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; lymphedoema; venous insufficiency or chronic leg oedema.
INVESTIGATIONS
FBC: raised 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
FBC: raised 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.
Necrotising 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; hospitalisation.
INVESTIGATIONS
FBC: raised WBC.
Urea and creatinine: raised.
Sodium: normal or reduced.
Serum creatinine phosphokinase: raised.
Blood cultures: positive.
ABG analysis: hypoxaemia 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-coloured 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 tumours with rolled borders; small crusts and non-healing wounds; non-healing 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 ionising radiation/arsenic/tar, actinic keratosis, male sex. Unlike bites and stings, these lesions do not appear acutely.
Erythematous papules or plaques; thin flesh-coloured or erythematous plaques/dome-shaped nodules; exophytic, fungating verrucous nodules or plaques.
INVESTIGATIONS
Biopsy: keratinocyte atypia.
Kaposi's 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, non-pruritic, varying in size (ranging from several millimetres to centimetres in diameter) and colour (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 ischaemia, gastroenteritis, bowel obstruction, etc.
INVESTIGATIONS
X-ray: may show dilated bowel loops.
FBC may be raised in infectious process. Lactate may be raised in mesenteric ischaemia.
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), haemoperitoneum, ileus, abdominal distention, oliguria.
INVESTIGATIONS
Trans-bladder measurement of intra-abdominal pressure: raised.
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), paraesthesia, pulselessness, pallor, paralysis.
INVESTIGATIONS
Compartment pressure: differential pressure ≤20 mmHg.
Serum creatinine kinase: raised.
Urine myoglobin: raised.
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: ischaemic changes.
Cardiac enzymes: positive.
Toxic plant ingestion
SIGNS / SYMPTOMS
Could resemble anaphylaxis or neurological reaction to black widow spider bite.
History of recent ingestion of plant matter.
INVESTIGATIONS
Clinical diagnosis.
Organophosphate poisoning
SIGNS / SYMPTOMS
Could resemble anaphylaxis or neurological reaction to black widow spider bite.
History of recent ingestion of insecticides; distinctive odour; 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 hypovolaemic.
History of recent myocardial infarction, recent surgery or immobilisation, severe infection, or haemorrhage.
INVESTIGATIONS
Lactate >2 mmol/L (>18 mg/dL) 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, GORD, use of oral corticosteroids, or non-compliance 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: hyper-inflation, flattened diaphragm, 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 hyper-expanded lung field.
Viral syndrome
SIGNS / SYMPTOMS
Could resemble anaphylaxis or neurological 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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