Warts are benign proliferations of skin and mucosa caused by the human papillomavirus (HPV). Currently, more than 100 types of HPV have been identified. Certain HPV types tend to infect skin at particular anatomic sites; however, warts of any HPV type may occur at any site. The primary clinical manifestations of HPV infection include common warts, genital warts, flat warts, and deep palmoplantar warts (myrmecia). Less common manifestations of HPV infection include focal epithelial hyperplasia (Heck disease),  epidermodysplasia verruciformis, and plantar cysts. Warts are transmitted by direct or indirect contact, and predisposing factors include disruption to the normal epithelial barrier.
Treatment is difficult, with frequent failures and recurrences. Many warts, however, resolve spontaneously within a few years even without treatment.
A small number of high-risk HPV subtypes are associated with the development of malignancies, including types 6, 11, 16, 18, 31, and 35. Malignant transformation most commonly is seen in patients with genital warts and in immunocompromised patients. HPV types 5, 8, 20, and 47 have oncogenic potential in patients with epidermodysplasia verruciformis.
Warts can affect any area on the skin and mucous membranes. The HPV virus infects the epithelium, and systemic dissemination of the virus does not occur. Viral replication occurs in differentiated epithelial cells in the upper level of the epidermis; however, viral particles can be found in the basal layer.
Warts are caused by HPV, which is a double-stranded, circular, supercoiled DNA virus enclosed in an icosahedral capsid and comprising 72 capsomers. More than 100 types of HPV have been identified. Note the following wart types and HPV types:
Common warts – HPV types 2 and 4 (most common), followed by types 1, 3, 27, 29, and 57
Deep palmoplantar warts (myrmecia) – HPV type 1 (most common), followed by types 2, 3, 4, 27, 29, and 57
Flat warts – HPV types 3, 10, and 28
Butcher’s warts – HPV type 7 (although some data suggest the association may be weak)
Focal epithelial hyperplasia (Heck disease) – HPV types 13 and 32
Cystic warts – HPV type 60
Warts are widespread in the worldwide population. Although the frequency is unknown, warts are estimated to affect approximately 7-12% of the population. In school-aged children, the prevalence is 10-20%. An increased frequency also is seen among immunosuppressed patients and meat handlers.
Although warts may affect any race, common warts appear approximately twice as frequently in whites as in blacks or Asians.  Focal epithelial hyperplasia (Heck disease) is more prevalent among American Indians and Inuit. 
Male-to-female ratio approaches 1:1.
Warts can occur at any age. They are unusual in infancy and early childhood, increase in incidence among school-aged children, and peak at 12-16 years. 
Approximately 65% of warts disappear spontaneously within 2 years. When warts resolve on their own, no scarring is seen. However, scarring can occur as a result of different treatment methods. Growth of periungual or subungual warts may result in permanent nail dystrophy.
Treatment failures and wart recurrences are common, more so among immunocompromised patients. Normal appearing perilesional skin may harbor HPV, which helps explain recurrences.
Common warts are usually asymptomatic, but they may cause cosmetic disfigurement or tenderness. Plantar warts can be painful, and extensive involvement on the sole of the foot may impair ambulation. Malignant change in nongenital warts is rare but has been reported and is termed verrucous carcinoma. [4, 5, 6] Verrucous carcinoma is considered to be a slow-growing, locally invasive, well-differentiated squamous cell carcinoma that may be easily mistaken for a common wart. It can occur anywhere on the skin but is most common on the plantar surfaces. Although this type of cancer rarely metastasizes, it can be locally destructive.
Alert patients to the risk factors for transmission of warts. These include trauma or maceration of the skin, frequent wet work involving hands, hyperhidrosis of feet, swimming pools, and nail biting. Butchers and slaughterhouse workers also are at increased risk for developing warts.
Alert patients that some warts may require multiple treatments and may be resistant to several treatment modalities. In addition, some warts may regress spontaneously without treatment.
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Philip D Shenefelt, MD, MS Professor, Department of Dermatology and Cutaneous Surgery, University of South Florida College of Medicine; Past Chief, Section of Dermatology, James A Haley Veteran Affairs Medical Center
Philip D Shenefelt, MD, MS is a member of the following medical societies: American Academy of Dermatology, Florida Medical Association, Noah Worcester Dermatological Society, Society for Clinical and Experimental Hypnosis, American Contact Dermatitis Society, American Association for Physician Leadership, American Medical Association, American Society of Clinical Hypnosis
Disclosure: Nothing to disclose.
Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference
Disclosure: Received salary from Medscape for employment. for: Medscape.
Lester F Libow, MD Dermatopathologist, South Texas Dermatopathology Laboratory
Disclosure: Nothing to disclose.
William D James, MD Paul R Gross Professor of Dermatology, Vice-Chairman, Residency Program Director, Department of Dermatology, University of Pennsylvania School of Medicine
Disclosure: Received income in an amount equal to or greater than $250 from: Elsevier; WebMD.
Joshua A Zeichner, MD Assistant Professor, Director of Cosmetic and Clinical Research, Mount Sinai School of Medicine; Chief of Dermatology, Institute for Family Health at North General
Disclosure: Received consulting fee from Valeant for consulting; Received grant/research funds from Medicis for other; Received consulting fee from Galderma for consulting; Received consulting fee from Promius for consulting; Received consulting fee from Pharmaderm for consulting; Received consulting fee from Onset for consulting.
The authors and editors of Medscape Reference gratefully acknowledge the contributions of previous author, Margaret H. Rinker, MD, to the development and writing of this article.
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