Contains the full lesson along with a supporting toolkit, including teachers’ notes.
What is human papillomavirus (HPV) and how is it spread?
Viruses are usually regarded as non-living because they do not have a cellular structure and normally have only one type of nucleic acid ― either RNA or DNA. However, they have a major effect on most living species, causing diseases, including influenza, polio, AIDS and measles. Viruses including human papillomavirus (HPV), hepatitis B, hepatitis C and Epstein-Barr virus cause 10% of all cancers in humans.
HPV (human papillomavirus) is a small spherical, double stranded DNA type virus, of which there are more than 200 different types (strains). The DNA is surrounded by a protein coat. Some HPV types are responsible for common warts (verrucae).
HPV is contagious and is spread by skin to skin contact infecting the mucous membranes. This occurs mainly by sexual means i.e. vaginal, anal and oral contact.
Around 40 types can infect the genital tract. Some of these are low risk types which cause genital warts. Most HPV infections clear naturally but some, caused by high risk types, can cause cancer.
Of the hundreds of strains of HPV thirteen types are known to cause cancer. Two specific strains are responsible for 70% of cervical cancer cases, 90% of anal cancers, 65% of vaginal cancers, up to 70% of oropharyngeal (tonsil and base of the throat) and 35% of penile cancers: these are types 16 and 18. These are described as oncogenic types. About 90% of all genital warts are caused by HPV types 6 and 11. These strains also cause benign tumours to grow in air passages leading to the lungs.
How does HPV cause cervical cancer?
When HPV infects an epithelial cell, the virus directs the production of specific proteins. Two particular proteins made by high-risk HPVs interfere with cell functions that normally prevent excessive multiplication. This allows the cell to grow in an uncontrolled manner and also to avoid apoptosis (programmed cell death). Most HPV infections are symptomless and go away within two years due to the immune response of the body. However, some infections are persistent and, if these are the result of a high-risk strain, they can lead to a series of cellular changes that produce abnormal looking cells, especially in the cervix.
These infected cells are usually recognized by the immune system and destroyed. If they are not destroyed a persistent infection occurs and as the cells continue to grow they may develop mutations that cause even more cell growth. In this way an area of pre-cancerous cells can develop, over a period of ten to twenty years, into a malignant cancerous tumour. It is important to remember that not all infections with high-risk HPV strains lead to cancer.