Current concepts on human papillomavirus infections in children

Abstract

Syrjänen S. Current concepts on human papillomavirus infections in children. APMIS 2010; 118: 494–509.

Current evidence is strong enough to conclude that human papillomavirus (HPV) can be transmitted both sexually and non-sexually. The debate on HPV infections in children still continues but it is more focused on HPV prevalence than on transmission modes. HPV DNA detection in amniotic fluid, foetal membranes, cord blood and placental trophoblastic cells all suggest HPV infection in utero, i.e. prenatal transmission. Based on recent meta-analysis, vertical transmission occurs in approximately 20% of cases. Most of the mucosal HPV infections in infants are incident, persistent infections in oral and genital mucosa being found in less than 10% and 2% respectively. The mother seems to be the main transmitter of HPV to her newborn, but subsequent HPV infections are acquired horizontally via saliva or other contacts. Bimodal peak prevalence is seen for skin warts, oral papillomas and recurrent respiratory papillomatosis (RRP) in younger and older age groups, suggesting similar epidemiology. Of the clinical HPV diseases, juvenile-onset-RRP and genital condylomata are problematic; the former because of its life-threatening potential and the latter because of possible sexual abuse. HPV6 and 11 are the most common genotypes in both the lesions. Early in life, infections by the high-risk HPV genotypes may also remain persistent for a considerable period, and should be of considerable importance for HPV vaccination strategies.

Human papillomaviruses (HPVs) are common pathogens in adults but can also cause a variety of mucosal and cutaneous infections in children. Skin warts and laryngeal papillomas are the most widely recognized benign HPV lesions in the paediatric population. Both legal and clinical implications are involved in anogenital warts, and the possibility of sexual abuse should always be excluded. Recently, asymptomatic HPV infections in children, their clinical significance as well as transmission modes have become important subjects in HPV research, especially in the emerging era of HPV vaccines. In this communication, a short overview is made on paediatric HPV infections, their risk factors, natural history and potential modes of transmission.

HPV-associated diseases

Skin warts

Current evidence suggests that HPV infections with multiple skin HPV types are acquired during early infancy (1). These cutaneous HPV types can persist over a long time in healthy skin (2, 3), with hair follicles being suggested as possible reservoirs. The prevalence of beta and gamma HPV species seems to increase with age from 5% at <20 years to 76% at >60 years (4). It has been shown that 26–32% of HPV-positive children shared at least one HPV type in common with their mother or father at the same time point (1, 2, 4). However, donors other than parents also need to be considered as sources of viral dispersal (4).

Skin warts are caused most frequently by the cutaneous HPV types 1, 2, 3, 4, 27 and 57. These different skin types are associated with distinct histology of the warts (5). Warts are rare in children before 5 years of age. Common warts (verruca vulgaris) represent 70% of all skin warts and occur more commonly in younger children than plantar (myrmecias) and flat warts (verruca plana). This might suggest different modes of transmission for these distinct HPV genotypes (6).

There are few recent epidemiological studies on skin warts. According to the older literature, the peak prevalence of childhood warts is between 10 and 14 years, followed by a rapid decline by the age of 20 years (7). Based on medical records from a national birth cohort of 9263 children, skin warts were found in 3.9% and 4.9% of the British school children at the age of 11 and 16 years respectively (8). Residence in the south of Britain, having a father with non-manual occupation, being the only child and belonging to an ethnic group other than white European were all associated with decreased risk of manifest warts. No sex differences were found in the prevalence of skin warts (8). In a recent study from Turkey, 6300 paediatric patients under 16 years were analysed, admitted to hospital between 2004 and 2006 (9). The prevalence of skin warts increased from 5.4% in the age group of 3–5 years to 7.1% among children aged 6–11 years. No warts were reported in children younger than 2 years. As in the British study (8), no gender difference was found (9). In a community-based survey, hand warts were found in 2.8% (95% CI 2.24–3.38%) of school children in Taiwan (10).

Skin warts might predict some failure in immunity and thus also susceptibility to genital HPV lesions. It has been shown that common warts and eczema at the age of 11–16 years were significantly associated with subsequent cervical cancer (11). When common warts and eczema were combined into a single variable, those with either common warts or eczema had an adjusted OR for cervical cancer of 2.5 (95% CI 1.2–5.14), and those with both common warts and eczema had an adjusted OR = 10.21 (95% CI 1.22–85.54) (11). Consistent with this, we previously demonstrated that the presence of HPV DNA in oral mucosa was significantly associated with skin warts among women with genital HPV infections (12).

Oral papillomas

Based on the old literature, oral papillomas are the most common (7.5%) epithelial tumours of the oral mucosa in children. However, there are no population-based or other systematic studies on oral papillomas in adults or children. Based on our survey of the literature published until 1998, approximately 50% of the 223 reported lesions (both children and adults) are caused by HPV, mostly by HPV 6 or HPV 11 (13). Oral papillomas cannot be reliably distinguished from oral condylomas either histologically or clinically. In this literature, an additional 116 lesions classified as oral condylomas were found, 75% being HPV positive, either HPV 6 or HPV 11. In our cohort of children born to mothers with genital HPV infection, we correlated HPV DNA status (PCR and oral scrapings) with the clinical appearance of oral mucosa (14). Clinically, minor hyperplastic growths were found in 22.4% of the 98 children, with the mean age of 4.0 years (range 0.6–11.6 years). Eight of these children with clinical findings tested HPV positive (36.4%). Interestingly, a 7-year-old girl had an HPV16-positive oral papilloma, and the same HPV type was also detected in the genital tract of her mother at delivery (14).

Recurrent respiratory papillomatosis

Recurrent respiratory papillomatosis (RRP) has a bimodal age distribution, which forms the basis of their classification as juvenile- (JO) or adult-onset (AO). Recently, a comprehensive review on RRP literature was published (15). Juvenile-onset RRP (JO-RRP) is presented in prepubertal children usually before 5 years of age, while in adults the typical age is 20–40 years. The younger the age of onset, the more severe is the disease (15, 16). RRP presents with multiple squamous cell papillomas on the vocal cords, followed by transmission to the false cords, epiglottis and sub-glottic area, and more rarely into the trachea and even bronchi (15, 17–19). JO-RRP is a potentially life-threatening disease, because it shows a tendency to grow in size and number of lesions causing total respiratory obstruction. The symptoms include hoarseness, chronic dyspnoea and cough, present from 2 months to >2 years before definitive diagnosis has been settled (15). Similar to oral papilloma, laryngeal papilloma also represents the most common benign tumour of the larynx in infants and children. In a Danish population, the incidence and prevalence of JO-RRP were 0.6 and 0.8/100 000 respectively (18, 20). In a paediatric population of the United States, the incidence was approximately 1.7–4.3 per 100 000 (15, 21). JO-RRP is almost invariably associated with HPV type 6 or 11, and HPV11 is more likely to cause a more severe disease with earlier onset (21–23).

In 1956, Hajek wrote: ‘multiple laryngeal papillomata are found in small children and adolescents. They are not hereditary, but in 20% of cases can be found at birth’ (24). Since then, several studies have demonstrated a relationship between JO-RRP and maternal genital condylomata in 30–50% of the patients (17, 25–27). However, the prevalence of genital condylomata among women of childbearing age far exceeds the reported number of new cases of JO-RRP. The risk of transmission from an HPV-infected mother to her newborn has been estimated to range from 1:80 to 1:1500 (28). In a retrospective study on a cohort of Danish births between 1974 and 1993 by Silverberg et al. (29), seven of every 1000 births with maternal history of genital warts during pregnancy resulted in laryngeal lesions. In women with genital warts, delivery exceeding 10 h was associated with a twofold risk of disease.

However, the majority of children who subsequently developed JO-RRP have been born to mothers with no history of genital warts during pregnancy, and these mothers might have HPV as a subclinical infection. Children whose mothers had a history of genital warts have been reported to develop JO-RRP at an earlier median age than children without such a history (4.3 vs 5.9 years) (29). Other risk factors associated with JO-RRP include maternal age <20 years, first-order births and vaginal delivery (30). Recently, the susceptibility to AO-RRP was associated with DRB1*0301, while HLA-DRB1*14 increases the risk of JO-RRP (31). A prospective study of Stern et al. (32) brought new evidence that cellular immunity is compromised in children with JO-RRP. However, it remained unclear whether HPV causes this impaired response or whether children with impaired immunity will develop JO-RRP. Interestingly, there are no documented cases of RRP occurring among siblings, marital partners or family members, suggesting the importance of impaired mucosal or systemic immunity in the development of RRP.

Malignant transformation of laryngeal papilloma to carcinoma has been reported in 3–5% of the RRP patients, but nearly all cases are associated with previous irradiation of the papillomas or history of heavy smoking (33, 34).

Anogenital warts

Children with HPV infections, especially in the anogenital tract, will often raise the suspicion of sexual abuse (35–44). However, several studies indicate that the origin of paediatric anogenital HPV infections remains often untraced, with no indication of sexual abuse.

Until 1990, only 174 children with anogenital warts had been reported (13). Nearly 500 additional cases were published by 1998, however, indicating a rapid increase in awareness of this disease (13), although one cannot exclude the possibility that the prevalence of anogenital warts in children is truly increasing. Interpretation of these data is still difficult, because most studies are based on small series or selected individuals.

HPV DNA was found in 78% of the 254 anogenital warts identified in the literature by 1998. HPV6 and 11 were the two most common (56%) types, but HPV16 and 18 were also identified in 4% of the warts. Cutaneous HPV types 1–4 were present in 12% of the samples. A wide variation in HPV detection rates and genotypes is evident among the individual studies, not unlike in most other HPV lesions at different anatomical sites. These divergent results can be explained by several factors including the sensitivity of the HPV test, limited number of cases, quality of the analysed samples and the sampling method itself (13).

Detection of HPV types 6, 11, 16 or 18 has often been interpreted as an indication of sexual or vertical transmission, whereas the presence of skin HPV types would rather implicate hetero- or autoinoculation. As HPV6, 11 and 16 are commonly found in oral lesions, it should be kept in mind that these types can also be transmitted by inoculation. There is a chance that the high-risk types HPV16 and 18 in anogenital warts might predispose the child to the development of genital neoplasia in the future, but no such follow-up studies are available. Handley et al. (45) followed up 42 prepubertal children with anogenital warts (15 boys and 27 girls) for 15.9 months. Most (73.8%) of the children had perianal condyloma-type warts and 26.2% had concurrent non-genital warts. None had any other anogenital infections or STDs. In 31 children, the warts were treated with a combination of scissor excision and electrocautery. During the follow-up, 31.4% of the warts recurred, all within 4 months after treatment (45). Spontaneous resolution of the warts was seen in 21.4% of cases. Of these 42 children, 23.8% had at least one adult family member with anogenital warts, 36.9% one adult family member with another anogenital infection or STD, 62.2% had a mother with CIN and 47.6% had a family member with extra-genital warts.

Asymptomatic HPV infections

It is currently agreed that newborns can be exposed to HPV mostly via maternal genital HPV infection (46, 47). The frequency of vertical HPV infection remains controversial (48, 49). Only one systematic review on vertical transmission of HPV is available (50). The concordance of HPV types detected in infants and their mothers is in the range of 57–69% (46, 47). Prospective studies with several time points confirmed that a substantial number of newborns are infected with mucosal HPVs, and HPV might even persist both in oral and genital mucosa of the children. This raises concern first with regard to the development of intraepithelial neoplasia in these sites and second with the optimal time point of HPV vaccinations. The transmission modes of HPV in children are discussed in more detail later. Before that, a short overview is made on HPV detection rates at different time points after delivery.

Genital tract

HPV DNA has been also found in the genital tract of newborns. Within 3 days after delivery, HPV DNA detection varied from 0% (51) to 53% (52). However, in the latter study, the overall HPV detection rate of oral and genital mucosa was reported simultaneously (52). Specific HPV genotyping data from prospective studies are still scanty, but the results favour an idea of transient and early incident HPV infections in most cases (52, 53). High-risk types are frequently detected in genital samples of newborns; HPV16 seems to be the most prevalent genotype, but HPV6, 18, 54 and 61 have also been found.

Follow-up studies have shown that HPV detection declines with the increasing age of the infant. At the age of 6 weeks, HPV detection rate varies from 0% to 33% (52–55), at 6 months, from 0% to 3% (53, 55), and up to 33% (54). In most studies, HPV DNA in the infant’s genital mucosa seems to clear within 1 year after delivery. In the majority of these studies, HPV DNA detection rate varies between 2.7% and 4.9% (55–60). As in younger infants, HPV 6, 16, 18, 31 and 33 have been detected. In our study, HPV DNA detection decreased to 4% at 36 months of age (60), which could implicate virus clearance by HPV antibodies as suggested by coexistent seroconversion.

Watts et al. (55) followed up infants for up to 3 years, but the detection rates of HPV DNA were low. At 12, 18, 24 and 36 months of follow-up, only 2, 1, 1 and 0 infants had an HPV-positive genital sample. Koch et al. (58) studied 708 children aged 0–17 years and reported four HPV-positive anal samples; two in 2-year-old children, one in a 5-year-old boy and one in a 11-year-old boy.

Finnish Family HPV Study –  We found HR-HPV DNA in 15% of genital mucosa in the newborn. Sixteen per cent of pregnant mothers were also HR-HPV DNA positive in their third trimester of pregnancy (59, 60). After delivery, HPV DNA detection slightly decreased during the first month of follow-up, followed by peaking at 6 months and another decline to 10% at 24 months (60). The increase in genital HPV prevalence at the age of 6 months might be due to diminished protection by maternal antibodies and/or due to acquired new HPV infections.

Almost half of the infants tested always HPV negative in the genital mucosa during the follow-up, while 36% acquired an incident infection and 14% cleared their HPV (60). Persistent genital HPV infection was detected only in 1.5% of the infants during the follow-up (60). These persistent genital HPV infections were predicted by mother initiating smoking at 18–21 years of age, and by the history of genital warts.

Oral mucosa

HPV DNA can also be detected in the healthy oral mucosa of children. The detection rate of asymptomatic oral HPV infection (or presence of HPV DNA) varies from 0% to 47% in children aged 0.3–11.6 years (14, 55, 58, 61–63). Moreover, these asymptomatic infections show a bimodal age distribution, similar to skin warts, oral papillomas and RRP. The highest prevalence is detected before 1 year of age, and the second peak at the age of 13–20 years (64–66).

The detection rate of HPV in nasopharyngeal aspirates taken immediately after delivery varies from 1.5% to 37% (14, 54, 55, 59, 60, 68–72). The following HPV types were detected: HPV6, 11, 16, 18, 31, 33 and some unknown types.

There are seven studies on HPV DNA detection in the buccal swabs of infants at the age of 1–4 days (51–54, 65, 66, 73). HPV DNA detection rates show much variation from 0.9% to 56%. The detection rate was only 0.9–1% in the two studies of Smith et al. (51, 65), while four other studies found HPV in 40–56% of the samples (52–54, 73). One obvious source of variation is the method of sampling. Buccal swabs, which were used in most of these studies, might not always give representative samples. Moreover, the methods used have different sensitivity (Southern blot hybridization, Vira-Pap which is a dot blot hybridization method and PCR). Also the size of the cohorts was different, with a range from 16 to 571 infants. The following HPV types were detected: HPV6, 11, 16, 18, 31, 33, 35, 51 and 61. While comparing these data from infants at the age of 1–4 days with those immediately after delivery, it seems that HPV DNA detection increases a few days after delivery, possibly because of new incident HPV infections or just reflecting a hospital contamination.

HPV DNA seems to persist in the oral mucosa of some infants at least 6 weeks after delivery, with detection rates varying between 0% and 62% (49, 52, 53, 55, 74). In these studies, the following HPV types were detected: HPV6, 11, 16 and 18. Contradictory to the highest detection rate of 62% by Cason and Mant (49), two other studies reported completely negative results (52, 55). This could implicate either clearance of a true oral HPV infection or merely disappearance of a passenger.

At 6 months, Cason and Mant (49) reported that 59% of oral swab samples were still HPV16 positive, while Kaye et al. and Watts et al. reported totally negative results (52, 55). In the Finnish Family HPV Study, HPV detection in oral scrapings was the highest (21%) at 6 months of age (60), using nested PCR and hybridization of the PCR products with a cocktail of 12 different HR-HPV types.

The prevalence of HPV DNA among 3-year-old children was reported to vary between 10% and 40% in the oral samples (14, 69, 75, 76). Thereafter, HPV prevalence even increased up to 50% among children aged 2.5–11 years (61, 63, 76). However, a much lower HPV prevalence (<10%) has been reported in children under 11 years of age (58, 64). Overall, HPV seems to persist longer in the oral cavity than in the genital mucosa of children. The most prevalent HPV types in asymptomatic oral infections seem to be HPV16, followed by HPV6/11 (13, 46, 47).

Acquisition and clearance of HPV in the buccal mucosa of 4- to 9-year-old children were studied recently (77). During the follow-up of 30 months, 63% of 19 initially HPV-negative children acquired new HPV16 infection, while 40% of 22 initially HPV-positive children cleared the virus. These acquisition and clearance rates were higher than those detected in our cohort, but the children were older and HPV DNA was detected by a more sensitive assay for HPV E5 open reading frame (sensitivity <10 copies in nested PCR).

Finnish Family HPV Study –  In our cohort, the prevalence of HPV DNA in the oral mucosa fluctuated from 14% at delivery to 21% at 6 months, and lowered to 12% at 36 months (59, 60). Our figures present oral HPV infections and not oropharyngeal HPV, because all scrapings were taken only from the oral mucosa avoiding the posterior part of the tongue. During the 3-year follow-up, 42% of the infants acquired incident HPV infection, while 11% cleared their HPV and 10% had persistent oral HPV infection (60). HPV persistence was associated with oral HPV in the parents, hand warts in mothers, young age at the onset of sexual activity by the mother and mother’s use of oral contraceptives (60). Interestingly, father’s sexual habits were not related to infant’s persistent oral HPV infection, but father’s oral HPV increased the risk.

HPV and tonsils

In adults, HPV has been detected in 50–80% of oropharyngeal cancers, mostly attributed to HPV in tonsillar carcinomas (78). Thus, it is important to clarify the natural history of HPV infections in the tonsils, prevalence of HPV in children and the time when tonsillar HPV infections are acquired. Unfortunately, only few studies on HPV infection in tonsillar and adenoid hyperplasia exist, with HPV detection rates varying from 0% to 8.5% (79–82). Chen et al. (2005) found HPV in 6.3% of tonsillitis and hypertrophic tonsillar tissues, and in 0.6% of exfoliated cells from normal tissues (81). Importantly, they found only HPV16, but it did appear to lead to L1 antibody response. Similarly, Sisk et al. (80) also found predominantly HPV16 in tonsillar samples testing HPV positive.

HPV serology in children

Currently, very little is known about the HPV serostatus of the foetus or newborn. It is known that the foetus can generate IgM and IgG as a response to intrauterine infections. However, antigen-specific immunity is mainly provided by maternally derived IgG antibodies (83, 84). Indeed, IgG antibodies cross the placenta after 16 weeks of gestation by an active placental transport mechanism and reach similar concentrations as in the mother by 26 weeks of gestation. IgM and IgA cannot cross the placenta. Recently, Heim et al. (84) showed that neonatal IgG antibodies to HPV detected with virus-like particle (VLP)-ELISA were transferred from their mothers. Persistence of IgG antibodies was found in one infant. Also a few HPV11- and HPV31-positive IgM or IgA in newborn sera were found but their significance was not discussed. Thus, these results indicate that neonatal IgG antibodies to HPV are not a sign of intrauterine HPV infection but rather derived from maternal–foetal antibody transmission.

There are several studies on HPV seroprevalence in children (84–94), which were reviewed by Cason and Mant (49). All studies except that of Andersson-Ellstrom et al. (94) indicate that prepubertal children have been exposed to HPV16 or other HR-HPV types. A bimodal distribution of seropositivity was found to peak between 2 and 5 years, as well as between 13 and 16 years of age (49). The detection rate of serological response seems to be related to HPV antigen used in the assay. When HPV16 peptides were used, detection was much higher, with a range of 16–44%, than obtained using HPV16 VLPs (4.5–15%). Seroprevalence is also related to the number of VLPs included in the analyses. Marais et al. (91) showed that combining the results of the tests to individual VLP types, 22.6% of the children were seropositive to at least one of the following VLP types: 16, 18, 31, 33 and 45. This means that a relatively large number of children have been exposed to HR-HPV types. They also showed that seropositivity drops off with age, and infection occurs early in life. Furthermore, seropositivity was more prevalent among boys (18/54) than in girls (8/61), but the reasons for this difference remain unclear. af Geijersstam et al. (95) reported much lower antibody positivity for HPV16, 18 and 33 (5.2%, 5.2% and 8.6% respectively) among 58 children aged between 0 and 0.5 years. Manns et al. (87) screened HPV16 IgG antibodies in 92 two-year-old children and found only three children testing seropositive. These studies provide serological evidence on vertical transmission of HPV. Moreover, Kawana et al. in 2003 demonstrated neutralizing capacity of both maternal and newborn HPV-positive IgG sera in two cases where maternal condylomata occurred during pregnancy (93).

Recently, an extensive study on HPV sero-epidemiology was published in Germany (96). Simultaneously, antibodies to 34 HPV types were analysed by multiplex serology, which uses the Luminex technology. HPV antibody prevalence in children was mostly low. The highest seroprevalence was found for HPV types 3 (8.6%), 1 (6.4%) and 4 (4.8%). Antibodies to HR mucosal α-papillomaviruses were rare in children (HPV16 0.5%, other HR types 0.0–2.1%) (96). In line with that study, Dunne et al. reported an overall HPV16 seroprevalence in 2.4% of 1316 children aged 6–11 years (97). Seropositivity against any mucosal types was more common in boys than in girls in the German study (boys 7.6%, girls 2.1%) (96) and in the study of Dunne et al. (97) (boys 3.5%, girls 1.2%). Contradictory to that, seropositivity to cutaneous HPV types was less common in boys than in girls, i.e. 29.3% and 34.7% respectively (96). Seropositivity to mucosal HPV types was more prevalent in children over 7 years of age than in younger children (3.3% vs 0.4%) (97). Interestingly, in studies reporting HPV antibodies in children, there was no correlation between seropositivity and HPV DNA detection in either oral or genital samples (13, 46, 47, 98).

Taken together, detection of seropositivity for mucosal HPV types in children raises several questions: (i) do these antibodies neutralize the virus, or (ii) do they help the virus escape the immunological defence system, (iii) what is the age when prophylactic vaccinations should be given and finally (iv) is there any need to identify seropositive individuals before vaccination.

Modes of HPV transmission

Infections with α-papillomaviruses have been regarded as a sexually transmitted disease. Detection of HPV in virgins, infants and children suggests that vertical transmission exists, but the exact rates and routes have not been well established (46–49, 72). Possible non-sexual transmission modes include vertical or horizontal transmission and autoinoculation. Vertical transmission can be divided into three categories according to the assumed time of HPV transmission: (i) peri-conceptual transmission (time around fertilization), (ii) prenatal (during pregnancy) and (iii) perinatal (during birth and immediately thereafter).

Peri-conceptual transmission

Peri-conceptual transmission could theoretically occur via the infected oocyte or spermatozoon. HPV DNA has been detected in 8–64% of the semen samples from asymptomatic men (99–106). Both seminal plasma and spermatozoa have been shown to contain HPV DNA (103). Moreover, HPV16 has been transcriptionally active in spermatozoa (102, 103). Consistent with this, we found HPV DNA in vas deferens biopsies (105). It cannot be excluded that HPV was present on the endometrium already at the stage of trophoblast invasion. HPV DNA has been detected high up in the female genital tract up to the endometrium and even ovaries, but the significance of these findings is uncertain (107–109). Currently, no studies exist on HPV detection in oocytes. Thus, theoretically, subsequent virus transmission might originate from the embryos soon after fertilization.

Prenatal transmission

There are also data favouring intrauterine transmission, because HPV-induced lesions are occasionally present at birth (24, 37, 110–112). HPV DNA-positive infants have been born to HPV-negative mothers (4, 49, 68, 69). However, hospital contamination in these cases cannot be excluded. Caesarean section does not completely protect newborns against HPV (29, 37, 69, 73, 113).

Further evidence has been provided by studies reporting HPV DNA in amniotic fluid, placenta and cord blood samples. During the viraemic phase, chorionic and placental tissues may be infected by the direct transmission of certain viruses to amniotic cells that are subsequently ingested by the foetus (114–118). However, the viraemic phase has not been confirmed for HPV, and haematogenous transmission of HPV to foetus remains unknown. Some early studies have reported the presence of HPV DNA in peripheral blood mononuclear cells (PBMCs) (114, 119, 120). HPV DNA has also been detected in banked, frozen blood cells from paediatric HIV patients and in fresh blood cells from healthy donors (120). Tseng et al. (114) noted that HPV DNA in umbilical cord blood was more closely related to the status of HPV DNA in maternal peripheral blood samples and maternal cervico-vaginal cells. However, we failed to detect any HPV in maternal peripheral blood samples (121).

Other possibilities for intrauterine HPV transmission could be an ascending HPV infection from the maternal genital tract through micro-tears in foetal membranes or with blood through the placenta (114, 115). Armbruster-Moraes et al. reported positive correlation between the grade of cervical lesions and the presence of HPV DNA in the amniotic fluid and suggested ascending infection (115).

Trophoblastic cells have been shown to be broadly permissive for HPV (122, 123). HPV11, 16, 18 and 31 are able to complete their life cycle in cultured placental trophoblasts (122–124). HPV has been shown to decrease both the trophoblast cell numbers (in a dose-dependent manner via apoptosis) and the trophoblast–endometrial cell adhesion (125). These in vitro studies support the hypothesis that part of the spontaneous abortions could be caused by HPV infection of the trophoblasts (122, 125, 126). Previous studies have implicated HPV infection of the placenta as contributing to some miscarriages (127–129), genetic abnormalities of the foetus (128) and spontaneous preterm delivery (126). We have recently localized HPV6 and 16 in syncytiotrophoblasts using tyramine-amplified in situ hybridization, which detects one HPV copy per cell (121).

Detection rates of HPV DNA in placental samples have varied from 0% to 42.5%. In the Finnish Family HPV Study, we found HPV DNA in 4.2% (n = 13) of the placental samples, and HPV types 6, 16 and 83 were identified. Two of the HPV+ placental samples were obtained from caesarean sections and the remaining 11 from vaginal deliveries. The mean time from rupture of the membranes to delivery was the same for HPV+ and HPV− mothers. HPV detection in the placenta was significantly associated with Pap smear abnormality (ASCUS) at enrolment (OR = 5.3, 95% CI 1.63–17.35, p = 0.011). If the placenta was HPV+, the risk of the neonate of being a carrier of oral HPV at delivery increased 8.6-fold (p = 0.001, 95% CI 2.73–27.13) (121). In our study, all HPV-positive placentas were from normal pregnancies. The gestational age, history of preterm births or previous miscarriages did not differ between mothers with HPV+ and HPV− placenta. All HPV+ placentas appeared normal upon examination by midwives in the delivery ward.

Substantiating our data, Rombaldi et al. found placental infections in 23.3% and trans-placental transmission in 12.2% of their cases (130). They also reported a significant association between placental HPV and immunosuppressive status of the mother. Interestingly, in a very recent study by Fedrizzi et al., HPV DNA was 3.5 times more frequently present in the normal endometrium of smokers than in non-smokers (25% vs 7%), although this difference was not statistically significant (109). We found a similar trend in our placental samples, where HPV DNA was three times more prevalent among women who had ever smoked compared with never-smokers (6.4% vs 2.1%) (121).

Cord blood and HPV –  Previous studies have found HPV DNA also in cord blood with a prevalence varying from 0% to 13.5% (114, 121, 129–132). In our series, HPV DNA was present in 3.5% of the 311 cord blood samples. HPV genotypes 6, 16 and 39 were detected (121). As with the placentas, HPV positivity of cord blood was significantly associated with maternal Pap smear abnormality at enrolment (OR = 5.0, 95% CI 1.39–18.18, p = 0.024). When umbilical cord blood was HPV positive, the risk of the neonate being a carrier of genital HPV DNA at delivery increased 4.0-fold (p = 0.048, 95% CI 1.08–14.83) and of oral HPV DNA 4.4-fold (p = 0.039, 95% CI 1.17–16.14). In a univariate analysis, HPV positivity of cord blood was significantly associated with mother’s marital status, the number of past sexual partners and a history of genital warts. In a multivariate regression model, a history of genital warts proved to be the only independent predictor of cord blood HPV (OR = 4.0, 95% CI 1.09–14.54, p = 0.036). This was not confounded by maternal age. Peripheral blood samples from all mothers with HPV-positive placenta or cord blood (n = 19) tested negative for HPV DNA (121).

In general, the HPV DNA detection rate in neonates did not differ by mode of delivery. After vaginal birth, 14.4% (35/243) of the neonates tested positive for genital HPV and 10% (23/242) for oral HPV. After caesarean delivery, the detection rates were 18% (12/65) and 6% (4/64) respectively. HPV detection in cord blood or placenta was associated with a maternal history of productive genital HPV infection, but not with maternal HPV DNA detection before delivery (121). This may be due to inadequate sampling or false-negative DNA scrapings. Moreover, HPV may have infected the placenta at an earlier stage of pregnancy and could have cleared from the cervix by the time of delivery.

Amniotic fluid and HPV –  Other studies have also confirmed the detection of HPV DNA in the amniotic fluid, with detection rates varying from 15% to 65% (115, 116, 133).

Perinatal transmission

Vertical transmission is thought to result mainly from a close contact of the foetus with infected cervical and vaginal cells of the mother during delivery. Confirmation of virus acquisition from the mother has been obtained from several studies by detecting HR-HPV DNA in cervical samples of the mother before the delivery and in the nasopharyngeal aspirates or genital swab samples of the neonate. However, the debate continues on the detection rate of HPV and whether HPV positivity reflects passive contamination or true infection of the infant (46–48, 55, 62, 98, 134). The concordance between HPV infections of the mother and the infant has been 39%, with a range between 0.2% and 73%. There is some evidence that mothers who transmitted infection to their infants had significantly higher viral load in cervical samples than those who did not (52, 119). Currently, the only way of estimating the persistence of HPV infection is to detect HPV DNA in subsequent samples during the follow-up. Some studies indicate that HPV DNA is detectable only for 2–4 days after delivery implicating a passive contamination (65, 71). However, some authors report that HPV DNA has been detected up to 6 weeks (52, 53, 74), 6 months (54), 12 months (55) and even 3 years after delivery (14, 69, 75, 76). Our recent results have already been discussed in the section on asymptomatic HPV infections.

The first systematic review on vertical transmission of HPV included 2111 pregnant women and their 2113 newborns (50). Pooled mother-to-child HPV transmission was 6.5% and was shown to be higher after vaginal delivery than after caesarean section (18.3% vs 8%) (RR = 1.8; 95% CI 1.3–2.4). The combined relative risk of mother-to-child HPV transmission was 7.3 (95% CI 2.4–22.2). However, there was extensive clinical heterogeneity as in most meta-analysis (50).

Horizontal transmission

Recent studies also suggest that oral HPV infections in infants of HPV-negative mothers could be explained by the horizontal transmission of HPV (14, 54, 69). Additionally, infants might theoretically acquire HPV infection from breast milk during breast-feeding, from siblings via kissing, or from other householders, relatives and friends via digital contacts (13, 46, 47, 49, 134–138). A recent study examined the presence of HPV DNA in the secretion of mammary ductal epithelium (135). This study included 25 breast milk and 10 colostrum samples from lactating Japanese mothers, and the detection rate of HPV DNA in these milk samples was ∼8% (3/35). In breast milk, HPV DNA was detected in 4.5% and 19.7% of samples obtained 3 days and 2 months after delivery respectively. We also found HPV16 DNA in 223 breast milk samples taken 3 days postpartum, but did not find any association with the HPV status of the infants (139).

The increased incidence of cutaneous warts in children more than 5 years of age is believed to be due to exposure to common showers during the first years of school (136). However, Puranen et al. did not find HPV DNA in the floors and seats of bathing resorts, indoor swimming pools, schools and private homes (140). Anogenital warts in children can occur as a result of hand warts in the child or relatives (141, 142). The proportion of cutaneous HPV types found in genital warts of children is higher than in adults (15% vs 2% respectively) (135).

Autoinoculation

Autoinoculation of HPV, e.g. by scratching from one site of the body to another, is also possible (46, 47). Sonnex et al. found that 27% of subjects had the same HPV type detected in both genital and finger samples (138). HPV type 2 is frequently detected in lesions of the oral mucosa or lips, and it might be acquired by chewing of common warts present on hands (46, 47).

Sexual abuse

Sexual abuse has been regarded as a possible cause of childhood genital warts associated with mucosal HPV types. The forms of sexual abuse are oral–genital contact, genital–genital contact, genital–anal contact, fondling and digital penetration of the vagina or anus [reviewed in (13, 46, 47)]. Reported frequency of sexual abuse varies considerably depending on the series studied (13, 47). Since 2001, 217 of the 1211 children with anogenital warts were reported to be sexually abused, 54 anogenital warts were transmitted vertically, and the rest of the children had unknown transmission mode of their genital warts (47). Only three of the 409 children with anogenital warts were proved to be sexually abused (144–146). In line with these findings are the results of three other studies with large cohorts (n = 310, 1538, 3040) reporting genital warts only in 1.3%, 1.8% and 1.7% of sexually abused children (147–149). Sexually abused children also often have other signs of abuse. The likelihood of possible abuse as a source of HPV infection seems to increase with age. The positive predictive value of HPV for possible sexual abuse was 36% (95% CI 1.13–1.65) among children 4–8 years of age, and 70% (95% CI 1.35–1.93) in children >8 years of age (150).

Most of these data suggest that anogenital and laryngeal HPV infections among preadolescent children result from non-sexual transmission acquired either perinatally or postnatally (47). Indications of suspected sexual abuse in children should always include behavioural indications of abuse, medical examination to identify physical indications of abuse, microbiological assessment of other STDs, and age-appropriate interviews of the child and caretakers by skilled personnel (143–146).

Finnish Family HPV Study

The Finnish Family HPV Study was initiated in 1998 to assess the dynamics of HPV infections in regular Finnish families. This study is being conducted in cooperation between the Institute of Dentistry, Faculty of Medicine and Department of Obstetrics and Gynaecology, Turku University Hospital (TUH). The Finnish Family HPV Study was designed to be a continuation of our two previous studies, assessing oral HPV infections in women with genital HPV infections as well as HPV transmission from the mother to newborn. Several papers have been published from the study (59, 60, 105, 106, 121, 138, 151–153).

Between 1998 and 2002, 329 pregnant women in their third trimester were enrolled at the Maternity Unit of TUH. Fathers-to-be (n = 131) and all newborn babies (n = 331) were also enrolled in the study. The Research Ethics Committee of TUH approved the study design before initiation (#3/1998), and all parents gave a written consent. The oral and genital mucosal scrapings of mothers, fathers and newborns were sampled for DNA testing at baseline, and at 2, 6, 12, 24 and 36 months. In addition, samples from the placenta, umbilical cord blood, semen, breast milk, saliva and blood were taken for HPV testing, and serum samples for HPV serology were collected at each follow-up visit.

Transmission –  Our results from the ongoing study suggest that oral HPV infections play an important role in the transmission of HPV DNA between the family members. Part of HPV infections in the newborn is contamination from an infected birth canal, but part of them can still be detected up to 3 years after delivery, as reported by us and others (14, 59, 60, 69, 76). Intrauterine transmission of HPV seems to be true in a minority of cases, and HPV detection in the placenta and cord blood seems to be an important predictor of HPV detection in the newborn. The evidence accumulated so far suggests that the role of father as a transmitter of HPV to the infant seems to be less important than that of the mother.

Conclusions

The current evidence supports the view that HPV can be transmitted from the mother to the child. HPV positivity of the newborn immediately after delivery can indicate either an HPV infection acquired in utero or a contamination with infected maternal cells. A minority of infants might have a persistent, asymptomatic infection with mucosal types similarly as with cutaneous types. The critical question is how often infants are infected with HPV and how frequently high-risk HPVs can persist. To answer this, future prospective studies should be designed, where sampling and DNA testing methods have to be standardized. Moreover, the cohorts should be large enough to acquire reliable data on HPV type-specific natural history.