Papillomaviruses are a diverse family of small, non-enveloped viruses that infect epithelial cells, which line the skin and mucous membranes. Human Papillomavirus (HPV) is the type relevant to human health. HPV targets and replicates within human epithelial cells.
Human Papillomavirus Types and Risk
Over 200 types of Human Papillomavirus exist, with about 40 infecting mucosal surfaces of the genital tract, mouth, and throat. These types are categorized into two main groups: low-risk and high-risk HPV.
Low-risk HPV types cause benign conditions and are rarely associated with cancer. HPV types 6 and 11, for example, cause most genital warts. These types can lead to visible growths on or around the genitals, anus, mouth, or throat.
High-risk HPV types can cause cellular changes that may progress to cancer if the infection persists. HPV types 16 and 18 are the most recognized high-risk types, accounting for a significant percentage of HPV-related cancers globally. Other high-risk types include HPV 31, 33, 35, 39, 45, 51, 52, 56, 58, 59, 66, and 68. These types can cause sustained infections that interfere with normal cell growth and division.
Associated Health Conditions
Low-risk HPV types cause various warts. Common warts appear on the hands and feet, while plantar warts grow on the soles of the feet.
Genital warts, appearing on or around the genitals and anus, are a common outcome of low-risk HPV types 6 and 11, which cause about 90% of cases. These warts can be small, skin-colored bumps or larger, cauliflower-shaped lesions. They may be itchy or uncomfortable. In rare instances, HPV types 6 and 11 can also cause recurrent respiratory papillomatosis (RRP), where warts form in the respiratory tract, potentially leading to breathing problems.
Persistent high-risk HPV infections can lead to precancerous lesions. These are abnormal cell changes that, if untreated, may develop into cancer over time. The progression from initial infection to cancer typically takes many years.
High-risk HPV infections cause several cancers. Cervical cancer is almost entirely caused by HPV, with types 16 and 18 responsible for about 70% of cases. The virus also causes oropharyngeal cancer, affecting the middle part of the throat, with HPV 16 being a major contributor.
High-risk HPV is also linked to anal cancer, primarily due to types 16 and 18. Additionally, HPV can cause penile, vaginal, and vulvar cancers. The presence of high-risk HPV types in these tissues indicates a potential for the development of these specific malignancies.
Transmission and Viral Lifecycle
Human Papillomavirus is highly contagious and spreads through direct skin-to-skin contact. The most common route for genital and oral HPV types is sexual contact, including vaginal, anal, and oral sex. Transmission can occur even without visible warts or symptoms.
Non-penetrative sexual contact, like hand-to-genital contact, can also facilitate transmission. In rare instances, HPV can pass from a birthing parent to a baby during childbirth, potentially leading to respiratory papillomatosis. The virus typically enters the basal layer of epithelial cells through small abrasions or micro-wounds in the skin or mucous membranes.
Upon entry, the viral DNA establishes itself as an episome, a circular DNA molecule, within the nucleus of the infected basal cells. In most cases, the body’s immune system recognizes and clears the HPV infection within one to two years without causing any health problems or symptoms. However, if the immune system does not successfully clear the infection, particularly with high-risk types, the virus can persist.
Persistent high-risk HPV infections can lead to the continued expression of viral proteins, such as E6 and E7, which interfere with normal cellular processes. This sustained viral activity can promote abnormal cell growth and division over many years. This disruption in cellular regulation is what can eventually lead to the development of precancerous lesions and, if unchecked, invasive cancers.
Prevention and Screening Methods
Prevention against HPV infection is primarily achieved through vaccination. The HPV vaccine, such as Gardasil 9, provides protection against the most common high-risk HPV types, including 16, 18, 31, 33, 45, 52, and 58, which cause a significant majority of HPV-related cancers. It also protects against low-risk types 6 and 11, which are responsible for most genital warts.
The vaccine is most effective when administered before an individual is exposed to HPV, ideally before becoming sexually active. It is routinely recommended for adolescents, typically around 11 to 12 years of age, but can be given starting at age 9 and up to age 26. Vaccination can significantly reduce the risk of developing HPV-related cancers and genital warts.
In addition to vaccination, screening methods are available for early detection of HPV-related cellular changes, particularly for cervical cancer. The Pap test, also known as a Pap smear or cervical cytology, involves collecting cells from the cervix to examine them for any abnormal changes. This test can identify precancerous cells that might develop into cancer if not treated.
The HPV test directly checks for the presence of high-risk HPV DNA in cervical cells. This test can detect the virus itself, indicating an infection that has the potential to cause cellular changes. Often, the Pap test and HPV test are used together in a “cotest” to provide a more comprehensive assessment of cervical health. Regular screening allows for the detection and treatment of precancerous lesions, which can prevent cervical cancer from developing.