Incidence of CMV infection

Population-based incidence data for CMV infection are not available. Cohort studies in pregnant women, in blood donors, and in hospital employees suggest that the incidence of CMV infection in young adults is around 2 percent per year, with higher rates in low-income young women (Balfour and Balfour, 1986; Stagno et al., 1986; Balcarek et al., 1990). Cohort studies performed in groups thought to be at higher risk for CMV infection show markedly higher rates (see Table 6.2). Patients in sexually transmitted disease clinics have very high rates of CMV infection. The incidence of CMV infection is also increased in day-care workers and parents of young children.

Table 6.2 Incidence of CMV infection in various groups

Group

Rate* (%/year)

Reference

Blood donors

1.57

Balfour and Balfour, 1986

Hospital employees:

• Richmond, VA

2

Adler, 1989

• Birmingham, AL

2.2

Balcarek et al., 1990

Pregnant women:

• Middle income

2.5

Stagno et al., 1986

• Low income

6.8

Women in STD clinic

37

Chandler et al., 1985

Parents of CMV-shedding child:

• 0-12 months

47

Yeager, 1983

• ^18 months

32

Pass and Hutto, 1986

• 19 months-6 years

13

Pass and Hutto, 1986

Day-care workers:

• Iowa

7.9

Murph et al., 1991

• Richmond

11

Adler, 1989

• Toronto

12.5

Ford-Jones et al., 1996

• Birmingham

20

Pass et al., 1990

*Percent per year who seroconverted from antibody negative to antibody positive. Reproduced, with permission, from Fields Virology, 4th edn, p. 2683 (Pass, 2001).

*Percent per year who seroconverted from antibody negative to antibody positive. Reproduced, with permission, from Fields Virology, 4th edn, p. 2683 (Pass, 2001).

The Social Ecology of Infectious Disease Transmission in Day-care Centers Vertical transmission

Cytomegalovirus is transmitted from mother to baby by three routes - transplacental, intrapartum contact with maternal secretions, and through breast milk (see Table 6.3). Although transplacental transmission is of great medical significance because of the morbidity associated with congenital CMV infection, it is infrequent (around 0.5-1.5 percent) compared with intrapartum and breastmilk transmission, which occur in a sizeable proportion of babies born to seropositive mothers. If CMV is in the cervico-vaginal canal at the time of delivery, the transmission rate to the newborn is around 50 percent (Reynolds et al., 1973). Overall, approximately 10 percent of women have CMV in the cervix or vagina at the time of delivery, but rates of shedding as high as 28 percent have been observed (Stagno et al., 1982a). Epidemiologically, breast milk is the most important route for transmission of CMV from mother to baby. Numerous studies of human milk have found CMV (or viral DNA) in milk from seropositive mothers. Older studies using virus culture recovered CMV from the milk of from 30 to 70 percent of seropositive mothers with repeated sampling (Dworsky et al., 1983; Ahlfors and Ivarsson, 1985). More recent studies using polymerase chain reaction (PCR) to detect CMV DNA report finding virus in around 95 percent of milk samples (Hotsubo et al., 1994; Asanuma et al., 1996; Vochem et al., 1998; Jim et al., 2004). Virolactia is uncommon in colostrum or after more than a few months post-partum; a Swedish study reported no detection (by virus culture) of CMV in mothers' milk before nine days or later than three months post-partum (Ahlfors and Ivarsson, 1985). Transmission of CMV to nursing infants is related to duration of nursing, and the presence or quantity of

Table 6.3 Transmission of CMV from mother to infant: three routes of vertical transmission

Setting

Proposed

Maternal

Estimated rate of

mechanism

infection

infection in infant

Pregnancy

Viremia/

Unknown

1%

placental infection

(population sample)

Primary infection

Viremia/placental

Seroconversion or

20%-40%

during pregnancy

infection

CMV-IgM positive

Birth, intrapartum

Virus in genital tract

Seropositive

5%

Birth, intrapartum

Virus in genital tract

Positive cervical or

50%

vaginal culture

Breastfeeding

Virolactia

Seropositive mother

25%

Breastfeeding

Virolactia

Milk positive for

65%-70%

virus by culture

or PCR

virus in milk. A cohort study of infants nursed by CMV seropositive mothers found that those nursed for less than one month were not infected, compared with a CMV infection rate of 39 percent in infants nursed for more than one month; 25 percent of infants nursed by seropositive mothers acquired CMV by one year of age (Dworsky et al, 1983). When CMV was isolated from milk, 69 percent of infants were infected, compared with 10 percent of infants nursed by seropositive mothers whose milk was negative by virus culture. Review of rates of CMV infection during the first year of life from diverse geographic areas showed a consistent correlation between high prevalence of CMV at one year of age and high rates of both maternal seropositivity and breast feeding (Stagno et al, 1980).

Infants who acquire CMV vertically shed the virus in their body fluids for years (Pass and Hutto, 1986). They will amplify the epidemiologic effect of vertical transmission because they can transmit CMV to susceptible age peers with whom they have contact. In addition, they will spread CMV to their susceptible caregivers, including their parents and child-care workers.

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