Papillomaviruses are a diverse group of DNA-based viruses that infect the skin and mucous membranes of humans and a variety of animals. More than 100 different human papillomavirus (HPV) types have been characterized.
Introduction
Some HPV types cause benign skin warts, or papillomas, for which the virus family is named. HPVs associated with the development of common warts are transmitted environmentally or by casual skin-to-skin contact.
A separate group of HPVs are typically transmitted through sexual contact. Some sexually-transmitted HPVs, such as types 6 and 11, can cause genital warts. However, most HPV types that infect the genitals tend not to cause noticeable symptoms. Persistent infection with a subset of about a dozen sexually-transmitted HPVs, including types 16 and 18, can lead to the development of cancer of the cervix. HPV infection is a necessary factor in the development of nearly all cases of cervical cancer (Walboomers 1999). Some cancer-causing HPV types (particularly HPV-16) are also believed to cause a substantial fraction of other genital cancers, as well as some cancers of the mouth, throat and anus (Parkin 2006).
Genital HPV infection is very common, with estimates suggesting that up to 75% of women will become infected with one or more sexually-transmitted HPV types at some point during adulthood (Baseman 2005). Although condoms are highly effective for preventing the transmission of other sexually-transmitted diseases (STDs), recent studies have concluded that condoms only offer limited protection against the transmission of genital HPVs (Holmes 2004). This may be due to the fact that HPVs can infect genital skin areas that are not covered by condoms.
Cervical Pap smear testing is used to detect HPV-induced cellular abnormalities. This allows surgical removal of pre-cancerous lesions prior the development of invasive cervical cancer. In the absence of Pap testing or treatment, about 1% of women with genital HPV infections will eventually go on to develop cervical cancer. Although the widespread use of Pap testing has reduced the incidence and lethality of cervical cancer in developed countries, the disease still kills several hundred thousand women per year worldwide. It is hoped that a recently-developed HPV vaccine that blocks initial infection with several of the most common sexually-transmitted HPVs will lead to further decreases in the incidence of HPV-induced cancer (Lowy and Schiller 2006).
Effects
Common skin warts
Some strains of HPV, called cutaneous strains, (e.g., HPV 1) cause common skin warts. They are most commonly found on the hands and the feet. These strains of HPV do not cause genital warts.
- Plantar warts: Plantar warts are found on the soles of the feet.
- Subungual or periungual warts: Warts forming under the fingernail (subungual) and around the fingernail or on the cuticle (periungual) are a subtype of the common skin wart. They may be more difficult to cure than warts in other locations.
- Flat warts: Flat warts are most commonly found on the face or forehead, and are most common in children and teens.
Genital warts
Genital warts (condylomata acuminata or venereal warts) are the most easily recognised sign of genital HPV infection. Of the multiple strains of genital HPV, strains 6, 11, 30, 42, 43, 44, 45, 51, 52, and 54 can cause genital warts (Gearhart et al, 2004). Most people who acquire those strains never develop warts or any other symptoms.
It is important to note that the strains of HPV that can cause genital warts are NOT the same ones that can cause cervical cancer. These strains are called low-risk types.
Cancer
 HPV-induced cancers
Cervical cancer
A history of HPV infection is believed to be a prerequisite for the development of cervical cancer; according to the American Cancer Society, women with no history of the virus do not develop this type of cancer. However, most HPV infections do not progress to cervical cancer. Because the process of transforming normal cervical cells into cancerous ones is slow, cancer occurs in people who have been infected with HPV for a long time, usually over a decade or more.(Greenblatt, 2005; Sinal and Woods, 2005).
High-risk HPV types 16 and 18 are together responsible for over 70% of cervical cancer cases (Baseman and Koutsky, 2005; Cohen, 2005). Type 16 causes 41 to 54% of cervical cancers (Noel et al., 2001; Baseman and Koutsky, 2005) and is also the usual cause of VIN (Edwards et al., 2005) and oropharyngeal cancers (Bolt et al., 2005).
If a woman has cervical intraepithelial neoplasia, or abnormal (possibly precancerous) cervical cells, a Pap smear can detect them. It is particularly important for women who have abnormal cervical cells to undergo colposcopy so that precancerous and cancerous lesions can be detected and treated early.
Throat cancers and tumors
HPV can also cause laryngeal papillomatosis, or warts in the respiratory tract (Wu et al., 2003; Sinal and Woods, 2005). These warts often recur frequently, may require repetitive surgery (Moore et al., 1999; Sinal and Woods, 2005), may interfere with breathing, and in rare cases can progress to cancer (Moore et al., 1999). HPV types 30 and 40 cause laryngeal carcinoma.
Head and neck cancers
Recent scientific evidence, as mentioned by Paul Ewald in his book Plague Time, has suggested that human papillomavirus infections of the head and neck areas, as a result of oral sex, may lead to head and neck cancers.
Prevention
Viral sites may be distributed widely over the pelvic area, and transmission can occur even when there are no visible symptoms; thus, the only sure way to prevent genital HPV infection is to abstain from any contact with the genitals of another.
The American Cancer Society advises that condoms offer only limited protection against HPV. Both Planned Parenthood and the Centers for Disease Control recommend condom use to reduce the risk of HPV, and the latter advises that condom use has been associated with a lower rate of cervical cancer. Since the virus infects the skin and is transmitted by skin-to-skin contact, covering even part of the skin (as with a condom) reduces the opportunity for infection, as the area of contact would be reduced for both partners.
Vaccine
At least two pharmaceutical companies are currently testing vaccines against the human papillomavirus (HPV), in hopes of preventing this very common viral infection and eradicating cervical cancer in women.
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