Bumps on Forearm & Shoulder: Spider Bite? Differential Diagnosis & When to Seek Care

Spider bites can cause localized reactions, but many "spider bites" are actually other conditions (MRSA abscess, insect bites, contact dermatitis, folliculitis, herpes, shingles, lymphoma).

Typical Spider Bite Signs

• Two tiny puncture marks (often not visible).
• Pain, swelling, redness, warmth.
• Possible central blanching/necrosis (brown recluse) or target lesion (Lyme-like).
• Systemic (rare): fever, chills, nausea, muscle cramps, hemolysis (black widow).

Differential for Bilateral Forearm/Shoulder Bumps

• Insect bites (mosquito, flea, bed bug, chigger) – often multiple, itchy.
• Contact dermatitis – Poison ivy, nickel, detergent; linear/geometric, vesicular.
• Folliculitis – Pustular, hair-centered; bacterial/fungal.
• Viral exanthem – Herpes zoster (dermatomal, painful), herpes simplex, molluscum.
• Drug eruption – New medication.
• Autoimmune/granulomatous – Sarcoidosis, granuloma annulare.
• Lymphoma/leukemia cutis – Rare; firm, non-tender nodules.

When to Seek Care Immediately

• Spreading redness, heat, red streaks (cellulitis).
• Fever >100.4°F (38°C).
• Severe pain disproportionate to appearance.
• Bullae, necrosis, black eschar.
• Immunocompromised / diabetic.

Diagnosis & Treatment

• Clinical exam ± culture, biopsy, dermoscopy.
• Antibiotics if bacterial.
• Antihistamines/topical steroids if inflammatory.
• Antivenom (black widow) – rare, hospital-based.

Photograph lesions daily. See a dermatologist or primary care provider for accurate diagnosis.