Viral respiratory infections : diagnosis and epidemiology

Abstract

Background: Respiratory viral infections are common causes of human morbidity and mortality in children as well as in adults. Adenovirus, influenza virus, parainfluenza virus and respiratory syncytial virus (RSV) have been recognized for many years. During recent years two main events have influenced both the diagnosis and our knowledge of respiratory virus epidemiology: (1) Five new viruses have been described; (2) the use of molecular methods for the diagnosis of respiratory tract infections has been introduced.

Objectives: The first objective of this thesis was to study the diversity of respiratory virus infections in Stockholm in different patient groups and using different diagnostic methods. The second objective was to study the molecular epidemiology of RSV. The third objective was to evaluate and compare the conventional diagnostic methods for respiratory viruses with new molecular diagnostic methods.

Materials and Methods: In paper I the diagnostic results from 7303 respiratory specimens were analyzed retrospectively. The results were obtained by virus isolation and antigen detection with immunofluorescence. In paper II RSV strains from 234 frozen respiratory specimens were sequenced and genotyped. In paper III a real-time diagnostic platform for 15 respiratory viruses was developed and evaluated based on 585 frozen nasopharyngeal aspirates and after that evaluated based on diagnostic samples. In paper IV 37 infection episodes with multiple viral findings were compared with 94 infection episodes with single findings.

Results: In paper I one or two viruses were found in 43% of the samples, with the highest proportion of positive samples among the oldest patients. RSV was the most common finding among children under five years of age, and influenza was the most common in adults and children over five years old. RSV was only found in 2% of patients over 81 years old. The genotyping of 234 strains in paper II showed that both RSV subgroups A and B were circulating during the 2002 2003 season. The two subgroups were further subdivided into two genotypes each. A comparison with available strains from other parts of the world showed a high degree of similarity. In paper III the diagnostic yield increased from 37% with conventional methods to 57% with the new diagnostic real-time PCR platform. The children with multiple findings were significantly older than those with single findings in the comparison between the two groups in paper IV. There was no difference between the groups regarding hospitalization time, diagnosis, CRP levels, signs on x-ray or oxygen treatment.

Conclusions: By using a real-time PCR platform for viral respiratory tract infections, the diagnostic yield increased compared to virus isolation and antigen detection. Besides older age, no apparent differences between children with multiple findings and single findings were observed, indicating that prolonged sheddings from sequential infections rather than true multiple infections were detected. The low findings of RSV among the elderly indicate either that RSV is not common among the elderly in Sweden or that RSV infection appears before admission to hospital or that more sensitive methods than IF have to be used in the elderly. The high degree of similarity between strains from Sweden and other parts of the world supports the current theories on RSV epidemiology suggesting that local factors, such as local immunity in the population, are more important for the outcome of epidemics than globally circulating strains.