By Dr Nevein Girgis
The Cancer Centre Bahamas
According to the National Cancer Institute, human papillomaviruses (HPVs) are a group of more than 200 related viruses. More than 40 HPV types can be easily spread through direct sexual contact, from the skin and mucous membranes of infected people to the skin and mucous membranes of their partners. They can be spread by vaginal, anal and oral sex.
HPV infections are the most common sexually transmitted infections in the United States. In fact, the Centers for Disease Control and Prevention estimates that more than 90 per cent and 80 per cent, respectively, of sexually active men and women will be infected with at least one type of HPV at some point in their lives. Most high-risk HPV infections occur without any symptoms, go away within one to two years, and do not cause cancer. Some HPV infections, however, can persist for many years. Persistent infections with high-risk HPV types can lead to cell changes that, if untreated, may progress to cancer.
Which cancers are caused by HPV?
High-risk HPVs cause several types of cancer.
• Cervical cancer: Virtually all cases of cervical cancer are caused by HPV, and just two HPV types, 16 and 18, are responsible for about 70 per cent of all cases.
• Anal cancer: About 95 per cent of anal cancers are caused by HPV. Most of these are caused by HPV type 16.
• Oropharyngeal cancers (cancers of the middle part of the throat, including the soft palate, the base of the tongue, and the tonsils): About 70 per cent of oropharyngeal cancers are caused by HPV. In the United States, more than half of cancers diagnosed in the oropharynx are linked to HPV type 16.
• Rarer cancers: HPV causes about 65 per cent of vaginal cancers, 50 per cent of vulvar cancers, and 35 per cent of penile cancers. Most of these are caused by HPV type 16.
Who gets HPV infections?
Anyone who has ever been sexually active (that is, engaged in skin-to-skin sexual conduct, including vaginal, anal, or oral sex) can get HPV. HPV is easily passed between partners through sexual contact. HPV infections are more likely in those who have many sex partners or have sex with someone who has had many partners. Because the infection is so common, most people get HPV infections shortly after becoming sexually active for the first time. A person who has had only one partner can get HPV.
Someone can have an HPV infection even if they have no symptoms and their only sexual contact with an HPV-infected person happened many years ago.
Can HPV infections be prevented?
People who are not sexually active almost never develop genital HPV infections. In addition, HPV vaccination before sexual activity can reduce the risk of infection by the HPV types targeted by the vaccine.
The Food and Drug Administration has approved three vaccines to prevent HPV infection. These vaccines provide strong protection against new HPV infections, but they are not effective at treating established HPV infections or disease caused by HPV.
Correct and consistent condom use is associated with reduced HPV transmission between sexual partners, but less frequent condom use is not. However, because areas not covered by a condom can be infected by the virus, condoms are unlikely to provide complete protection against the infection.
What are treatment options for HPV-infected individuals?
There is currently no medical treatment for persistent HPV infections that are not associated with abnormal cell changes. However, the genital warts, benign respiratory tract tumors, precancerous changes at the cervix, and cancers resulting from HPV infections can be treated.
Methods commonly used to treat precancerous cervical changes include cryosurgery (freezing that destroys tissue), LEEP (loop electrosurgical excision procedure, or the removal of cervical tissue using a hot wire loop), surgical conisation (surgery with a scalpel, a laser, or both to remove a cone-shaped piece of tissue from the cervix and cervical canal), and laser vaporization conization (use of a laser to destroy cervical tissue).
Treatments for other types of benign respiratory tract tumours and precancerous changes caused by HPV (vaginal, vulvar, penile, and anal lesions) and genital warts include topical chemicals or drugs, excisional surgery, cryosurgery, electrosurgery, and laser surgery.
HPV-infected individuals who develop cancer generally receive the same treatment as patients whose tumours do not harbour HPV infections, according to the type and stage of their tumors. However, people who are diagnosed with HPV-positive oropharyngeal cancer may be treated differently than people with oropharyngeal cancers that are HPV-negative. Recent research has shown that patients with HPV-positive oropharyngeal tumours have a better prognosis and may do just as well on less intense treatment.
How does high-risk HPV cause cancer?
HPV infects epithelial cells. These cells, which are organized in layers, cover the inside and outside surfaces of the body, including the skin, the throat, the genital tract, and the anus.
Once HPV enters an epithelial cell, the virus begins to make the proteins it encodes. Two of the proteins made by high-risk HPVs (E6 and E7) interfere with cell functions that normally prevent excessive growth, helping the cell to grow in an uncontrolled manner and to avoid cell death.
Many times these infected cells are recognised by the immune system and eliminated. Sometimes, however, these infected cells are not destroyed, and a persistent infection results. As the persistently infected cells continue to grow, they may develop mutations in cellular genes that promote even more abnormal cell growth, leading to the formation of an area of precancerous cells and, ultimately, a cancerous tumor.
Other factors may increase the risk that an infection with a high-risk HPV type will persist and possibly develop into cancer. These include:
• Smoking or chewing tobacco (for increased risk of oropharyngeal cancer)
• Having a weakened immune system
• Having many children (for increased risk of cervical cancer)
• Long-term oral contraceptive use (for increased risk of cervical cancer)
• Poor oral hygiene (for increased risk of oropharyngeal cancer)
• Chronic inflammation
Researchers believe that it can take between 10 and 30 years from the time of an initial HPV infection until a tumour forms. However, even when severely abnormal cells are seen on the cervix (a condition called cervical intraepithelial neoplasia 3, or CIN3), these do not always lead to cancer. The percentage of CIN3 lesions that progress to invasive cervical cancer has been estimated to be 50 per cent or less.