Module 4: Skin Infections

Learning Objectives

By the end of this module, students should be able to:

1. Describe the pathophysiology of common bacterial, viral, fungal, and parasitic skin infections.

2. Identify key clinical features and differentiate between types of skin infections.

3. Apply appropriate diagnostic methods for each condition.

4. Formulate evidence-based management plans, including pharmacological and non-pharmacological strategies.

5. Recognize complications and indications for referral or escalation of care.

6. Educate patients on prevention, hygiene, and treatment adherence.

Bacterial Infections

1. Impetigo

• Pathophysiology:

  • Superficial epidermal infection.

  • Caused by Staphylococcus aureus (bullous) or Streptococcus pyogenes (non-bullous).

  • Spread via direct contact or fomites.

• Diagnosis:

  • Clinical: honey-coloured crusts, especially around the nose and mouth.

  • Swab for culture if resistant or recurrent.

Fig. 4.1: Impetigo

1. “Kissing lesions” on both sides of the axilla in impetigo
2. Impetigo on the leg

Image sourced from DermNet.

• Management:

  • Topical mupirocin or fusidic acid (mild).

  • Oral antibiotics (e.g., cloxacillin, cephalexin) for extensive or systemic involvement.

  • Hygiene education; avoid school/daycare until non-infectious.

Fig. 4.2: Impetigo

2. Cellulitis

• Pathophysiology:

  • Infection of the dermis and subcutaneous tissue.

  • Common organisms: Streptococcus pyogenes, S. aureus.

  • Entry via a skin breach (e.g., trauma, tinea pedis).

• Diagnosis:

  • Clinical: erythema, warmth, swelling, tenderness, fever.

  • Blood cultures if febrile or immunocompromised.

• Management:

  • Oral antibiotics (e.g., amoxicillin-clavulanate).

  • IV antibiotics for severe/systemic cases.

  • Elevation, rest, and treat the underlying cause (e.g., tinea).

Viral Infections

3. Herpes Simplex (HSV-1/2)

• Pathophysiology:

  • Primary infection followed by latency in sensory ganglia.

  • Reactivation triggered by stress, illness, UV light.

• Diagnosis:

  • Clinical: grouped vesicles on an erythematous base, painful.

  • Tzanck smear (multinucleated giant cells); PCR for confirmation.

Fig. 4.3: Multinucleated giant cell (Tzanck smear)

Fig. 4.4: (i) Herpes labialis and (ii) herpes genitalis

Image sourced from DermNet.

• Management:

  • Oral acyclovir, valacyclovir (early initiation reduces duration).

  • Topical antivirals for mild cases.

  • Education on recurrence and transmission.

4. Varicella (Chickenpox)

• Pathophysiology:

  • Primary infection with varicella-zoster virus (VZV).

  • Spread via respiratory droplets or direct contact.

• Diagnosis:

  • Clinical: crops of vesicles at different stages, pruritic.

  • PCR or serology if atypical.

• Management:

  • Supportive: antihistamines, calamine lotion, paracetamol.

  • Acyclovir for high-risk groups (e.g., immunocompromised, adults).

  • Isolation until lesions have crusted.

Fungal Infections

5. Tinea (Dermatophytosis)

• Pathophysiology:

  • Infection of keratinized tissue by dermatophytes (Trichophyton, Microsporum).

  • Spread via direct contact, fomites, animals.

• Diagnosis:

  • KOH prep: hyphae visible.

  • Fungal culture for confirmation.

Fig. 4.5: Tinea (ringworm on a human leg)

Fig. 4.6: Tinea corporis

Fig. 4.7: Tinea capitis

Image sourced from DermNet.

• Management:

  • Topical antifungals (clotrimazole, terbinafine) for skin.

  • Oral antifungals (griseofulvin, terbinafine) for scalp/nails.

  • Treat contacts and decontaminate shared items.

6. Candidiasis

• Pathophysiology:

  • Overgrowth of Candida albicans, especially in moist areas or the immunosuppressed.

• Diagnosis:

  • Clinical: erythematous patches with satellite lesions.

  • KOH prep or culture.

Fig. 4.8: Candidal vulvovaginitis

Image sourced from DermNet.

• Management:

  • Topical nystatin or azoles (e.g., clotrimazole).

  • Oral fluconazole for systemic or recurrent cases.

  • Address predisposing factors (e.g., diabetes, antibiotics).

Parasitic Infection

7. Scabies

• Pathophysiology:

  • Infestation by the Sarcoptes scabiei mite.

  • Hypersensitivity reaction to mite proteins.

• Diagnosis:

  • Clinical: intense nocturnal pruritus; burrows in web spaces, axillae, and genitals.

  • Skin scraping with microscopy to confirm mites/eggs.

Fig. 4.9: Interdigital scaling in the first web space of the fingers

Image sourced from DermNet.

• Management:

  • Topical permethrin 5% cream (apply overnight, repeat in 1 week).

  • Oral ivermectin for crusted or resistant cases.

  • Treat all close contacts simultaneously.

  • Wash bedding and clothing in hot water.

Essential Learning Notes

Table 4.1: Essential Learning Notes — Skin Infections

Category	Condition	Pathophysiology	Clinical Features	Diagnosis	Treatment
Bacterial	Impetigo	Superficial infection of the epidermis, commonly by Staphylococcus aureus or Streptococcus pyogenes	Honey-coloured crusts, often on face and limbs; non-bullous (more common) or bullous types	Clinical; Gram stain/culture if needed	Topical mupirocin (mild); oral antibiotics (e.g., cloxacillin, cephalexin) for extensive cases
	Cellulitis	Infection of dermis and subcutaneous tissue, usually by Streptococcus pyogenes or S. aureus	Erythema, warmth, swelling, tenderness; may have fever, lymphangitis	Clinical; blood cultures if febrile/systemically ill	Oral/IV antibiotics (e.g., amoxicillin-clavulanate, clindamycin); elevation and rest
Viral	Herpes Simplex	Reactivation of latent HSV-1 or HSV-2 in sensory ganglia	Grouped vesicles on erythematous base; painful; may recur	Tzanck smear (multinucleated giant cells), PCR, serology	Oral acyclovir, valacyclovir; topical antivirals for mild cases
	Varicella (Chickenpox)	Primary infection with varicella-zoster virus (VZV)	Crops of vesicles on erythematous base (“dew drop on a rose petal”); pruritic; systemic symptoms	Clinical; PCR or serology if uncertain	Supportive care; acyclovir for high-risk or immunocompromised patients
Fungal	Tinea (Dermatophytosis)	Infection by dermatophytes (Trichophyton, Microsporum, Epidermophyton) affecting keratinized tissue	Annular, scaly plaques with central clearing; varies by site (e.g., tinea corporis, pedis, capitis)	KOH prep, fungal culture	Topical antifungals (e.g., clotrimazole); oral terbinafine or griseofulvin for scalp/nail involvement
	Candidiasis	Overgrowth of Candida albicans, especially in moist areas or immunocompromised hosts	Erythematous patches with satellite lesions; oral thrush, intertrigo, genital involvement	KOH prep, culture	Topical nystatin or azoles; oral fluconazole for systemic or recurrent cases
Parasitic	Scabies	Infestation by Sarcoptes scabiei mite; hypersensitivity reaction to mite proteins	Intense pruritus (worse at night), burrows, papules in web spaces, wrists, axillae, genitals	Clinical; skin scraping with microscopy	Topical permethrin 5%; oral ivermectin for severe cases; treat close contacts and decontaminate linens

Key Learning Points

• Recognize the pattern of lesion distribution and evolution (e.g., vesicles in HSV vs. crusts in impetigo).

• Understand host factors (e.g., immunosuppression, hygiene, exposure) that influence infection risk.

• Emphasize infection control: hand hygiene, contact precautions, and treatment of contacts (e.g., scabies).

• Know when to escalate care: systemic symptoms, immunocompromised host, treatment failure.

Multiple Choice Questions

Tap an answer to check yourself — the correct option and an explanation appear once you choose.

Q1 A 6-year-old boy presents with honey-coloured crusted lesions around his mouth. He is otherwise well. What is the most appropriate first-line treatment?

• A Oral flucloxacillin
• B Topical mupirocin
• C Oral acyclovir
• D Topical hydrocortisone

Q2 Which of the following features would most likely indicate a need for hospital admission in a patient with cellulitis?

• A Mild erythema and warmth
• B No systemic symptoms
• C Rapid progression with fever and hypotension
• D Localized itching only

Q3 Which of the following is the most appropriate diagnostic test for confirming herpes simplex infection?

• A KOH preparation
• B Tzanck smear
• C Gram stain
• D Wood’s lamp examination

Q4 A 22-year-old athlete presents with an annular, scaly lesion with central clearing on his thigh. What is the most likely diagnosis?

• A Psoriasis
• B Tinea corporis
• C Eczema
• D Candidiasis

Q5 Which of the following is the most appropriate management for scabies?

• A Topical clotrimazole
• B Oral fluconazole
• C Topical permethrin 5%
• D Oral acyclovir

OSCE Stations

OSCE 1: Impetigo Counselling

OSCE

Scenario: You are a medical student in a GP clinic. A mother brings her 5-year-old child with crusted lesions around the nose. The GP has diagnosed impetigo.

Task: Counsel the mother on the nature of the condition, the treatment plan, infection control at home and school, and when to return for follow-up.

Self-assess against checklist

Tick each point you covered, then check your score.

• Introduces self and confirms patient/carer identity; uses clear, simple language appropriate for the parent.
• Explains that impetigo is a superficial bacterial infection of the outer layer of the skin (epidermis), most commonly caused by Staphylococcus aureus (which causes the bullous type) or Streptococcus pyogenes (which causes the more common non-bullous type).
• Describes the characteristic appearance: honey-coloured crusted lesions, most frequently around the nose and mouth; explains the two forms — non-bullous (crusted) and bullous (fluid-filled blisters).
• Explains how impetigo spreads: through direct contact with the lesions or via contaminated objects (fomites) such as towels, flannels, and clothing.
• Outlines the treatment plan: for mild, localized disease, topical mupirocin or fusidic acid applied directly to the lesions is first-line; for extensive involvement or systemic symptoms, oral antibiotics (e.g., cloxacillin or cephalexin) are required.
• Explains that a swab for culture should be taken if the infection is resistant to initial treatment or if it is recurrent, to guide antibiotic choice.
• Advises the child must stay away from school or daycare until the lesions are non-infectious — typically when crusts have dried and healed, or after 48 hours of antibiotic treatment.
• Advises strict handwashing for the child and all household members, especially before and after touching the lesions or applying treatment.
• Advises against sharing towels, flannels, clothing, or any items that come into contact with the lesions.
• Explains that siblings or close household contacts should be monitored for new lesions and assessed promptly if they develop similar symptoms.
• Advises to return if lesions are spreading, not improving after treatment, or if the child develops fever or systemic symptoms — which may indicate need for oral antibiotics.

OSCE 2: Scabies Diagnosis and Management

OSCE

Scenario: A 19-year-old hostel student presents with intense itching, especially at night. Examination reveals burrows and papules in the web spaces of the fingers.

Task: Take a focused history, explain the diagnosis, and outline treatment and prevention for the patient and contacts.

Self-assess against checklist

Tick each point you covered, then check your score.

• Introduces self and confirms patient identity; reassures in a non-stigmatizing manner.
• Takes a focused history: onset and character of itch (specifically asking whether it is worse at night — nocturnal pruritus is the hallmark), distribution of lesions, duration of symptoms.
• Asks about close contacts with similar symptoms: roommates, hostel residents, household members, or sexual contacts.
• Examines (or describes examination findings): characteristic burrows and papules in the web spaces of the fingers, wrists, axillae, and genital area.
• Explains the diagnosis: scabies is an infestation by the Sarcoptes scabiei mite, which burrows into the skin; the intense itching is caused by a hypersensitivity reaction to mite proteins, eggs, and faeces.
• Explains that the itch is typically worse at night because mite activity increases with warmth under bedclothes.
• Explains how scabies spreads: through prolonged direct skin-to-skin contact; living in close quarters such as hostels increases transmission risk.
• Explains that a skin scraping with microscopy can confirm the diagnosis by identifying mites or eggs, if clinical diagnosis is uncertain.
• Explains the treatment: topical permethrin 5% cream is first-line — applied from the neck down to the entire body surface, left on overnight, and repeated after one week.
• Explains that oral ivermectin is used for crusted (Norwegian) scabies or cases resistant to topical treatment.
• Emphasises that all close contacts must be treated simultaneously, even if they are not yet symptomatic, to prevent re-infestation.
• Advises that all bedding, clothing, and towels used in the previous 72 hours must be washed in hot water and thoroughly dried.
• Warns that itching may persist for several weeks after successful treatment due to the ongoing hypersensitivity reaction, and that this does not necessarily mean treatment failure.
• Advises to return if symptoms persist beyond 2–4 weeks despite correct treatment, suggesting possible re-exposure or treatment failure.

Tutorial & Group-Work Prompts

1. Case vignette: A 35-year-old diabetic woman presents with a red, warm, swollen leg and fever. Prompt: What is your differential diagnosis? What investigations and treatment would you initiate?

2. Visual diagnosis: Show images of tinea corporis, herpes simplex, and impetigo. Prompt: Identify each condition and justify your diagnosis based on lesion morphology.

3. Role play: One student plays a patient with varicella, another the doctor. Prompt: Practise explaining the diagnosis, home care, and when to seek medical attention.