Influenza viruses are important human pathogens because they cause both outbreaks of influenza that sicken and kill thousands of people each year as well as infrequent but devastating worldwide epidemics (pandemics).
Influenza viruses are the only members 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.1 The term myxo refers to the observation that these viruses interact with mucins (glycoproteins on the surface of cells).
In addition, the orthomyxoviruses are smaller (110 nm in diameter) than the paramyxoviruses (150 nm in diameter). See Table 39–1 for additional differences.
TABLE 39–1Properties of Orthomyxoviruses and Paramyxoviruses |Favorite Table|Download (.pdf) TABLE 39–1 Properties of Orthomyxoviruses and Paramyxoviruses
|Property ||Orthomyxoviruses ||Paramyxoviruses |
|Viruses ||Influenza A, B, and C viruses ||Measles, mumps, respiratory syncytial, and parainfluenza viruses |
|Genome ||Segmented (eight pieces) single-stranded RNA of negative polarity ||Nonsegmented single-stranded RNA of negative polarity |
|Virion RNA polymerase ||Yes ||Yes |
|Capsid ||Helical ||Helical |
|Envelope ||Yes ||Yes |
|Size ||Smaller (110 nm) ||Larger (150 nm) |
|Surface spikes ||Hemagglutinin and neuraminidase on different spikes ||Hemagglutinin and neuraminidase on the same spike1 |
|Giant cell formation ||No ||Yes |
Table 39–2 shows a comparison of influenza A virus with several other viruses that infect the respiratory tract. Table 39–3 describes some of the important clinical features of influenza virus and compares them with the clinical features of the other medically important viruses in this chapter.
TABLE 39–2Features of Viruses That Infect the Respiratory Tract1 |Favorite Table|Download (.pdf) TABLE 39–2 Features of Viruses That Infect the Respiratory Tract1
|Virus ||Disease ||Number of Serotypes ||Lifelong Immunity to Disease ||Vaccine Available ||Viral Latency ||Treatment |
|RNA viruses || || || || || || |
|Influenza virus ||Influenza ||Many ||No ||+ ||– ||Amantadine rimantadine, oseltamivir, zanamivir |
|Parainfluenza virus ||Croup ||Many ||No ||– ||– ||None |
|Respiratory syncytial virus ||Bronchiolitis ||Two ||Incomplete ||– ||– ||Ribavirin |
|Rubella virus ||Rubella ||One ||Yes ||+ ||– ||None |
|Measles virus ||Measles ||One ||Yes ||+ ||– ||None |
|Mumps virus ||Parotitis, meningitis ||One ||Yes ||+ ||– ||None |
|Rhinovirus ||Common cold ||Many ||No ||– ||– ||None |
|Coronavirus ||Common cold, SARS2 ||Three ||No ||– ||– ||None |
|Coxsackie virus ||Herpangina, pleurodynia, myocarditis ||Many ||No ||– ||– ||None |
|DNA viruses || || || || || || |
|Herpes simplex virus type 1 ||Gingivostomatitis ||One ||No3 ||– ||+ ||Acyclovir in immunodeficient patients |
|Varicella-zoster virus ||Chickenpox, shingles ||One ||Yes for varicella; no for zoster ||– ||+ ||Acyclovir in immunodeficient patients |
|Cytomegalovirus ||Pneumonia in immunocompromised ||One ||No3 ||– ||+ ||Ganciclovir |
|Epstein–Barr virus ||Infectious mononucleosis ||One ||Yes ||– ||+ ||None |
|Adenovirus ||Pharyngitis, pneumonia ||Many ||No ||+4 ||+ ||None...|