Skin Pathology: Infections Flashcards
1
Q
What are the three way in which infectious agents can affect the skin? Which is the most common? Give examples of each.
A
- 1) direct breaching of the skin, allowing infection from the outside (most common): impetigo, erysipelas, folliculitis
- 2) skin manifestations of systemic infections via hematogenous spread or direct extension: rose spots of enteric fever, petechial rash of meningococcal disease
- 3) toxin-mediated skin damage due to microbial toxins produced at another site: scarlet fever, scalded skin syndrome, toxic shock syndrome
2
Q
What are the major pathogens involved in direct invasion of the skin? What infection does each cause? Which are the most common causes of skin infections?
A
- dermatophyte fungi: ringworm infection (tinea)
- Propionibacterium acnes: acne
- Staph aureus: impetigo, cellulitis, folliculitis (boils), necrotizing fasciitis
- Strep pyogenes: impetigo, erysipelas, cellulitis, necrotizing fasciitis
- anaerobic bacteria: necrotizing fasciitis
- Clostridium perfringes: myonecrosis, gas gangrene
- S. aureus and dermatophyte fungi are the most common causes
3
Q
What is impetigo? Which patients are commonly affected? What do the lesions classically look like?
A
- impetigo is a highly contagious spreading infection of the superficial epidermis
- it is caused by Staph aureus and Strep pyogenes (group A beta-hemolytic Strep)
- it commonly occurs in children, and usually affects the face
- lesions are erythematous pustules that contain a honey-colored serum that becomes dry and crusty
- (note that bullous impetigo is only caused by Staph aureus)
4
Q
What is erysipelas?
A
- erysipelas is similar to impetigo, but is a slightly deeper infection, occurring in the dermis; fever is commonly present
- classically involves the face (butterfly rash) and ears
- essentially caused only by Strep pyogenes (and also Staph aureus to a much lesser degree)
- (erysipelas is not as deep as cellulitis)
5
Q
What is cellulitis? How do patients present? What can this disease progress to?
A
- cellulitis is a deep infection of the subcutaneous fat in the dermis
- caused by Strep pyogenes and Staph aureus, usually following trauma (some event is usually needed for the organism to penetrate this deeply)
- the affected area (major location is lower limbs) becomes red, tender, and swollen; the rash/lesions are less demarcated than what is seen in the more superficial infections (impetigo and erysipelas); patients will also have fever
- cellulitis can progress into a necrotizing fasciitis, which is a medical emergency
6
Q
What is scalded skin syndrome?
A
- scalded skin syndrome is a result of Staph aureus’ exfoliatin exotoxin (S. aureus’ other exotoxin causes toxic shock syndrome)
- it results in the sloughing off of skin, an erythematous rash, and fever
- the toxin separates the epidermis, which is very similar to what happens in toxic epidermal necrosis (TEN), but the separation here occurs more superficially, and is generally less severe than TEN
7
Q
What can follow a skin infection with Strep pyogenes? What is the mechanism behind this? How do patients present?
A
- acute post-streptococcal glomerulonephritis can follow a streptococcal skin infection (can also follow strep throat, but less so)
- this is due to the deposition of immune complexes in the glomerulus (it is an antibody mediated delayed disease caused by Strep pyogenes, rheumatic fever is another delayed disease, but it only follows strep throat not skin infections)
- patients present with a puffy face (because of fluid retention) and tea/coke colored urine (hematuria)
8
Q
What is toxic shock syndrome?
A
- TSS is a result of Staph aureus’ TSST-1 exotoxin (S. aureus’ other exotoxin causes scalded skin syndrome)
- the toxin is extremely pyrogenic, stimulating the release of the endogenous mediators of sepsis (TNF and IL-1) to cause septic shock (high fever, nausea, vomiting, diarrhea, diffuse rash)
9
Q
What do the exotoxins of Strep pyogenes cause?
A
- these exotoxins are pyrogenic and cause scarlet fever and a streptococcal toxic shock syndrome (which is more mild than Staph aureus’)
- scarlet fever: scarlet-red rash develops on the neck and trunk before spreading out; fever is also present
10
Q
What is necrotizing fasciitis?
A
- necrotizing fasciitis is an extremely acute and highly toxic infection of the fascia that results in widespread necrosis
- it has a very high mortality rate and is a medical emergency
- caused by Staph aureus, Strep pyogenes, and anaerobic bacteria (such as Pseudomonas aeruginosa)
11
Q
What is myonecrosis and gas gangrene?
A
- myonecrosis and gas gangrene are rapid necrotic infections caused by Clostridium perfringens; the anaerobic organism infects muscle and causes local necrosis (its lecithinase toxin destroys lipid cellular membranes, facilitating the rapid spread)
- gas is generated by the organism’s metabolism and can often be felt as crepitus in the affected tissues (gas gangrene)
12
Q
What is acne vulgaris? Explain the progression from comedone to pustule to nodule.
A
- acne vulgaris is an extremely common infection of a plugged hair follicle and its surrounding sebaceous glands
- the follicle gets plugged with keratin (this is a comedone AKA whiteheads and blackheads)
- the plugged follicle fills with sebum and provides an excellent area for Propionibacterium acnes to colonize; the bacteria act on the sebum to produce fatty acids and peptides, which trigger inflammation (infected comedones are pustules AKA pimples)
- if the pustule scars when healing it becomes a nodule
13
Q
What is tinea? What are the three groups of this organism? What do these organisms need to survive?
A
- tinea is a cutaneous infection of skin, hair, and/or nails with the dermatophyte fungi (Trichophyton, Epidermophyton, Microsporum)
- common sites: foot (athlete’s foot), groin (jock itch), scalp, etc.
- tinea is also known as ringworm
- these fungi require keratin in order to survive
14
Q
Where would cutaneous infections with the Candida fungi most likely occur?
A
- cutaneous candidiasis occurs in areas of moisture (because the organism needs moisture to survive)
- mouth, vagina, perianal area, axilla, underneath breasts, in skin folds, etc.
- cutaneous candidiasis usually develops more rapidly than tinea does
15
Q
Which major viruses can cause mucocutaneous lesions and do not spread systemically? Which can spread systemically?
A
- not systemic: papilloma (warts; via HPV), molluscum contagiosum (via poxvirus)
- systemic: HSV, VZV (varicella-zoster virus), coxsackievirus, measles, rubella
