Jawetz Chapter 39: Orthomyxoviruses (Influenza Viruses) PDF

39 C H A P T E R Orthomyxoviruses...

39 C H A P T E R Orthomyxoviruses (Influenza Viruses) Respiratory illnesses are responsible for more than half of Influenza virus particles contain nine different struc- all acute illnesses each year in the United States. The Ortho- tural proteins. The nucleoprotein (NP) associates with the myxoviridae (influenza viruses) are a major determinant of viral RNA to form a ribonucleoprotein (RNP) structure 9 nm morbidity and mortality caused by respiratory disease, and in diameter that assumes a helical configuration and forms outbreaks of infection sometimes occur in worldwide epi- the viral nucleocapsid. Three large proteins (PB1, PB2, and demics. Influenza has been responsible for millions of deaths PA) are bound to the viral RNP and are responsible for RNA worldwide. Mutability and high frequency of genetic reas- transcription and replication. The matrix (M1) protein, which sortment and resultant antigenic changes in the viral surface forms a shell underneath the viral lipid envelope, is impor- glycoproteins make influenza viruses formidable challenges tant in particle morphogenesis and is a major component of for control efforts. Influenza type A is antigenically highly the virion (∼40% of viral protein). variable and is responsible for most cases of epidemic influ- A lipid envelope derived from the cell surrounds the enza. Influenza type B may exhibit antigenic changes and virus particle. Two virus-encoded glycoproteins, hemag- sometimes causes epidemics. Influenza type C is antigeni- glutinin (HA) and neuraminidase (NA), are inserted into cally stable and causes only mild illness in immunocompe- the envelope and are exposed as spikes about 10 nm long on tent individuals. the surface of the particle. These two surface glycoproteins determine antigenic variation of influenza viruses and host immunity. The HA represents about 25% of viral protein and PROPERTIES OF ORTHOMYXOVIRUSES the NA about 5%. The M2 ion channel protein and the NS2 Three immunologic types of influenza viruses are known, protein are also present in the envelope but at only a few cop- designated A, B, and C. Whereas antigenic changes continu- ies per particle. ally occur within the type A group of influenza viruses and Because of the segmented nature of the genome, when a to a lesser degree in the type B group, type C appears to be cell is coinfected by two different viruses of a given type, mix- antigenically stable. Influenza A strains are also known for tures of parental gene segments may be assembled into prog- aquatic birds, chickens, ducks, pigs, horses, and seals. Some eny virions. This phenomenon, called genetic reassortment, of the strains isolated from animals are antigenically similar may result in sudden changes in viral surface antigens—a to strains circulating in the human population. property that explains the epidemiologic features of influ- The following descriptions are based on influenza virus enza and poses significant problems for vaccine development. type A, the best-characterized type (Table 39-1). Influenza viruses are relatively hardy in vitro and may be stored at 0–4°C for weeks without loss of viability. Lipid solvents, protein denaturants, formaldehyde, and irradiation Structure and Composition destroy infectivity. Both infectivity and hemagglutination Influenza virus particles are usually spherical and about are more resistant to inactivation at alkaline pH than at 100 nm in diameter (80–120 nm), although virions may dis- acid pH. play great variation in size (Figure 39-1). The single-stranded, negative-sense RNA genomes of influenza A and B viruses occur as eight separate segments; Classification and Nomenclature influenza C viruses contain seven segments of RNA, lacking Genus Influenzavirus A contains human and animal strains a neuraminidase gene. Sizes and protein-coding assignments of influenza type A, Influenzavirus B contains human strains are known for all the segments (Table 39-2). Most of the seg- of type B, and Influenzavirus C contains influenza type C ments code for a single protein. The first 12–13 nucleotides at viruses of humans and swine. each end of each genomic segment are conserved among all Antigenic differences exhibited by two of the internal eight RNA segments; these sequences are important in viral structural proteins, the nucleocapsid (NP) and matrix (M) transcription. proteins, are used to divide influenza viruses into types A, B, 581 Riedel_CH39_p581-p594.indd 581 04/04/19 5:08 PM  582   SECTION IV  Virology TABLE 39-1 Important Properties of human isolates, such as A/Hong Kong/03/68(H3N2), but it is Orthomyxovirusesa indicated for others, such as A/swine/Iowa/15/30(H1N1). So far, 18 subtypes of HA (H1–H18) and 11 subtypes of Virion: Spherical, pleomorphic, 80–120 nm in diameter (helical nucleocapsid, 9 nm) NA (N1–N11), in many different combinations, have been recovered from humans and animals. Composition: RNA (1%), protein (73%), lipid (20%), carbohydrate (6%) The Orthomyxoviridae family also contains the genus Genome: Single-stranded RNA, segmented (eight molecules), Thogotovirus, members of which are not known to cause dis- negative-sense, 13.6 kb overall size ease in humans. Proteins: Nine structural proteins, one nonstructural Envelope: Contains viral hemagglutinin and neuraminidase proteins Structure and Function of Hemagglutinin Replication: Nuclear transcription; capped 5′ termini of cellular RNA The HA protein of influenza virus binds virus particles to scavenged as primers; particles mature by budding from plasma susceptible cells and is the major antigen against which neu- membrane tralizing (protective) antibodies are directed. Variability in Outstanding characteristics: HA is primarily responsible for the continual evolution of Genetic reassortment common among members of the same genus new strains and subsequent influenza epidemics. HA derives Influenza viruses cause worldwide epidemics its name from its ability to agglutinate erythrocytes under certain conditions. a Description of influenza A virus, genus Influenzavirus A. The primary sequence of HA contains 566 amino acids (Figure 39-2A). A short signal sequence at the amino termi- nal inserts the polypeptide into the endoplasmic reticulum; and C. These proteins possess no cross-reactivity among the the signal is then removed. The HA protein is cleaved into three types. Antigenic variations in the surface glycoproteins, two subunits, HA1 and HA2, that remain tightly associated HA and NA, are used to subtype type A viruses. by a disulfide bridge. A hydrophobic stretch near the carboxyl The standard nomenclature system for influenza virus terminal of HA2 anchors the HA molecule in the membrane, isolates includes the following information: type, host of ori- with a short hydrophilic tail extending into the cytoplasm. gin, geographic origin, strain number, and year of isolation. Oligosaccharide residues are added at several sites. Antigenic descriptions of the HA and the NA are given in The three-dimensional structure of the HA protein has parentheses for type A. The host of origin is not indicated for been revealed by x-ray crystallography. The HA molecule is Envelope Viral Nucleocapsid glycoproteins A B FIGURE 39-1 Influenza virus. A: Electron micrograph of influenza virus A/Hong Kong/1/68(H3N2). Note the pleomorphic shapes and glycoprotein projections covering particle surfaces (315,000×). (Courtesy of FA Murphy and EL Palmer.) B: Schematic view of influenza. Virus particles have segmented genomes consisting of seven or eight different RNA molecules, each coated by capsid proteins and forming helical nucleocapsids. Viral glycoproteins (hemagglutinin and neuraminidase) protrude as spikes through the lipid envelope. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw Hill, 2008. © McGraw-Hill Education.) Riedel_CH39_p581-p594.indd 582 04/04/19 5:08 PM  CHAPTER 39 Orthomyxoviruses (Influenza Viruses)   583 TABLE 39-2 Coding Assignments of Influenza Virus A RNA Segments Genome Segment Encoded Polypeptide Approximate Predicted Number of Size (Number of Molecular Molecules Numbera Nucleotides) Designation Weightb per Virion Function 1 2341 PB2 85,700 30–60 RNA transcriptase components 2 2341 PB1 86,500 3 2233 PA 84,200 4 1778 HA 61,500 500 Hemagglutinin; trimer; envelope glycoprotein; mediates virus attachment to cells; activated by cleavage; fusion activity at acid pH 5 1565 NP 56,100 1000 Associated with RNA and polymerase proteins; helical structure; nucleocapsid 6 1413 NA 50,000 100 Neuraminidase; tetramer; envelope glycoprotein; enzyme 7 1027 M1 27,800 3000 Matrix protein; major component of virion; lines inside of envelope; involved in assembly; interacts with viral RNPs and NS2 M2 11,000 20–60 Integral membrane protein; ion channel; essential for virus uncoating; from spliced mRNA 8 890 NS1 26,800 0 Nonstructural; high abundance; inhibits pre-mRNA splicing; reduces interferon response NS2 14,200 130–200 Minor component of virions; nuclear export of viral RNPs; from spliced mRNA HA, hemagglutinin; M1, matrix protein; M2, integral membrane protein; NA, neuraminidase; NP, nucleoprotein; NS1 and NS2 are nonstructural proteins; PB2, PB1, and PA are polymerase proteins; RNP, ribonucleoprotein. a RNA segments are numbered in order of decreasing size. b The molecular weights of the two glycoproteins, HA and NA, appear larger (about 76,000 and 56,000, respectively) because of the added carbohydrate. Adapted with permission from Lamb RA, Krug RM: Orthomyxoviridae: The viruses and their replication. In Fields BN, Knipe DM, Howley PM (editors-in-chief). Fields Virology, 3rd ed. Lippincott-Raven, 1996. folded into a complex structure (Figure 39-2B). Each linked cleave HA are expressed only at those sites. Examples have HA1 and HA2 dimer forms an elongated stalk capped by a been noted of more virulent viruses that have adapted to use large globule. The base of the stalk anchors it in the mem- a more ubiquitous enzyme, such as plasmin, to cleave HA and brane. Five antigenic sites on the HA molecule exhibit exten- promote widespread infection of cells. The amino terminal sive mutations. These sites occur at regions exposed on the of HA2, generated by the cleavage event, is necessary for the surface of the structure, are apparently not essential to the viral envelope to fuse with the cell membrane, an essential molecule’s stability, and are involved in viral neutralization. step in the process of viral infection. Low pH triggers a con- Other regions of the HA molecule are conserved in all iso- formational change that activates the fusion activity. lates, presumably because they are necessary for the molecule to retain its structure and function. The HA spike on the virus particle is a trimer composed Structure and Function of Neuraminidase of three intertwined HA1 and HA2 dimers (Figure 39-2C). The antigenicity of NA, the other glycoprotein on the surface The trimerization imparts greater stability to the spike of influenza virus particles, is also important in determining than could be achieved by a monomer. The cellular receptor the subtype of influenza virus isolates. binding site (viral attachment site) is a pocket located at The NA spike on the virus particle is a tetramer com- the top of each large globule. The pocket is inaccessible to posed of four identical monomers (Figure 39-2D). A slender antibody. stalk is topped with a box-shaped head. There is a catalytic The cleavage that separates HA1 and HA2 is necessary site for NA on the top of each head, so that each NA spike for the virus particle to be infectious and is mediated by cel- contains four active sites. lular proteases. Influenza viruses normally remain confined The NA functions at the end of the viral replication to the respiratory tract because the protease enzymes that cycle. It is a sialidase enzyme that removes sialic acid from Riedel_CH39_p581-p594.indd 583 04/04/19 5:08 PM  584   SECTION IV  Virology Proteolytic Hydrophobic Signal activation membrane peptide cleavage domain NH2 COOH S–S HA1 HA2 Hydrophilic Hemagglutinin cytoplasmic Hydrophobic domain membrane domain NH2 COOH Conserved sequence Neuraminidase A Receptor site Site B A Large globule A A Site D Site A 13.5 nm A Protease cleavage site Site E Small globule Membrane Site C C C C Active site Fusion peptide 6 nm Stalk B D N FIGURE 39-2 Influenza virus hemagglutinin (HA) and neuraminidase (NA) surface glycoproteins. A: Primary structures of HA and NA polypeptides. The cleavage of HA into HA1 and HA2 is necessary for virus to be infectious. HA1 and HA2 remain linked by a disulfide bond (S–S). No posttranslational cleavage occurs with NA. Carbohydrate attachment sites are shown. The hydrophobic amino acids that anchor the proteins in the viral membrane are located near the carboxyl terminal of HA and the amino terminal of NA. B: Folding of the HA1 and HA2 polypeptides in an HA monomer. Five major antigenic sites (sites A–E) that undergo change are shown as shaded areas. The amino terminal of HA2 provides fusion activity (fusion peptide). The fusion particle is buried in the molecule until it is exposed by a conformational change induced by a low pH. C: Structure of the HA trimer as it occurs on a virus particle or the surface of infected cells. Some of the sites involved in antigenic variation are shown (A). Carboxyl terminal residues (C) protrude through the membrane. D: Structure of the NA tetramer. Each NA molecule has an active site on its upper surface. The amino terminal region (N) of the polypeptides anchors the complex in the membrane. (Redrawn with permission from [A, B] Murphy BR, Webster RG: Influenza viruses, pp. 1185 and 1186, and [C, D] Kingsbury DW: Orthomyxo-and paramyxoviruses and their replication, pp. 1163 and 1172. In Fields BN [editor-in-chief] Virology. Raven Press, 1985.) glycoconjugates. It facilitates release of virus particles from Antigenic Drift and Antigenic Shift infected cell surfaces during the budding process and helps Influenza viruses are remarkable because of the frequent prevent self-aggregation of virions by removing sialic acid antigenic changes that occur in HA and NA. Antigenic residues from viral glycoproteins. It is possible that NA helps variants of influenza virus have a selective advantage over the virus negotiate through the mucin layer in the respiratory the parental virus in the presence of antibody directed tract to reach the target epithelial cells. against the original strain. This phenomenon is responsible Riedel_CH39_p581-p594.indd 584 04/04/19 5:08 PM  CHAPTER 39 Orthomyxoviruses (Influenza Viruses)   585 Antigenic drift Antigenic shift Decreasing serologic relatedness pH 5-6 Coated Coated RNA Endosome pit vesicle nucleocapsid HA or NA HA or NA Cytoplasm 3 4 +ssRNA –ssRNAs NP NP PB1 Years One year 1 (+) 2 C Host mRNA FIGURE 39-3 Antigenic drift and antigenic shift account for 5′ C 3′ C antigenic changes in the two surface glycoproteins (hemagglutinin Viral mRNA C [HA] and neuraminidase [NA]) of influenza virus. Antigenic drift is a Nucleus gradual change in antigenicity caused by point mutations that affect major antigenic sites on the glycoprotein. Antigenic shift is an abrupt change caused by genetic reassortment with an unrelated strain. 5 Changes in HA and NA occur independently. Internal proteins of the virus, such as the nucleoprotein (NP), do not undergo antigenic changes. 5′ C 3′ HA N A Rough ER for the unique epidemiologic features of influenza. Other respiratory tract agents do not display significant antigenic variation. Golgi The two surface antigens of influenza undergo antigenic apparatus variation independent of each other. Minor antigenic changes 6 are termed antigenic drift; major antigenic changes in HA or Insertion of NA, called antigenic shift, result in the appearance of a new envelope proteins Budding subtype (Figure 39-3). Antigenic shift is most likely to result in an epidemic. Antigenic drift is caused by the accumulation of point FIGURE 39-4 Schematic diagram of the life cycle of influenza virus. After receptor-mediated endocytosis, the viral mutations in the gene, resulting in amino acid changes in ribonucleoprotein complexes are released into the cytoplasm and the protein. Sequence changes can alter antigenic sites on transported to the nucleus, where replication and transcription the molecule such that a virion can escape recognition by the take place (1). Messenger RNAs are exported to the cytoplasm host’s immune system. The immune system does not cause for translation. (2) Early viral proteins required for replication and the antigenic variation but rather functions as a selection transcription, including nucleoprotein (NP) and a polymerase protein force that allows new antigenic variants to expand. A variant (PB1), are transported back to the nucleus. RNA polymerase activity must sustain two or more mutations before a new, epidemio- of the PB1 protein synthesizes positive single-stranded RNA (ssRNA) logically significant strain emerges. from genomic negative single-stranded RNA (–ssRNA) molecules. Antigenic shift reflects drastic changes in the sequence (3) These +ssRNA templates are copied by the RNA polymerase of a viral surface protein, caused by genetic reassortment activity of the PB1 protein. (4) Some of these new genome segments between human, swine, and avian influenza viruses. Influ- serve as templates for the synthesis of more viral mRNA. Later in the infection, they become progeny genomes. Viral mRNA molecules enza B and C viruses do not exhibit antigenic shift because transcribed from some genome segments encode structural few related viruses exist in animals. proteins such as hemagglutinin (HA) and neuraminidase (NA). These messages are translated by endoplasmic reticulum-associated ribosomes and delivered to the cell membrane (5). Viral genome Influenza Virus Replication segments are packaged as progeny virions bud from the host cell The replication cycle of influenza virus is summarized in (6). ER, endoplasmic reticulum. (Reproduced with permission from Figure 39-4. The viral multiplication cycle proceeds rapidly. Willey JM, Sherwood LM, Woolverton CJ (eds): Prescott, Harley, There is the shut-off of host cell protein synthesis by about & Klein’s Microbiology. McGraw-Hill, 2008, p. 457. © McGraw-Hill 3 hours postinfection, permitting selective translation of Education.) viral mRNAs. New progeny viruses are produced within 8–10 hours. Riedel_CH39_p581-p594.indd 585 04/04/19 5:08 PM  586   SECTION IV  Virology A. Viral Attachment, Penetration, and Uncoating completely free RNAs are mRNAs. The first step in genome The virus attaches to cell-surface sialic acid via the receptor replication is production of positive-strand copies of each site located on the top of the large globule of the HA. Virus segment. These antigenome copies differ from mRNAs at particles are then internalized within endosomes through both terminals; the 5′ ends are not capped, and the 3′ ends receptor-mediated endocytosis. The next step involves fusion are neither truncated nor polyadenylated. These copies serve between the viral envelope and cell membrane, triggering as templates for synthesis of faithful copies of genomic RNAs. uncoating. The low pH within the endosome is required for Because there are common sequences at both ends of all virus-mediated membrane fusion that releases viral RNPs viral RNA segments, they can be recognized efficiently by into the cytosol. Acid pH causes a conformational change in the RNA-synthesizing machinery. Intermingling of genome the HA structure to bring the HA2 “fusion peptide” in cor- segments derived from different parents in coinfected cells rect contact with the membrane. The M2 ion channel protein is presumably responsible for the high frequency of genetic present in the virion permits the entry of ions from the endo- reassortment typical of influenza viruses within a genus. Fre- some into the virus particle, triggering the conformational quencies of reassortment as high as 40% have been observed. change in HA. Viral nucleocapsids are then released into the cell cytoplasm. D. Maturation The virus matures by budding from the surface of the cell. B. Transcription and Translation Individual viral components arrive at the budding site by Transcription mechanisms used by orthomyxoviruses differ different routes. Nucleocapsids are assembled in the nucleus markedly from those of other RNA viruses in that cellular and move out to the cell surface. The glycoproteins, HA and functions are more intimately involved. Viral transcription NA, are synthesized in the endoplasmic reticulum; are modi- occurs in the nucleus. The mRNAs are produced from viral fied and assembled into trimers and tetramers, respectively; nucleocapsids. The virus-encoded polymerase, consisting of and are inserted into the plasma membrane. The M1 protein a complex of the three P proteins, is primarily responsible for serves as a bridge, linking the nucleocapsid to the cytoplas- transcription. Its action must be primed by scavenged capped mic ends of the glycoproteins. Progeny virions bud off the and methylated 5′ terminals from cellular transcripts that cell. During this sequence of events, the HA is cleaved into are newly synthesized by cellular RNA polymerase II. This HA1 and HA2 if the host cell possesses the appropriate pro- explains why influenza virus replication is inhibited by dacti- teolytic enzyme. The NA removes terminal sialic acids from nomycin and α-amanitin, which block cellular transcription, cellular and viral surface glycoproteins, facilitating release of but other RNA viruses are not affected because they do not virus particles from the cell and preventing their aggregation. use cellular transcripts in viral RNA synthesis. Many of the particles are not infectious. Particles some- Six of the genome segments yield monocistronic mRNAs times fail to encapsidate the complete set of genome segments; that are translated in the cytoplasm into six viral proteins. frequently, one of the large RNA segments is missing. These The other two transcripts undergo splicing, each yielding two noninfectious particles are capable of causing hemagglutina- mRNAs that are translated in different reading frames. At tion and can interfere with the replication of intact virus. early times after infection, the NS1 and NP proteins are pref- Reverse-genetics systems that allow the generation of erentially synthesized. At later times, the structural proteins infectious influenza viruses from cloned cDNAs of viral are synthesized at high rates. The two glycoproteins, HA and RNA segments are available and allow for mutagenesis and NA, are modified using the secretory pathway. functional studies. The influenza virus nonstructural protein NS1 has a posttranscriptional role in regulating viral and cellular gene expression. The NS1 protein binds to poly(A) sequences, INFLUENZA VIRUS INFECTIONS IN inhibits pre-mRNA splicing, and inhibits the nuclear export HUMANS of spliced mRNAs, ensuring a pool of donor cellular mole- A comparison of influenza A virus with other viruses that cules to provide the capped primers needed for viral mRNA infect the human respiratory tract is shown in Table 39-3. synthesis. The NS2 protein interacts with M1 protein and is Influenza virus is considered here. involved in nuclear export of viral RNPs. C. Viral RNA Replication Pathogenesis and Pathology Viral genome replication is accomplished by the same virus- Influenza virus spreads from person to person by airborne encoded polymerase proteins involved in transcription. The droplets or by contact with contaminated hands or surfaces. mechanisms that regulate the alternative transcription and A few cells of respiratory epithelium are infected if deposited replication roles of the same proteins are related to the abun- virus particles avoid removal by the cough reflex and escape dance of one or more of the viral nucleocapsid proteins. neutralization by preexisting specific immunoglobulin A As with all other negative-strand viruses, templates (IgA) antibodies or inactivation by nonspecific inhibitors in for viral RNA synthesis remain coated with NPs. The only the mucous secretions. Progeny virions are soon produced Riedel_CH39_p581-p594.indd 586 04/04/19 5:08 PM  CHAPTER 39 Orthomyxoviruses (Influenza Viruses)   587 TABLE 39-3 Comparison of Viruses That Infect the Human Respiratory Tract Number of Lifelong Immunity Vaccine Viral Virus Disease Serotypes to Disease Available Latency RNA viruses Influenza A virus Influenza Many No + − Metapneumovirus Croup, bronchiolitis Several No − − Parainfluenza virus Croup Many No − − Respiratory syncytial virus Bronchiolitis, pneumonia Two No − − Rubella virus Rubella One Yes + − Measles virus Measles One Yes + − Mumps virus Parotitis, meningitis One Yes + − Rhinovirus Common cold Many No − − Coronavirus Common cold Many No − − Coxsackievirus Herpangina, pleurodynia Many No − − DNA viruses Herpes simplex virus type 1 Gingivostomatitis One No − + Epstein-Barr virus Infectious mononucleosis One Yes − + Varicella-zoster virus Chickenpox, shingles One Yes a + + Adenovirus Pharyngitis, pneumonia Many No − + a Lifelong immunity to reinfections with varicella (chickenpox) but not to reactivation of zoster (shingles). and spread to adjacent cells, where the replicative cycle is probably account for local symptoms. The fever and systemic repeated. Viral NA lowers the viscosity of the mucous film in symptoms associated with influenza reflect the action of the respiratory tract, laying bare the cellular surface receptors cytokines. and promoting the spread of virus-containing fluid to lower portions of the tract. Within a short time, many cells in the respiratory tract are infected and eventually killed. Clinical Findings The incubation period from exposure to virus and the Influenza attacks mainly the upper respiratory tract. It poses onset of illness varies from 1 day to 4 days, depending on the a serious risk for elderly adults, very young children, and peo- size of the viral dose and the immune status of the host. Viral ple with underlying medical conditions such as lung, kidney, shedding starts the day preceding onset of symptoms, peaks or heart problems, diabetes, cancer, or immunosuppression. within 24 hours, remains elevated for 1–2 days, and then declines over the next 5 days. Infectious virus is very rarely recovered from blood. A. Uncomplicated Influenza Interferon is detectable in respiratory secretions about Symptoms of classic influenza usually appear abruptly and 1 day after viral shedding begins. Influenza viruses are sensitive include chills, headache, and dry cough followed closely by to the antiviral effects of interferon, and it is believed that the high fever, generalized muscular aches, malaise, and anorexia. innate immunity response contributes to host recovery from The fever usually lasts 3–5 days, as do the systemic symp- infection. Specific antibody and cell-mediated responses can- toms. Respiratory symptoms typically last another 3–4 days. not be detected for another 1–2 weeks. The cough and weakness may persist for 2–4 weeks after Influenza infections cause cellular destruction of the major symptoms subside. Mild or asymptomatic infections superficial mucosa of the respiratory tract but do not affect may occur. These symptoms may be induced by any strain of the basal layer of epithelium. Complete reparation of cellular influenza A or B. In contrast, influenza C rarely causes the damage probably takes up to 1 month. Viral damage to the influenza syndrome, causing instead a common cold illness. respiratory tract epithelium lowers its resistance to secondary Coryza and cough may last for several weeks. bacterial pathogens, especially staphylococci, streptococci, Clinical symptoms of influenza in children are similar to and Haemophilus influenzae. those in adults, although children may have higher fever and Edema and mononuclear infiltrations in response to a higher incidence of gastrointestinal manifestations such as cytokine release and cell death caused by viral replication vomiting. Febrile seizures can occur. Influenza A viruses are Riedel_CH39_p581-p594.indd 587 04/04/19 5:08 PM  588   SECTION IV  Virology an important cause of croup, which may be severe, in children Protection correlates with both serum antibodies and younger than 1 year of age. Finally, otitis media may develop. secretory IgA antibodies in nasal secretions. The local secre- When influenza appears in epidemic form, clinical tory antibody is probably important in preventing infection. findings are consistent enough that the disease can be diag- Serum antibodies persist for many months to years; secre- nosed presumptively. Sporadic cases cannot be diagnosed on tory antibodies are of shorter duration (usually only several clinical grounds because disease manifestations cannot be months). Antibody also modifies the course of illness. A per- distinguished from those caused by other respiratory tract son with low titers of antibody may be infected but will expe- pathogens. However, those other agents rarely cause severe rience a mild form of disease. Immunity can be incomplete; viral pneumonia, which can be a complication of influenza reinfection with the same virus can occur. A virus infection. The three types of influenza viruses are antigenically unrelated and therefore induce no cross-protection. When a B. Pneumonia viral type undergoes antigenic drift, a person with preexisting antibody to the original strain may have only mild infection Serious complications usually occur only in elderly adults with the new strain. Subsequent infections or immunizations and debilitated individuals, especially those with underly- reinforce the antibody response to the first subtype of influ- ing chronic disease. Pregnancy appears to be a risk factor enza experienced years earlier, a phenomenon called “original for lethal pulmonary complications in some epidemics. The antigenic sin.” lethal impact of an influenza epidemic is reflected in the The primary role of cell-mediated immune responses excess deaths caused by pneumonia and cardiopulmonary in influenza is believed to be clearance of an established diseases. infection; cytotoxic T cells lyse infected cells. The cyto- Pneumonia complicating influenza infections can be toxic T lymphocyte response is cross-reactive (able to lyse viral, secondary bacterial, or a combination of the two. cells infected with any subtype of virus) and appears to be Increased mucous secretion helps carry agents into the lower directed against both internal proteins (NP, M) and the sur- respiratory tract. Influenza infection enhances susceptibility face glycoproteins. of patients to bacterial superinfection. This is attributed to loss of ciliary clearance, dysfunction of phagocytic cells, and provision of a rich bacterial growth medium by the alveolar Laboratory Diagnosis exudate. Bacterial pathogens are most often Staphylococcus aureus, Streptococcus pneumoniae, and H. influenzae. Clinical characteristics of viral respiratory infections can Combined viral–bacterial pneumonia is approximately be produced by many different viruses. Consequently, diag- three times more common than primary influenza pneumo- nosis of influenza relies on identification of viral antigens nia. A molecular basis for a synergistic effect between virus or viral nucleic acid in specimens, isolation of the virus, or and bacteria may be that some S. aureus strains secrete a pro- demonstration of a specific immunologic response by the tease able to cleave the influenza HA, thereby allowing pro- patient. duction of much higher titers of infectious virus in the lungs. Nasopharyngeal swabs and nasal aspirate or lavage fluid are the best specimens for diagnostic testing and should be obtained within 3 days after the onset of symptoms. C. Reye Syndrome Reye syndrome is an acute encephalopathy of children and A. Polymerase Chain Reaction adolescents, usually between 2 and 16 years of age. The mortality rate is high (10–40%). The cause of Reye syndrome Rapid tests based on detection of influenza RNA in clini- is unknown, but it is a recognized rare complication of cal specimens using reverse transcription polymerase chain influenza B, influenza A, and herpesvirus varicella-zoster reaction (RT-PCR) are preferred for diagnosis of influenza. infections. There is an epidemiologic relationship between RT-PCR is rapid (