Social organization and ecology of human CMV infection

In societies in which all or almost all mothers are seropositive to CMV and breastfeeding is widely practiced, it might be expected that half or more of children would acquire CMV during infancy and these children would infect their age peers during childhood. As a result, the proportion of women of childbear-ing age who had never had CMV infection would be very low, and disease due to congenital CMV infection would be uncommon. Observations in countries in which almost all young adults are seropositive and infants are routinely breastfed support this conclusion (Stagno et al, 1982b; Sohn et al, 1992). It is likely that this was also the situation in the US and Western Europe prior to the twentieth century. During the twentieth century rates of breastfeeding in the US decreased, and it is likely that this led to lower rates of CMV infection in infants and less opportunity for spread of CMV among young children (Wright and Schanler, 2001). In addition, most young children were cared for within the family, where they had relatively limited opportunity (compared with center care) for contact with other children. The use of group day-care centers increased dramatically in the US in the latter part of the twentieth century; the proportion of children in day-care centers increased from 6 percent in 1965 to 30 percent in 1993, and it has continued to rise (Hofferth, 1996). In the latter part of the twentieth century, many young mothers in the US had been raised under circumstances that allowed them to escape CMV infection during childhood; they had not been breastfed and they had not been in a group-care situation as young children. These CMV-susceptible young mothers were placing their own children in day-care centers, where they would very commonly acquire CMV, shed virus for years and possibly infect their mothers - who might well be pregnant. A recent case-control study of maternal risk found that contact with young children significantly increased the risk of having a newborn with congenital CMV infection (Fowler and Pass, 2006).

Although population-based studies aimed at determining whether use of day care has increased the prevalence of CMV infection in young children, or the rate of congenital CMV infection, have not been conducted, it is certainly plausible that widespread use of day-care centers could change the epidemiology of CMV infection in the US. Breast-milk transmission is the means by which CMV is transmitted from one generation to the next. In the US, day-care centers could be viewed as an amplifier of breast milk-acquired CMV infection. Perhaps 10 percent of children acquire CMV through breast milk, and these children will shed virus in their body fluids for years. If 10 percent of children who enter day care were shedding CMV, it might be expected that in most day-care centers there would be at least one infected child in each entering age cohort. Daily contact between toddler-age children would almost certainly increase the proportion of children infected several-fold, and children infected in the day-care center will also shed virus for years, continuing to infect their family members and others who have close contact with them.

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