Cockroaches could be the cause of children's asthma in cities.
Cockroaches have been identified as a possible explanation for dramatic neighbourhood variations in asthma rates among New York City children.
In some neighbourhoods, 19% - nearly 1 in 5 - of children have asthma, while in others the rate can be as low as 3%. Heavy traffic, industrial incinerators, and other outdoor air pollution sources have all been blamed in the past.
But researchers at Columbia University have now found that children living in neighbourhoods with high rates of asthma were twice as likely to carry antibodies against a cockroach protein in their blood, a sign the children had been exposed to the insects and were likely allergic to them.
In addition, homes in the neighbourhoods with high rates of asthma
contained more of the allergen produced by cockroaches in household dust.
The study provides "further evidence that cockroach exposure is part of the story," said study author Matthew Perzanowski.
"Cockroach allergen really could be contributing to disparities in asthma prevalence, even in an urban environment like New York City."
For the study, published in the Journal of Allergy and Clinical Immunology, Perzanowski and his team visited in the homes of 239 7- and 8-year-olds, half of whom lived in areas with high asthma rates.
Previous research has linked poverty to an increased rate of asthma in childhood, but to eliminate the influence of income or the results, the authors only included families with the same middle-income health insurance plan, to ensure they had the same income and access to health care.
More than half of the children already had asthma.
During the visits, the researchers collected dust from the children's beds, then took blood samples to look for antibodies against various allergens associated with asthma - including dog, cat, mouse, dust mite and cockroach proteins.
Nearly 1 in 4 children in neighbourhoods with high asthma rates appeared to be allergic to cockroaches, compared to 1 in 10 children living in areas where asthma is less common.
Cockroaches leave behind proteins that people inhale and can become allergic
to, which in turn increases the chance they will develop asthma, Perzanowski said.
Homes in high-asthma communities also had higher concentrations of the cockroach allergen, as well as allergens associated with mice and cats.
In addition, children who were allergic to cockroaches and mice were more likely to have asthma, noted Joanne Sordillo at the Channing Laboratory of Brigham and Women's Hospital, Boston, who reviewed the findings for Reuters Health.
"Mouse or cockroach allergen exposure may increase the risk of allergic sensitization (allergies), which is in turn related to the development of asthma in children," she said.
Although cockroach-protein sensitization was more common in children in high-asthma neighbourhoods, overall, children who were allergic to dust and cats were also more likely to have asthma.
Perzanowski said the issue of cat ownership is a bit murkier. Some previous research has found that children in homes with cats were more likely to be allergic
, but in this study, having a cat did not predispose children to asthma.
"It's complicated. Avoidance of cats doesn't seem to reduce your risk of developing asthma," he said.
Does your child have asthma? Do you think cockroaches could have affected them?