Washington: Researchers from Tufts University have suggested that your country of birth can decide your susceptibility to asthma.
The researchers pooled data from five previous epidemiological studies to investigate the prevalence of asthma in children in the Boston neighbourhoods of Chinatown and Dorchester.
Among children born in the United States, low socio-economic status (SES) and exposure to pests (mice and cockroaches) were both associated with having asthma.
Neither association was present in children born outside of the United States.
"In earlier studies, we found that country of birth was associated with asthma risk, which led us to the current analyses. Our current findings may help bring a new perspective to asthma research as they highlight the importance of studying foreign-born children," said Doug Brugge, senior author of the study.
"Much of the existing research follows U.S.-born children from birth to see if, and potentially why, they develop asthma.
"It might add to our understanding of what causes asthma if we knew why foreign-born children seem to be less likely to develop asthma," he said.
"Pooling data from these studies gave us a larger sample size and allowed us to conduct additional analyses. We found that, in addition to an association with place of birth; both low SES and exposure to pests are associated with asthma in U.S.-born children but not in foreign-born children," said first author Mark Woodin of the Tufts University.
"While this type of epidemiological study cannot establish causation, our findings may be explained by the fact that certain pathogens common in the developing world are nearly nonexistent in the U.S.
"If exposure to such pathogens confers some sort of protection against developing asthma, foreign-born children may be less susceptible than children born in the U.S.," said Brugge.
This idea, called the "hygiene hypothesis," suggested that children born in less-developed countries may have early exposure to intestinal worms, viruses and bacteria that affects immunity and makes them more resistant to asthma than U.S.-born children.
The five studies Brugge and colleagues examined were conducted from 2002 to 2007, sampling a total of 962 children aged 4 to 18 years.
There did not initially appear to be a significant relationship between pest exposure and asthma; but when the researchers took birthplace into account, they found that U.S.-born children who were exposed to pests were 60 percent more likely to have asthma than U.S.-born children not exposed to pests.
The study was published online in the Journal of Immigrant and Minority Health.