Respiratory Pathogens in Respiratory Tract Illnesses During the First Year of Life: A Birth Cohort Study
van der Zalm, Marieke M. MD; Uiterwaal , Cuno S.P.M. MD, PhD; Wilbrink, Berry PhD; de Jong, Brita M. MD; Verheij, Theo J.M. MD, PhD; Kimpen, Jan L. L. MD, PhD; van
der Ent, Cornelis K. MD, PhD

Abstract
Background:
Respiratory virus infections are the most important trigger of respiratory illnesses in childhood. Data on the occurrence and the clinical impact of respiratory pathogens in the general population of infants are scarce. Therefore, we described the occurrence and clinical impact of respiratory pathogens in infants with respiratory tract infections during the first year of life.

Methods: In a prospective birth cohort study, infants were followed from birth through the first year of life with daily questionnaires about respiratory symptoms. Nose and throat swabs were collected during episodes with respiratory symptoms. Polymerase chain reaction was used to detect an extensive panel of respiratory pathogens.

Results: The parents reported a median of 5 respiratory episodes per infant per year. A total of 668 respiratory samples were collected in 305 infants. One or more respiratory pathogens were detected in 85% of the samples. The most common respiratory pathogens were human rhinovirus (HRV) (73% of the samples), respiratory syncytial virus (RSV) (11%), and coronavirus (8%). HRV infections were associated with a prolonged period of symptoms compared with RSV (P = 0.03). Infections with RSV were associated with more physician visits than HRV infections (P = 0.06).

Conclusion: We found a high prevalence of respiratory pathogens among infants with parent -reported respiratory illnesses in the first year of life, with HRV being the most prevalent . Although RSV infections seemed to be responsible for the most severe symptoms compared with HRV, the overall burden of disease was highest for HRV infections.

Introduction
Respiratory virus infections are the most important trigger of respiratory illnesses in young childhood. Human rhinovirus (HRV) and respiratory syncytial virus (RSV)are reported as the most frequentcausing agents of respiratory illness in selected populations of hospitalized children and in children at high risk for atopic diseases.[1-4] Data on the occurrence and clinical impact of respiratory pathogens during respiratory tract infections in unselected infants are scarce. Most of these studies have used rather insensitive conventional methods for virus detection. The development and use of polymerase chain reaction (PCR) methods have markedly increased the identification of viral pathogens.[5,6]

Because of the scarcity of data on the prevalence of respiratory pathogens in young children, prospective data on the clinical impact of respiratory pathogens during respiratory tract infections are almost lacking. For a long time, it has been suggested that some respiratory pathogens cause more severe respiratory illness than others. RSV is most often mentioned to cause severe respiratory tract illness in young children, resulting in prolonged postviral wheezing.[7,8] HRV, on the contrary, was thought to be limited to the upper respiratory tract, causing only mild symptoms.[9] During the last decade the perspective on HRVs has changed, as HRVs seem to be able to infect the lower respiratory tract as well.[10 ] Moreover, some symptomatic HRV infections could play an important role in recurrent wheezing later on in life.[11]

In view of this, we questioned whether there is a difference in the clinical impact between the various respiratory pathogens. In this prospective population based birth cohort study, we described the occurrence and clinical impact of respiratory pathogens in infants whose parents reported respiratory infection episodes during the first year of life.

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