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INTRODUCTION

The viruses described in this chapter are the “professional” respiratory tract viruses whose primary clinical manifestations are in the upper and/or lower respiratory tract (Table 38–1). Many other viruses, such as measles virus, mumps virus, rubella virus, and varicella-zoster virus, initially infect the respiratory tract, but their characteristic clinical findings are seen elsewhere. These viruses are described in other chapters.

TABLE 38–1123Clinical Features of Respiratory Viruses

Almost all of the respiratory tract viruses have RNA as their genome; one has DNA. Most are enveloped viruses, whereas two, rhinovirus and adenovirus, are nonenveloped. In addition, the enveloped respiratory viruses belong to several different virus families, namely, orthomyxoviruses, paramyxoviruses, and coronaviruses. So they are quite varied in their structure and replication. The feature that unites all of these viruses is their ability to infect the mucosal cells of the respiratory tract and cause significant symptoms there.

In serious respiratory virus infections, a laboratory diagnosis can be made by using polymerase chain reaction (PCR)-based assays on respiratory tract secretions. A panel of PCR assays is used to diagnose infections caused by viruses such as influenza virus, parainfluenza virus, respiratory syncytial virus, rhinovirus, human metapneumovirus, and adenovirus.

INFLUENZA VIRUS

Influenza virus is an important human pathogen because it causes both outbreaks of influenza that sicken and kill thousands of people each year as well as infrequent but devastating worldwide epidemics (pandemics).

Influenza virus is the only member of the orthomyxovirus family. The orthomyxoviruses differ from the paramyxoviruses primarily in that the former have a segmented RNA genome (usually eight pieces), whereas the RNA genome of the latter consists of a single piece. The term myxo refers to the observation that these viruses interact with mucins (glycoproteins on the surface of cells). Table 38–2 shows a comparison of the properties of influenza A virus with several other viruses that infect the respiratory tract.

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