Human Papillomavirus, or HPV, is a group of more than 200 related viruses and is the most common sexually transmitted infection globally. Most sexually active people will contract some form of it during their lives. While many infections resolve on their own without causing any symptoms, certain types can lead to different health outcomes.
Categorizing HPV Strains
The numerous strains of HPV are identified by number and are primarily categorized based on their potential to cause cancer. This classification divides the virus into two main groups: low-risk and high-risk types.
Low-risk HPV types, such as HPV 6 and HPV 11, are associated with benign, or noncancerous, conditions. While they can cause cellular changes, they are rarely linked to the development of cancer and are the primary cause of genital warts.
Conversely, high-risk HPV types are those with the ability to cause cancer. About 14 types are identified as high-risk, including HPV 16, 18, 31, 33, and 45. These strains can integrate into a host’s cells and trigger changes that may eventually lead to malignant tumors.
High-Risk HPV and Associated Cancers
Persistent infection with a high-risk HPV strain is a cause of several types of cancer. HPV types 16 and 18 are the most significant, accounting for approximately 70% of all cervical cancers and a large percentage of other HPV-related malignancies. Beyond cervical cancer, these strains are linked to cancers of the vulva, vagina, penis, anus, and oropharynx, which includes the back of the throat, base of the tongue, and tonsils.
An infection with a high-risk HPV type is often asymptomatic, producing no noticeable signs. The virus can remain in the body for years before any cellular changes develop into cancer, damaging cells over time without being detected.
The development of cancer from a high-risk HPV infection is not immediate. The process involves the virus evading the immune system and integrating its genetic material into the host’s cells. This action disrupts the normal cell cycle, leading to uncontrolled cell growth and precancerous lesions that can progress to cancer if untreated.
High-risk HPV causes about 5% of all cancers worldwide. In the United States, it is responsible for 3% of all cancer cases in women and 2% in men. While nearly all cases of cervical cancer are caused by HPV, the virus also accounts for:
- Over 90% of anal cancers
- 75% of vaginal cancers
- 69% of vulvar cancers
- 63% of penile cancers
Low-Risk HPV and Warts
Low-risk HPV strains are known for causing warts, which are noncancerous skin growths. The most common are genital warts, with HPV types 6 and 11 responsible for approximately 90% of all cases. These warts can appear as small, flesh-colored bumps or have a cauliflower-like appearance. They emerge on the genital or anal areas weeks or months after sexual contact with an infected partner.
While genital warts are the most frequently discussed outcome, they can also appear on the cervix, vagina, scrotum, or even the mouth and throat. These benign growths are not associated with cancer but may require treatment to manage their spread.
Other types of low-risk HPV are responsible for common warts that are not on the genitals. These include plantar warts on the soles of the feet and common warts on the hands. The HPV strains that cause these warts are different from those that cause genital warts and do not lead to genital infections.
HPV Vaccines and Covered Types
Vaccination is a primary method of preventing infections from the most common types of HPV. The vaccine works by protecting against specific HPV strains before an individual is exposed to them and is not a treatment for existing infections.
In the United States, the available vaccine is Gardasil 9. This vaccine targets nine specific types of HPV: two low-risk and seven high-risk.
The covered low-risk types are HPV 6 and 11, the leading causes of genital warts. The seven high-risk types included are HPV 16, 18, 31, 33, 45, 52, and 58. Together, these seven strains account for about 90% of cervical cancers and also offer protection against other HPV-related cancers of the anus, vulva, vagina, and oropharynx.
Identification and Testing for HPV Types
Identifying high-risk HPV strains relies on specific screening tests, which are distinct from a Pap test, although they are often performed at the same time. A Pap test is designed to find precancerous cell changes on the cervix that could become cancer, but it does not detect the virus itself. An HPV test is a molecular test that detects the genetic material (DNA or mRNA) of the virus in cervical cells.
A primary HPV test is the recommended screening method for cervical cancer for individuals aged 25 to 65. When an HPV test comes back positive, it indicates the presence of one or more high-risk HPV types.
Some tests perform genotyping, which can identify specific high-risk strains like HPV 16 and 18. This information is valuable for risk assessment, as these two types carry a higher risk of leading to cancer compared to other high-risk strains.