients, with manifestations including disseminated infection, visceral involvement, and extensive mucocutaneous disease. Asymptomatic viral shedding occurs with both HSV-1 and HSV-2, facilitating transmission even in the absence of visible lesions.

The clinical distinction between primary and recurrent HSV infections has important implications for severity, duration, and patient counseling. Primary infection occurs in individuals without pre-existing antibodies and typically causes more severe disease with extensive lesions, prolonged duration of 2-3 weeks, and often systemic symptoms including fever and malaise. Recurrent infections, in contrast, are generally milder and shorter in duration (5-10 days), as pre-existing immune responses limit viral replication. Many patients experience a prodrome of tingling, burning, or itching at the site where lesions will appear. Triggers for reactivation include physical or emotional stress, ultraviolet light exposure, fever, immunosuppression, and local tissue trauma. The frequency of recurrences varies considerably among individuals and tends to decrease over time.

Diagnosis of HSV infections employs multiple modalities depending on clinical context and specimen availability. Clinical recognition of characteristic grouped vesicles on an erythematous base is often sufficient for mucocutaneous disease, though laboratory confirmation is valuable for initial diagnosis and in atypical presentations. PCR has become the preferred diagnostic method for HSV encephalitis, with cerebrospinal fluid analysis providing rapid and sensitive detection. Viral culture of vesicle fluid remains an option when vesicles are present. The Tzanck smear, a rapid bedside test, demonstrates multinucleated giant cells from scraped vesicle bases but cannot distinguish between HSV and VZV. Treatment with nucleoside analogs including acyclovir, valacyclovir, and famciclovir inhibits viral DNA polymerase, requiring initial phosphorylation by viral thymidine kinase. Resistance, occurring primarily in immunocompromised patients, typically results from thymidine kinase mutations.

<image> Panel A: HSV types clinical comparison - split panel showing HSV-1 (oral herpes lesions, temporal lobe encephalitis MRI, keratitis with dendritic ulcer) and HSV-2 (genital lesions, neonatal herpes manifestations), with arrows indicating overlapping presentations becoming more common.

Panel B: Latency and reactivation cycle - sensory neuron diagram showing primary infection at epithelium, retrograde transport to ganglion (trigeminal for HSV-1, sacral for HSV-2), latent genome in nucleus, and anterograde transport during reactivation triggered by stress/immunosuppression/UV.

Panel C: Primary versus recurrent infection comparison - two timelines showing primary (extensive lesions, 2-3 weeks duration, systemic symptoms, no prodrome) versus recurrent (limited lesions, 5-10 days, prodrome present, triggered by identifiable factors).

Panel D: Diagnostic and treatment algorithm - specimen types (vesicle fluid, CSF), diagnostic methods (PCR preferred for CNS, culture, Tzanck with multinucleated giant cell image), treatment medications (acyclovir, valacyclovir, famciclovir with mechanism of action), and resistance note (thymidine kinase mutations in immunocompromised). </image>

III. Varicella-Zoster Virus (VZV)

Varicella-zoster virus, designated human herpesvirus 3 (HHV-3), causes two distinct clinical syndromes reflecting primary infection and reactivation from latency. Primary infection causes varicella, commonly known as chickenpox, characterized by a generalized vesicular rash that was historically a near-universal childhood illness before vaccine introduction. Following resolution of primary infection, VZV establishes latency in dorsal root ganglia throughout the spinal cord and cranial nerve ganglia, remaining dormant for years to decades. Reactivation causes herpes zoster, commonly called shingles, presenting as a painful dermatomal rash reflecting the sensory distribution of the involved ganglion. Unlike HSV, which typically reactivates multiple times, most individuals experience only one or at most a few episodes of herpes zoster in their lifetime.

Varicella (chickenpox) presents following a 14-16 day incubation period with prodromal fever and malaise, followed by the characteristic rash. The rash begins on the trunk and spreads centripetally to the face and extremities, in contrast to the centrifugal distribution of smallpox. Individual lesions progress through stages over 24-48 hours: erythematous macules evolve to papules, then to vesicles with clear fluid described as "dew drops on a rose petal," which subsequently umbilicate, pustulate, and crust. The hallmark finding is lesions in all stages of development simultaneously present on the body, again distinguishing varicella from smallpox where lesions are synchronous. Complications include bacterial superinfection of skin lesions, pneumonia (more common in adults), cerebellar ataxia in children, and rarely encephalitis. Varicella during pregnancy poses risks including congenital varicella syndrome in early pregnancy and severe neonatal varicella when maternal infection occurs near delivery.

Herpes zoster (shingles) results from VZV reactivation in a single sensory ganglion, producing a unilateral vesicular rash limited to the corresponding dermatome. Pain often precedes the rash by days, sometimes leading to misdiagnosis as cardiac, pleuritic, or abdominal disease depending on the involved dermatome. The vesicular eruption typically appears as grouped vesicles on an erythematous base, remaining strictly unilateral and not crossing the midline. Postherpetic neuralgia, persistent pain lasting months to years after rash resolution, represents the most common complication and significantly impacts quality of life, particularly in elderly patients. Zoster ophthalmicus involves the first division of the trigeminal nerve and may cause serious ocular complications including corneal scarring and vision loss. Ramsay Hunt syndrome involves the geniculate ganglion of the facial nerve, presenting with ear pain, vesicles in the external auditory canal, and ipsilateral facial paralysis.

Prevention and treatment strategies for VZV have evolved substantially with the availability of vaccines and antiviral therapy. The live attenuated varicella vaccine, administered in two doses during childhood, has dramatically reduced the incidence of chickenpox and its complications. The recombinant zoster vaccine (Shingrix), containing VZV glycoprotein E with adjuvant, provides superior protection against herpes zoster compared to the older live vaccine and is recommended for adults over 50 years. Antiviral treatment with acyclovir, valacyclovir, or famciclovir accelerates rash healing and may reduce the incidence and severity of postherpetic neuralgia if initiated within 72 hours of rash onset. Varicella-zoster immune globulin (VZIG) provides passive immunization for post-exposure prophylaxis in high-risk individuals including immunocompromised patients, pregnant women without immunity, and newborns exposed perinatally.

<image> Panel A: Varicella (chickenpox) presentation - child with rash showing centripetal distribution (trunk-predominant), progression of individual lesion (macule to papule to vesicle "dew drop on rose petal" to crust), and demonstration of all stages present simultaneously with multiple lesion types labeled on same body region.

Panel B: Varicella versus smallpox distinction table - side-by-side comparison of rash distribution (centripetal vs centrifugal), lesion synchrony (different stages vs same stage), lesion depth (superficial vs deep-seated), and prodrome features, with body diagrams showing distribution pattern.

Panel C: Herpes zoster dermatomal pattern - body diagram showing unilateral dermatomal distribution along thoracic dermatome, cross-section of ganglion with reactivating virus, and special presentations: zoster ophthalmicus (trigeminal V1 with eye involvement), Ramsay Hunt syndrome (geniculate ganglion with facial palsy and ear vesicles).

Panel D: Prevention and treatment timeline - vaccination schedule (two-dose childhood varicella vaccine, Shingrix at age 50+), antiviral treatment window (most effective within 72 hours of rash), postherpetic neuralgia development curve, and VZIG indications for post-exposure prophylaxis in susceptible high-risk individuals. </image>

IV. Epstein-Barr Virus (EBV)

Epstein-Barr virus, designated human herpesvirus 4 (HHV-4), possesses unique biological properties that underlie its clinical manifestations and associations with multiple malignancies. The virus exhibits dual tropism for B lymphocytes and epithelial cells, with B cells serving as the primary site of latent infection and the reservoir for lifelong viral persistence. Entry into B cells occurs through binding of viral glycoprotein gp350 to CD21 (also known as complement receptor 2), the receptor for complement component C3d. Transmission occurs predominantly through saliva, earning infectious mononucleosis the colloquial designation "kissing disease." In developing countries with crowded conditions and poor sanitation, primary infection typically occurs asymptomatically in early childhood, while in developed countries infection is often delayed until adolescence or young adulthood when it more frequently manifests as symptomatic infectious mononucleosis.

Infectious mononucleosis represents the classic clinical manifestation of primary EBV infection in adolescents and young adults, presenting with the triad of fever, pharyngitis, and lymphadenopathy. Splenomegaly occurs in approximately 50% of patients and creates the risk of splenic rupture, necessitating avoidance of contact sports for several weeks. Mild hepatitis with elevated transaminases occurs commonly, though frank jaundice is unusual. A characteristic rash develops in patients who receive ampicillin or amoxicillin during the acute illness, occurring in up to 90% of such patients through an unclear mechanism possibly related to altered immune responses. The peripheral blood shows increased lymphocytes with characteristic atypical morphology, representing reactive CD8+ cytotoxic T cells responding to EBV-infected B cells rather than infected cells themselves.

Laboratory diagnosis of infectious mononucleosis employs both heterophile antibody testing and specific EBV serologies. The monospot test detects heterophile antibodies, IgM antibodies that agglutinate sheep or horse red blood cells; though convenient and specific, the monospot may be falsely negative in children under four years and early in the illness. Specific EBV serologies include viral capsid antigen (VCA) IgM indicating acute infection, VCA IgG indicating current or past infection, and Epstein-Barr nuclear antigen (EBNA) antibodies that appear weeks to months after acute infection and indicate past infection. Early antigen (EA) antibodies may appear during acute infection or reactivation. The serologic pattern helps distinguish acute infection (VCA IgM positive, EBNA negative), past infection (VCA IgG positive, EBNA positive), and reactivation (VCA IgG positive, EA positive).

The oncogenic potential of EBV underlies its association with several human malignancies through viral gene expression that promotes B cell proliferation and survival. Burkitt lymphoma, a highly aggressive B cell lymphoma, occurs in an endemic African form strongly associated with EBV and malaria co-infection, characterized by jaw involvement and the t(8;14) translocation placing c-myc under control of the immunoglobulin heavy chain promoter. Nasopharyngeal carcinoma shows high association with EBV, particularly in Southeast Asian and northern African populations, with both genetic and environmental cofactors. Hodgkin lymphoma, particularly mixed cellularity subtype, contains EBV genomes in the Reed-Sternberg cells in a subset of cases. Post-transplant lymphoproliferative disorder (PTLD) represents EBV-driven B cell proliferation in the setting of immunosuppression, ranging from benign polyclonal expansion to aggressive lymphoma. Primary CNS lymphoma in AIDS patients is nearly uniformly EBV-associated.

<image> Panel A: EBV biology and tropism - virus particle binding to CD21 on B lymphocyte, latent genome episome in B cell nucleus, lytic replication in epithelial cells, and transmission through saliva with age-dependent clinical outcome (asymptomatic childhood infection vs symptomatic adolescent mononucleosis).

Panel B: Infectious mononucleosis clinical features - teenager with classic triad (fever, exudative pharyngitis, cervical lymphadenopathy), splenomegaly with "avoid contact sports" warning, ampicillin rash image, and peripheral blood smear showing atypical lymphocytes (reactive CD8+ cells with abundant cytoplasm).

Panel C: EBV serology interpretation chart - timeline showing antibody appearance (VCA IgM early/acute, VCA IgG persistent, EBNA delayed appearance indicating past infection, EA variable), with clinical interpretation table showing patterns for acute infection, recent past infection, remote past infection, and reactivation.

Panel D: EBV-associated malignancies - four panels: Burkitt lymphoma (jaw mass, t(8;14) diagram, African child), nasopharyngeal carcinoma (nasal anatomy with tumor, Southeast Asian distribution map), Hodgkin lymphoma (Reed-Sternberg cell illustration), and immunocompromised disease (PTLD in transplant, CNS lymphoma in AIDS). </image>

V. Cytomegalovirus (CMV)

Cytomegalovirus, designated human herpesvirus 5 (HHV-5), represents the largest human herpesvirus with correspondingly complex biology and diverse clinical manifestations that vary dramatically based on host immune status. The large genome encodes numerous proteins dedicated to immune evasion, enabling CMV to persist lifelong despite robust immune responses. Transmission occurs through contact with infected body fluids including saliva, urine, genital secretions, breast milk, and blood products. CMV can also transmit through organ transplantation and vertically from mother to fetus. Seroprevalence increases with age and varies by socioeconomic status, ranging from 50% to over 80% in adult populations. Following primary infection, CMV establishes latency in monocytes and CD34+ hematopoietic progenitor cells, with the capacity for reactivation under conditions of immune suppression.

The clinical manifestations of CMV infection vary dramatically based on host immune competence. In immunocompetent individuals, primary CMV infection is usually asymptomatic or causes a mild mononucleosis-like syndrome with fever, mild hepatitis, and atypical lymphocytosis, distinguished from EBV mononucleosis by less prominent pharyngitis and typically negative heterophile antibody testing. In transplant recipients, CMV represents one of the most important opportunistic pathogens, causing pneumonitis, hepatitis, colitis, and a CMV syndrome of fever and cytopenias. In AIDS patients with CD4 counts below 50 cells per microliter, CMV retinitis presents with progressive visual loss from necrotizing retinal lesions visible on fundoscopy, while CMV colitis causes diarrhea and CMV esophagitis causes odynophagia. The widespread use of antiretroviral therapy has dramatically reduced the incidence of CMV end-organ disease in HIV-infected patients.

Congenital CMV infection represents the most common congenital infection worldwide and the leading infectious cause of sensorineural hearing loss and neurodevelopmental disability. Approximately 1% of newborns are infected, with primary maternal infection during pregnancy carrying the highest risk of severe fetal disease, though reactivation and reinfection with new strains can also transmit to the fetus. Only 10-15% of infected newborns are symptomatic at birth, presenting with petechiae, hepatosplenomegaly, jaundice, microcephaly, and periventricular intracranial calcifications. However, an additional 10-15% of initially asymptomatic infants develop late sequelae, primarily sensorineural hearing loss. Diagnosis requires detection of CMV in urine or saliva by PCR within the first three weeks of life, as later positive tests cannot distinguish congenital from postnatal infection. Treatment with ganciclovir or valganciclovir in symptomatic infants may improve hearing and neurodevelopmental outcomes.

Diagnosis of CMV infection utilizes PCR quantification, antigenemia testing, histopathology, and serology depending on the clinical context. Quantitative PCR of blood (CMV viral load) serves as the primary method for monitoring transplant recipients, with rising viral loads triggering preemptive therapy before clinical disease develops. The pp65 antigenemia assay detects CMV protein in circulating leukocytes and offers rapid results. Histopathology of affected tissues reveals characteristic large cells containing both nuclear and cytoplasmic inclusions, with the nuclear inclusions described as "owl's eyes" for their central inclusion surrounded by a clear halo. Treatment with ganciclovir (intravenous) or valganciclovir (oral prodrug) inhibits viral DNA polymerase but carries significant toxicity including bone marrow suppression, particularly neutropenia. Alternative agents include foscarnet (nephrotoxic) and cidofovir for ganciclovir-resistant infection.

<image> Panel A: CMV characteristics and transmission - large herpesvirus particle with complex envelope, transmission routes (saliva, urine, genital secretions, blood products, transplant, vertical), latency sites (monocytes and CD34+ progenitors), and seroprevalence graph by age and socioeconomic status.

Panel B: CMV disease spectrum by immune status - three columns: immunocompetent (mild/asymptomatic, mono-like illness), transplant recipients (pneumonitis, hepatitis, colitis images), AIDS with CD4 <50 (retinitis fundoscopy showing pizza pie appearance, colitis and esophagitis diagrams).

Panel C: Congenital CMV features - symptomatic newborn with petechiae, hepatosplenomegaly, and microcephaly, brain imaging showing periventricular calcifications (distinguishing from toxoplasmosis which has diffuse calcifications), hearing loss icon indicating leading infectious cause, and diagnostic timing requirement (PCR within 3 weeks of birth).

Panel D: Diagnosis and treatment panel - diagnostic methods (quantitative PCR for viral load monitoring, pp65 antigenemia, histopathology with owl's eye inclusions), treatment drugs (ganciclovir/valganciclovir as first-line, foscarnet/cidofovir as alternatives), and toxicity warnings (neutropenia from ganciclovir, nephrotoxicity from foscarnet). </image>

VI. Other Herpesviruses

Human herpesvirus 6 (HHV-6) causes roseola infantum, also known as exanthem subitum or sixth disease, a common childhood illness with a distinctive clinical pattern that facilitates recognition. Primary infection typically occurs between 6 months and 2 years of age, presenting with abrupt onset of high fever (often 40 degrees Celsius or higher) lasting 3-5 days. Parents and physicians may observe febrile seizures during this phase, representing the most common cause of febrile seizures in this age group. The characteristic feature is the sudden defervescence followed within hours by the appearance of a rose-pink maculopapular rash, primarily on the trunk and spreading to the face and extremities. The rash typically lasts 1-2 days and resolves without sequelae. HHV-6 can integrate into human chromosomes and is transmitted vertically in this integrated form, leading to chromosomally integrated HHV-6 (ciHHV-6) that may cause diagnostic confusion when detected by PCR.

Human herpesvirus 7 (HHV-7) shares many biological features with HHV-6 and causes a similar though less well-characterized clinical syndrome. Primary infection occurs slightly later in childhood than HHV-6, typically between 2 and 5 years of age. HHV-7 can cause roseola-like illness indistinguishable from HHV-6 infection and may account for some cases of recurrent roseola. A possible association with pityriasis rosea has been suggested but not definitively established. In transplant recipients, HHV-7 may reactivate and has been implicated as a cofactor for CMV disease, though its clinical significance in immunocompromised patients is less well defined than that of CMV or HHV-6. The close relationship between HHV-6 and HHV-7, including shared cellular receptor usage, creates potential for complex interactions during co-infection.

Human herpesvirus 8 (HHV-8), also known as Kaposi sarcoma-associated herpesvirus (KSHV), causes Kaposi sarcoma and primary effusion lymphoma through oncogenic mechanisms involving viral proteins that promote cell proliferation and angiogenesis. Kaposi sarcoma presents as violaceous or red-brown plaques and nodules affecting skin, mucous membranes, and visceral organs including the gastrointestinal tract and lungs. The disease manifests in distinct epidemiological settings: classic KS in elderly Mediterranean and Eastern European men, endemic KS in sub-Saharan Africa, iatrogenic KS in transplant recipients, and epidemic or AIDS-associated KS in HIV-infected individuals, particularly men who have sex with men. The dramatic decline in AIDS-associated KS following the introduction of effective antiretroviral therapy demonstrates the importance of immune control in preventing disease. Primary effusion lymphoma represents a rare B cell lymphoma occurring in AIDS patients, presenting as pleural or peritoneal effusions without solid tumor masses.

Herpes B virus (Cercopithecine herpesvirus 1) represents a serious occupational hazard for individuals working with macaque monkeys. While herpes B virus causes asymptomatic latent infection in its natural macaque hosts, analogous to HSV in humans, transmission to humans through bites, scratches, or mucosal exposure to monkey secretions can cause severe and frequently fatal encephalomyelitis. Initial symptoms may include local wound findings followed by neurologic deterioration. The mortality rate exceeds 70% in untreated cases, with survivors often experiencing permanent neurologic sequelae. Post-exposure prophylaxis with valacyclovir is recommended following high-risk exposures, and treatment of established infection uses high-dose acyclovir, though efficacy data are limited given the rarity of human cases.

<image> Panel A: HHV-6 roseola clinical pattern - fever chart showing high temperature (40 degrees) for 3-5 days, abrupt defervescence with simultaneous rash appearance, toddler-age child (6 months to 2 years), rose-pink maculopapular trunk rash photograph, and febrile seizure association noted.

Panel B: Herpesvirus 6-8 comparison chart - three rows showing HHV-6 (roseola, infant, seizures), HHV-7 (roseola-like, toddler, less defined), and HHV-8 (Kaposi sarcoma, AIDS/transplant/Mediterranean elderly, oncogenic), with age of acquisition and key features for each.

Panel C: Kaposi sarcoma presentations - skin lesions showing violaceous plaques and nodules, four epidemiological forms (classic in elderly Mediterranean, endemic African, iatrogenic transplant, AIDS-associated), visceral involvement diagram (GI tract, lungs), and treatment response to ART shown as declining lesions.

Panel D: Herpes B virus danger panel - macaque monkey as reservoir, transmission routes (bite, scratch, mucosal exposure), human disease progression (wound to encephalomyelitis), mortality rate indicator (>70%), and post-exposure prophylaxis protocol with valacyclovir. </image>

VII. Papillomaviridae (HPV)

Human papillomaviruses comprise a large family of small, non-enveloped DNA viruses with strict tropism for squamous epithelium, causing a spectrum of benign and malignant proliferative lesions depending on viral type and anatomic site. The HPV genome consists of circular double-stranded DNA of approximately 8,000 base pairs, encoding early (E) proteins involved in replication and transformation and late (L) proteins comprising the viral capsid. Over 200 HPV types have been identified and are classified as high-risk or low-risk based on their association with malignancy. High-risk types, particularly HPV-16 and HPV-18, are causally implicated in cervical cancer and account for approximately 70% of cases, as well as oropharyngeal, anal, penile, vaginal, and vulvar cancers. Low-risk types, especially HPV-6 and HPV-11, cause benign genital warts (condylomata acuminata) and rarely progress to malignancy.

The clinical manifestations of HPV infection depend on viral type and anatomic site. Common warts (verruca vulgaris) typically occur on hands and fingers, caused by HPV types 1, 2, and 4, presenting as raised, rough papules. Plantar warts occur on the soles of feet, often painful due to pressure during walking, caused by similar types. Flat warts (verruca plana) appear as smooth, flat-topped papules on the face and hands, associated with HPV types 3 and 10. Genital warts present as soft, verrucous growths on the penis, vulva, vagina, cervix, or perianal region, caused predominantly by low-risk types 6 and 11. Cervical dysplasia and cancer represent the most significant HPV-associated disease, with high-risk HPV detection in virtually all cervical cancers. Oropharyngeal squamous cell carcinoma associated with HPV-16 has increased dramatically in incidence, particularly among younger individuals, now exceeding cervical cancer in some populations.

The oncogenic mechanism of high-risk HPV types involves viral proteins E6 and E7, which inactivate cellular tumor suppressor proteins, leading to uncontrolled cell proliferation. E6 protein binds and promotes ubiquitin-mediated degradation of p53, the guardian of the genome, eliminating the normal cellular response to DNA damage. E7 protein binds and inactivates retinoblastoma protein (Rb), releasing E2F transcription factors and driving cell cycle progression. In productive infection, the viral genome remains episomal and E6/E7 expression is regulated by E2 protein. Integration of HPV DNA into the host chromosome, which typically disrupts E2, removes this regulatory control and results in high-level E6/E7 expression, promoting transformation. The characteristic koilocyte, a squamous cell with perinuclear clearing and nuclear atypia, represents the cytologic hallmark of HPV infection.

Prevention and screening strategies have dramatically impacted HPV-associated disease. Prophylactic vaccines containing virus-like particles (VLPs) assembled from L1 capsid protein generate neutralizing antibodies that prevent initial infection. The current 9-valent vaccine (Gardasil 9) targets HPV types 6, 11, 16, 18, 31, 33, 45, 52, and 58, providing broad protection against both warts and cancer-associated types. Vaccination is recommended at ages 11-12 years before sexual debut, with catch-up vaccination available through age 26 (and approved to age 45). Cervical cancer screening employs cytology (Pap smear) to detect dysplastic cells, HPV testing to detect high-risk types, or combined approaches. Colposcopy with biopsy follows abnormal screening results to guide management. The integration of vaccination and screening has made cervical cancer potentially the first cancer eliminated through public health intervention.

<image> Panel A: HPV type and disease correlation chart - two columns showing low-risk types (6, 11 causing genital warts and rarely recurrent respiratory papillomatosis) and high-risk types (16, 18, 31, 33, etc. causing cervical, oropharyngeal, anal, and other cancers), with percentage contribution to cervical cancer indicated.

Panel B: Clinical manifestation gallery - common warts on hand, plantar wart on sole, flat warts on face, genital condylomata acuminata, cervical lesion colposcopy image, and oropharyngeal tumor with rising incidence graph showing HPV-positive cases increasing.

Panel C: Oncogenic mechanism diagram - normal cell with p53 and Rb maintaining cell cycle control, HPV E6 binding and degrading p53 (via ubiquitin pathway), E7 binding and inactivating Rb (releasing E2F), episomal versus integrated viral DNA with E2 regulation difference, and koilocyte cytology image with perinuclear clearing.

Panel D: Prevention and screening strategy - vaccine vial (Gardasil 9 with types covered listed), age recommendation timeline (11-12 years ideal, catch-up through 26-45), screening methods (Pap smear showing normal vs dysplastic cells, HPV DNA testing, co-testing), and colposcopy/biopsy for abnormal results. </image>

VIII. Other Non-Enveloped DNA Viruses

The Polyomaviridae family includes JC virus and BK virus, which establish asymptomatic persistent infection in the majority of the population but cause significant disease in immunocompromised patients. JC virus causes progressive multifocal leukoencephalopathy (PML), a devastating demyelinating disease of the central nervous system. The virus infects and destroys oligodendrocytes, the cells responsible for maintaining myelin in the CNS. PML classically occurred in AIDS patients with CD4 counts below 200 cells per microliter but has emerged as a significant risk with certain immunomodulatory therapies, particularly natalizumab used for multiple sclerosis and Crohn's disease. MRI reveals multifocal areas of T2 hyperintensity in the white matter without mass effect or enhancement. There is no specific antiviral therapy; treatment relies on immune reconstitution, which may paradoxically cause worsening through immune reconstitution inflammatory syndrome (IRIS). Prognosis remains poor, with significant mortality and disability among survivors.

BK virus causes disease primarily in the setting of kidney and hematopoietic stem cell transplantation. BK-associated nephropathy (BKVAN) represents a major cause of kidney allograft dysfunction and loss, resulting from uncontrolled viral replication in renal tubular epithelium in the immunosuppressed transplant recipient. Diagnosis involves detection of BK viremia by quantitative PCR, with biopsy showing characteristic viral inclusions and the pathognomonic finding of "decoy cells" (infected cells with enlarged nuclei) in urine. Management requires reduction of immunosuppression to allow immune reconstitution while balancing rejection risk. Hemorrhagic cystitis caused by BK virus occurs in hematopoietic stem cell transplant recipients, presenting with hematuria and bladder symptoms. Merkel cell polyomavirus, more recently discovered, is associated with Merkel cell carcinoma, an aggressive neuroendocrine skin cancer.

Adenoviruses comprise a large family of non-enveloped DNA viruses with over 50 serotypes causing diverse clinical syndromes affecting the respiratory tract, gastrointestinal tract, and eyes. The non-enveloped nature confers environmental stability, enabling transmission through fomites, contaminated water, and respiratory secretions. Pharyngoconjunctival fever presents in children as fever, pharyngitis, and follicular conjunctivitis, often associated with swimming pool exposure. Epidemic keratoconjunctivitis, caused by types 8, 19, and 37, is highly contagious and causes severe eye disease with keratitis that may lead to permanent corneal scarring. Adenovirus pneumonia can be severe, particularly in military recruits (historically addressed through vaccination) and immunocompromised patients. Adenovirus types 40 and 41 cause gastroenteritis, particularly in young children. Hemorrhagic cystitis and disseminated disease occur in transplant recipients.

Parvovirus B19 is the only member of the Parvoviridae family known to be pathogenic in humans, with distinctive clinical manifestations reflecting its unique tropism for erythroid progenitor cells. The virus uses the P antigen (globoside) as its receptor, explaining both its erythroid tropism and the rare individuals lacking P antigen who are resistant to infection. Erythema infectiosum, or fifth disease, affects school-age children with the characteristic "slapped cheek" facial rash followed by a lacy, reticular rash on the trunk and extremities that may wax and wane for weeks. In patients with underlying hemolytic anemias or shortened red cell survival (sickle cell disease, hereditary spherocytosis), temporary cessation of erythropoiesis during infection causes transient aplastic crisis with sudden, severe anemia. Infection during pregnancy, particularly the second trimester, can cause fetal anemia leading to hydrops fetalis and fetal death. Adults, particularly women, may develop symmetric polyarthropathy involving small joints of the hands.

<image> Panel A: JC virus and PML - oligodendrocyte infection diagram showing demyelination, risk factors (AIDS with CD4 <200, natalizumab therapy), brain MRI showing multifocal white matter T2 hyperintensities without mass effect, and prognosis bar indicating high mortality with "immune reconstitution is treatment" note.

Panel B: BK virus disease - two pathways: kidney transplant (BK nephropathy with tubular epithelium involvement, decoy cells in urine, graft loss risk) and stem cell transplant (hemorrhagic cystitis with bladder diagram), management strategy (reduce immunosuppression vs rejection risk balance).

Panel C: Adenovirus syndromes - respiratory (pharyngoconjunctival fever in child at pool, military recruit pneumonia), eye (epidemic keratoconjunctivitis with highly contagious warning), GI (gastroenteritis with types 40/41), and immunocompromised (disseminated disease, hemorrhagic cystitis).

Panel D: Parvovirus B19 clinical spectrum - erythroid progenitor cell tropism diagram using P antigen receptor, four clinical presentations: fifth disease (slapped cheek and lacy rash in child), aplastic crisis (sudden Hgb drop in sickle cell patient), hydrops fetalis (edematous fetus with anemia), and adult arthropathy (symmetric small joint involvement in woman). </image>

IX. Poxviridae

The Poxviridae family includes the largest and most complex DNA viruses, distinguished by their cytoplasmic replication (unique among DNA viruses) and complex structure with multiple membrane layers. Poxviruses possess a large double-stranded DNA genome encoding all proteins necessary for replication in the cytoplasm, including DNA-dependent RNA polymerase, capping enzymes, and poly-A polymerase. The distinctive "brick-shaped" or dumbbell-shaped morphology is visible by electron microscopy. The prototype member, variola virus (smallpox), was responsible for hundreds of millions of deaths throughout human history before its eradication through a global vaccination campaign, with the last natural case occurring in 1977 and WHO certification of eradication in 1980. Variola remains a bioterrorism concern, classified as a Category A agent.

Variola major caused classic smallpox, presenting with a prodrome of high fever and malaise followed by a distinctive rash that progressed synchronously through macular, papular, vesicular, and pustular stages. The centrifugal distribution, with lesions most dense on the face and extremities (including palms and soles), distinguished smallpox from chickenpox, which has a centripetal distribution concentrated on the trunk. All lesions on a given body area were in the same stage of development, contrasting with the pleomorphic lesions of varicella. The lesions were deep-seated and firm to palpation. Mortality ranged from 30% for variola major to less than 1% for variola minor. Vaccination with vaccinia virus (cowpox-related) provided protection and represented the first successful vaccine, developed by Edward Jenner in 1796. Following eradication, routine vaccination ceased, leaving large populations susceptible should variola re-emerge through bioterrorism.

Molluscum contagiosum, caused by a poxvirus distinct from variola, produces characteristic skin lesions transmitted through direct contact and remains common in contemporary practice. The lesions are smooth, dome-shaped papules with characteristic central umbilication, typically 2-5 mm in diameter. In children, lesions commonly appear on the trunk, extremities, and face, often spreading through autoinoculation and resolving spontaneously over months. In sexually active adults, genital lesions indicate sexual transmission. In HIV-infected individuals, especially those with low CD4 counts, molluscum lesions may be numerous, large (giant molluscum), and refractory to treatment. Treatment is often observation for spontaneous resolution, though curettage, cryotherapy, and other destructive modalities can accelerate clearance.

Monkeypox has emerged as a disease of global public health importance following outbreaks outside its endemic African regions. The virus causes a smallpox-like illness with fever, lymphadenopathy, and a pustular rash, typically milder than smallpox but with significant morbidity. Endemic in West and Central Africa with rodent reservoirs, monkeypox historically caused sporadic human cases through contact with infected animals. The 2022 global outbreak, primarily affecting men who have sex with men through close physical contact, represented a novel epidemiological pattern requiring updated public health responses. Vaccination with modified vaccinia Ankara (MVA) vaccine provides protection for high-risk individuals. Other poxviruses causing human disease include orf virus (transmitted from sheep and goats, causing single nodular lesions on hands) and cowpox (rare, localized lesions from contact with infected animals).

<image> Panel A: Poxvirus unique features - large brick-shaped virion by electron microscopy, cytoplasmic replication diagram (own DNA-dependent RNA polymerase), genome size comparison to other DNA viruses (largest), and complex multiple-membrane structure illustration.

Panel B: Smallpox historical and clinical features - historical photograph of patient with characteristic rash, centrifugal distribution diagram (face/extremities > trunk), synchronous lesion staging (all same stage in given area), comparison table with varicella, eradication timeline (last case 1977, certification 1980), and bioterrorism Category A warning.

Panel C: Molluscum contagiosum presentation - dome-shaped papule with central umbilication close-up, typical locations in children (trunk, face) versus adults (genital), HIV-associated giant molluscum, and treatment options (observation for self-resolution, curettage, cryotherapy).

Panel D: Monkeypox emergence - African endemic regions with rodent reservoir, 2022 outbreak spread map, clinical presentation (fever, lymphadenopathy, pustular rash), epidemiological shift in affected population, and MVA vaccine for prevention in high-risk groups. </image>

X. Hepadnaviridae (Hepatitis B)

Hepatitis B virus (HBV) possesses unique molecular characteristics that distinguish it from other DNA viruses and inform both pathogenesis and treatment strategies. The HBV genome consists of a partially double-stranded circular DNA of approximately 3,200 base pairs, the smallest of any human DNA virus. Replication proceeds through an RNA intermediate, with the viral polymerase functioning as both reverse transcriptase (to synthesize DNA from RNA) and DNA polymerase, classifying HBV as a pararetrovirus. This replication strategy creates opportunities for nucleoside analog therapy similar to antiretroviral drugs used for HIV. The viral envelope contains hepatitis B surface antigen (HBsAg), the target of protective antibodies and the antigen used in recombinant vaccines. HBV demonstrates strong hepatotropism, infecting hepatocytes where chronic infection can lead to cirrhosis and hepatocellular carcinoma through both direct and indirect mechanisms.

The serologic markers of HBV infection provide critical information for diagnosis, determining infection status, and monitoring treatment response. Hepatitis B surface antigen (HBsAg) indicates active infection (acute or chronic), while anti-HBs indicates immunity from resolved infection or vaccination. Hepatitis B core antigen (HBcAg) is not detected in serum, but anti-HBc antibodies are useful markers: IgM anti-HBc indicates recent or acute infection, while IgG anti-HBc persists for life and indicates current or past infection. The "window period" during early acute infection may show positive IgM anti-HBc as the only marker when HBsAg has declined but anti-HBs has not yet appeared. Hepatitis B e antigen (HBeAg) indicates active viral replication with high infectivity, while development of anti-HBe suggests lower replication and reduced infectivity. Quantitative HBV DNA levels directly measure viral replication and guide treatment decisions.

The clinical outcomes of HBV infection vary dramatically based on age at acquisition, immune status, and viral factors. Acute HBV infection is subclinical in approximately 70% of cases, with the remainder developing symptomatic hepatitis; fulminant hepatic failure occurs in approximately 1% of acute infections. The critical determinant of chronic infection is age at acquisition: 90% of perinatally infected infants develop chronic infection, compared to only 5% of adults with acute infection, reflecting the immature immune response in neonates and development of immune tolerance. Chronic HBV infection progresses through phases of immune tolerance, immune clearance, inactive carrier state, and potential reactivation. Complications of chronic infection include cirrhosis and hepatocellular carcinoma (HCC), with HBV responsible for the majority of HCC cases worldwide. Extrahepatic manifestations include polyarteritis nodosa and membranous glomerulonephritis through immune complex deposition.

Prevention and treatment strategies for HBV have evolved substantially. The recombinant HBV vaccine contains HBsAg and is administered as a three-dose series, with universal infant vaccination now standard in most countries; the vaccine is highly effective, providing over 95% protection in responders. Hepatitis B immune globulin (HBIG) provides passive immunization for post-exposure prophylaxis and is combined with vaccination for infants born to HBsAg-positive mothers. Treatment indications for chronic HBV include elevated ALT, positive HBeAg with high viral load, and evidence of hepatic fibrosis. First-line antiviral agents are the nucleos(t)ide analogs entecavir and tenofovir, which suppress viral replication with high barrier to resistance. Pegylated interferon alpha is an alternative that offers finite treatment duration but lower response rates and significant side effects. Treatment goals include viral suppression, normalization of ALT, and HBeAg seroconversion; complete HBsAg loss (functional cure) is rare. Most patients require indefinite antiviral therapy, as the covalently closed circular DNA (cccDNA) in hepatocyte nuclei persists despite treatment.

<image> Panel A: HBV genome and replication - circular partially double-stranded DNA structure (gapped circle), replication through RNA intermediate (pgRNA), reverse transcriptase/DNA polymerase enzyme, comparison to retroviruses, and hepatocyte tropism with nuclear cccDNA persistence.

Panel B: Serologic marker interpretation table - columns for HBsAg, anti-HBs, anti-HBc (IgM/IgG), HBeAg, anti-HBe, with rows for acute infection, window period, resolved infection, vaccinated immunity, chronic active infection, chronic inactive carrier, and interpretation for each pattern.

Panel C: Clinical outcomes by age of acquisition - pediatric/adult split showing 90% chronic in infants versus 5% chronic in adults, chronic infection phases diagram (immune tolerance, immune clearance, inactive carrier, reactivation), and complications (cirrhosis and HCC with surveillance recommendation).

Panel D: Prevention and treatment algorithm - vaccination schedule (three doses, universal infant, at-risk adults), post-exposure prophylaxis (HBIG + vaccine for neonates of HBsAg+ mothers), treatment indications (elevated ALT, HBeAg+, fibrosis), first-line drugs (entecavir, tenofovir), and treatment goals (viral suppression, rare HBsAg loss). </image>

Summary

• DNA viruses possess double-stranded genomes (except parvovirus), replicate in the nucleus (except poxvirus), and demonstrate genomic stability enabling latency and oncogenesis
• Herpes simplex viruses cause orolabial and genital herpes with latency in sensory ganglia; HSV-1 causes most encephalitis; treatment is acyclovir
• Varicella-zoster virus causes chickenpox (primary) with centripetal rash and all lesion stages present, and shingles (reactivation) with dermatomal distribution and postherpetic neuralgia
• Epstein-Barr virus causes infectious mononucleosis (fever, pharyngitis, lymphadenopathy, splenomegaly, atypical lymphocytes) and is associated with Burkitt lymphoma, nasopharyngeal carcinoma, and Hodgkin lymphoma
• Cytomegalovirus causes the most common congenital infection with periventricular calcifications and leading cause of infectious deafness; causes retinitis and colitis in AIDS patients; treatment is ganciclovir
• Human papillomavirus high-risk types (16, 18) cause cervical and oropharyngeal cancer through E6 (degrades p53) and E7 (inactivates Rb); vaccine prevents infection
• Parvovirus B19 targets erythroid progenitors, causing fifth disease in children, aplastic crisis in hemolytic anemias, and hydrops fetalis in pregnancy
• Smallpox (eradicated) had synchronous, centrifugal rash; molluscum contagiosum causes umbilicated papules; monkeypox emerged as global concern
• Hepatitis B virus replicates via RNA intermediate; chronic infection (90% neonates, 5% adults) causes cirrhosis and HCC; treatment is entecavir or tenofovir

Key Terms

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