Kiss a Toad: Plantar Warts

Warts (also known as verrucae) are skin infections caused by human papillomaviruses (HPV). Different strains of HPV tend to infect specific body sites and cause characteristic lesions at these sites. For example, HPV types 6 and 11 infect the anogenital area and cause condyloma acuminata while HPV type 1 commonly affects the soles of the feet and causes plantar warts. Some strains of HPV have malignant potential, like HPV 6, 11, 16, and 18 which can cause cervical cancer and are protected against by the recently introduced Gardasil vaccine for young women. Plantar warts can be caused by HPV 1, 2, 4, or 63 and are benign. The Gardasil vaccine does not protect against plantar warts.

Plantar warts are very common – around 10 percent of the U.S. population is infected. They are most common in children and young adults. Infection may be transmitted by walking on moist surfaces such as showers, locker rooms or swimming pools or by wearing infected shoes. Warts can spread through ‘autoinoculation’ by infecting nearby skin or by infecting walking surfaces. Once the virus has direct contact with the skin, it can enter though tiny cuts and abrasions in the outermost layer of skin. After infection, the wart may lay dormant and invisible under the skin for several weeks or months.

A plantar wart typically resembles a cauliflower with tiny, black petechiae (or tiny blood vessels resembling specks) in the center. Small amounts of bleeding may occur when these petechiae are scratched. The part of the wart on the surface of the skin is only a small part of the entire wart which might be several times as large as the part that you can see.

The plantar wart may be painful when standing or walking. If allowed to grow without treatment, the wart may grow up to one inch in circumference and may spread into clusters called mosaic warts. In severe cases, the wart may cause a change in posture, leading to back or leg pain.

Your doctor is often able to diagnose a plantar wart simply by its general appearance. If there is any doubt, he or she might use a small scalpel to scrape off a thin layer of hardened dead skin on the top of the lesion. Because of the small petechiae or vessels supplying the wart, it will probably bleed slightly when scraped. On the other hand, a corn or callus is composed of dead skin and is not likely to bleed or reveal evidence of petechiae.

Most plantar warts spontaneously resolve within two years and so many patients will not seek aggressive treatment. On the other hand, warts left untreated might spread and enlarge. The virus is microscopic, so treatment must involve removal of even healthy-looking skin to reduce the risk of recurrence. Treatment may take weeks or months and requires patience and perseverance. Often, treatment will involve multiple applications of a mild acid (e.g., salicylic acid) to the wart over the course of several weeks. This treatment should disintegrate viral cells and allow healthy skin cells to replace them. Other treatments include laser treatments and cryotherapy (e.g., liquid nitrogen). Surgical removal is only used for small, isolated warts.

A controversial approach to plantar wart treatment is the use of duct tape. In a 2002 study, participants covered their warts with duct tape for six days, then soaked their warts and gently rubbed them with a pumice stone. This process was repeated for up to two months until the warts went away. This treatment is often combined with the application of salicylic acid.

The best approach to plantar warts is prevention. Avoid walking barefoot, particularly in areas likely to spread warts – e.g., swimming pools and locker rooms. Change shoes and socks every day. Keep feet clean and dry. Avoid direct contact with warts on other people or on other parts of your own body. Do not ignore growths on your skin.

Dr. McMahon, a graduate of Yale University and the University of Pittsburgh School of Medicine, is board certified in Internal Medicine. She is a member of the American College of Physicians and the Allegheny and Pennsylvania Medical Societies. She is in solo practice in the North Hills.