Smallpox differential diagnosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]

Overview

Prior to its eradication, smallpox would need to be differentiated from other diseases that cause a vesicular rash and a fever including chickenpox (which was often mistaken for smallpox), herpes zoster and erythema multiforme.

Differentiating Smallpox from other Diseases

Different rash-like conditions may be misdiagnosed with smallpox, particularly during the initial maculopapular phase, including:

Disease	Findings
Insect bites	In an insect bite, the insect injects formic acid, which can cause an immediate skin reaction often resulting in a rash and swelling in the injured area, often with formation of vesicles
Kawasaki disease	Commonly presents with high and persistent fever, red mucous membranes in mouth, "strawberry tongue", swollen lymph nodes and skin rash in early disease, with peeling off of the skin of the hands, feet and genital area
Monkeypox	Presentation is similar to smallpox, although it is often a milder form, with fever, headache, myalgia, back pain, swollen lymph nodes, a general feeling of discomfort, and exhaustion. Within 1 to 3 days (sometimes longer) after the appearance of fever, the patient develops a papular rash, often first on the face. The lesions usually develop through several stages before crusting and falling off.
Rubella	Commonly presents with a facial rash which then spreads to the trunk and limbs, fading after 3 days, low grade fever, swollen glands, joint pains, headache and conjunctivitis. The rash disappears after a few days with no staining or peeling of the skin. Forchheimer's sign occurs in 20% of cases, and is characterized by small, red papules on the area of the soft palate
Coxsackievirus	The most commonly caused disease is the Coxsackie A disease, presenting as hand, foot and mouth disease. It may be asymptomatic or cause mild symptoms, or it may produce fever and painful blisters in the mouth (herpangina), on the palms and fingers of the hand, or on the soles of the feet. There can also be blisters in the throat or above the tonsils. Adults can also be affected. The rash, which can appear several days after high temperature and painful sore throat, can be itchy and painful, especially on the hands/fingers and bottom of feet.
Acne	Typical of teenagers, usually appears on the face and upper neck, but the chest, back and shoulders may have acne as well. The upper arms can also have acne, but lesions found there are often keratosis pilaris, not acne. The typical acne lesions are comedones and inflammatory papules, pustules, and nodules. Some of the large nodules were previously called "cysts"
Syphilis	Commonly presents with gneralized systemic symptoms such as malaise, fatigue, headache and fever. Skin eruptions may be subtle and asymptomatic Classically described as 1) non-pruritic bilateral symmetrical mucocutaneous rash; 2) non-tender regional lymphadenopathy; 3) condylomata lata; and 4) patchy alopecia
Molluscum contagiosum	Lesions are commonly flesh-colored, dome-shaped, and pearly in appearance. They are often 1-5 millimeters in diameter, with a dimpled center. Generally not painful, but they may itch or become irritated. Picking or scratching the lesions may lead to further infection or scarring. In about 10% of the cases, eczema develops around the lesions. They may occasionally be complicated by secondary bacterial infections
Measles	Commonly presents with high fever, coryza and conjunctivitis, with observation of oral mucosal lesions (Koplik's spots), followed by widespread skin rash
Atypical measles	Symptoms commonly begin about 7-14 days after infection and present as fever, cough, coryza and conjunctivitis. Observation of Koplik's spots is also a characteristic finding in measles
Mononucleosis	Common symptoms include low-grade fever without chills, sore throat, white patches on tonsils and back of the throat, muscle weakness and sometime extreme fatigue, tender lymphadenopathy, petechial hemorrhage and skin rash
Erythema toxicum	Common rash in infants, with clustered and vesicular appearance
Rat-bite fever	Commonly presents with fever, chills, open sore at the site of the bite and rash, which may show red or purple plaques.
Parvovirus B19	The rash of fifth disease is typically described as "slapped cheeks," with erythema across the cheeks and sparing the nasolabial folds, forehead, and mouth
Cytomegalovirus	Common symptoms include sore throat, swollen lymph nodes, fever, headache, fatigue, weakness, muscle pain and loss of appetite
Scarlet fever	Commonly includes fever, punctate red macules on the hard and soft palate and uvula (Forchheimer's spots), bright red tongue with a "strawberry" appearance, sore throat and headache and lymphadenopathy
Rocky Mountain spotted fever	Symptoms may include maculopapular rash, petechial rash, abdominal pain and joint pain
Stevens-Johnson syndrome	Symptoms may include fever, sore throat and fatigue. Commonly presents ulcers and other lesions in the mucous membranes, almost always in the mouth and lips but also in the genital and anal regions. Those in the mouth are usually extremely painful and reduce the patient's ability to eat or drink. Conjunctivitis of the eyes occurs in about 30% of children. A rash of round lesions about an inch across, may arise on the face, trunk, arms and legs, and soles of the feet, but usually not on the scalp.
Varicella-zoster virus	Commonly starts as a painful rash on one side of the face or body. The rash forms blisters that typically scab over in 7-10 days and clears up within 2-4 weeks.
Chickenpox	Commonly starts with conjunctival and catarrhal symptoms and then characteristic spots appearing in two or three waves, mainly on the body and head, rather than the hands, becoming itchy raw pox (small open sores which heal mostly without scarring). Touching the fluid from a chickenpox blister can also spread the disease.
Meningococcemia	Commonly presents with rash, petechiae, headache, confusion, and stiff neck, high fever, mental status changes, nausea and vomiting.
Rickettsialpox	First symptom is commonly a bump formed by a mite-bite, eventually resulting in a black, crusty scab. Many of the symptoms are flu-like including fever, chills, weakness and muscle pain but the most distinctive symptom is the rash that breaks out, spanning the person's entire body.
Meningitis	Commonly presents with headache, nuchal rigidity, fever, petechiae and altered mental status.
Impetigo	Commonly presents with pimple-like lesions surrounded by erythematous skin. Lesions are pustules, filled with pus, which then break down over 4-6 days and form a thick crust. It's often associated with insect bites, cuts, and other forms of trauma to the skin