Furuncle

Acute, deep, painful perifollicular abscess (a boil) that arises from a hair follicle infection, almost always S. aureus.

• Typical sites are hair bearing and friction prone areas: neck, face, axillae, buttocks, thighs and groin.
• Risk factors: nasal S. aureus carriage (20 to 30% of adults), diabetes, obesity, atopic dermatitis, iron deficiency, immunosuppression, hyperhidrosis, close contact with an infected person, shared towels or razors and sports contact.
• Strains producing Panton-Valentine leucocidin cause recurrent, severe boils and necrotising pneumonia, and are usually community acquired MRSA or MSSA.
• Furuncles in the nose, upper lip and central face (the danger triangle) can spread through the facial veins to the cavernous sinus.
• Recurrent furunculosis is defined as 3 or more episodes in 6 to 12 months.

• Starts as a tender red nodule, 1 to 2 cm, that enlarges, becomes fluctuant over 3 to 7 days and develops a central yellow pustular head, then ruptures with drainage. Healing occurs in 1 to 3 weeks and may leave a scar.
• Fever and malaise are mild or absent. Surrounding cellulitis, lymphangitis or satellite lesions suggest spread.
• Danger signs: facial furuncle with swelling of the eyelid, headache, proptosis or reduced consciousness (cavernous sinus thrombosis); fever with rigors (bacteraemia); spinal or hip pain in a patient with boils (deep abscess).
• Differential diagnosis: carbuncle, hidradenitis suppurativa, epidermoid cyst with infection, acne cystic lesions, pilonidal abscess, deep fungal or mycobacterial infection, anthrax, tularaemia, myiasis and tungiasis.
• Complications: cellulitis, bacteraemia, osteomyelitis, septic arthritis, endocarditis and, rarely, cavernous sinus thrombosis. Recurrence is the usual problem.

• Clinical. Culture pus from any furuncle that is recurrent, large, in a patient with MRSA risk or treatment failure, or in an immunocompromised host.
• Check fasting glucose or HbA1c. In recurrent disease check full blood count, ferritin, and consider immunoglobulin levels and HIV testing.
• Nasal and perineal swabs identify carriers in recurrent disease.
• Ultrasound shows a drainable collection when clinical fluctuance is uncertain.
• Blood cultures if febrile or systemically unwell.

• Small furuncle without fluctuance: warm moist compresses for 10 to 15 minutes, 3 to 4 times daily, to promote localisation and spontaneous drainage. Do not squeeze, as this can force infection into deeper tissue.
• Fluctuant furuncle: incision and drainage is the primary treatment. Use a small stab incision, break up any loculations, and irrigate. Packing is not required for small cavities. Antibiotics alone do not cure an abscess.
• Antibiotics are not needed for a single furuncle under 5 cm that is fully drained in a well patient. Give them when there is:
  • Fever or systemic signs.
  • Surrounding cellulitis, or lesions on the face, hands, genitals or spine.
  • Multiple lesions, or an incompletely drained abscess.
  • Immunosuppression, diabetes, extremes of age or an indwelling device.
  • Failure to respond to drainage.
• Oral options, for 5 to 7 days:
  • Flucloxacillin 500 mg orally 6 hourly (child 12.5 to 25 mg/kg 6 hourly).
  • Cefalexin 500 mg orally 6 hourly.
  • Doxycycline 100 mg orally 12 hourly (not in pregnancy or under 8 years).
  • Co-trimoxazole 160/800 mg, 1 to 2 tablets orally 12 hourly (child trimethoprim 4 to 6 mg/kg 12 hourly). Avoid at term.
  • Clindamycin 300 to 450 mg orally 8 hourly (child 10 mg/kg 8 hourly).
• Facial danger triangle furuncle, or systemic illness: admit and give flucloxacillin 1 to 2 g IV 6 hourly (or vancomycin 15 to 20 mg/kg IV 8 to 12 hourly if MRSA is likely), and arrange imaging if cavernous sinus thrombosis is suspected.
• Recurrent furunculosis:
  • Obtain a pus culture and nasal swab.
  • Treat predisposing causes: control diabetes, correct iron deficiency, treat eczema and hyperhidrosis.
  • Decolonise: mupirocin 2% to each nostril twice daily for 5 days plus chlorhexidine 4% wash daily for 5 to 14 days, repeated monthly for 3 to 6 months if recurrences continue.
  • Wash bedding, towels and clothes hot, and avoid sharing razors, towels and sports equipment.
  • Treat household contacts when they are colonised.
  • For refractory disease, a 10 to 14 day course of rifampicin 600 mg daily with clindamycin 300 mg 8 hourly is sometimes used, but evidence is limited and rifampicin has many drug interactions.
• Special groups:
  • Pregnancy: use flucloxacillin, cefalexin or clindamycin. Avoid doxycycline.
  • Renal impairment: adjust co-trimoxazole and cefalexin doses.
  • Immunosuppressed and diabetic patients: lower threshold for antibiotics and for review at 48 hours.
• Follow up: review at 48 to 72 hours after drainage. Failure to improve suggests an undrained deeper abscess, MRSA, or an alternative diagnosis. Refer recurrent disease for immunology or dermatology review.