Human papillomaviruses (HPVs) are an ancient and highly successful group of viruses that have co-evolved with their host to replicate in specific anatomical niches of the stratified epithelia. They replicate persistently in dividing cells, hijack key host cellular processes to manipulate the cellular environment and escape immune detection, and produce virions in terminally differentiated cells that are shed from the host.

Some HPVs cause benign, proliferative lesions on the skin and mucosa, and others are associated with the development of cancer. However, most HPVs cause infections that are asymptomatic and inapparent unless the immune system becomes compromised.

Human papillomaviruses (HPVs) belong to family of pox viruses.. HPVs are divided into three main groups: cutaneous, mucocutaneous and those associated with the rare autosomal recessive disorder, epidermodysplasia verruciformis (EV). 

They can also be grouped according to the areas of the body where infection is found – external skin, anogenital and oral regions. The mucocutaneous HPV types can be further sub-divided into low risk (LR-HPV) mainly associated with benign warts and high risk (HR-HPV) defined by their risk of progression to malignancy.

HPV infection can be productive, subclinical or latent in both skin and mucosa. Warts can be seen clinically, while sub-clinical mucosal infections need additional tools such as microscopic examination with the aid of topically applied acetic acid, as in colposcopic examination of the cervix or anoscopy of the anal canal.

Latent papillomaviruses are detectable only through the demonstration of HPV DNA in clinically and histologically normal skin and mucosa.

Approximately 70% of HPV infections resolve spontaneously in 1 year and 90% in 2 years, while HPV persistence develops in the remainder.

Clearance requires an effective cell mediated immune response, while persistent infection with HPV represents a failure of the immune response and increases the risk of progression to cancer.

Histologically, warts are benign lesions, with hypertrophy of all layers of the skin, resulting in acanthosis (thickening), papillomatisis (folding) and hyperkeratosis (increase in the horny layer) often with abnormal keratohyaline granules.

Vacuolation of cells occurs in the upper layers and inclusion bodies are sometimes observed. Resolution using the electron microscope reveals densely packed virus particles in crystalline array in the upper stratum granulosum and stratum corneum.

Warts usually disappear spontaneously but occasionally may be resistant to treatment. Regrowth of lesions after treatment is frequently due to persistence of the virus in the skin surrounding the original wart.

HPV is an epitheliotropic virus that exclusively attacks epithelial cells and primarily spreads and acts locally in this type of tissue. In the cervix, HPV infects the keratinocytes in the basal layer of the cervical epithelium in the transformation zone of the cervix. Viral particles are assembled when differentiated keratinocytes die and the virus is released during desquamation into the ambient environment. Downregulated protein expression in the lower epithelial layers, high non-cytolytic genomic replication, and the absence of a viremic phase enable the virus to evade immune system recognition.

HPV-infected patients have a weakened immune response caused by the HPV infection. A direct effect of depressed CD8 T cell activity is delayed or poor genital wart clearing while adequate T cell immune responses regress HPV-induced lesions and clear local HPV infection. Inactive T cell immune responses are associated with long-term viral persistence and HPV induced cancers.

As HPV lacks a viremic phase, immune cells cannot easily enter into contact with it. HPV attacks basal epithelial layers of the skin as immunocompetent Langerhans cells occur only in the surface epithelial layers and avoid invoking the T cell immune response. HPV does not cytolyse the cervical epithelium. In this way, it attenuates inflammatory and immune signal responses and can persist incognito. Evasion of a local immune response is required for the development of HPV infection. Therefore, the therapeutic agents stimulating innate immunity may effectively prevent the onset of local HPV infections.


Persistence and malignant conversion of cutaneous warts

HPV infections are normally controlled by intact cell-mediated and humoral immune systems. Regression was shown to be largely driven by cytotoxic T cells and NK cells while protection from subsequent infection with the same HPV type results from stimulation of the adaptive response and production of antibodies. However, sequestration of HPV in epithelial cells provides protection for the virus, resulting in inefficient activation of innate immunity, poor priming of the adaptive response and persistence of infection

Patients with cell-mediated immunodeficiencies, whether primary (as in or familial CD4 lymphopaenia), secondary (as in haematological malignancies, HIV positivity) or iatrogenic (as a result of immunosuppressive therapy) are all therefore at increased risk of developing extensive, persistent and recurrent warts.


HPV in mucosal and anogenital skin

 external genital warts (EGW) are the most common sexually transmitted infection with a huge increase in incidence reported in recent years . Incidence and prevalence peaks in late teens/early 20s as a result of both behavioural and biological factors. Prevalence declines until around age 50–55, when there is a secondary peak in many countries, associated either with reactivation of latent virus or incident new infection due to new sexual partners.

External Genital Warts (Ano-genital warts also known as condylomata acuminata) are found predominantly at sites traumatised during sexual intercourse. This includes the glans penis, coronal sulcus and inner aspect of the foreskin in uncircumscribed men, while infection in circumcised men is less common and generally restricted to the penile shaft. In women, the labia, clitoris, vulva, vagina and ectocervix can all be infected, with vulvar and vaginal warts usually plainly visible. In both sexes, HPV can be detected in the pubis, perineum, urethra and peri-anal area. Intra-anal warts are most commonly, but not always, a consequence of receptive anal intercourse. Symptoms of EGW include itching, bleeding, fissuring and painful intercourse.

Subclinical lesions may increase in size and number in pregnancy, resulting in increased shedding of HPV from the genital tract, followed by spontaneous regression after delivery. Vaginal warts may occasionally grow so large that the birth canal is obstructed.

External Genital Warts cause significant psychosocial stress and impact on quality of life, whether by the creation of feelings of guilt and anxiety, loss of self-esteem or the embarrassment of, and delays in, seeking treatment. Topical treatment at home and/or removal by ablation in the clinic setting are often effective, but only surgical interventions have clearance rates approaching 100%.


Transmission requires close contact, but not necessarily penetrative sex. Indeed indirect transmission can also occur by innocent contact such as sharing baths or towels. HPV DNA has been isolated from underwear and towels of infected individuals and virus can be spread through the use of inadequately sterilised instruments and from laser plumes when inadequate extraction facilities exist. Condoms can protect against acquisition of External Genital Warts.


Urethral warts which are rare; complications include reduced urinary flow and infertility due to obstruction of urethral and ejaculatory ducts.



Histologically, condylomata show epithelial thickening due to severe acanthosis and papillomatosis, but without the hyperkeratosis of external cutaneous warts. A chronic inflammatory infiltrate is usually observed. The majority of EGW are associated with HPV 6 and 11, although other HPV serotypes such as HPV 42 and 81 are also common.  The presence of vacuolated cells with enlarged, hyperchromatic nuclei described as koilocytes is pathognomonic of productive HPV infection,


Acanthosis -  Acanthosis is term pathologists use to describe an increased number of specialized squamous cells on the surface of a tissue. Acanthosis is commonly seen in the top layer of the skin called the epidermis. Acanthosis can also be seen in the epithelium that lines the inside of the mouth and throat. The increased number of squamous cells causes the epidermis or epithelium to look thicker than normal when examined under the microscope. By itself, acanthosis is a non-cancerous change

Papillomatosis - Papillomatosis refers to the projection of dermal papillae above the surface of the skin, resulting in an irregular undulating configuration of the epidermis. Often associated with epidermal hyperplasia, papillomatosis is also seen with chronic inflammatory and neoplastic dermatoses. 

Koilocytosis - The presence of vacuolated cells with enlarged, hyperchromatic nuclei described as koilocytes is pathognomonic of productive HPV infection, while the consequences of persistent infection are reflected in increasingly severe nuclear changes, mitotic figures and clumps of pyknotic cells.







Bowenoid papulosis (low grade Carcinoma in Situ), which was originally described in 1977 by Kopf and Bart as reddish-brown verrucous papules on the penis. Bowenoid papulosis is now known to occur in sexually active people of both sexes, is now usually referred to as a low grade carcinoma in situ and can be treated by locally destructive methods. While usually benign, they are associated with HR-HPV types (Hight risk HPV types), dysplasia is full thickness, recurrences are a potential problem and transformation to malignancy occurs in 2–3%.

Giant condyloma acuminatum (Bushcke-Löwenstein tumours), which is a large exophytic lesions with a verrucous, fungating surface, frequently found on the penis or in the perianal area and usually associated with HPV 6 and11. It too is characterised by local invasion and its distinctive histological appearance, in which koilocytes and cellular atypia are largely absent, suggest it is a well-differentiated variant of verrucous carcinoma. 


HPV associated anogenital pre-cancers and cancers

HPV associated pre-cancers present as intraepithelial neoplasia and are named after the site: cervical intraepithelial neoplasia (CIN) for the most commonly recognised and extensively studied consequence of persistent HPV infection, together with VIN and VAIN (vulval and vaginal intraepithelial neoplasia respectively), PIN (penile intraepithelial neoplasia) and AIN (anal intraepithelial neoplasia).


Cervical intraepithelial neoplasia and cancer


Colposcopy - Clinically, precancerous cervical lesions can usually only be seen with the aid of a colposcope to magnify the epithelium and application of 5% acetic acid to reveal subclinical lesions which contain a high level of nuclear material, observed as areas of densely white whorls known as flat warts.

PAP Smear - Microscopic examination of exfoliated cells scraped with a wooden spatula from the cervix, fixed, and stained appropriately to reveal dysplastic changes in cells of mucocutaneous epithelium was developed in the 1940s by Papanicolau (Papanicolaou, 1942). The technique is most common screening method used nowadays for cervical screening for perneoplastic conditions and cervical cancers. 

Observations are classified in different ways although the Bethesda classification is the most widely accepted, with low grade squamous intraepithelial neoplasia (LSIL) used for samples containing cells with larger, irregular and more detailed nuclei and high grade Squamous Intraepithelial Neoplasia (HSIL) for samples containing immature cells, small cells in sheets and syncytial groupings and with distinct cytoplasmic borders

Liquid based cytology (LBC), which allows the creation of a uniform and consistent cell monolayer in the laboratory is used preferentially to improve quality and consistency and reduce the number of inadequate samples. LBC has the added advantage of providing suitable material for HPV testing,

CIN lesions are graded according to the proportion of epithelium affected by abnormalities (as shown in table below). In CIN3 there is minimal maturation, nuclear abnormalities throughout the full thickness and numerous mitotic figures. ASCUS (Atypical squamous cells of unknown significance)


Papanicolau classification Dysplasia classification Bethesda classification Histology classification
I Negative NILM (Negative for intraepithelial lesion or malignancy) Negative
II Squamous atypia ASCUS (Atypical squamous cells of unknown significance) Squamous atypia
    ASC-H (Atypical squamous cells – cannot exclude HSIL)  
  Mild LSIL (Low-grade squamous intra-epithelial lesions encompassing HPV, mild dysplasia and CIN1) CIN1 (Abnormal cells in 1:3 of layers; very unlikely to progress)
  Moderate HSIL (High-grade squamous intra-epithelial lesions, encompassing; encompassing moderate and severe dysplasia, carcinoma in situ, CIN2 and CIN3) CIN2 (Abnormal cells including mitotic figures in 2:3 of layers of epithelium with loss of stratification and differentiation)
IV Severe CIS (Carcinoma in situ)   CIN3 (Abnormal cells across all layers of epithelium; can progress to invasive cancer if untreated)
V Carcinoma Carcinoma Carcinoma



Most cervical cancers develop in the transformation zone, which is the junction between squamous cells of the ectocervix and columnar, glandular cells of the endocervical canal.

More than 99% of cervical cancers contain HPV DNA, although the proportion associated with specific high-risk HPV types (HR-HPV). The IARC study of over 30,000 cervical cancers showed HPV 16, 18, 58, 33, 45, 31, 52, 35, 59, 39, 51, 56, to be the most common types associated with invasive cervical cancer with HPV 16 accounting for over 50% and HPV 16 and 18 for >70% worldwide. 


Intraepithelial neoplasia and cancer at other anogenital sites

Asymptomatic anal and perianal carriage of HPV is common  with a prevalence of 42% reported in HIV− women with high risk behaviours. A similar squamous columnar junction exists between the anus and rectum and approximately 10% of AIN will progress to cancer within 5–10 years, comparable to the progression of CIN3 in the same time frame.

As with CIN, HPV DNA can be demonstrated in all grades of premalignant anal lesions. In low grade AIN, multiple types are found in over 50% with roughly one-third containing HPV 16 and one-third containing HPV 6, and HPV 18, 33, 58 and 45 making up the remainder.

The co-factors associated with anogenital malignancies are similar to those for HPV infection in general, with smoking the most significant.

Other risk factors include receptive anal intercourse, age of first exposure to HPV and number of sexual partners, immune status and genetic background, long-term use of oral contraceptives and history of External Genital Warts.

Vaginal, vulval and penile cancers are all rare. This is thought to be because these sites lack a cellular transformation zone in contrast to both the cervix and anus.

Most penile cancers occur in men over aged 50 and about 50% are HPV associated. Most are SCC, but a few are verrucous carcinomas of the Buschke–Lowenstein type. Symptoms and signs may include the painless appearance of flat blue-brown growths, red rash or warty papules under the foreskin, or on the glans or shaft of the penis; discharge or bleeding may be present. Penile cancer is less common in males circumcised at birth.

Vulvar cancer represents only 3% of gynaecological cancers worldwide (Rumbold et al., 2012). Over the last two decades however, there has been an increase in vulvar cancer in young women, usually arising in areas of basaloid or warty VIN. The peak of vulvar cancer incidence has now shifted from women over 50 to women under 20, with two different aetiological entities being recognised. Vulvar cancer in young women is associated with persistent HR-HPV, usually HPV 16 as in cervical cancer. In post-menopausal women, cancer can arise in vulvar skin following squamous hyperplasia or lichen sclerosus. Interestingly, white women in the US are more often affected by vulvar cancer, while blacks and Hispanics present with more vaginal cancers (Jemal et al., 2013).

Co-infection with HIV and HPV is well known to increase the risk of cancer at several sites. The greater risk of in situ cancers in HIV+ people is associated with increasing immunosuppression.


Head and neck HPV

Oral papillomas

Oral papillomas have been recognised since the 17th century and are the most common benign tumours of the oral mucosa. Common warts can spread from the hands to lips by finger sucking and nail biting, with cutaneous HPV types identified in the lesions. They can become warty on the tongue and hard palate which has a stratum corneum .

Other HPV-induced oral lesions include common warts on the oral mucosa associated with HPV 2; the multiple soft papules of Heck's disease (focal epithelial hyperplasia) found most commonly in Eskimos and North American Indians and associated with HPV 13 or 32; and oral florid papillomatosis which presents as multiple lesions on the buccal mucosa.

The incidence of oral HPV infection and oral warts in adults is low with a peak incidence around aged 20 and acquisition is thought to occur through orogenital contact with an infected partner. While the most common site of infection is the larynx, the oral cavity, trachea, bronchi, lung parenchyma and oesophagus can also be infected


HPV associated oral cancers

There are about 650,000 new cases of head and neck cancer annually worldwide, the majority of which (75–80%) occur in men and are associated with alcohol and smoking in the West and chewing of tobacco and betel nuts. 

 HPV 16 is even more dominant in oral cancers than in cervical cancers and the increase in HPV positive oral cancers is presumed to be associated with changes in sexual behaviours, including oral sex.


Anogenital warts

External warts HPV 6, 11, 40, 42, 43, 44, 54, 61, 72, 81, 89

Buschke–Lowenstein tumour


Bowenoid papulosis

HPV 16, 55

Anogenital cancers and precancers

Group 1: Carcinogenic to humans HPV 16,18, 31, 33, 45, 51, 52
  Group 2A: Probably carcinogenic to humans HPV 68
  Group 2B: Possibly carcinogenic to humans HPV 26, 53, 64, 65, 66, 67, 69, 70, 73, 82
Oral lesions Oral papillomas HPV 2,6,7,11,16,18,32,57
  Laryngeal papillomas HPV 6,11
  Focal hyperplasia (Heck's disease) HPV 13, 32
  Oropharyngeal carcinoma HPV 16 predominantly,18
Common warts   HPV 2, 4, 7; occasionally other types in immunosuppressed (e.g. HPV 75–77)
Flat plane warts   HPV 3, 10, occasionally HPV 26–29 and 41
Plantar warts   HPV 1, 2, 4
Epidermodysplasia verruciformis Plane warts HPV 3, 10
  Pityriasis-like plaques HPV 5, 8; less commonly 9,12,14,15, 17, 19,20, 21–25, 36–39, 47,49
  Squamous cell carcinomas of sun-exposed skin HPV 5, 8, less commonly 14,17,20 and 47



  • Common warts are associated with HPV types 2, 4 (most common), followed by types 1, 3, 27, 29, and 57
  • Flat warts are caused by types 3, 10, and 28
  • Deep palmoplantar warts are caused by types 1 (most common) followed by types 2, 3, 4, 27, and 57
  • Cystic warts are caused by type 60
  • Focal epithelial hyperplasia is caused by types 13 and 32
  • Butcher's warts are caused by type 7


Warts are a common medical problem,


Warts are common worldwide and affect approximately 10% of the population. In school-aged children, the prevalence is as high as 10% to 20%. They are more common among immunosuppressed patients and meat handlers. 


Warts can occur at any age. Although rare in infancy and early childhood, prevalence increases among school-aged children and peaks at 12 to 16 years.


Warts are twice as common in Whites as in Blacks or Asians. Focal epithelial hyperplasia (Heck disease) is more prevalent in Inuit and American Indians.


The male-to-female ratio is approximately equal.



Out of the 100 subtypes of HPV, a few of them have the propensity to induce cancer. These subtypes include HPV strains 6, 11, 16, 18, 31, and 35. Malignant transformation tends to occur in individuals with genital warts and those who are immunocompromised. HPV strains 5, 8, 20, and 47 also have malignant potential in individuals with epidermodysplasia verruciformis.



Common wart (Verrucca vulgaris)

Histopathologic features include acanthosis, digitated epidermal hyperplasia, papillomatosis, compact orthokeratosis, hypergranulosis, tortuous capillaries within the dermal papillae, and vertical tiers of parakeratotic cells with red blood cells entrapped above the tips of the digitations. Elongated rete ridges may point radially toward the center of the lesion. In the granular layer, cells infected with HPV have coarse keratohyalin granules and vacuoles surrounding wrinkled-appearing nuclei. Koilocytic cells are pathognomonic.


Butcher's warts have acanthosis, hyperkeratosis, and papillomatosis. Small vacuolized cells are seen, and centrally located shrunken nuclei may be identified in clusters within the granular layer rete ridges.


Filiform warts appear similar to common warts, but they may have prominent papillomatosis.

Focal Epithelial Hyperplasia (Heck disease)

Focal epithelial hyperplasia is characterized by acanthosis, blunting, hyperplastic mucosa with thin parakeratotic stratum corneum, anastomosis of rete ridges, and whiteness of epidermal cells due to intracellular edema. Some may have prominent keratohyalin granules, and vacuolated cells may be present.

Deep Palmoplantar

Deep palmoplantar warts are similar to common warts except the lesion lies deep to the plane of the skin surface. The endophytic epidermal growth has the distinctive feature of polygonal, refractile-appearing, eosinophilic, cytoplasmic inclusions made up of keratin filaments, forming ringlike structures. Basophilic parakeratotic cells loaded with virions and basophilic nuclear inclusions and may be in the upper layers of the epidermis.


Flat warts are similar to common warts in light microscopy. Cells with prominent perinuclear vacuolization around pyknotic, basophilic, centrally located nuclei can be in the granular layer. These are referred to as "owl's eye cells."


A cyst wart is filled with horny material. The wall is composed of basal, granular, and squamous cells. Many epithelial cells have large nuclei and clear cytoplasm with eosinophilic inclusion bodies. The cyst may rupture causing a foreign body granuloma.




  1. Al Aboud AM, Nigam PK. Wart. [Updated 2021 Aug 11]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from:
  2. Diseases associated with human papillomavirus infection, Virology, Volume 445, Issues 1–2, 2013, Pages 21-34, ISSN 0042-6822,
  3. Krause KA, Neelon D, Butler SL. Koilocytosis. [Updated 2021 Aug 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from:


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