Skin infections

Question 1

Summarise the epidemiology of organisms that commonly cause skin infections

Answer 1

• Skin disease is common – 15% of GP appointments are skin-related (25% due to skin infection).
  
  • 6% of the hospital outpatient attendances are skin-related
  • 5% of dermatologist appointments are due to skin infections.
• Skin conditions are more common in hot, humid climates amongst poorer populations (opposite in dry/rich).

Question 2

Recall the biology and main clinical features of common skin infections and infestations, including Staphylococcus aureus

Answer 2

• Features of SA:
  
  • A gram+ bacteria, commensal in approx. 30% of humans with colonisation in the nose, axilla and groin.
  • SA is the most common bacterial cause of skin infections including – impetigo (and bullous (blistering) impetigo), folliculitis, ecthyma, boils and carbuncles.

Diseases caused – via the production of toxins, SA can cause:

• Staphylococcal Scalded Skin Syndrome – Exfoliative toxin.
• Toxic shock Syndrome – Toxic Shock Syndrome Toxin 1 (TSST-1).
• Food poisoning – Enterotoxin.
• Necrotizing soft tissue infections – Panton-Valentine Leucocidin virulence factor (toxin causing blisters).
• Treatment – infections all treated with antibiotics, there is no vaccine.
• Diagnosed: by swob (gram +, staph, and resistance)

Manifestations:

1. Impetigo: top layer of epidermis (golden appearance)
   
   • can produce exfoliating toxin causing spitting of the skin - bullous/ blister formation
2. Ecthyma: infection of the epidermis
   
   • thick adherent scar formation
3. Folliculitis - abscess
4. Staphylococcal scalded skin syndrome - treated in hospital with AB

Question 3

Treponema pallidum

Answer 3

Features of TP:

• A gram- spirochaete and the cause of Syphilis (which is usually an STI).
• 12 million new cases per year worldwide

Stages of Syphilis:

1. Primary – painless ulcers (chancre) at site of inoculation. (3-8 weeks after initial inoculation)
2. Secondary – disseminated and rapid proliferation of infection with rash (maculopapular) and lymphadenopathy.
3. Latent – asymptomatic period (it can still be transmitted and reactivate as tertiary)
4. Tertiary – skin (gummatous skin lesions, bone lesions), neurological and vascular manifestations, gummatous skin lesion, thoracic aneurysm

Neurosyphilis: one of the primary causes of dementia

• Syphilis can be VERTICALLY transmitted and cause congenital syphilis (paced through the placenta)
  
  • Linked to miscarriage, stillbirth, prematurity, rashes, brain and neurological problems, bone disease
• No vaccine is available but the infection can be treated with antibiotics.

Diagnosis: Dark-field microscopy, swob, serology

Question 4

Herpes Simplex Viruses 1 & 2

Answer 4

• Features of HSV:
  
  • Members of the human herpes virus (HHV) family which are DNA viruses.
  • Type 1 – causes oral infections.
  • Type 2 – causes genital infections.
  • Type 3 - Varicella-zoster- chicken pox

Pathogenesis:

• Transmission by direct contact.
• Clinical features involve – painful vesicular rash (heals over 2-4 weeks), eczema herpeticum, herpes encephalitis
• It resides in nerves
• Due to latency in sensory involvement, there can be reactivation with reoccurrences of infection.
• No vaccine is available, outbreaks treated with anti-viral medication such as acyclovir.

Question 5

Varicella Zoster Virus

Answer 5

Features of VZV:

• Another human herpes virus as seen above.
• Pathogenesis:
  
  • Primary infection – causes chicken pox – prodrome of fever and malaise followed by development of widespread vesicular rash. Usually lasts ~2 weeks and then becomes latent. Can reactivate in shingles.
• Herpes Zoster or Shingles where there is reactivation of the VZV and a painful vesicular rash appears along the course of a dermatome – usually heals in 2-4 weeks.
  
  • There can be serious consequences if CN V1 (ophthalmic division of trigeminal).
• malaise, fever, sore throught, rash
• Vaccine is available and anti-viral medication can be given.

Question 6

Trichophytum rubrum

Answer 6

Dermarophytes – e.g. Trichophytum rubrum

• Features of Trichophytum:
  
  • A common cause of superficial fungal infections.
  • It is a dermatophyte – a type of fungus that particularly affects parts of the body that have keratin.
  • The names of the clinical infections are prefixed with the Latin “tinea” followed by the body part:
    
    • Tinea capitis – Kerion is a type of tinea capitis.
    • Tinea manuum – dorsum of hand.
    • Tinea cruris - scrotum
    • Tinea facei -
    • Tinea pedis

Yeasts are another form of fungal infection distinct from dermatophytes – e.g. candida (grow in warm/wet places).

• Pathogenesis:
  
  • Clinical manifestations – eryhthromatous scaly rash on skin/scalp, discoloured or crumbly nails.
• Treated with topical or systemic anti-fungal medications such as Terbinafine.

Question 7

Sarcoptes scabei - scabies

Answer 7

• Features:
  
  • A skin infestation by the mite Sarcoptes scabei.
  • Scabei burrow within the epidermis - 4 mm long s shaped, one end is black dot the head of the mite
• Pathogenesis:
  
  • The mite burrows into the surface of the skin and exposure to the mite faeces and eggs cause a delayed-type allergic reaction resulting in widespread eczematous rash occurring ~4 weeks after first infestation.
  • Usually very itchy.
  • The burrow sites are usually at – genital regions, nipples, wrists, finger webs, instep of feet, axilla.
  • Secondary bacterial infection is common.
• Transmission is by skin-skin contact.
• Treatment is with topical systemic insecticides.