Children who attended a day care center for at least three months before the age of 2 were found to have a lower risk of developing childhood leukemia than children who were not placed in such an environment, a study to be reported Saturday in the British Medical Journal says.
In the study of 136 children with childhood leukemia and a slightly larger number of healthy children, conducted in Greece, children who attended a day care center before the age of 2 had a 70 percent lower risk for childhood leukemia than children who had never been in day care. Children who had been in day care at any age had a 30 percent lower risk. The statistical association was "startlingly strong," the authors of the study said.
The study was conducted by Dr. Eleni Petridou of Athens and Dr. Dimitrios Trichopoulos, who heads the department of epidemiology at the Harvard School of Public Health in Boston. In 1981, Dr. Trichopoulos did research in Greece linking exposure to passive cigarette smoke and lung cancer.
Dr. Trichopoulos's latest findings strengthen a thesis advanced in recent years by scientists in England and Scotland that children exposed to an undetected virus or other infectious agent very early in life may gain immunity to childhood leukemia. Childhood leukemia is the most common cancer among children under 15. Timing of Infection
The thesis holds that the timing of infection is a critical determinant of whether a child will develop leukemia.
In developing the thesis, scientists have used poliomyelitis as a model. Poliomyelitis is a paralyzing form of an infection caused by a virus spread by fecal-oral contamination. In most cases, the polio virus causes widespread but symptomless infection. Cases of paralysis are rare, with the ratio of symptomless infection exceeding cases of paralysis by 100 to 1. Scientists don't know why some people develop the paralytic form, which is now prevented by immunizations.
The older an individual is when infected with the polio virus, researchers have found, the more susceptible the individual is to the paralytic form.
Dr. Trichopoulos's team theorized that childhood leukemia might similarly take a more severe form if the infection was contracted at a later age. It is known from epidemiologic studies that crowding enhances transmission of infectious agents. The team reasoned that children in day care were more likely to be exposed to the possible infectious agent at an early age, when theoretically they have a greater ability to respond to the infection in a benign and symptomless way.
The most common form of childhood leukemia is known as acute lymphoblastic leukemia, and scientists have long suspected that this form might be caused by a virus. Suspicions rest mainly on the fact that the incidence of acute lymphoblastic leukemia peaks at ages 2 and 3 in both girls and boys.
Such a peak suggests that exposure to whatever causes childhood leukemia ordinarily occurs in the first year or two of life or is something to which immunity develops rapidly, Dr. Brian MacMahon, an epidemiologist at Harvard, wrote last year. Dr. MacMahon reviewed the evidence for and against a viral cause of lymphoblastic leukemia in the American Journal of Epidemiology, which is published by Johns Hopkins University in Baltimore.
Proponents of a viral cause of childhood leukemia also point to evidence that viruses cause leukemia in cats and mice. Also, a virus known as HTLV-1 has been found to cause a rare form of leukemia among adult Japanese.
But the lack of a known virus causing childhood leukemia has hampered epidemiological studies of the disease. Clusters of Leukemia
Renewed research interest in the viral cause of childhood leukemia stems from reports of small clusters of childhood leukemia in the neighborhoods of nuclear plants in England.
In 1987, Dr. Sarah C. Darby and Sir Richard Doll said that an alternative explanation for such clusters might be that young workers moved to an isolated area and became infected with a virus they had not been exposed to previously. Leo J. Kinlen of the University of Edinburgh then reported evidence supporting such a theory from studies of Glenrothes, a town in Scotland, and from other areas in England.
However, other studies have found no change in the incidence of childhood leukemia following the mass migration of children during World War II and in the Greek isles in recent decades.
Dr. Trichopoulos said in an interview that, having become intrigued with the viral theory from these reports, he began the study now being reported before he left Greece to join the Harvard faculty in 1989.
The study involved 136 cases of childhood leukemia in Athens and the surrounding area and on the island of Crete over the four-year period from 1987 to 1991. For purposes of scientific comparison, the team also studied 187 children of similar age and sex who were treated at the same hospitals but who did not have leukemia.
The study was supported by the Greek Ministry of Health and a gift to Harvard University by the family of Stavros Niarchos, the Greek shipping magnate.