From the Guidelines
Smallpox and chickenpox can be differentiated based on their distinct clinical presentations, with smallpox characterized by a centrifugal rash, synchronous lesion development, and a severe prodrome, whereas chickenpox presents with a centripetal rash, asynchronous lesion development, and a mild or absent prodrome. The key differences between smallpox and chickenpox lie in their rash distribution and lesion development. Smallpox typically presents with a rash that is most prominent on the face and extremities, with lesions developing at the same time 1. In contrast, chickenpox has a rash that is most prominent on the trunk, with lesions in various stages of development and resolution 1.
The clinical features of smallpox and chickenpox can be summarized as follows:
- Smallpox:
- Centrifugal rash (more prominent on extremities and face)
- Synchronous lesion development
- Severe prodrome with high fever and prostration for 2-4 days before rash appears
- Deep-seated, firm, and well-circumscribed vesicles that become pustular
- Chickenpox:
- Centripetal rash (concentrated on the trunk)
- Asynchronous lesion development
- Mild or absent prodrome with rash often being the first sign
- Superficial vesicles described as "dewdrops on a rose petal"
It is essential to note that smallpox carries a higher mortality rate (30% in unvaccinated individuals) compared to chickenpox's generally benign course in immunocompetent children 1. These distinctions are crucial for accurate diagnosis and treatment, and are frequently tested concepts in medical board examinations.
From the Research
Smallpox vs Chickenpox Presentation
- Smallpox and chickenpox are two distinct infectious diseases with different clinical presentations 2, 3.
- Chickenpox, caused by the varicella-zoster virus, is characterized by a maculopapular rash that progresses to papules, vesicles, and crusts, accompanied by fever, fatigue, and loss of appetite 4, 5.
- Smallpox, caused by the variola virus, is marked by a more severe clinical course, with high fever, back pain, malaise, and prostration, followed by a maculopapular rash that progresses to papules, vesicles, and pustules 2, 3.
- The mortality rate for smallpox is approximately 30%, whereas chickenpox has a minimal morbidity and mortality in healthy patients 4, 2.
Differential Diagnosis
- The key differential diagnostic characteristics of chickenpox and smallpox include the presence of lesions in different stages in chickenpox, whereas smallpox lesions are all in the same stage and evolve at the same rate 6, 3.
- Physicians' ability to diagnose smallpox is limited, with only 17% reporting comfort in diagnosing the disease 6.
- Diagnostic tests can be useful in differentiating between monkeypox, smallpox, and chickenpox, which are frequently mistaken for one another due to similar manifestations 3.
Prevention and Treatment
- Vaccination is the most effective method for preventing smallpox epidemic progression, with a live vaccinia virus preparation administered by scarification with a bifurcated needle 2.
- Treatment of chickenpox with oral acyclovir appears to decrease the number of pox lesions and shorten the duration of new lesion formation, with children treated with acyclovir beginning to feel better soon and having fewer systemic signs and symptoms 4, 5.