Poxvirus Infections (Smallpox, Monkeypox, Molluscum Contagiosum)

Poxvirus Family

Largest, most complex human viruses. Light-microscope visible, brick-shaped. UNIQUE: replicates in CYTOPLASM (not nucleus) — carries own complete transcriptional apparatus, independent of nuclear enzymes. Members: Variola (smallpox), Monkeypox virus (mpox), Molluscipoxvirus (molluscum contagiosum) — very different clinically despite shared architecture.

Smallpox (Variola)

ONLY human disease ever ERADICATED globally (WHO 1980). Last natural case: Somalia 1977. 2 repositories remain (USA, Russia). Bioterrorism concern = main current relevance.

Why eradicable (explains why not repeatable for most diseases):

NO animal reservoir (human-only host)
STABLE single serotype (unlike flu drift — one vaccine worked everywhere)
Distinctive rash (case ID without lab confirmation)
NO asymptomatic carrier/prolonged subclinical shedding (every case identifiable → ring vaccination worked)

Clinical (historical): respiratory droplet/contact. ~12d incubation. Severe prodrome (fever, malaise, headache) 2-4d BEFORE rash. Rash: SYNCHRONOUS — all lesions in a region SAME STAGE simultaneously (contrast varicella’s asynchronous successive crops). Distribution: CENTRIFUGAL — denser face+extremities > trunk (REVERSE of varicella’s central/truncal predominance). Case-fatality (variola major): 20-30%.

Monkeypox (Mpox)

CURRENT ongoing zoonosis (not historical). Rodents = reservoir (NOT primates, despite name — just first identified species). Central/West Africa endemic. 2022 global outbreak: close/sexual contact transmission (atypical vs prior endemic outbreaks) → broad international attention.

Clinical: similar to smallpox but MILDER. Case-fatality << smallpox, varies by clade (Clade I Central African = more severe; Clade II West African = 2022 outbreak strain). KEY DISTINGUISHER: LYMPHADENOPATHY — prominent in mpox, ABSENT in both smallpox AND varicella. Useful bedside discriminator. Rash: similar synchronous sequential evolution to smallpox.

Diagnosis: PCR lesion swab = DOC. Management: supportive. Tecovirimat (developed for smallpox bioterrorism threat) — activity vs orthopoxviruses generally, severe/high-risk cases. Smallpox (vaccinia) vaccine → cross-protects against mpox (shared genus).

Molluscum Contagiosum

DIFFERENT entity entirely. Benign, self-limited, PURELY cutaneous (never systemic/life-threatening in immunocompetent host). Molluscipoxvirus. Direct skin contact (± sexual if genital) or fomites.

Clinical: discrete, firm, dome-shaped, flesh-colored/pearly papules, CENTRAL UMBILICATION (small dimple) = distinctive, usually CLINICAL diagnosis (no lab needed). Multiple lesions typical. Children: self-inoculation spread (scratching). Adults (sexually active): genital/lower-abdominal = sexual transmission. Immunocompromised (advanced HIV): unusually large/numerous/treatment-resistant — clue to underlying immunosuppression.

Treatment: usually SELF-LIMITED, resolves months, observation reasonable default. If treated: curettage, cryotherapy, topical agents (cantharidin) — patient preference/location driven, not medical necessity.

The Poxvirus Family

Poxviruses are the largest and structurally most complex viruses infecting humans — large enough to be seen by light microscopy, brick-shaped, and, uniquely among DNA viruses of medical importance, replicating entirely within the host cell cytoplasm rather than the nucleus (every other major DNA virus family needs the nucleus’s machinery; poxviruses instead carry their own complete transcriptional apparatus packaged into the virion itself, making them functionally independent of nuclear enzymes). The family includes the historically pivotal Variola (smallpox), Monkeypox virus (mpox), and Molluscipoxvirus (molluscum contagiosum) — three diseases with almost nothing in common clinically beyond the shared viral architecture.

Smallpox (Variola)

Historical and Present Status

Smallpox holds a unique place in medical history as the only human infectious disease ever eradicated globally (WHO certified eradication in 1980), the culmination of a targeted vaccination and ring-containment campaign built specifically on the properties described below. The last natural case occurred in Somalia in 1977. Two official virus repositories remain (USA, Russia) under WHO oversight, and residual bioterrorism concern is the main reason smallpox retains any contemporary clinical relevance at all.

Why Eradication Was Achievable

Several biological features made smallpox a genuinely favourable eradication target, worth naming explicitly since they explain why this success has not been repeatable for most other diseases: no animal reservoir (humans are the only host, so there was no wildlife source to reseed transmission); a stable, single serotype (unlike influenza’s constant drift, one vaccine formulation worked everywhere); a distinctive, hard-to-miss rash (making case identification straightforward without laboratory confirmation, which mattered enormously in resource-poor settings); and no asymptomatic carrier state or prolonged subclinical shedding (every infected person was clinically identifiable, letting ring vaccination around each case actually interrupt chains of transmission).

Clinical Features (Historical)

Transmission was by respiratory droplet or direct contact. After a roughly 12-day incubation, illness began with a severe prodrome (high fever, malaise, headache) 2–4 days before rash onset, followed by a rash that was — in sharp contrast to varicella (see Herpes Simplex/VZV topic) — synchronous: all lesions in a given body region progressed through the same stage (macule → papule → vesicle → pustule → crust) at the same time, rather than varicella’s asynchronous “successive crops.” Distribution was classically centrifugal, denser on the face and extremities than the trunk — again the reverse of varicella’s more central/truncal predominance. Case-fatality for the more severe variola major strain reached 20–30%.

Monkeypox (Mpox)

Current Relevance

Unlike smallpox, mpox is an ongoing, actively circulating zoonosis — genuinely relevant today rather than a historical footnote — caused by Monkeypox virus, with rodents (not primates, despite the name, which reflects only the species it was first identified in) as the presumed natural reservoir in endemic Central/West African settings. The 2022 global multi-country outbreak, driven substantially by close/sexual contact transmission in a way not typical of prior endemic-region outbreaks, brought mpox to much broader international attention and case counts than any prior episode.

Clinical Features

Clinically similar to smallpox but generally milder, with a case-fatality rate well below smallpox’s historical figures (varying meaningfully by clade — the Central African/clade I strain is more severe than the West African/clade II strain that drove the 2022 outbreak). A key clinical distinguishing feature from both smallpox and varicella is lymphadenopathy, prominent in mpox and characteristically absent in both of the other two — a genuinely useful bedside discriminator. The rash follows a broadly similar synchronous, sequential evolution to smallpox.

Diagnosis and Management

PCR of lesion swab material is the diagnostic method of choice. Management is largely supportive; tecovirimat (an antiviral originally developed against the smallpox-eradication bioterrorism threat) has activity against orthopoxviruses generally and is used for severe or high-risk cases. The existing smallpox (vaccinia-based) vaccine provides meaningful cross-protection against mpox, a direct consequence of the two viruses’ shared genus.

Molluscum Contagiosum

An entirely different clinical entity from the two above — a benign, self-limited, purely cutaneous infection (never systemic, never life-threatening in an immunocompetent host) caused by Molluscipoxvirus, transmitted by direct skin contact (including sexual contact when genital) or fomites.

Clinical Features

Discrete, firm, dome-shaped, flesh-coloured to pearly papules, typically with a characteristic central umbilication (a small central dimple) — a distinctive enough appearance that diagnosis is almost always clinical, without laboratory confirmation. Lesions are usually multiple, and can occur anywhere on the skin; in children, spread is often by simple self-inoculation (scratching) across the body; in sexually active adults, genital/lower-abdominal distribution reflects sexual transmission. In immunocompromised patients (notably advanced HIV), lesions can become unusually large, numerous, and treatment-resistant — a pattern worth recognizing as a possible clue to underlying immunosuppression in an adult with extensive molluscum.

Treatment

Usually self-limited, resolving spontaneously over months in an immunocompetent host, so observation alone is a reasonable default. Where treatment is wanted (cosmetic concern, risk of spread, or genuine discomfort), options include curettage, cryotherapy, or topical agents (e.g. topical cantharidin), chosen more for patient preference and lesion location than any difference in fundamental necessity.