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Paediatr Child Health. Sep 2006; 11(7): 401–407.
PMCID: PMC2528629

Language: English | French

Cytomegalovirus as an occupational risk in daycare educators

Abstract

BACKGROUND

Cytomegalovirus (CMV) infection continues to be an important occupational risk in the daycare setting. A comprehensive update of scientific evidence is timely to inform and promote appropriate preventive measures.

METHODS

A review of the literature was conducted to examine the evidence for an occupational risk of CMV infection in daycare educators. Sources included Medline, government documents and additional references from published bibliographies. The key words used for searches were ‘child day care centres’ or ‘nurseries’ and ‘cytomegalovirus’ or ‘cytomegalovirus infection’.

RESULTS

Eight CMV seroprevalence studies on daycare educators in industrialized countries were found: four in the United States, three in Canada and one in Italy. Risk factors for seropositivity were older age, nonwhite race, foreign birth, birth in a low- or middle-income country, diaper changing, having children at home, and a child to educator ratio greater than 6:1 in children 18 to 35 months of age. Risk factors for seroconversion were younger age and working with young children. These studies suggest that daycare centres may be a high-risk setting for CMV infection.

DISCUSSION

Recommendations to prevent CMV infection in this setting include handwashing, selective serological screening, avoiding work with younger children if pregnant and, in some cases, preventive leave from work. Evaluation and expert opinion of the effectiveness of various preventive options for CMV acquisition are needed to ensure that recommendations are evidence-based.

Keywords: Child care, Cytomegalovirus, Daycare, Daycare educators, Occupational risk

Résumé

HISTORIQUE

L’infection à cytomégalovirus (CMV) continue de représenter un important risque professionnel en service de garde. Une mise à jour complète des données scientifiques arrive à point pour informer et promouvoir des mesures préventives pertinentes.

MÉTHODOLOGIE

Une analyse bibliographique a été menée pour examiner les données probantes de risque professionnel d’infection à CMV pour les éducatrices en service de garde. Les sources consultées étaient Medline, des documents gouvernementaux et des références tirées de bibliographies publiées. Les mots clés utilisés pour la recherche étaient child day care centres ou nurseries et cytomegalovirus ou cytomegalovirus infection.

RÉSULTATS

Huit études de séroprévalence du CMV chez les éducatrices en service de garde dans des pays industrialisés ont été colligées : quatre aux États-Unis, trois au Canada et une en Italie. Les facteurs de risque de séropositivité étaient un âge plus avancé, une autre race que blanche, une naissance à l’étranger, une naissance dans un pays à faible revenu ou à revenu intermédiaire, le fait de changer des couches, la présence d’enfants à la maison et un ratio enfants-éducatrice de plus de six pour un dans les groupes d’enfants de 18 à 35 mois. Les facteurs de risque de séroconversion étaient un âge plus jeune et le fait de travailler avec de jeunes enfants. D’après ces études, les services de garde pourraient être un lieu à haut risque pour contracter une infection à CMV.

DISCUSSION

Les recommandations pour prévenir l’infection à CMV en service de garde sont le lavage des mains, le dépistage sérologique sélectif, le fait d’éviter de travailler avec de jeunes enfants pendant la grossesse et, dans certains cas, des retraits préventifs. Il faut obtenir une évaluation et l’avis de spécialistes quant à l’efficacité des diverses options afin de prévenir l’acquisition du CMV pour s’assurer que ces recommandations sont fondées sur des faits probants.

Cytomegalovirus (CMV), a member of the herpesvirus group, is a double-stranded DNA virus (1,2). As with other herpesvirus, such as herpes simplex, varicella-zoster and Epstein-Barr, recurrence of infection can occur throughout the lifespan, leading to re-emergence of viral shedding (2,3). After infection, CMV disseminates to various organs of the body and can be found in bodily fluids such as blood, tears, feces, cervical secretions, semen and breast milk (4). However, CMV excretion occurs mainly in urine and saliva (5). The exact mode of transmission is not well understood, but both vertical (mother to infant) and horizontal (person to person) transmission are common. It is generally accepted that horizontal transmission of CMV requires intimate contact with secretions and that mucosal contact with infectious secretions or tissue may be required (4,6). However, this is based on epidemiological evidence and has not been proven experimentally (4). Vertical transmission can occur prenatally or perinatally from an infected mother to her child (6).

Infection in healthy adults and children is usually asymptomatic (4). However, symptomatic infection characterized by a mononucleosis-like illness with fever, malaise and lymphadenopathy can develop (5). In immunocompromised individuals with impaired cell-mediated immunity, CMV can lead to more severe sequelae and is one of the most common opportunistic pathogens (4). However, the major public health concern for CMV focuses on congenital infection. During maternal infection in pregnancy at any gestational age, CMV can cross the placenta, leading to in utero transmission of the virus to the fetus (4). Whereas pathology can occur in the lungs, liver, spleen, heart and brain, the most severe damage occurs in the developing nervous system, with possible outcomes including seizures, mental disabilities, deafness and blindness (5). The risk of delivering an infant with symptomatic CMV infection after in utero exposure to the virus is between 10% and 20% (7). An additional 10% to 15% of fetuses exposed to CMV in utero will be asymptomatic at birth, but will develop hearing loss and other adverse neurological sequelae later in life (5). Congenital CMV occurs in 0.3% to 1.0% of all live births worldwide and is the most common congenital infection (2,8,9).

CMV disease can result from primary or recurrent infection. Recurrent infection can occur in one of two ways: reactivation of latent virus, or reinfection from a new strain of CMV (1). Pre-existing maternal antibodies in response to CMV have long been considered protective against congenital infection in the offspring, both in terms of likelihood of in utero transmission (10) and of severity of adverse effects (11,12). However, it has been suggested that the relationship between recurrent infection and congenital CMV has been underestimated due to diagnostic limitations (13). Recent studies have shown the occurrence of congenital CMV in women with recurrent infection (1418), occasionally with similar health outcomes in both primary and recurrent infection (7). However, it is generally understood that the risk of congenital CMV infection is greater in the offspring of women with a primary infection.

CMV infection occurs worldwide, although patterns of infection and transmission can vary regionally and among different patient populations. In industrialized countries, CMV rates are high in young children, with disease acquisition occurring in utero, during childbirth through exposure to genital secretions, in infancy through breastfeeding from seropositive mothers, or in childhood through contact with infected children (19). The development of antibodies from exposure to CMV increases steadily after childhood, and then plateaus in adulthood, where seroprevalence is approximately 40% in industrialized countries (8,19,20). Children are thought to be a common source of CMV for women of childbearing age (5,21,22). In addition, sexual transmission of CMV occurs among adults and adolescents (8,23). However, the epidemiology of CMV appears to be changing due to an increased use of daycare centres, which has increased the transmission of CMV among children and adult contacts (24). This is of concern for daycare educators who work in these settings and are in frequent contact with young children. Because a higher rate of female employment has resulted in greater demand for outside-of-home care (25), increasing attention is being directed to infectious disease transmission in the daycare centre.

METHODS

A review of the literature was conducted to examine the evidence for an occupational risk of CMV infection in day-care educators. Sources included Medline (1966 to November 2005), government documents and additional references from published bibliographies. Key words for searches were ‘child day care centres’ or ‘nurseries’ and ‘cytomegalovirus’ or ‘cytomegalovirus infection’. Studies were included in the review if the research focused on CMV infection (including excretion and transmission) in children attending daycare centres, as well as on seroprevalence and seroconversion in daycare educators. Only studies conducted in developed countries were included in the review due to the differing epidemiology of CMV in developed and developing countries.

RESULTS

CMV infection in children

In 1971, Weller (26) first suggested that the higher CMV seropositivity in young children in Sweden compared with that of similar children in the United States and United Kingdom may be due to the differential use of daycare centres. Since Weller’s paper, studies in industrialized countries have confirmed that children attending daycare have higher excretion rates of CMV than children not attending day-care and that horizontal transmission is common between children in daycare and their adult contacts (5,2747).

A high prevalence of CMV excretion has been found in case-control studies in children in the daycare setting. Adler (27) found CMV excretion in 24.2% of children attending daycare in Richmond, Virginia. This was significantly higher than the 6.8% prevalence of CMV excretion found in age-matched hospital controls not attending daycare. In a study by Pass et al (28) in Birmingham, Alabama, 57.0% of children from a daycare centre were shedding CMV compared with 11.0% of controls who were recruited from a paediatric office who attended some form of home care, out-of-home care or school. Other types of studies in children attending daycare have found prevalences of CMV excretion ranging from 9.0% to 60.0%, with most over 20.0% (2935).

Although these studies have documented high CMV excretion in all age groups, the highest prevalence of excretion is in children under the age of 24 months. Murph et al (34) found the highest level of urinary CMV excretion in those aged 10 to 24 months. Murph and Bale (29) found that CMV acquisition was more likely in children entering daycare at a younger age, with the highest prevalence of excretion found in children younger than 24 months of age. In the Pass et al study (28), approximately 80% of infants between the ages of 13 and 24 months were excreting CMV in urine. This percentage did not vary over a 12-month follow-up period. However, CMV excretion in children younger than 13 months of age increased from 10% to 80% at the end of a 12-month follow-up period, indicating the importance of exposure time for CMV acquisition. In an earlier study by Pass et al (35), CMV excretion in children between 13 and 24 months of age was significantly higher than in those who were younger than 12 months of age, with a prevalence of excretion of 83.0% and 9.0%, respectively.

Studies using molecular biology analysis have shown strong evidence that child-to-child transmission of CMV is common in the daycare setting. Excretion of similar strains has corresponded with the physical clustering of children of the same age group or the same room assignment (27,34). In addition, Bale et al (5) found that recurrence of infection, attributed to reinfection with a new strain of CMV, is common among children in the daycare centre. These molecular studies have provided evidence that horizontal transmission is an important mechanism by which CMV infection is sustained within the daycare setting.

In addition to direct contact with infectious children, environmental contamination with CMV and the subsequent contact of susceptible individuals with contaminated objects or fomites have also been implicated in the transmission of CMV in the daycare centre. Pass et al (35) examined four plastic toys that were placed in the mouths of CMV-excreting children in daycare. All four toys were positive for CMV immediately after contact with these children. Hutto et al (36) also isolated CMV from toys that had been mouthed by CMV-excreting children. CMV was found on toys up to 30 min after contact with the children. In addition, Faix (37) determined that CMV could be recovered from absorptive surfaces and nonabsorptive surfaces inoculated with CMV for up to 2 h and 8 h, respectively, in a hospital nursery. Although this study took place in a hospital setting, contaminated objects included diapers, bedding and gloves, which are common in the daycare setting. An additional study looked at the survival duration of CMV that was experimentally inoculated into saliva and urine (38). CMV was found for up to 2 h in saliva on environmental surfaces such as plastic surfaces, and for up to 48 h in paper diapers soaked with CMV-contaminated urine. These studies show the potentially important role of fomites in the transmission of CMV in daycare centres, which is particularly relevant because preschool-aged children frequently contaminate their environment by putting their hands or toys in their mouth (39).

Overall, the evidence has shown that horizontal transmission of CMV between children is common in the day-care centre. Although child-to-child transmission of CMV may be common in this setting, a more serious implication of these studies is that a high potential for CMV acquisition may exist for daycare educators who have close contact with infected children, the majority of whom are of child-bearing age (40).

CMV infection in daycare educators

Studies examining the occupational risk of CMV acquisition in daycare educators in industrialized countries have estimated seroprevalence and seroconversion rates (Table 1). The risk factors found to be significant in these studies are listed in Table 2.

TABLE 1
Cytomegalovirus seroprevalence and annual seroconversion rates from published studies in daycare centre educators in the United States and Canada
TABLE 2
Risk factors for cytomegalovirus infection in the daycare educator population in published reports from the United States and Canada

The first seroprevalence study for CMV in educators in the daycare setting took place in Birmingham, Alabama, from 1980 to 1988 (41). A total of 509 educators from 32 daycare centres were enrolled in the study. The CMV seroprevalence of educators at enrollment was 62.5%. Independent risk factors for CMV seropositivity were older age (P<0.0001) and nonwhite race (P<0.01). A marginally significant association (P=0.06) was found for educators working with children two years of age or younger for 8 h or more per week. An annual seroconversion rate of 20% was obtained with a median of 14 months of follow-up time. Educators who worked with children younger than three years of age for 20 h or more per week were significantly more likely to have seroconverted (P<0.03).

A second study in daycare educators took place in Richmond, Virginia, from 1986 to 1988 (32). Women from 34 daycare centres participated. A control group of women employed by the Medical College of Virginia Hospitals (unspecified duties) was used for comparison. For the 565 control subjects, the seroprevalence was 47.0%, which was not significantly different from the daycare educators. In the 610 daycare educators, CMV seroprevalence was 41.0%. Independent risk factors for CMV seropositivity were age (P<0.0001), race (P<0.0004) and the ages of the children cared for (P<0.026). However, when educators were stratified according to the age of the children cared for (two years or younger versus older than two years of age), a significantly higher seropositivity was found for those day-care educators looking after younger children (46.0% versus 35.0%). In addition, an annual seroconversion rate of 11.0% was found for the 202 seronegative daycare educators, with an average follow-up time of 305 days. This was five times higher than the 2.2% annual seroconversion rate found among 229 hospital employees. A total of 357 women were seronegative, of whom 202 were followed up. Although the proportion of women who were retested (56.6%) is high compared with other studies, the characteristics of the individuals not available for follow-up were not known. In addition, it appears that the control group may not have been ideal because hospital employees differed significantly from daycare educators in terms of socioeconomic status (education and household income), which is known to be related to seropositivity. In addition, the job responsibilities of the members of the control group were not specified, and it is not clear what their potential exposure to CMV may have been.

A CMV seroprevalence study in daycare centres was conducted in Cedar Rapids-Iowa City, Iowa (42). Participants included 252 adult educators from six daycare centres. A seroprevalence of 38.0% was found. Significant risk factors for seropositivity included older age (P<0.001) and nonwhite race (P=0.032). The median length of follow-up for seronegative educators was 13 months, with an annual seroconversion rate of 7.9%. However, loss to follow-up was almost 50%. The rate of CMV acquisition in educators paralleled the rate of CMV acquisition in children, suggesting that CMV seroconversion in educators was related to CMV transmission dynamics in children.

An additional study on the seroprevalence of CMV in daycare centres took place in King County, Washington, in 1994 (43). The study population included 560 educators from 49 participating daycare centres. A seroprevalence of 62.0% was found. The independent risk factors for CMV seropositivity were nonwhite race (OR=2.4; 95% CI 1.2 to 2.5), changing diapers three or more times per week (OR=1.8; 95% CI 1.1 to 2.8) and having a child living at home (OR=1.8; 95% CI 1.1 to 2.9).

The first Canadian study took place in Montreal, Quebec, from 1989 to 1991 (44). A convenience sample of 166 educators was recruited from 29 daycare centres. A CMV seroprevalence of 47.3% was observed among 148 Canadian-born educators. Risk factors for CMV seropositivity included daycare employment for more than five years, and schooling of less than 15 years. A 23.8% seroconversion rate after 12 months was observed in 42 educators available for follow-up. Educators caring for children younger than three years of age showed higher conversion rates than educators caring for children of other age groups (50.0% and 8.0%, respectively). The internal validity and generalizability of this study was limited because of the convenience sample and the high loss to follow-up. In addition, although small in number, the group of educators born outside of Canada (n=18) was excluded from the analysis.

A second Canadian study took place in Toronto, Ontario, from 1993 to 1994 (45). The study population included a convenience sample of 206 educators from 38 daycare centres within the city of Toronto. A seroprevalence of 67.0% was observed. Risk factors for CMV seropositivity were reported as birth outside of Canada, the presence of children younger than five years of age in the educator’s home, and four or more people living in the educator’s home. A seroconversion rate of 12.5% was found one year later in 56 available seronegative educators tested. The investigators concluded that working at a centre where gloves were not worn, working with infants (compared with working with both infants and toddlers) and an age of less than 30 years were independent risk factors for seroconversion. Although this study used a convenience sample of educators, differences in terms of risk factors and seroprevalence and seroconversion rates, compared with the earlier Canadian study, highlight the variability in the epidemiology of CMV in two large urban centres.

The most recent CMV seroprevalence study in daycare educators was conducted in Montreal (46). A population of 473 female educators from 81 randomly selected daycare centres on the island of Montreal was included in the study. The investigators found an overall seroprevalence of 56.9%. Risk factors at the educator level included increasing age (per five years) (OR=1.19; 95% CI 1.05 to 1.35), birth in a low-income country (OR=10.23; 95% CI 2.64 to 39.50) or middle-income country (OR=4.99; 95% CI 2.39 to 10.40), and having two or more of their own children (OR=1.98; 95% CI 1.19 to 3.31). At the daycare level, a child to educator ratio greater than 6:1 for children 18 to 35 months of age (OR=1.87; 95% CI 1.25 to 2.81) was found to be a significant risk factor for CMV seropositivity. The generalizability of the results was strengthened by the heterogeneity of the population studied, the random sample of daycare centres, the large sample size of educators and centres, and a multivariable model that took into account potential clustering of educator results by centre.

Studies conducted in other industrialized countries have examined the occupational risk of CMV acquisition in day-care educators. In 1988, a study by Volpi et al (47) in Rome, Italy, looked at excretion rates in children and the seroprevalence of the educators. Seroprevalence was 96.4% in 82 female educators, which was not significantly higher than the 84.1% found in 82 housewife controls. In addition, 229 female students in training, between the ages of 15 and 20 years, were examined and had a seroprevalence of 85.2%, which was similar to that found in adult participants. CMV acquisition in childhood and adolescence may be higher in Rome than in similarly large, urban centres in Canada or the United States, leading to higher seropositivity in adulthood. Therefore, the occupational risk to educators in Rome may be lower than the risk to educators in the Canadian and American studies.

DISCUSSION

The previous research in daycare educators demonstrates that both educator- and daycare-level risk factors are associated with CMV seropositivity. The higher seroconversion rates in daycare educators compared with controls, in addition to daycare-specific risk factors for seropositivity and seroconversion (eg, caring for younger children and diaper changing), suggest that educators are at an increased risk of acquiring CMV from children in the daycare setting. Therefore, it is important to identify measures to prevent CMV infection in female daycare educators. Current recommendations include adequate hygiene behaviours, selective serological screening, reassignment of work duties and preventive leave.

Improvements in hygiene behaviours, especially handwashing, have been the most important preventive measures reported in the literature for reducing CMV transmission (8,19,39,48,49). Although hygiene interventions focusing on handwashing and glove use have not been shown to reduce CMV transmission (50,51), the evidence in daycare educators demonstrates that glove use is a protective factor for seroconversion (45). In addition, maintaining a clean daycare environment may be essential to prevent CMV survival on daycare toys and surfaces (3538). Proper hygiene is an important part of any preventive strategy for daycare educators; however, its effectiveness in reducing CMV infection in daycare educators has not yet been demonstrated.

Previous guidelines from the Canadian Paediatric Society have suggested that it may be prudent to provide CMV screening of daycare educators who are pregnant or intending to become pregnant to determine susceptibility (52,53).

It is recommended that those who are found to be seronegative avoid working with children younger than three years of age (54). This recommendation is supported by evidence showing both higher excretion rates in younger children (24,28,34) and increased seroconversion and seroprevalence in educators who care for younger children (32,41,44,45). However, it is not clear whether CMV contamination in the daycare environment (eg, fomites) would continue to be a risk for daycare educators who are reassigned to other duties within the daycare centre. In addition, the evidence suggests that an increased amount of time spent caring for children (41) and caring for a greater number of children (46) in the younger age groups may also increase the potential for CMV infection. This relationship may need to be evaluated further.

The exclusion of infected individuals from the daycare centre has been suggested as a means to prevent CMV infection. No recommendations have been made for the exclusion of CMV-infected children from daycare centres due to the large number of asymptomatic children shedding the virus (54). However, the possibility of exclusion policies for female educators has been suggested. In Quebec, the Act Respecting Occupational Health and Safety allows for the reassignment of tasks, or leave from the workplace, of pregnant educators who have a certificate of proof that working conditions pose a threat to the unborn child or herself (55). Due to the increased occupational risk of CMV infection that has been demonstrated in daycare educators, and the paucity of evidence on the effectiveness of other preventive measures, the usefulness of this measure for preventing CMV infection should be further investigated. Although it is clear that the risk of acquiring CMV infection in the daycare centre would be negligible if the daycare educator is absent from the setting, the cost-effectiveness of this preventive option needs to be established.

Although certain preventive measures for reducing CMV infection in daycare educators are supported by previous research, it is clear that there are methodological issues that limit the usefulness of these studies. Therefore, seroconversion studies are needed with high compliance during follow-up to increase the validity of the results. The generalizability of results will also be increased by using studies with random sampling techniques and large sample sizes of educators. Additionally, there is a lack of research demonstrating a direct link between an increased risk of CMV infection in daycare educators and an increased incidence of congenital CMV infection. An evaluation of the effectiveness of different preventive options for CMV infection in daycare educators is also required. Research that focuses on these knowledge gaps and that incorporates sound methodology, in combination with expert opinion, is necessary to ensure that recommendations for CMV prevention in daycare educators are evidence-based.

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