Respiratory infections provide urban asthma clue

Urban infants have a different pattern of viral respiratory illness from suburban infants, which could help explain why they are more likely to develop asthma, say US researchers.

"We compared the viral etiology of respiratory infection and illness in urban and suburban cohorts and found significant differences in the percentage of illnesses that were attributed to viral infections, as well as the patterns of specific viruses that were detected," say James Gern (University of Wisconsin, Madison) and colleagues.

The researchers compared a group of 515 infants from four inner-city areas with a group of 285 infants from a predominantly suburban area. During the first year of life, the researchers took nasal swabs from the infants when their parents reported respiratory illness, as well as at 1 year of age.

Overall, viral infection was detected in 89.2% of swabs from suburban infants compared with 67.5% of urban infants, which was an unexpected finding.

"Reasons for this observation are of considerable interest because of the high prevalence, morbidity, and mortality associated with asthma in poor urban communities," say Peter Heymann and Thomas Platts-Mills (University of Virginia, Charlottesville, USA) in an accompanying editorial.

The researchers say that the lower rate of viral infection in urban infants raises questions of the influence of other factors that cause respiratory illness, such as bacterial infections and pollutant exposure, in the development of asthma.

Additionally, they found that viral infections showed a different pattern in urban areas compared with suburban. Human rhinovirus alone was detected in 24.1% of samples from urban infants, which was significantly lower compared with 36.0% of samples from suburban infants. There was also a trend toward a greater percentage of respiratory syncytial virus infection in suburban infants.

Both human rhinovirus and respiratory syncytial virus infection have been associated with an increased risk for developing asthma, notes the team.

Interestingly, urban infants had significantly greater rates of adenovirus-only infections, which were found in 4.8% of swabs compared with 0.7% of swabs from suburban infants.

"Adenovirus infections, either as a single pathogen or when detected in concert with other viruses, were significantly more common in the urban population, and this held true for each of the four urban locations where our study was conducted," write Gern and colleagues in the Journal of Infectious Diseases. They add that adenovirus infections can become chronic, which could alter the development of the respiratory system during early life.

The authors say that the different pattern of respiratory infections in urban infants could help explain the high prevalence of asthma among these children in comparison to their suburban counterparts.

They conclude: "This approach is likely to provide novel insights that will serve to guide the development of treatment interventions to decrease the prevalence and severity of asthma during childhood."

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