Migrant Studies

Migrant studies are an extension of the ecologic study design, which compares disease rates in different locations. In migrant studies, the disease rate among persons who have migrated from one location to another is compared with the disease rate in persons who did not migrate. Ideally, the rate for migrants is compared both with persons remaining in the country of origin and with lifelong residents of the destination or host country. Migrants share their genetic makeup and early life environment with persons remaining in the country of origin. They share recent environmental exposures with residents of the host country. Thus, the comparison of disease rates between the migrants and the nonmigrating populations is used to generate hypotheses about the relative importance of genetics and environment in determining disease risk. Migrant [Page 677]disease rates that remain similar to those in the country of origin suggest that genetic factors play a role in geographic variation. Migrant disease rates that converge on that of the host country suggest an important role for the environment. Comparisons between persons migrating at different ages, or between migrants and their offspring, may identify a critical age of exposure for environmental factors.

Migrant studies are likely to be informative when there is marked geographic variation in disease and little is known about the etiologic factors responsible for the variation. The diseases most extensively studied with migrant populations are multiple sclerosis and cancer. Multiple sclerosis has an unusual geographic pattern, with prevalence generally higher with increasing distance from the equator, in both hemispheres. Migrants from higher to lower risk areas, such as Europeans to Israel, South Africa, or Australia, or internal migrants from higher to lower latitudes within the United States, have disease risk intermediate between their place of origin and destination. However, there is little change in risk for migrants moving from lower to higher risk areas, such as Africa to Israel or southern to northern United States. Studies able to ascertain age at migration suggest disease risk is largely determined by age 15 or 20. These studies have been interpreted as most compatible with an environmental exposure in childhood being an important etiologic factor, such as a protective effect from early infection by an agent endemic in areas closer to the equator.

Most cancers show significant geographic variations. Breast and stomach cancers offer two contrasting patterns. Breast cancer rates are highest in the most developed Western countries. Rates for white migrants to the United States converge to rates for U.S.-born whites within 20 years of migration. However, although breast cancer rates for Asian migrants to the United States and their U.S.-born daughters are higher than rates for women in Asia, they are not as high as U.S.-born whites. This suggests the importance of environmental factors in breast cancer risk, but it does not exclude the possibility that there might be genetic differences in risk for Asian women, since even U.S.-born Asian women have lower rates of breast cancer than whites.

The international pattern for stomach cancer is quite different: Japan has one of the highest rates and the United States one of the lowest. Migrant studies, including those of Japanese migrants to Hawaii and their offspring, found that migrants had rates somewhat lower than persons remaining in Japan, while their offspring had much lower stomach cancer rates. This pattern suggests the importance of environmental factors, both in childhood and adulthood.

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