While chickenpox and mpox share some symptomatic similarities, it’s critical for emergency room physicians to quickly differentiate between the two to ensure proper patient care and public health safety. Here’s a breakdown of the key distinctions, focusing on the practical aspects:

Incubation periods

• Mpox: WHO describes symptom onset within 1–21 days of exposure, usually within a week; CDC lists 3–17 days. These overlapping estimates cannot reliably distinguish mpox from varicella.
• Chickenpox: symptoms usually begin 14–16 days after exposure, with a 10–21-day range. This is the incubation period, not the rash duration. CDC overview.

Lymph nodes and diagnostic limits

• Mpox: lymphadenopathy supports suspicion, but an absent prodrome or a localized rash does not exclude it. CDC clinical features.
• Chickenpox: lymph node findings alone cannot safely separate the two infections. Interpret them with the exposure history, skin examination and lesion testing.

Lesion patterns

• Mpox: lesions are often firm, deep and painful, sometimes with central umbilication. A single genital, perianal or oral lesion, or prominent proctitis, may be the presenting problem; palms and soles need not be involved. Lesions often evolve together within one body area, but lesion stage or distribution alone is not a diagnostic rule. CDC clinical features.
• Chickenpox: successive crops of itchy vesicles commonly produce mixed-stage lesions, often concentrated on the trunk. After vaccination, breakthrough disease may have few lesions, little fever and a mainly maculopapular rash. These are clues, not confirmation. CDC clinical features.

Transmission

• Mpox: close skin or mucosal contact, including sexual or household contact, contaminated materials and infectious respiratory particles during close contact can transmit infection. Ask about actual contacts without assuming risk from identity. WHO transmission guidance.
• Chickenpox: VZV spreads through contact and infectious aerosols. Transmission begins 1–2 days before the rash; breakthrough cases remain contagious. CDC overview.

Exposure and immunity

• Mpox: assess recent close contacts and travel in the context of current local alerts. Anyone with a relevant exposure can be affected. Other infections, including herpes, syphilis and varicella, may coexist; finding one does not automatically exclude another. WHO diagnostic guidance.
• Chickenpox: ask about prior disease, documented vaccination and household or healthcare exposure. Vaccination reduces risk but does not exclude breakthrough infection. CDC clinical features.

Severity and high-risk patients

• Mpox: severe immunocompromise, especially advanced HIV, and conditions disrupting skin integrity increase concern for uncontrolled disease. Children and pregnant or breastfeeding patients also warrant early specialist assessment. CDC clinical care.
• Chickenpox: infants, adolescents, adults, pregnant patients and immunocompromised patients have greater complication risk. Watch for bacterial skin infection, pneumonia and neurologic disease. CDC clinical features.

Emergency assessment and precautions

• Isolate before testing. Under US CDC guidance, suspected mpox requires a single room, gown, gloves, eye protection and an N95-or-higher respirator; use an airborne-infection isolation room for intubation, extubation or procedures likely to spread oral secretions. Varicella requires standard, airborne and contact precautions. If both remain plausible, retain varicella-level airborne precautions while infection control advises. Mpox precautions; varicella precautions.
• Examine beyond the visible rash. Assess vital signs, hydration, pain, immune status and medications. Ask sensitively about oral, anorectal, genital, urinary and eye symptoms, then examine relevant sites with consent. Offer HIV testing and appropriate testing for other sexually transmitted infections when evaluating mpox. WHO diagnostic guidance.
• Sample safely. For MPXV PCR, confirm the receiving laboratory’s requirements, then vigorously swab lesion surfaces with synthetic swabs. CDC recommends two swabs per lesion from 2–3 lesions when available. Do not unroof or aspirate suspected mpox lesions or use sharps. CDC sampling instructions. Request VZV PCR if varicella remains possible; scabs, vesicular material or appropriately sampled maculopapular lesions can be useful. Coordinate collection when both infections are possible rather than applying a VZV unroofing technique to suspected mpox. CDC VZV testing; VZV collection methods.
• Treat the patient while results are pending. Provide analgesia, hydration and wound care. Escalate suspected mpox with eye involvement, organ dysfunction, severe mucosal disease or immunocompromise to infectious-disease and relevant specialty teams promptly. Do not let a small rash obscure severe illness. CDC clinical care.
• Coordinate follow-up. Contact infection control and the responsible public-health service for reporting, contact assessment and time-sensitive post-exposure prevention under local guidance. Specify who will review results and contact the patient.

Before discharge, check that pain is controlled, oral intake is adequate and the patient can follow the isolation plan. Give explicit return instructions for breathing difficulty, worsening systemic illness, eye symptoms or inability to drink or pass urine.

Complications and treatment context

• Escalation and disposition: hypoxia, confusion, sepsis, inability to drink, urinary obstruction, uncontrolled pain or ocular symptoms require urgent reassessment and consideration of admission. These are practical escalation triggers, not a validated admission score. Mpox complications; varicella complications.
• Varicella treatment: oral acyclovir or valacyclovir may be appropriate for selected higher-risk patients, with greatest benefit when started within 24 hours of rash onset; routine treatment is not recommended for otherwise healthy children with uncomplicated disease. Severe or disseminated varicella and varicella in immunocompromised patients require IV acyclovir assessment. Pneumonia in pregnancy needs urgent specialist care and IV therapy. Use a current age-, weight- and renal-function-adjusted protocol. CDC treatment guidance.
• Mpox antiviral evidence: PALM007 and STOMP did not show faster lesion resolution with tecovirimat in their randomized populations. STOMP’s randomized group excluded severe mpox and severe immunocompromise, so benefit in these patients remains unresolved. Tecovirimat is not routine therapy for uncomplicated disease; US access for selected eligible patients remains investigational through CDC’s expanded-access protocol. Seek specialist advice early for severe or high-risk cases. CDC evidence and access criteria.
• Jurisdiction matters: the European Commission’s 29 May 2026 decision removed mpox from Tecovirimat SIGA’s authorised indications in the EU. Do not export US access arrangements to another country; check the treating jurisdiction’s current requirements. EMA regulatory review.

Explain when isolation ends: for mpox, all lesions must heal, scabs separate and intact new skin form; for varicella, lesions must crust, or, with non-crusting breakthrough lesions, no new lesions appear for 24 hours. Arrange result review and follow-up rather than relying on the patient to interpret the rash. CDC mpox guidance; CDC varicella guidance.