Young adults who attended day care or nursery school when they were children were more than a third less likely to develop Hodgkin's disease, according to a new study by Harvard School of Public Health researchers.
Ellen Chang, a postdoctoral fellow and a researcher in the School of Public Health's Department of Epidemiology and at Karolinska Institute in Sweden, said the reduced risk is likely because kids in day care get exposed to many common bacterial and viral infections through contact with other children.
Chang said early infections can function to "prime" a still-developing part of the immune system responsible for defense against bacterial and viral invasion. Children whose bodies have to fight bacteria and viruses early develop more robust protection against infection than those who are more sheltered.
In contrast, adolescents who are exposed to certain common childhood infections when they're older tend to develop more severe cases of the illnesses.
One virus in particular, Epstein-Barr, is suspected in this case, Chang said. The Epstein-Barr virus causes a mild illness or no symptoms at all in young children but often causes mononucleosis in adolescents. A history of mononucleosis is linked to a three-fold increase in the risk of developing Hodgkin's disease.
Hodgkin's disease is a type of lymphoma, or a cancer of the immune system. It tends to strike two distinct age groups: early adulthood, usually between age 15 and 40, and after age 55. Chang said her results show that the link between day-care experience and a reduced risk of developing Hodgkin's disease applies in the younger group, but not in those who develop the disease later in life. The indication, she said, is that Hodgkin's disease in that older group develops in a different way, perhaps because of a decline of the immune system as people age.
Chang's research, published in the August issue of Cancer Epidemiology, Biomarkers and Prevention, was part of a project on the viral roots of Hodgkin's disease led by Professor of Epidemiology Nancy Mueller of the Harvard School of Public Health. Other collaborators on the project included Professor Donna Spiegelman from the School of Public Health, Tongzhang Zheng of Yale Medical School, and Edward Weir, Michael Borowitz, and Risa Mann of the Johns Hopkins Medical Institute.
The research looked at the personal histories of 565 Hodgkin's cases and 679 control subjects living in the Boston metropolitan region and in the state of Connecticut. Researchers had expected to find a link, first observed by Mueller in the 1970s, between a family's socioeconomic status and Hodgkin's disease. Those studies showed that children raised in low-density housing like that found in suburbs, whose mothers had high education levels, and children who had few siblings had a higher risk of developing Hodgkin's.
In this study, however, the results didn't show an association with socioeconomic status, but rather with day-care experience. Researchers found that those who attended day care for a year or more were 36 percent less likely to develop Hodgkin's disease. No significant effect was found for those attending for less than a year.
The research on day care led to a study on the impact that having multiple siblings has on developing Hodgkin's disease. Though conducted later, the results of that study were published in the same journal in July. That research shows that Hodgkin's is less common in children who have several older siblings, presumably for the same reason found in the day care study: Older siblings bring bacteria and viruses home from school and infect their younger siblings.
Chang said the results of the current study make sense when one considers that decades ago far fewer children were in day care. Wealthier families with the mother at home kept children from being exposed to viruses and bacteria. Rather than keeping them safe, however, the lack of exposure made the children's immune systems' responses to infections develop more slowly.
Today, after decades of change in the American family, with more mothers in the workforce, day-care attendance is much more common among all socioeconomic groups.
"Nursery school exposes children to other children, but it's beneficial for other reasons: It promotes the development of the ... immune system," Chang said. "Postponed exposure to common infections can lead to a worse outcome when [children] get older."