A human papillomavirus (HPV) is a member of the papillomavirus family of viruses that is capable of infecting humans. Like all papillomaviruses, HPVs establish productive infections only in the stratified epithelium of the skin or mucous membranes. While the majority of the nearly 200 known types of HPV cause no symptoms in most people, some types can cause warts (verrucae), while others can – in a minority of cases – lead to cancers of the cervix, vulva, vagina, and anus in women or cancers of the anus and penisin men.
More than 30 to 40 types of HPV are typically transmitted through sexual contact and infect the anogential region. Some sexually transmitted HPV types may cause genital warts. Persistent infection with “high-risk” HPV types—different from the ones that cause warts—may progress to precancerous lesions and invasive cancer. HPV infection is a cause of nearly all cases of cervical cancer; however, most infections with these types do not cause disease.
Most HPV infections in young females are temporary and have little long-term significance. 70% of infections are gone in 1 year and 90% in 2 years. almost all are gone after that but when infection persists—in 5% to 10% of infected women—there is high risk of developing cervical pre-cancer (lesions on the cervix), which can progress to invasive cervical cancer. This process usually takes 15–20 years, providing many opportunities for detection and treatment of the pre-cancerous condition, often with high cure rates.
In the US and other high-resource countries, a cervical Papanicolaou (Pap) test is used to detect abnormal cells which may develop into cancer. A cervical examination also detects warts and other abnormal growths which become visible as white patches of skin after they are washed with acetic acid (visual inspection). Abnormal and cancerous areas can be removed with a simple procedure, typically with a cauterizing loop or—more common in the developing world—by freezing (cryotherapy). New HPV DNA tests are more sensitive than Pap or visual inspection and a lower-cost HPV test suitable for low-resource settings may become available soon, potentially making high-sensitivity screening feasible where it currently does not exist in Africa, Asia,and Latin America.
Pap smears have reduced the incidence and fatalities of cervical cancer in the developed world, but even so there were 11,000 cases and 3,900 deaths in the U.S. in 2008. Cervical cancer has substantial mortality in resource-poor areas; worldwide, there are 490,000 cases and 270,000 deaths. In large part because Pap is difficult to sustain in low-resource settings, eighty to 85 percent of cervical cancer deaths occur in the developing world.
HPV In The United States of America
HPV is estimated to be the most common sexually transmitted infection in the United States. Most sexually active men and women will probably acquire genital HPV infection at some point in their lives. The American Social Health Association reported estimates that about 75-80% of sexually active Americans will be infected with HPV at some point in their lifetime. By the age of 50 more than 80% of American women will have contracted at least one strain of genital HPV.
It was estimated that in the year 2000, there were approximately 6.2 million new HPV infections among Americans aged 15–44; of these, an estimated 74% occurred to people between ages 15–24. Of the STDs studied, genital HPV was the most commonly acquired.
Estimates of HPV prevalence vary from 14% to more than 90%. One reason for the difference is that some studies report women who currently have a detectable infection, while other studies report women who have ever had a detectable infection. Another cause of discrepancy is the difference in strains that were tested for.
One study found that, during 2003–2004, at any given time, 26.8% of women aged 14 to 59 were infected with at least one type of HPV. This was higher than previous estimates. 15.2% were infected with one or more of the high-risk types that can cause cancer. However only 3.4% were infected with one or more of the four types prevented by the Gardasil vaccine, which was lower than previous estimates.
Note that prevalence decreases with age. This may be due to HPV infection being cleared by the immune system, or sinking to undetectable levels while still present in the body. HPV will probably remain in the infected person’s cells for an indefinite time—most often in a latent state, but occasionally producing symptoms or disease. Recent studies from the Albert Einstein College of Medicine and from the University of Washington suggest that HPV may eventually be cleared in most people with well functioning immune systems. It appears that in some cases the virus does remain in the body indefinitely, producing symptoms if the immune system weakens.
The American Cancer Society estimates that in 2008, about 11,070 women in the United States will be diagnosed with invasive cervical cancer, and about 3,870 US women will die from this disease.
HPV infection is limited to the basal cells of stratified epithelium, the only tissue in which they replicate. The virus can not bind to live tissue; instead it infects epithelial tissues through micro-abrasions or other epithelial trauma that exposes segments of the basement membrane. The infectious process is slow, taking 12–24 h for initiation of transcription. It’s believed that involved antibodies play a major neutralizing role while the virions still reside on the basement membrane and cell surfaces.
Once an HPV viron invades a cell, an active infection occurs, and the virus can be transmitted. Several months to years may elapse before squamous intraepithelial lesions (SIL) develop and can be clinically detected. The time from active infection to clinically detectable disease makes it difficult for someone who has become infected to establish which partner was the source of infection.
Some HPV infections can cause warts (verrucae), which are noncancerous skin growths. Infection with these types of HPV causes a rapid growth of cells on the outer layer of the skin. Types of warts include:
- Comon warts: Some “cutaneous” HPV types, such as HPV-1 and HPV-2, cause common skin warts. Common warts are often found on the hands and feet, but can also occur in other areas, such as the elbows or knees. Common warts have a characteristic cauliflower-like surface and are typically slightly raised above the surrounding skin. Cutaneous HPV types can cause genital warts but are not associated with the development of cancer.
- Plantar warts are found on the soles of the feet. Plantar warts grow inward, generally causing pain when walking.
- Subungual or periungual wartsform under the fingernail (subungual), around the fingernail or on the cuticle (periungual). They may be more difficult to treat than warts in other locations.
- Flat warts: Flat warts are most commonly found on the arms, face or forehead. Like common warts, flat warts occur most frequently in children and teens. In people with normal immune function, flat warts are not associated with the development of cancer.
Genital warts are quite contagious, while common, flat, and plantar warts are much less likely to spread from person to person.
Genital or anal warts (condylomata acuminata or venereal warts) are the most easily recognized sign of genital HPV infection. Although a wide variety of HPV types can cause genital warts, types 6 and 11 account for about 90% of all cases.
Most people who acquire genital wart-associated HPV types clear the infection rapidly without ever developing warts or any other symptoms. People may transmit the virus to others even if they do not display overt symptoms of infection.
HPV types that tend to cause genital warts are not those that cause cervical cancer. However, since an individual can be infected with multiple types of HPV, the presence of warts does not rule out the possibility of high-risk types of the virus also being present.
The types of HPV that cause genital warts are usually different from the types that cause warts on other parts of the body, such as the hands or inner thighs.