Infections of the Skin
- Nonbullous: yellowish crusted plaques.
- Bullous: bullae, with minimal surrounding erythema, rupture to leave a shallow ulcer.
Impetigo is a bacterial infection of the skin. More than 70% of cases are of the nonbullous variety.
Most cases of nonbullous impetigo are caused by Staphylococcus aureus. Group A β-hemolytic streptococci are found in some cases. Coagulase-positive S aureus is the cause of bullous impetigo. Methicillin-resistant S aureus (MRSA) has been isolated from patients with bullous impetigo. MRSA should be considered when selecting antibiotics for treatment of this infection. Impetigo can develop in traumatized skin, or the bacteria can spread to intact skin from its reservoir in the nose.
Nonbullous impetigo usually starts as a small vesicle or pustule, followed by the classic small (<2 cm) honey-colored crusted plaque. The infection may be spread to other parts of the body by fingers or clothing. There is usually little surrounding erythema, itching occurs occasionally, and pain is usually absent. Regionally lymphadenopathy is seen in most patients. Without treatment, the lesions resolve without scarring in 2 weeks.
Bullous impetigo is usually seen in infants and young children. Lesions begin on intact skin on almost any part of the body. Flaccid, thin-roofed vesicles develop, which rupture to form shallow ulcers.
Nonbullous impetigo is unique in appearance. Bullous impetigo is similar in appearance to pemphigus and bullous pemphigoid. Growth of staphylococci from fluid in a bulla confirms the diagnosis.
Cellulitis follows about 10% of cases of nonbullous impetigo but rarely follows bullous impetigo. Either type may rarely lead to septicemia, septic arthritis, or osteomyelitis. Scarlet fever and post-streptococcal glomerulonephritis, but not rheumatic fever, may follow streptococcal impetigo.
Localized disease may be treated with mupirocin ointment. Patients with widespread lesions or evidence of cellulitis should be treated with systemic antibiotics effective against staphylococci and streptococci. If infection with MRSA is a possibility, intravenous vancomycin is the preferred drug for hospitalized patients. Trimethoprim-sulfamethoxazole is almost always effective against community-acquired MRSA and may be considered a preferred agent where S aureus is a likely causative agent.
Fungal infections of the skin and skin structures may be generally grouped into three categories: dermatophyte infections, other tinea infections, and candidal infections.
Dermatophytoses are caused by a group of related fungal species—primarily Microsporum, Trichophyton, and Epidermophyton species—that require keratin for growth and can invade hair, nails, and the stratum corneum of the skin. Some of these organisms are spread from person to person, some from animals to people, and some infect people from the soil. Other fungi can also cause skin disease, such as Malassezia furfur in tinea versicolor. Finally, Candida albicans, a common resident of the gastrointestinal tract, can cause diaper dermatitis and thrush.
Infection of the skin produces one or more characteristic gradually spreading lesions with an erythematous raised border and central areas that are generally scaly but relatively clearer and less indurated than the margins of the lesions. The central clearing helps to differentiate these lesions from those of psoriasis. Small lesions may resemble those of nummular eczema. The lesions may have a somewhat serpiginous border, but they are usually more or less round in shape, hence the common name of “ringworm.” They can range in size from one to several centimeters.
Fungal infection of the scalp and hair is the most common dermatophytosis in children. This presents as areas of alopecia with more or less regular borders. Typically, the hair shafts break off a few millimeters from the skin surface, distinguishing this from alopecia areata. The infection may also produce a sterile inflammatory mass in the scalp, called a kerion, which may be confused with a bacterial infection.
Tinea versicolor is normally seen in adolescents and adults. The causative organism, M furfur, is part of the normal skin flora. The infection most often becomes evident during warm weather, when new lesions develop. A warm, humid environment, excessive sweating, and genetic susceptibility are important factors for developing this infection. Because treatment does not eradicate the fungus from the skin, it often recurs annually, during the summer months, in susceptible individuals. The lesions are characteristically scaly macules, usually reddish brown in light-skinned people but often hyper- or hypopigmented in people of color. They can be found almost anywhere on the body but are seen most commonly on the torso. The lesions are rarely pruritic. The individual lesions may enlarge and coalesce to form larger lesions with irregular borders.
Thrush is a common oral infection in infants. Isolated incidents of this disease are common in immunocompetent infants, but recurrent infections in infants or infections in children and adolescents may indicate an underlying immune deficiency. The infection presents as thick white plaques on the tongue and buccal mucosa. These can be scraped off only with difficulty, revealing an erythematous base.
This infection is most common in infants from 2 to 4 months of age. Candida is a common colonist of the gastrointestinal tract, and infants with diaper dermatitis should be examined for signs of thrush. The fungus does not ordinarily invade the skin, but the warm, humid environment of the diaper area provides an ideal medium for growth. The infection is characterized by an intensely erythematous plaque with a sharply demarcated border. Advancing from the border are numerous satellite papules, which enlarge and coalesce to enlarge the affected area.
Dermatophyte and other tinea infections are usually diagnosed clinically. Examination of potassium hydroxide preparations of scrapings from the affected area, which show hyphae, confirms the diagnosis. Fungal cultures may be helpful when the diagnosis is suspected but cannot otherwise be confirmed. Diagnosis of candidal infections is generally made by clinical findings.
Tinea corporis is treated with topical antifungal medications. Nystatin, miconazole, clotrimazole, ketoconazole, and terbinafine creams are all effective. Rarely, widespread infection requires systemic therapy.
Topical therapy is ineffective in tinea capitis. The gold standard for treatment has long been griseofulvin, but because of the 6-week duration of therapy required, other treatments of shorter duration are becoming more popular. Fluconazole, itraconazole, and terbinafine can be given for 2 weeks, with an additional week of treatment if the response is incomplete. Ketoconazole is not recommended due to rare incidents of hepatotoxicity.
Tinea versicolor can be treated with topical selenium sulfide lotion or any of the previously listed topical creams. In older children, systemic treatment can also be given, either with ketoconazole or itraconazole for 5 days.
Candidal infections are most often treated with nystatin. Diaper dermatitis responds well to topical nystatin cream. If intense inflammation is present, topical steroids for a few days may be helpful. Thrush is usually treated with nystatin suspension. Up to 2 weeks may be needed for complete resolution of the infection. In resistant cases, the mouth may be painted with gentian violet.
All these infections in immunocompetent children respond well to treatment. However, left untreated, they can cause widespread and significant skin disease.
Parasitic Infestations
- Intense pruritus.
- Small erythematous papules.
- Burrows are pathognomonic but may not be seen.
Scabies is a common infestation caused by the mite Sarcoptes scabei. The disease is acquired by physical contact with an infected person. Transmission of the disease by contact with infested linens or clothing is less common, because the mites can only live off the body for 2-3 days. The female mite burrows between the superficial and deeper layers of the epidermis, laying eggs and depositing feces as she goes along. After 4-5 weeks, her egg laying is complete, and she dies in the burrow. The eggs hatch, releasing larvae which move to the skin surface, molt into nymphs, mature to adults, mate, and begin the cycle again. Pruritus is caused by an allergic reaction to mite antigens.
Diagnosis is based primarily on clinical suspicion, as physical findings are highly variable and the disease can mimic a wide variety of skin conditions. The classic early symptom is intense pruritus. The usual finding is 1- to 2-mm erythematous papules, often in a linear pattern. The finding of burrows connecting the papules is diagnostic but is not always seen. In infants, the disease may involve the entire body—including the face, scalp, palms, and soles—and pustules and vesicles are common. In older children and adolescents, the lesions are most often seen in the interdigital spaces, wrist flexors, umbilicus, groin, and genitalia. Severe infestation may produce widespread crusted lesions.
Potassium hydroxide preparations of skin scrapings may show entire mites, eggs, or fecal pellets. However, success in finding these is limited and a negative examination does not rule out the disease.
Permethrin cream, applied to the entire body (excluding the face in older children) is the preferred treatment. Treatment will kill mites and eliminate the risk of contagion within 24 hours. However, pruritus may continue for several days to 2 weeks after treatment. The entire family should be treated at the same time, and all clothing and bedding should be washed. Many reports also demonstrate the effectiveness of oral ivermectin (200 mcg/kg, given once, then repeated in1-2 weeks), although it is not approved by the FDA for this indication. When used to treat scabies, it has not been conclusively shown to have any serious adverse effects. Although no definite toxicity has been shown, data on safety and effectiveness are lacking in children less than 5 years of age or less than 15 kg and in pregnant women. It is recommended that ivermectin not be given to these patients.
Lice (Pediculosis)
Three varieties of lice cause human disease. Pediculus humanus corporis causes infestations on the body, and Pediculus humanus capitis causes infestation on the head. Phthirus pubis, or crab lice, infests the pubic area. All are spread by physical contact, either with an infested person or with clothing, towels, or hairbrushes that have been in recent contact with an infested person. Symptoms are caused by an allergic reaction to louse antigens that develop after a period of sensitization. Body lice can be a vector for other disease, such as typhus, trench fever, and relapsing fever. Infestation with pubic lice is highly correlated with infection by other sexually transmitted diseases. Nits are the eggs of the louse. They are cemented to hairs, are usually less than 1 mm in length, and are translucent. Body lice lay their nits in the seams of clothing. The nits can remain viable for up to 1 month and will hatch when exposed to body heat when the clothing is worn again.
Body lice are associated primarily with poor hygiene and can be prevented by regular bathing and washing of clothing and bedding. There are no specific measures for prevention of infestation by other types of lice.
The cardinal symptom of louse infestation is pruritus, which develops as the person becomes sensitized. Excoriations in the infested area are common. The lice themselves can usually be seen easily. Head and pubic lice are easily seen, but body lice are only present on the body when feeding.
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