25/12/2021
Skin Infections :
Skin serves as a barrier to fluid loss and protects internal organs against mechanical injury, infections,temperature changes, noxious agents, and trauma.
When the skin’s defenses are altered or destroyed,
bacteria, viruses, fungi, and parasitic organisms can
infect or infest it.
PATHOGENESIS :
Clinical infection results from breaks in the skin (i.e.,abrasions, needle punctures, and catheters), loss of local immunity, and changes in the skin flora. Althoughmore than 100 bacteria are known to cause cellulitis,two gram-positive cocci, Staphylococcus aureus and
group A beta-hemolytic streptococcus, account for themajority of skin and soft tissue infections. S. aureus cancause folliculitis, cellulitis, and furuncles (abscess/boil).
Toxins elaborated by S. aureus can result in bullous impetigo and staphylococcal scalded skin syndrome.
Streptococci are usually secondary invaders of traumaticskin lesions and can cause impetigo, erysipelas, cellulitis, and lymphangitis.
Viruses damage host cells by entering the cell
and replicating at the host’s expense. HSV infections can occur anywhere on the skin and are
caused by two types of the virus: HSV-1 and HSV-
2. HSV-1 is usually seen in oral infections, while
HSV-2 is associated with ge***al infections. HSV
infections have two phases: the primary phase representing infection transmitted by respiratory
droplets or by direct contact with an active lesion
or infected secretions, and the secondary phase representing a reactivation of latent virus from dorsalroot ganglia.
The varicella virus, which causes chickenpox, is a
highly contagious viral infection transmitted by airborne droplets or vesicular fluid. Patients are contagious from 2 days before onset of the rash until alllesions have crusted. Varicella can lay dormant in thedorsal root ganglion for many years. It reactivates andpresents as herpes zoster (shingles) typically involving a painful vesicular rash that only involves a singledermatome.
Warts are benign skin tumors confined to
the epidermis resulting from HPV, which is transferred by touch and commonly occurs at sites oftrauma. Molluscum contagiosum is caused by a
poxvirus and produces an umbilicated skin lesion
that is spread by autoinoculation, scratching, or
touching a lesion.
The dermatophytes, or ringworm fungi, infect and
survive only on dead keratin, namely, the top layer ofthe skin (stratum corneum), the hair, and the nails.
Dermatophyte infections are clinically classified by
body region with varying disease responses. “Tinea”means “fungus infection,” so the term “tinea capitis”refers to a fungal infection of the scalp.
The yeast-like fungus Candida albicans and other
Candida species live normally in the mouth, vaginal
tract, and gut. They may become pathogenic and produce budding spores, pseudohyphae (elongated
cells), or true hyphae. In individuals with altered defenses against yeast (e.g., due to pregnancy, oral contraceptives, antibiotics, diabetes, skin maceration,
topical steroid therapy, and some endocrinopathies),Candida can infect the stratum corneum of mucousmembranes (mouth, anoge***al tract) and warm,
moist intertriginous skin areas (axillae, groin, breastfolds, digit spaces).
Scabies infestation begins when a fertilized female
mite burrows through the stratum corneum to begina 30-day life cycle of egg laying and deposition of f***l matter (scybala). After eggs have hatched, themites can migrate to other areas such as the finger
webs, wrists, extensor surfaces of the elbows and
knees, axillae, breasts, waist, sides of hands and feet,ankles, p***s, buttocks, sc***um, and palms and solesof infants, causing symptoms to intensify.
The diseaseis transmitted by direct skin contact with an infectedpatient.
Three kinds of lice infest humans: Pediculus humanus capitis (head louse), P. humanus corporis (bodylouse), and Phthirus p***s (p***c or crab louse).
Pediculosis capitis is most common in children. Live
nits fluoresce and can be detected by Wood light.
Pediculosis corporis is a disease of poor hygiene,
where the lice live and lay their nits in the seams of
clothing and return to the skin surface only to feed.
Pediculosis p***s is an extremely contagious sexuallytransmitted disease and may involve not only thegroin but also other hairy areas of the body. Eyelashinfestation in a child may be a sign of sexual abuse byan infested adult .
CLINICAL MANIFESTATIONS :
HISTORY :
The onset of the skin lesions and associated symptoms—such as fever, warmth, or pruritus—should be
part of the history. Tenderness, pain, mild paresthesias, or
burning may occur at the site of inoculation with herpesvirus infections. A prodrome of localized pain, tenderlymphadenopathy, headache, generalized aching, and
fever may occur. Shingles may also present prior to theeruption with a prodrome of itching, pain, and burningin the affected dermatome. Associated underlying skinconditions or trauma should be noted. Local trauma orsystemic changes like me**es, fatigue, or fever may trigger a recurrence of herpes simplex infections. Known
contact with cases of scabies, lice, viral, or fungal infection may suggest that transmission has occurred.
Medications and medication allergies may be important in identifying other potential causes for therash and in determining therapy. An attack of chickenpox usually confers lifelong immunity to chickenpox, but a previous varicella infection can reactivateand cause shingles. Unlike chickenpox, an episode ofshingles does not confer lifelong immunity.
PHYSICAL EXAMINATION :
The lesions of impetigo are superficial and are characterized by honey-colored crusts. Erythema, warmth,edema, pain, and sometimes fever characterize cellulitis. Folliculitis is characterized by a pustule in association with a hair follicle. Furuncles are larger fluctuant
erythematous lesions that also occur in associationwith hairy regions. Nikolsky sign aids in the diagnosisof staphylococcal scalded skin syndrome and is elicitedwhen local skin separation occurs after minor pressure.
Herpes simplex appears as grouped vesicles on an
erythematous base and is uniform in size, unlike thevesicles seen in herpes zoster or chickenpox. Thechickenpox rash has a centripetal distribution, startingat the trunk and spreading to the face and extremities.
Lesions appear as a “dewdrop on a rose petal,” with athin-walled vesicle, clear fluid, and a red base; they appear as constellations of lesions in different stages atthe same time. Warts are small tumors of the skin thatobscure normal skin lines, have a mosaic surface pattern, and may have thrombosed vessels appearing as
black dots on the surface. The lesions of molluscumcontagiosum are discrete 2- to 5-mm slightly umbilicated flesh-colored, dome-shaped papules occurringon the face, trunk, axillae, and extremities in childrenand in the p***c and ge***al areas in adults.
Fungal infections are characterized by erythematousas well as hypo- or hyperpigmented lesions associatedwith scaling.
They occur on various parts of the body.
The classic ringworm lesion has a central clear area.
Lice are suspected when a patient itches without
an apparent rash. Lice and nits may be identified onclose visual examination. Scabies are associated withlinear burrows on the distal extremities and occur as
scattered pruritic papules on the rest of the body.
DIFFERENTIAL DIAGNOSIS :
The differential diagnosis for bacterial infections includes other forms of dermatitis, such as eczema andcontact or stasis dermatitis.
Herpesvirus infections—including shingles, chickenpox, and herpes simplex—
may be confused with eczema, impetigo, or contactdermatitis. The lesions of molluscum contagiosum
may mimic warts or herpes simplex. Both warts andmolluscum may be confused with skin tags, dermatofibromas, or nevi. The differential diagnosis forfungal infections includes pityriasis alba, pityriasisrosea, eczema, or in some instances psoriasis or seborrheic dermatitis. Scabies lesions may form vesicles,leading to the consideration of diagnoses such as herpes and contact dermatitis.
DIAGNOSTIC EVALUATION :
Skin infections are commonly diagnosed clinically.
Additional diagnostic measures obtained to assist withdiagnosis include blood cultures, wound cultures, viralcultures of suspicious lesions, and microscopic examination of skin scrapings or suspected organisms .
Blood cultures are usually negative, but
bacteremia can occur with extensive cellulitis. Woundcultures are in general not helpful, though some advocate obtaining “leading edge” cultures by injecting andaspirating from the edge of the infection. More helpful is a sterilely obtained culture from a purulent infection such as an abscess or furuncle. Viral culture isthe most definitive method for diagnosing herpes infections. The diagnosis of fungal infections is made byKOH wet-mount preparations, which allow direct visualization under the microscope of the branching hyphae of dermatophytes in keratinized material.
Culture is necessary for scalp, hair, and nail fungal infections to identify the true source of infection and
determine proper treatment. Mycosel agar, dermatophyte test medium, and Sabouraud dextrose agar arethe most common fungal culture media.
TREATMENT : always consult your doctor
