LOWER RESPIRATORY TRACT INFCTION - HOW TO INVESTIGATE?

Question:
Since viral infection is suspected and preliminary investigations do not show a bacterial infection, how to work up for the viral infection?

Answer:
Viruses are common organisms that affect the respiratory tract and can cause variety of diseases depending on which part of the respiratory system is affected. They may lead to rhinitis, rhinosinusitis, nasopharyngitis, tonsillopharyngitis, otitis media, croup, bronchitis, bronchiolitis, viral pneumonia and wheeze associated lower respiratory tract infections (WALRI).

Common viruses that can cause respiratory tract infections are Influenza A and B; respiratory syncytial virus (RSV) A and B; parainfluenza 1,2 and 3; rhinovirus (many subtypes) and adenovirus (subtypes A to E). Other viruses recently isolated are metapneumovirus. Recent outbreak of novel influenza virus H1N1 (swine flu) has been one of the commonest cause of respiratory infections in recent times. Additional viruses that can cause symptoms or respiratory tract infection as part of a systemic viral infection include measles, Epstein-Barr virus, enteroviruses, human herpesvirus-6 and herpes virus types 1 and 2.

Laboratory methods available for respiratory viruses include: tissue (cell) culture, both standard and shell vial; serology and direct examination, which can include electron microscopy; indirect and direct immunofluorescent antibody tests (IFA/DFA); enzyme immunoassays (EIA); enzyme activity assays (neuraminidase assays); and nucleic acid amplification (e.g. PCR). Tissue culture used to be the gold standard but has been replaced during the last 10 years with molecular methods as it takes longer time to get result (average 4-5 days), is not very sensitive and is difficult to do. Serology is not very useful to detect acute infection as antibodies may take about 2-4 weeks to develop and thus may not be detected if tested in early stages. However with PCR being not accessible everywhere and also being expensive, is not cost-effective to be carried out routinely. Multiplex PCRs are available that detect 7-11 viruses at a time. PCR is now being offered for diagnosis of H1N1 to identify patients early so that they can treated and isolated early.

A number of significant advancements in rapid diagnostic methods have appeared. Antigen based assay such as enzyme immunoassays (EIAs), optical immunoassays and neuraminidase activity assays are available for rapid diagnosis of respiratory viruses. They offer the advantage of being cheap and easy to use. However, they are not very specific and sensitive in areas of low prevalence and in immunocompromised patients. These tests can be used in outpatient pediatric practice, school age children and young adults during peak viral season. These antigen assays are available as multiplex kits which can detect 7-11 types of viruses at one time itself.

Both PCR and rapid antigen assays can be done on naso-pharyngeal aspirate that is transported to the laboratory in viral transport medium (VTM).