DNA Viruses: Adenoviruses, Papillomaviruses, Polyomaviruses, Parvovirus and Poxviruses

Genome. Double-stranded DNA. Envelope. Absent, nonenveloped, contributing to notable environmental stability and resistance to standard alcohol-based disinfection, relevant to efficient spread via fomites and in settings such as swimming pools.

General features. A large family with numerous distinct serotypes, each with somewhat different characteristic tissue tropism and associated clinical syndrome, making this a genuinely diverse family rather than a single uniform clinical entity.

Respiratory disease. A common cause of upper and lower respiratory tract infection across all age groups, particularly significant in young children and in closed community settings. Military recruit barracks are a classically described setting for outbreak respiratory adenovirus disease. Disease ranges from mild upper respiratory illness to severe pneumonia in some cases.

Conjunctivitis. Epidemic keratoconjunctivitis is a highly contagious, often prolonged conjunctival infection, transmitted via direct contact and fomites, including inadequately disinfected ophthalmology equipment, a recognised healthcare-associated transmission route specific to this presentation. Pharyngoconjunctival fever combines conjunctivitis with pharyngitis and fever and is classically associated with swimming pool exposure.

Gastroenteritis. Specific enteric adenovirus serotypes, distinct from the respiratory-tropic serotypes, cause gastroenteritis, particularly in young children.

Haemorrhagic cystitis. Adenovirus is a recognised cause of haemorrhagic cystitis, particularly in immunocompromised hosts, notably haematopoietic stem cell transplant recipients.

Diagnosis. PCR, increasingly standard across the range of clinical presentations, antigen detection, available for rapid conjunctivitis and gastroenteritis diagnosis in some settings, and, less commonly now, viral culture.

Genome. Double-stranded, circular DNA. Envelope. Absent, nonenveloped.

HPV types. Human papillomavirus comprises over one hundred distinct types, broadly divided by their clinical and oncogenic behaviour into low-risk types, associated with benign warty lesions and not meaningfully oncogenic, and high-risk types, associated with malignancy,. This distinction carries major clinical and vaccination-strategy significance.

Warts. Low-risk HPV types, notably types 6 and 11, cause cutaneous warts (common, plantar, and flat warts depending on specific type and site) and anogenital warts. Anogenital warts, condylomata acuminata, are sexually transmitted. They are distinct in both appearance and causative type from the flat, moist condylomata lata of secondary syphilis, an important distinction given the similar anatomical location and superficially similar name.

Oncogenic strains. High-risk HPV types, notably types 16 and 18, which together account for the substantial majority of HPV-associated malignancy, encode the E6 and E7 oncoproteins. E6 inactivates the host tumour suppressor protein p53, and E7 inactivates the retinoblastoma protein, together removing two of the cell's central checks on uncontrolled proliferation.

Cervical and other malignancy associations. HPV is now recognised as a necessary, though not independently sufficient, cause of essentially all cervical cancer, the basis of both HPV vaccination programmes and cervical cytology and HPV-based screening strategies.

High-risk HPV also causes a range of other anogenital and oropharyngeal malignancies, including anal cancer, vulvar, vaginal, and penile cancer. It also causes a substantial and increasing proportion of oropharyngeal squamous cell carcinoma, particularly of the tonsil and base of tongue. This is now recognised as HPV-associated in many cases rather than exclusively tobacco and alcohol-related as classically taught.

Diagnosis. Clinical diagnosis for typical warts. HPV DNA testing, often combined with cervical cytology as co-testing, for cervical screening. Histopathology confirms malignant or premalignant dysplastic change.

Genome. Double-stranded, circular DNA. Envelope. Absent, nonenveloped.

BK virus. Establishes latency in the renal-urinary tract epithelium following typically asymptomatic primary infection in childhood.

Reactivation, specifically in the context of significant immunosuppression, is a well-recognised cause of BK virus nephropathy in renal transplant recipients. This is a leading cause of transplant graft dysfunction and loss from this specific viral cause, monitored via routine post-transplant blood and urine PCR surveillance in many transplant programmes.

BK virus is also a cause of haemorrhagic cystitis in haematopoietic stem cell transplant recipients.

JC virus. Similarly establishes latency, in the kidney and, notably, within cells of the central nervous system in a proportion of infected individuals, following typically asymptomatic primary infection.

Reactivation in the setting of severe immunosuppression, whether advanced HIV or certain immunomodulatory biologic therapies such as natalizumab for multiple sclerosis, a specific and clinically important drug-associated risk, causes progressive multifocal leukoencephalopathy. This is a severe, often fatal demyelinating disease of the central nervous system resulting from JC virus infection of oligodendrocytes.

Diagnosis. PCR, blood and urine for BK virus surveillance in transplant recipients, and cerebrospinal fluid PCR for JC virus in suspected progressive multifocal leukoencephalopathy.

Genome. Single-stranded DNA, the unique single-stranded DNA structure distinguishing Parvovirus B19 from the other major human DNA viruses, which carry double-stranded genomes, requiring host-cell-mediated synthesis of a complementary second strand before conventional transcription can proceed. Envelope. Absent, nonenveloped, among the smallest and structurally simplest viruses in virology.

Parvovirus B19. Transmitted via respiratory droplets, and, given its specific tropism below, also transmissible transplacentally and via blood or blood product transfusion.

Erythroid tropism. The virus specifically targets erythroid precursor cells in the bone marrow, binding the P antigen, globoside, on their surface, and directly lyses infected erythroid precursors, transiently halting red cell production. This specific tropism directly explains the entire clinical spectrum of B19 disease.

Major diseases. Erythema infectiosum, fifth disease, is the classic presentation in immunocompetent children: a mild febrile illness followed by a distinctive slapped cheek facial rash and a lacy, reticular rash on the trunk and limbs. The rash itself is believed to be immune-complex-mediated, occurring after the period of viraemia and erythroid suppression has already passed, an immune-mediated rather than direct cytopathic mechanism.

Transient aplastic crisis occurs in patients with an underlying chronic haemolytic condition, sickle cell disease being the classic example, who depend on a high baseline rate of red cell production to compensate for ongoing haemolysis. B19-induced temporary cessation of erythropoiesis can precipitate a severe, symptomatic aplastic crisis in these patients.

Hydrops fetalis follows transplacental transmission, particularly in the second trimester, and reflects severe fetal anaemia from erythroid suppression during a period of high physiological red cell demand.

Chronic pure red cell aplasia occurs in immunocompromised hosts unable to mount an adequate antibody response to clear the virus.

Diagnosis. Serology, IgM for recent infection, in immunocompetent hosts. PCR, particularly in immunocompromised hosts, who may fail to mount a reliable antibody response, and in suspected fetal infection.

Genome. Double-stranded DNA. Envelope. Present, though of a somewhat different, more complex structural arrangement than the simple envelope of most other enveloped viruses.

Complex structure. The largest and structurally most complex viruses in virology, with a brick-shaped or ovoid virion incorporating multiple internal structural layers rather than a simple, discrete geometric capsid. Reflecting their large genome size, poxviruses have an unusually extensive coding capacity for nonessential, host-interaction-focused genes, including an extensive repertoire of immune modulatory proteins.

Cytoplasmic replication. The defining and unusual replication feature distinguishing poxviruses from every other DNA virus family: poxviruses replicate entirely within the cytoplasm, carrying their own virus-encoded DNA-dependent RNA polymerase, and other necessary enzymes, packaged within the virion. Cytoplasmic replication means the virus cannot rely on host nuclear transcriptional machinery in the way other DNA viruses do, making this a specific exception to the general pattern of double-stranded DNA viruses replicating in the nucleus.

Molluscum contagiosum. Caused by molluscum contagiosum virus, a distinct poxvirus genus, producing characteristic small, firm, umbilicated, centrally dimpled, skin papules, transmitted via direct skin contact, including sexual contact when involving the genital area, and autoinoculation. Generally self-limited in immunocompetent hosts, though lesions can be more extensive and persistent in immunocompromised patients, notably advanced HIV infection.

Orthopoxvirus concepts. The genus containing variola virus, the cause of smallpox, declared globally eradicated following a successful vaccination campaign, the only human infectious disease to have achieved this status to date, and vaccinia virus, the live virus historically used in smallpox vaccination, exploiting cross-protective immunity between related orthopoxvirus species.

Mpox, monkeypox, virus, a further orthopoxvirus, causes a vesiculopustular illness with systemic symptoms and lymphadenopathy. It is maintained in an African rodent and animal reservoir with periodic zoonotic spillover. In recent years, sustained human-to-human transmission has occurred in wider outbreaks beyond its traditionally endemic range.

Diagnosis. Predominantly clinical for typical molluscum contagiosum lesions. PCR is used for confirming orthopoxvirus infections, including mpox, given the public health significance of accurate, prompt identification.