Mpox and Smallpox
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Direct answer
Mpox is a zoonotic infection caused by an orthopoxvirus, and its skin lesions evolve synchronously, so every lesion passes through the same stage at the same time. The rash moves from macule to papule, vesicle, pustule and crust, with lesions often deep, firm and umbilicated. Prominent lymphadenopathy and centrifugal spread to the face, palms and soles distinguish it from varicella, while smallpox, caused by variola virus, was eradicated in 1980.
What you must remember
- Causative agents: mpox is caused by an orthopoxvirus, while smallpox was caused by variola virus, which is now held only in two WHO-authorised repositories.
- Synchronous evolution: all lesions progress together through macule, papule, vesicle, pustule and crust, unlike the asynchronous crops of varicella.
- Distribution: lesions are most dense on the face, palms, soles and genitals, with a centrifugal pattern in contrast to the centripetal spread of varicella.
- Lesion depth: mpox lesions are often deep-seated, firm and umbilicated, whereas varicella vesicles are superficial and dew-drop-like.
- Lymphadenopathy: cervical and inguinal nodes are prominent in mpox and are a key point of difference from varicella.
- Diagnosis: PCR for orthopoxvirus on fluid from a vesicle or pustule, or on a crust, is the main laboratory test.
- Smallpox eradication: the World Health Organization declared smallpox eradicated in 1980, and routine smallpox vaccination was then discontinued.
- Public health control: patients are isolated until lesions have crusted and re-epithelialised, and close contacts are traced.
Comparing poxvirus-like eruptions
Mpox, smallpox and varicella all produce vesicular rashes, so examiners test whether the student can separate them by timing, distribution and depth. The table sets out the features that distinguish them.
| Feature | Mpox | Smallpox | Varicella |
|---|---|---|---|
| Lesion timing | Synchronous | Synchronous | Asynchronous crops |
| Distribution | Face, palms, soles and genitals | Face and extremities, centrifugal | Trunk first, centripetal |
| Lesion depth | Deep, firm, often umbilicated | Deep, firm | Superficial vesicles |
| Lymph nodes | Prominent | Usually not prominent | Not a feature |
| Prodrome | Fever and nodes, often before the rash | High fever, headache and back pain for two to four days | Mild or absent fever before the rash |
Mpox is the clinically important look-alike of smallpox, so examiners test whether the student separates them by lymph nodes and epidemiology. Smallpox no longer occurs naturally, so a smallpox-like rash today is a public-health alert, while mpox is the disease clinicians must manage. A quick sequence of questions helps at the bedside: when the rash began, whether lesions share one stage, whether nodes are enlarged, and what exposures occurred.
Where the exam plants its traps
Stems on poxvirus rashes test lesion timing, the lymph node sign and the correct public-health response.
- Trap: calling mpox chickenpox because both are vesicular Correct: mpox lesions are synchronous, deep and often umbilicated with prominent lymphadenopathy, whereas varicella is asynchronous and superficial.
- Trap: managing mpox lesions without isolation Correct: isolation until the crusts have fallen and the skin has re-epithelialised, with contact tracing, is essential.
- Trap: believing smallpox vaccination is still offered routinely Correct: smallpox was eradicated and routine vaccination was discontinued, so mpox vaccines are a separate, targeted public-health tool.
Frequently asked questions
How do mpox lesions differ from varicella lesions?
Mpox lesions evolve synchronously, so all lesions are at the same stage, whereas varicella lesions appear in asynchronous crops. Mpox lesions are also deeper and firmer, and they are accompanied by prominent lymphadenopathy, while varicella vesicles are superficial.
Why is lymphadenopathy important in mpox?
Prominent lymphadenopathy, particularly cervical and inguinal, is a characteristic feature of mpox and helps distinguish it from varicella. Its presence should prompt testing for orthopoxvirus in a patient with a suggestive rash.
When was smallpox declared eradicated?
Smallpox was declared eradicated by the World Health Organization in 1980, after a global vaccination campaign. Variola virus is now held only in two WHO-authorised repositories, so a smallpox-like rash is treated as a public-health alert.
How is mpox diagnosed in the laboratory?
Mpox is diagnosed by PCR for orthopoxvirus on fluid from a vesicle or pustule, or on a crust, which is the main laboratory test. Varicella-zoster PCR is used when the differential needs to be excluded, because the two rashes can look similar.
What public health measures are used for mpox?
Patients are isolated until lesions have crusted and re-epithelialised, and close contacts are traced and offered vaccination when they are at high risk. Antiviral therapy is reserved for severe disease and is used under specific protocols.