HPV, anal dysplasia and anal cancer


Anal cancer typically develops over a period of years, beginning with a precancerous condition called anal dysplasia.

Anal dysplasia occurs when clusters of abnormal cells form lesions in the mucosa (“wet lining”) of the anal canal (between the anus and the rectum). These lesions may also form just outside the anal opening.

A sexually transmitted virus called human papillomavirus (HPV) causes most cases of anal dysplasia and anal cancer. There are different types of HPV. Only some types cause anal dysplasia and anal cancer.

Screening tests can sometimes detect anal dysplasia. If treated early, anal dysplasia is less likely to develop into anal cancer. Treatments are used to remove the lesions before they turn into cancer.

Anal cancer is usually treated with radiation and chemotherapy or with surgery, to remove the cancer, slow its growth and/or prevent it from spreading.

People living with HIV have a higher risk of developing anal cancer.

There are several vaccines that can prevent acquiring the most common types of HPV that can lead to anal cancer.

Consistent and correct condom use reduces, but does not eliminate, the risk of getting HPV or passing it to someone else.

Quitting smoking can help reduce the risk of developing anal dysplasia and anal cancer.

Getting regular anal Pap tests and digital rectal exams may help to catch anal dysplasia or anal cancer early, improving treatment outcomes.

The words we use here – CATIE is committed to using language that is relevant to everyone. People use different terms to describe their genitals. This text uses medical terms, such as vagina and penis, to describe genitals. Cisgenderi people can often identify with these terms. Some trans peopleii may use other terms, such as front hole and strapless. CATIE acknowledges and respects that people use words that they are most comfortable with.

Key messages on HPV for clients are available here.

What are anal dysplasia and anal cancer?

Anal cancer starts as a precancerous condition called anal dysplasia (abnormal changes in cells) in the lining (mucosa) of the anal canal (the area connecting the anus to the rectum). Groups of these abnormal cells form areas of abnormal tissue called lesions. Over time, these lesions can develop into cancer.

Some lesions form but then shrink or disappear; some lesions return after disappearing; and some remain present without changing. Other lesions progress from low-grade to high-grade lesions, which may then progress to cancer. Anal cancer happens when cells in the anus or anal canal grow and multiply uncontrollably, spreading into and damaging surrounding tissue.1–4

What causes anal dysplasia and anal cancer?

A sexually transmitted virus called human papillomavirus (HPV) causes almost all cases of anal dysplasia and anal cancer. This makes HPV the most important risk factor for anal cancer.

Some types of HPV can be transmitted sexually, through bodily fluids, such as semen and vaginal secretions. HPV can also be transmitted through skin-to-skin contact (such as genital-to-genital contact) even when bodily fluids are not present.

Only some types of HPV cause anal dysplasia and anal cancer. HPV types 16 and 18, in particular, account for most cases of anal cancer, as well as most cases of cervical cancer. Other types can cause warts on, in or around the anus or genitals (anogenital warts).

Most sexually active people acquire HPV at some point in their lives. In most cases, an HPV infection will clear from the body on its own without causing any health problems. However, this does not make someone immune to future infections with other types of HPV.

One way in which HPV may cause cancer is by interfering with how the body prevents cancer from developing. The human body produces cells that make proteins, which help prevent dysplasia and cancer. In some cases, HPV can shut off these proteins.1,5–9

Who is at risk?

Anyone who is sexually active can get HPV, including the types that cause anal cancer.  

HPV can be passed through vaginal sex, anal sex and oral sex (mouth on penis, mouth on vagina). It can also be passed through oral-anal contact (rimming) and the sharing of sex toys.

Engaging in receptive anal intercourse can increase the risk of anal cancer later because it increases the risk of getting an HPV infection in or around the anus.

Other factors can contribute to the development of anal dysplasia and anal cancer, when HPV is present.

People with weakened immune systems are at greater risk for anal dysplasia and anal cancer. This includes people living with HIV.

Other important risk factors for anal dysplasia and anal cancer include cigarette smoking, diets lacking in fruit and vegetables, being over the age of fifty, and history of other sexually transmitted infections.

Symptoms or history of other HPV-related conditions, such as cervical cancer, can also indicate that a person is at risk for anal cancer, because the HPV types that cause these conditions might also cause anal cancer.

People living with HIV have a higher risk of acquiring an HPV infection and of developing precancerous lesions. They also have higher rates of anal cancer, particularly among gay, bisexual and other men who have sex with men (gbMSM) living with HIV. GbMSM living with HIV who also have a history of hepatitis B infection may be at still higher risk of developing anal cancer.1,7,8,10–13


Individuals with anal dysplasia often do not experience any clear symptoms until it is quite advanced or has become anal cancer. Early stage anal cancer may not produce any signs or symptoms either. In more advanced anal cancer, bleeding from the anus is the most common symptom, and usually the first. The blood may be visible in the stool (poo). The amount of blood may be very small.  Other possible symptoms include lumps around the anus or groin, abnormal discharge from the anus or changes in bowel habits (such as narrow stool, constipation or diarrhea). ​​​​Because not all of these symptoms are specific to anal cancer, they may be mistaken for other conditions.

Having anogenital warts may be a sign that a person should be tested for anal dysplasia or cancer. Even though anogenital warts and anal cancer are caused by different types of HPV, people with the type(s) that cause anogenital warts are more likely to also have the type(s) that cause anal cancer. Anogenital warts usually consist of a series of bumps or mini-cauliflower-like growths. These may be easily visible if they are located in, on or around the vagina or anus. Warts on the cervix or in the anal canal may not be detected prior to internal examination.1,2,6,14,15

Development of anal cancer

The abnormal cells that develop as a result of anal dysplasia can eventually develop into anal cancer, particularly if not detected and treated early. Because anal dysplasia is hard to detect with routine screening, it may not be diagnosed before it becomes anal cancer.

If the cancer has spread deeply into the tissues of the anal canal and/or rectum, removing the cancer or preventing its spread may make the bowels not function properly.

If anal cancer is not diagnosed and treated early, the cancer is more likely to spread to other parts of the body. More aggressive cancer treatments may then be required.1,5,6

Testing and diagnosis

Routine medical check-ups with anal examinations by healthcare providers may help to detect early cancers. These routine examinations involve a digital anorectal exam (DARE) to feel for tumours (lumps that might be cancerous) in the anal canal. DARE usually cannot detect anal dysplasia. An annual DARE may be useful for gbMSM who are living with HIV, as well as gbMSM who engage in receptive anal sex (bottoming). PHAC recommends DARE for people living with HIV who have anogenital warts.    

Another type of examination, called an anoscopy, can detect anal dysplasia. An anoscopy is a visual examination of the anal canal using a device called an anoscope. An anoscope is a small hollow plastic tube with a light on the end of it, that is inserted a few centimetres into the anal canal to inspect for lesions. The anoscope is inserted with lubricant to minimize discomfort.

There is a special kind of anoscopy, called high-resolution anoscopy (HRA), which is better still at detecting anal dysplasia. HRA uses a magnifier to provide more detailed images of the mucosa. During the HRA procedure, lesions are enhanced by first applying a thin layer of dilute vinegar to the mucosa and then iodine to highlight any abnormal or precancerous areas.

During an HRA, a biopsy (tissue sample) may be taken of an abnormal area, using forceps (a special kind of pinchers or tweezers). The tissue sample is then examined in a laboratory to determine if a high-grade lesion is present. Pain is rare. No bowel preparation is necessary before this procedure. The drawback of HRA is that it is not widely available. It must be performed by a trained and specialized healthcare provider.

Another, more widely available test, called an Anal Pap test, may be useful as a screening tool to detect anal dysplasia. Anal Pap tests take around five minutes to complete. Cells collected from a swab inserted in the anus are examined under a microscope.

One drawback of Anal Pap tests is that they often fail to diagnose the correct degree of dysplasia (low-grade/high-grade). They also have a high rate of false negatives for the highest risk groups (false negatives are when a test says someone does not have a disease, when they actually do). Among gbMSM living with HIV, rates of false negatives for Anal Pap tests can be particularly high. For these reasons, it may be useful to screen using an anal Pap test as well as an HRA (if available).

CT (computed tomography) scans or MRIs (magnetic resonance imaging) do not detect dysplasia, but may detect cancer. Other examinations, such as sigmoidoscopy and colonoscopy, do not adequately examine the anal canal. Having a colonscopy does not necessarily mean a person has been screened for anal cancer.

Some anal cancers occur just outside of the anal canal (perianal area). These may be visible by spreading the buttock cheeks. A biopsy of the perianal skin will confirm the diagnosis.1,6,12,16–19

Notification of partners

HPV is not a reportable infection in Canada. This means that when an infection is confirmed by a clinic, healthcare provider or laboratory it is not required to be reported to public health authorities. Partner notification is not required as a public health measure, unlike with a diagnosis of chlamydia, gonorrhea, syphilis or HIV.20


HPV itself cannot be treated, but the immune system is able to clear most cases of HPV. A variety of treatments are used for anal dysplasia and anal cancer. Treatments vary, depending on severity, location and size, and whether any cancer present has spread to other parts of the body.

There are several ways that anal dysplasia may be treated:

  • Cryotherapy destroys the lesion by freezing. This procedure can be done in the doctor’s office. There can be some discomfort or pain.
  • Laser treatment destroys the lesion with an intense beam of infrared light. This procedure is often done in a day-surgery clinic. It can be uncomfortable and can cause slight bleeding afterwards.  
  • Electrocautery (EC): The lesion is destroyed using heat from an electric current, applied using an electrode. Using a gentle brushing technique, the dead tissue is then removed with forceps.
  • TCA (trichloroacetic acid): The lesion is treated by being touched with acid-soaked cotton. This is simple and painless but four or more treatments may be needed over several weeks.
  • Surgery: The lesion is cut out by a surgeon. Surgery may also involve removing part of the lining of the anal canal.
  • Watch and wait: Sometimes the dysplasia is too widespread to remove without causing damage to the anus. In this case, healthcare providers may just observe it for months or years. If cancer does develop, it can be treated very early and with good results.
  • Topical treatments: A medication called imiquimod is sometimes prescribed in a cream formulation. Applied regularly to an affected area, this medication can help the immune system to attack lesions.  An anti-cancer drug called fluorouracil may also be prescribed in cream formulation.

If cancer is present, treatment may involve surgery, chemotherapy, radiation therapy or a combination of these.  

Surgery called “local excision” may be used to remove small, early-stage tumours. If the cancer is small, this may be enough. If the cancer has already spread more deeply into the tissues, trying to remove the cancer may impair the function of the anus (which is to help regulate the passage of stools). In this case, the anus and part of the rectum are removed and stool is diverted to a bag attached to the body (this procedure is called a colostomy).

Early diagnosis and treatment is important, because people with advanced forms of anal cancer are at increased risk of the cancer spreading to other areas of the body.3,6,12,14,21,22

What about HIV? 

HIV weakens the immune system which can make a person more vulnerable to some cancers.

Effective HIV treatment (also known as antiretroviral therapy or ART) can lower the risk of developing some cancers. However, even with effective HIV treatment, people living with HIV have a higher risk of HPV infection, more often carry multiple HPV types and have a higher risk of HPV-related disease - including faster progression to cancer. Therefore, people with HIV should get regular care, including screening for HPV-related disease and talk with a doctor or nurse about getting vaccinated against HPV if necessary.1,23,24


HPV vaccines are widely available and highly effective at preventing certain types of HPV, including those that most often cause anal dysplasia and anal cancer.

The vaccines do not provide protection against HPV types that people are already infected with but provide excellent protection against HPV types the person has not been exposed to.

It is also important to remember that even if someone has received one of the vaccines, they are only protected against the cancer-causing HPV types covered by the vaccine they have received.

Different vaccines prevent different types of HPV. Although several vaccines are approved to reduce the risk of HPV infection (and related cancer) in Canada, the most commonly used today reduces the risk of infection with nine types of HPV. This vaccine is called Gardasil-9.

It is estimated that, in Canada, immunization against HPV types 16 and 18, can prevent between 70% and 84% of anal and genital cancers.

Because the types of HPV that cause anal dysplasia and anal cancer are transmitted sexually, prevention benefits are greatest if a person is vaccinated before they have had their first sexual encounter. But even if a person is sexually active or has already acquired an HPV infection, vaccination can protect them from getting types of HPV they have not yet acquired.

People living with HIV are at reduced risk from HPV infection when given HPV vaccination. However, it is unclear whether HPV vaccination is as effective among people living with HIV as it is among people who are not living with HIV.

All provinces and territories provide school-based immunization programs for young people, starting in grades four to seven. “Catch-up” programs are also available in all provinces and territories for people who did not receive vaccination through school-based programs, but availability varies, based on age or sex.

The correct and consistent use of condoms during insertive vaginal and anal sex can reduce the risk of transmitting HPV, but does not eliminate the risk completely. This is because HPV can be transmitted from areas of skin not covered by a condom to the skin of a sexual partner.

There are two types of condoms available. The external condom (sometimes called the “male” condom) is a sheath made from polyurethane, latex or polyisoprene that covers the penis during sex. The internal condom (sometimes called the insertive or “female” condom) is a pouch made of polyurethane or a synthetic latex material called nitrile that can be inserted into the vagina or rectum. Some trans men may cut a condom or oral dam to fit their genitals.

The use of condoms or oral dams can reduce the risk of passing on HPV during oral sex or rimming.

When sharing sex toys, using a new condom and cleaning the toy between each use can reduce the risk of HPV transmission.

Quitting or reducing smoking reduces the risk of developing anal dysplasia and anal cancer.

Screening for anal dysplasia can increase the chances of catching the dysplasia early, so that it can be treated before cancer develops.1,23,25–29


This fact sheet was developed in partnership with the Sex Information and Education Council of Canada (SIECCAN).


Condoms for the prevention of HIV transmissionfact sheet

Safer Sex Guide – client resource

Oral Sex – client resource

Viral STI Basics – fact sheet

Sexually Transmitted Infections booklet (Public Health Agency of Canada)


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Author: Miller D

Published: 2023