Differentials
Cellulitis
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
Appropriate time frame; history of previous episodes of cellulitis; ulcer or wound; dermatosis; tinea pedis interdigitalis; lymphedema; venous insufficiency or chronic leg edema.
INVESTIGATIONS
CBC: elevated WBC.
Blood cultures: positive.
Purulent focus culture: positive.
Periorbital and orbital cellulitis
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting or anaphylaxis.
History of previous sinus infection, lack of Haemophilus influenzae type b vaccination; stye or chalazion; recent eye trauma.
INVESTIGATIONS
CBC: elevated WBC.
Blood cultures: positive.
Microbiology eye swab: positive.
Abscess
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
Red, tender, fluctuant mass; fever; purulent drainage.
INVESTIGATIONS
Incision and drainage yields pus/necrotic material. Culture can identify infectious organism.
Necrotizing fasciitis
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting or anaphylaxis.
History of immunosuppression due to chronic illness (e.g., diabetes mellitus, alcoholism); cutaneous trauma or ulcerative skin conditions; varicella zoster infections; hospitalization.
INVESTIGATIONS
CBC: elevated WBC.
BUN and creatinine: elevated.
Sodium: normal or reduced.
Serum creatinine phosphokinase: elevated.
Blood cultures: positive.
ABG analysis: hypoxemia and acidosis.
Folliculitis
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
Recent history of immersion in spa water; darkly pigmented male with curly hair; recent history of shaving; umbilicated flesh-colored papules.
INVESTIGATIONS
Gram stain: gram-positive cocci typical of Staphylococcus aureus infection.
Potassium hydroxide preparation: presence of hyphal forms suggestive of dermatophyte infection.
Basal cell carcinoma
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
History of ultraviolet radiation, sun exposure, x-ray exposure, arsenic exposure, xeroderma pigmentosa, Gorlin-Goltz syndrome, or transplant.
Papules with associated telangiectasias; plaques, nodules, and tumors with rolled borders; small crusts and nonhealing wounds; nonhealing scabs; pearly papules or plaques. Unlike bites and stings, these lesions do not appear acutely.
INVESTIGATIONS
Shave/punch biopsy: growth of nest(s) of varying size and shape.
Squamous cell carcinoma of the skin
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
History of ultraviolet exposure, older age, immunosuppression, fair skin, human papillomavirus, hereditary skin conditions, exposure to ionizing radiation/arsenic/tar, actinic keratosis, male sex. Unlike bites and stings, these lesions do not appear acutely.
Erythematous papules or plaques; thin flesh-colored or erythematous plaques/dome-shaped nodules; exophytic, fungating verrucous nodules or plaques.
INVESTIGATIONS
Biopsy: keratinocyte atypia.
Kaposi sarcoma
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
HIV infection; immunosuppressive therapy; transplantation, Central African ethnicity (e.g., from Uganda, Malawi, Zambia, Zimbabwe); human herpesvirus-8 infection.
Skin lesions may be multifocal, asymmetrically distributed, nonpruritic, varying in size (ranging from several millimeters to centimeters in diameter) and color (pink, red, purple, brown, or blue), papular, nodular, plaque-like, bullous-like, fungating with skin ulceration and secondary infection, indurated (woody), or hyperkeratotic.
INVESTIGATIONS
HIV test: positive.
Biopsy: characteristic vascular lesion.
Local trauma
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
History of trauma.
INVESTIGATIONS
X-ray: may show evidence of trauma.
Intra-abdominal processes (differential of black widow spider bite)
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
Abdominal tenderness with guarding or rebound tenderness. Bowel sounds may be absent.
May have risk factors for mesenteric ischemia, gastroenteritis, bowel obstruction, etc.
INVESTIGATIONS
X-ray: may show dilated bowel loops.
CBC may be elevated in infectious process. Lactate may be elevated in mesenteric ischemia.
Ultrasound/CT/MRI may show pathology.
Compartment syndrome of the abdomen
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
History of excessive fluid resuscitation (>5 L in 24 hours), massive blood transfusion (>10 units in 24 hours), recent abdominal infection (especially peritonitis), hemoperitoneum, ileus, abdominal distention, oliguria.
INVESTIGATIONS
Transbladder measurement of intra-abdominal pressure: elevated.
ABG analysis: metabolic acidosis or mixed metabolic and respiratory acidosis.
Compartment syndrome of extremities
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting.
History of trauma, bleeding disorder, compression support, thermal injury, intravenous infusion, venous obstruction, sports playing. Loss of muscle function, pain, pressure (tightness), paresthesia, pulselessness, pallor, paralysis.
INVESTIGATIONS
Compartment pressure: differential pressure ≤20 mmHg.
Serum creatinine kinase: elevated.
Urine myoglobin: elevated.
Muscle spasm
SIGNS / SYMPTOMS
Could resemble reaction to black widow bite.
Episodic occurrence. History of muscle strain or trauma.
INVESTIGATIONS
Clinical diagnosis.
Acute myocardial infarction
SIGNS / SYMPTOMS
Cardiac risk factors, chest pain, shortness of breath, nausea, diaphoresis, eliciting factors.
INVESTIGATIONS
ECG: ischemic changes.
Cardiac enzymes: positive.
Toxic plant ingestion
SIGNS / SYMPTOMS
Could resemble anaphylaxis or neurologic reaction to black widow spider bite.
History of recent ingestion of plant matter.
INVESTIGATIONS
Clinical diagnosis.
Organophosphate poisoning
SIGNS / SYMPTOMS
Could resemble anaphylaxis or neurologic reaction to black widow spider bite.
History of recent ingestion of insecticides; distinctive odor; incontinence; visual disturbances.
INVESTIGATIONS
Atropine therapeutic trial: lack of anticholinergic effects.
Plasma cholinesterase: reduced activity.
Shock
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
May be cardiogenic, septic, or hypovolemic.
History of recent myocardial infarction, recent surgery or immobilization, severe infection, or hemorrhage.
INVESTIGATIONS
Lactate >18 mg/dL (2 mmol/L) is suggestive of tissue hypoperfusion.
ABG analysis: pH <7.35 indicates acidosis.
Acute asthma
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
History of viral infection, exposure to cigarette smoke, exposure to allergens, atopic eczema, environmental irritants, GERD, use of oral corticosteroids, or noncompliance to asthma medication.
INVESTIGATIONS
Peak expiratory flow rate: <60% of predicted value if severe.
Acute COPD
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
History of bacterial infection; viral infection; exposure to pollutants; change in weather.
INVESTIGATIONS
Chest x-ray: hyperinflation, flattened diaphragms, bullae, and a small vertical heart.
ABG analysis: respiratory acidosis and compensatory metabolic alkalosis.
Foreign body aspiration
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
Sudden-onset stridor or choking; history of foreign body in mouth.
INVESTIGATIONS
Chest x-ray may show air-trapping and hyperexpanded lung field.
Viral syndrome
SIGNS / SYMPTOMS
Could resemble anaphylaxis or neurologic reaction to black widow spider bite.
Fever, chills, myalgias, upper respiratory infection symptoms, gastrointestinal symptoms.
INVESTIGATIONS
Clinical diagnosis.
Drug reaction
SIGNS / SYMPTOMS
Could resemble anaphylaxis.
Usually diffuse rash developing after beginning new medication.
INVESTIGATIONS
Symptom improvement after removal of drug.
Atopy
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting or anaphylaxis.
Recurrent sensitivity reactions following certain exposures.
INVESTIGATIONS
Allergy testing: positive.
Chemical exposure or sensitivity
SIGNS / SYMPTOMS
Could resemble local reaction to bite/sting or anaphylaxis.
History of recent exposure.
INVESTIGATIONS
Clinical diagnosis.
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