Nail Infection — Symptoms, Causes & Treatment | MyMedicPlus

Nail infections are divided into onychomycosis (fungal nail infection) and paronychia (infection of the nail fold). Onychomycosis accounts for 50% of all nail disorders, affecting 10% of adults and rising to 50% of those over 70. Paronychia may be acute (bacterial, typically Staphylococcus aureus) or chronic (candidal, common in those with wet work exposure). Nail infections encompass two distinct conditions with different pathogens, presentations, and treatments. Onychomycosis (fungal nail infection) is the most common nail disorder, affecting approximately 10% of the adult population globally — rising to 20% in adults over 60 and 50% over 70 — and accounting for 50% of all nail disorders. Toenails are 3-4 times more commonly affected than fingernails, with the hallux (big toe) most frequently involved. Dermatophyte fungi (Trichophyton species) cause 90% of toenail onychomycosis. Paronychia (infection of the nail fold) is distinct: acute paronychia is typically bacterial (Staphylococcus aureus in 60%), while chronic paronychia represents a chronic inflammatory condition of the nail fold associated with Candida species and repeated moisture exposure. Correct differentiation between onychomycosis, trauma, psoriatic nail disease, and lichen planus is essential — nail dystrophy is frequently misdiagnosed as fungal infection when aetiology is non-infectious, leading to unnecessary antifungal treatment.

Onychomycosis: caused by dermatophytes (Trichophyton rubrum in 70%), yeasts (Candida), and non-dermatophyte molds. Risk factors include age, diabetes, peripheral vascular disease, immunosuppression, nail trauma, public swimming pools, and tight footwear. Paronychia: acute form caused by S. aureus after nail trauma; chronic caused by Candida with repeated moisture exposure (dishwashers, healthcare workers). Onychomycosis pathogenesis requires both exposure to the dermatophyte and host susceptibility factors. Dermatophytes (T. rubrum in 70%, T. interdigitale in 20%, T. violaceum) enter through the distal-lateral nail groove, establishing infection in the hyponychium and proliferating within the nail plate. Risk factors: advancing age (cumulative nail trauma, slower nail growth — toenails grow 1-2 mm/month versus 3-4 mm/month for fingernails); diabetes mellitus (impaired peripheral circulation, immune dysfunction, and moisture retention); peripheral arterial disease (reduced antifungal drug delivery to the nail); immunosuppression (HIV, transplant recipients, corticosteroid use); tinea pedis (athlete's foot — acts as a reservoir for nail inoculation; 30% of tinea pedis cases develop nail infection). Occupational exposure: swimming pool workers, athletes, military personnel in shared environments. Trauma (microtrauma from tight footwear) creates entry points. Acute paronychia follows direct trauma (nail biting, manicure, dishwashing), introducing S. aureus or GAS (Group A Streptococcus) into the nail fold. Chronic paronychia is an occupational dermatosis in healthcare workers, food handlers, and cleaners with repeated hand immersion.

Onychomycosis: yellow, white, or brown nail discoloration, nail thickening (onychauxis), subungual debris and hyperkeratosis, nail fragility, and distortion (onychodystrophy). Toe nails more commonly affected than fingernails. Acute paronychia: painful red swelling of the nail fold with abscess formation. Chronic paronychia: persistent painful swelling with nail ridging and discoloration over months. Onychomycosis presents in four clinical patterns: distal-lateral subungual onychomycosis (DLSO — most common; yellowing and thickening beginning at the distal nail edge advancing proximally), proximal subungual onychomycosis (less common; white discoloration at the nail base — more common in immunosuppressed individuals, particularly HIV), superficial white onychomycosis (chalky white patches on the surface of the nail plate — T. interdigitale, Fusarium), and total dystrophic onychomycosis (entire nail affected — represents end-stage of any pattern). Clinical features: yellow, white, or brown discoloration; subungual hyperkeratosis (debris accumulation under the nail); onycholysis (nail plate separation from the nail bed); nail plate thickening (onychauxis) up to 2 mm; and nail fragility with crumbling. Acute paronychia: exquisitely tender, red, swollen nail fold with fluctuant abscess formation — pus visible at the nail margin. Chronic paronychia: swollen, indurated nail fold with loss of the cuticle, nail plate ridging, and Beau's lines (transverse grooves) from repeated nail fold inflammation.

Nail clippings and subungual debris sent for direct microscopy (KOH preparation showing hyphae) and culture identify the causative organism and guide antifungal selection. Culture takes 4-6 weeks. PCR-based testing provides rapid species identification. Dermoscopy identifies characteristic patterns. Nail biopsy with PAS staining is the most sensitive test for onychomycosis confirmation. Nail clipping specimen collection technique: clip the most proximal affected portion of the nail plate (greatest fungal burden) plus any subungual debris; collect from 3-4 nails if multiple affected. Laboratory tests: potassium hydroxide (KOH) direct microscopy (40% KOH preparation) demonstrates fungal hyphae and/or spores — sensitivity 80%, rapid result; fungal culture on Sabouraud's dextrose agar — species identification enabling appropriate antifungal choice (3-6 week turnaround); PCR-based multiplex testing (Onychodiag, NailPro PCR) provides rapid species identification within 24-48 hours with higher sensitivity than culture (90% vs 70%). Dermoscopy (non-polarised dermoscopy of the nail) shows the 'aurora borealis' pattern in onychomycosis — jagged proximal margin with spikes. Nail biopsy with PAS (periodic acid-Schiff) staining — most sensitive test (sensitivity 92%) — identifies hyphae within the nail plate when non-invasive tests are negative despite clinical suspicion.

Onychomycosis: oral terbinafine (250mg daily for 6 weeks for fingernails, 12 weeks for toenails) achieves mycological cure in 70-80% and is superior to oral itraconazole. Topical amorolfine lacquer or ciclopirox lacquer for mild-moderate distal-lateral subungual onychomycosis without matrix involvement. Acute paronychia: incision and drainage of abscess; oral antibiotics (flucloxacillin or trimethoprim-sulfamethoxazole for MRSA). Chronic paronychia: topical antifungals, emollients, and avoiding wet work. Oral terbinafine is first-line for toenail onychomycosis: 250 mg daily for 12 weeks achieves mycological cure in 70-80% and clinical cure in 50-60% of cases. The treatment course targets the nail at the site of infection during the active treatment period, but complete nail clearance requires 9-12 months of nail regrowth after completion. Hepatic function monitoring (LFTs) before treatment and at 4-6 weeks is recommended, particularly in patients with liver disease or on hepatotoxic medications. Itraconazole pulse therapy (200 mg twice daily for 1 week per month for 3 months) is an alternative for dermatophyte-resistant cases or candidal onychomycosis — oral itraconazole has more drug interactions than terbinafine (P450 inhibitor). Topical antifungals (amorolfine 5% nail lacquer twice weekly; ciclopirox 8% nail lacquer daily; tavaborole 5% solution daily; efinaconazole 10% solution daily) are used for mild-moderate DLSO without matrix involvement — cure rates of 15-20% for toenail onychomycosis versus 70-80% for oral terbinafine. Acute paronychia: incision and drainage of the abscess with a pointed scalpel; oral flucloxacillin 500 mg four times daily for 5 days (or clarithromycin if penicillin-allergic; trimethoprim-sulfamethoxazole for MRSA risk).

Untreated onychomycosis causes progressive nail dystrophy, discomfort with footwear, gait abnormalities, and risk of secondary bacterial infections especially in diabetics. Cellulitis of the foot from fungal nail disease is a recognized complication. Recurrence of onychomycosis after treatment occurs in 20-50% within 3 years. Acute paronychia can progress to felon (fingertip abscess) or osteomyelitis if untreated. Untreated onychomycosis causes progressive nail destruction — end-stage total dystrophic onychomycosis results in complete nail plate loss, permanent nail bed damage, and significant pain with footwear use and ambulation. In diabetic patients, nail infection creates an entry point for secondary bacterial infection — cellulitis and necrotising fasciitis can arise from periungual skin breakdown, representing a limb-threatening and potentially life-threatening complication in those with peripheral vascular disease. Recurrence after successful treatment is common: 20-50% of patients redevelop onychomycosis within 3 years, primarily from reinfection from tinea pedis reservoir rather than treatment failure. Acute paronychia progressing untreated causes felon (closed-space infection of the fingertip pulp) — presenting with intense throbbing pain under pressure from accumulating pus; untreated felon can progress to osteomyelitis of the distal phalanx and flexor tendon sheath infection, potentially requiring amputation.

Keep feet clean and dry; change socks daily and dry between toes thoroughly. Wear protective footwear in public pools, changing rooms, and gyms. Wear well-fitting shoes that don't cause nail trauma. Treat tinea pedis (athlete's foot) promptly as it is a reservoir for nail fungal infection. For paronychia prevention: wear gloves during wet work, avoid nail biting and cuticle manipulation. Onychomycosis prevention: treat tinea pedis simultaneously and promptly — athlete's foot is a reservoir for nail inoculation and must be eradicated with topical terbinafine or butenafine cream for 2-4 weeks. Keep feet clean and dry; use antifungal powder in footwear. Wear breathable synthetic-fibre socks (changed daily) and well-fitting footwear avoiding nail microtrauma. Wear waterproof sandals in communal changing rooms, swimming pools, and gymnasiums. Nail instruments (clippers, files) must be sterilised (soaking in 70% isopropyl alcohol for 30 minutes or boiling) between uses. In diabetics: podiatric care with regular professional nail trimming prevents microtrauma. Paronychia prevention: wear waterproof gloves for wet work; avoid nail biting and cuticle manipulation; use moisturising barrier cream to protect the cuticle.

See a GP or dermatologist for: nail fungal infection (onychomycosis) that is spreading to multiple nails, causing significant thickening or separation, or not responding to 12 weeks of regular topical antifungal treatment (amorolfine lacquer, ciclopirox) — systemic oral antifungal therapy (terbinafine 250 mg daily for 6 weeks for fingernails, 12 weeks for toenails) is far more effective than topical alone and requires prescription. Seek prompt medical assessment for: rapidly spreading redness, swelling, warmth, and pain around the nail fold (paronychia) — acute bacterial paronychia requires drainage and antibiotics; spreading cellulitis from nail infection (red streak extending up the finger or toe); or systemic symptoms (fever, red streaking — possible septicaemia from nail infection). People with diabetes should see a podiatrist or GP for any nail infection or foot condition promptly — impaired circulation and immune response means even minor nail infections can progress to serious limb-threatening infections. Confirm the diagnosis before treating — nail dystrophy from psoriasis, trauma, or lichen planus mimics fungal infection and does not respond to antifungals.