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Chapter Outlines

Chapter 12      Influenza Viruses

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12.1 History Influenza has been described as early as 400 B.C. Major epidemics and pandemics of influenza have occurred in history. Man virologists think another influenza pandemic that could kill millions of humans is inevitable. 12.2 Epidemiology Epidemics become unmanageable at alarming speeds because: Short incubation period (1-4 days) One droplet can contain 100,000 to 1,000,000 virus particles! Symptomatic people do not stay home = spread! Lack of herd immunity Absenteeism form schools is the best indicator of the scale of an epidemic. Influenza Statistics 1957-1987: there were 20 influenza epidemics recorded in the U.S. 36,000-50,000 people have died as a direct or indirect consequence of an influenza infection The 1918 Influenza Pandemic Also called the Spanish flu Killed 675,000 Americans 4.39/1000 national average Death rates peaked in October of 1918 Unique epidemic: healthy adults ages 20-40 died of the flu, in addition to children and elderly (who are usually high-risk) It decreased the life expectancy in the U.S. by 11 years. 12.3 Clinical Features of Influenza: Uncomplicated Infection Onset of symptoms: Headache Aching in the limbs and back Fever (100-103 oF) Malaise Dry cough Tickling throat Sore throat Myalgia Chest X-ray is normal Usually resolves itself after 7 days Influenza: Complicated Infection Age-dependent Young children: croup, secondary bacterial pneumonia, middle ear infections Elderly: life threatening secondary bacterial pneumonia, pre-existing conditions like congestive heart disease exacerbated Immune compromised individuals: at risk of death duirng an influenza epidemic Reye's Syndrome Rare condition Risk of developing Reye's Syndrome (R.S.) increases with aspirin use R.S. affects all organs of the body Most harmful to brain and liver Causes pressure in the brain and massive accumulation of fat in the liver and other organs 12.4 Classification of Influenza Viruses Orthomyxoviridae family 3 Types of Influenza: A, B and C All 3 can infect and cause similar symptoms in humans Infection with one type does not confer immunity to another type of influenza 12.5 Laboratory Diagnosis Office-based rapid tests - ELISA assays Some can distinguish between Influenza A and B Fast results - 1- to 20 minutes Some false-positive or negative results Other Tests Cell Culture: inoculate MDCK cells, most accurate test Serology Analyze convalescent serum and analyze for an increase in antibody titer "Flu chips" or microarrays 12.6 Cellular Pathogenesis Droplet transmission Virus enters respiratory tract Attaches to ciliated columnar epithelial cells lining the sinuses and airways Primary site of infection - tracheobronchial tree, involving nasopharynx As virus replicates, cilia are destroyed Cleaning of the lungs system does not work as well More mucus stays in the airway, clogging them, causing coughing Destruction of cilia contribute to secondary bacterial pneumonia infections, sinusitis, otitis 12.7 Immunity Infected individuals develop antibodies against the outer proteins of Influenza virus - neuraminidase (N) and hemagglutinin (H) Antibodies against H neutralize the virus Antibodies against N do not but they do reduce the release of virus from infected cells If a person has been infected in the past couple of years by a closely related strain of influenza H subtype - Their antibodies may intercept and neutralize the virus, protecting the lungs. Cytokine Storm Also referred to as systemic inflammatory response syndrome (SIRS). SIRS may explain the devastating nature of the 1918 strain of influenza. The immune system overreacts toward the pathogen. Cytokines signal macrophages to travel to the site of infection, causing damage to the body and organ failure. H5N1 Avian Influenza virus also causes SIRS and is at least 50% lethal in humans. 12.8 Viral Replication Properties of the Influenza A Particle Immediately after isolation, particles are filamentous. After several passages in cell cultures, the particles become spherical. Influenza A Virus Particle Enveloped Contains hemagglutinin (H) antigen glycoprotein spikes on the surface and neuraminidase (N) antigen spikes. M2 ion channel protein 8 segments of ssRNA of negative polarity Virus Nomenclature Influenza type/ Species isolated form (unless human) Place of Isolation Strain designation Year isolation H#N# subtypes Example: A/Hong Kong/1/68(H3N2) Influenza A Life Cycle Influenza virus H binds to sialic acid present on glycoproteins of ciliated cells lining the sinuses and airways. Virions enter by endocytosis. Inside of the endosome, the virion is exposed to a low pH (from 7 to 5). The low pH causes H to undergo a conformational change H protein Must be cleaved by cellular proteases inside of the trans golgi network for the virus to be infectious. Influenza A Uncoating Step M2 ion channel in the viral envelop allows H+ ions to penetrate the virion, Weakens the viral M1 matrix protein from the viral RNA, NP, and transcriptase complex (RNP) RNPs released into the cytoplasm Amantidine (sold as Symmetrel) and rimantidine (sold as Flumadine) block the M2 ion channel function, interfering with uncoating. mRNA Synthesis and Replication of Virion RNA After the viral RNPs enter the nucleus, mRNA synthesis begins Cap-snatching process Viral PB2 protein binds to the cap structure along with a short string of nucleotides of host mRNA in the nucleus of the cell. Viral mRNAs are not Cannibalized for Their 5' Caps The viral PB2 polymerase proteins selectively "snatch" caps. Binds to a specific sequence that is complementary to nucleotides 1-12 of the 3' ends of each vRNA segment. Influenza Exploits Host - Nuclear-Splicing Machinery to Splice Viral mRNA Segments 7 and 8 NS1 Prevents Nuclear Export of Cellular Pre-mRNAs, Facilitating Cap-Snatching Translational Control Mechanisms Influenza A suppresses the interferon system in host cells. Virion Maturation and Assembly Capped mRNAs exported to nucleus are translated by ribosomes in the cytoplasm H, N, and M2 are folded and glycosylated and transported to the trans Golgi network and cell surface where assembly takes place. One copy of each genome segment is packaged into the virion. Release of Influenza Virions Virions are released by budding. The viral N protein cleaves the sialic acid on host cells to prevent clumping of viral particles at the host surface. Zamanivir (sold as Relenza) and oseltamivir phosphate (sold as Tamiflu) inhibit the function of N. 12.9 Genetic Variation Mutations are common during viral replication. Viral RNA dependent RNA polymerase lacks proofreading and correction ability. Two processes, antigenic drift and antigenic shift mediate genetic variation that causes new strains of influenza to appear. Antigenic Drift - Season Influenza Strains Antigenic Shift - Responsible for Pandemic Strains Influenza Reassortments Is China the Incubator for Flu Viruses? Close association with humans with animals and birds. Influenza A is constantly circulating in birds, pigs and horses. Why is Influenza More Prevalent - During the Winter? Aerosol spread of influenza virus is dependent upon humidity and temperature. The virus was best transmitted at low humidity (20%) and colder temperatures (5 oC or 41oF). Supporting evidence that weather conditions play a role in influenza virus transmission. Lowen, A. C. et al. 2007. "Influenza Virus Transmission is Dependent on Relative Humidity and Temperature." PloS Pathogens 3(10):1470-1476. 12.10 Pandemics in History The 1918 Spanish Flu Stats Killed more people in 25 weeks than AIDS killed in 25 years. Kill more people in a year than the plagues of the Middle Ages killed in a century. Estimated deaths - 20 to 50 million, including 675,000 Americans. In NYC, 21,000 children were orphaned by influenza. 7 times as many people died of the 1918 Spanish Flu than in WWI. True estimates will never be known. It's Effects were Unique It killed 20-40 year old adults. It killed quickly (2-3 days). Infected individuals suffered from hemorrhagic symptoms. Why was the 1918 Spanish Flu So Deadly? Johan Hultin exhumed bodies of flu victims buried in frozen ground at Brevig Mission, Alaska. Removed frozen lung tissues and provided them to Jeffrey Taubenberger (Armed Forces Institute of Pathology in Washington D. C.) who sequenced the viral genome of the 1918 strain. 1918 was an avian - not swine strain! 1918 Influenza Strain was Re-Created in the Laboratory, 2005 Terrence Tumpey, CDC researcher. Reverse genetics. Work done in an enhanced BSL-3 laboratory. 2007 Yoshihiro Kawaoka - University of Wisconsin-Madison Infected macaques with the reconstructed 1918 Influenza virus. Carried out experiments in a BSL-4 laboratory Macaques died of SIRS (cytokine storm) 1957 Asian Flu 70,000 Americans died. Asian flu first identified in northern China (February, 1957) and spread to the U.S. by June, 1957. Vaccine was made and available by August, 1957. 1968 Hong Kong Flu First detected in Hong Kong during early months of 1968. Reached the U.S. by September, 1968. Deaths in the U.S. peaked in December, 1968-January, 1969 (mostly elderly). Mildest pandemic in the century, killing only 33,800 people in the U.S. 12.11 The Pandemic Scares 1976 Swine Flu Scare: A/New Jersey/76/H1N1 February, 1976 Cadets at Fort Dix, NJ came down with the flu, one private died. CDC investigated the outbreak. 4 out of 19 throat washings tested positive for H1N1 influenza A (at the time believed to be a "swine" flu closely related to the 1918 Spanish flu virus). 150 million doses of vaccine prepared in the U.S. 46 million doses were administered within a few months. 1976 Swine Flu Vaccine Vaccine was fast-tracked Congress passed a liability protect bill to protect manufacturers of the vaccine Early problems of the vaccine: Guillain Barre Syndrome (532 people within 10 weeks) 32 deaths Vaccine campaign suspended in the late fall of 1976 U. S. government paid more than $90 million on claims cases. 1977 Russian Flu Scare: A/USSR/77/H1N1 May, 1977 Influenza A H1N1 isolated in northern China. Primarily children and younger adults became ill. This strain was similar to the 1957 strain Individuals born before 1957 would have no cross-reacting/partial immunity toward the virus. 1997 Avian Flu Scare: H5N1 May, 1997, 3 year old boy in Hong Kong died. Influenza A isolated from the boy but the H subtype could not be identified. Later confirmed to be H5 H5 not known to infect humans before (subtype isolated from birds only) The "jump" of an avian strain directly to humans had never happened before. Same H5N1 strain was killing chickens. 1997 Hong Kong H5N1 Scare Veterinary authorities slaughtered chickents to prevent the spread of H5N1 to chickens and people! Lessons Learned from the 1997 H5N1 Hong Kong Flu Only 18 human cases in 1997 33% fatality rate! (1918 flu was about 4% fatal in the U.S.) Avian Strains Continue to Plague Eastern Asia 12.12 Antivirals for Influenza Treatment First drugs: M2 inhibitors (prevent uncoating step) Amantidine (sold as Symmetrel) Rimantidine (sold as Flumadine) New class of antivirals: N inhibitors (prevents neuraminidase from cleaving sialic acid during budding) Causes viruses to clump at the cell surface, reducing viral spread Oseltamivir (sold as Tamiflu, pill form) Zanamivir (sold as Relenza, must be inhaled) Treatment must begin within 36 hours of onset of symptoms Used prophylactically in chronic care facilities 12.13 Influenza Vaccines - Against Seasonal Flu Strains Most effective way to prevent influenza Flu vaccination time in the U.S.: October and November Vaccine grown in eggs Inactivated trivalent vaccine Live, attenuated vaccine, LAIV Licensed in 2003 Only approved for healthy people ages 5-49 years Trivalent Vaccine Composition Recommendations made based on antigenic analyses of recently isolated influenza viruses, epidemiologic data and post-vaccination serologic studies in humans Vaccine is a cocktail of 3 virus strains 2 strains of Influenza A 1 Influenza B strain Effectiveness of the Vaccine Age of the vaccine recipient Immunocompetence of the recipient Degree of similarity between the viruses in the vaccine and those in circulation Target Groups for Vaccination Persons aged 50 or older Individuals in chronic care facilities; including nursing homes Individuals with pre-existing chronic problems Asthma other pulmonary or cardiovascular problems immunosuppression Children on long-term aspirin therapy Pregnant women Healthcare workers Travelers 12.14 International Influenza Surveillance 1946 WHO established influenza surveillance program Influenza Sentinel Physicians in the United States U.S. sentinel physicians send flu statistics to the CDC Patient visits for influenza-like illness Age groups Morbidity and mortality stats Will There Be Another Killer Flu?: Are We Preparing? Virologists say it is inevitable Many pandemic planning teams at the international, national, state and local levels Will the Current Arsenal of Influenza Antivirals Work Against New Pandemic Strains of Influenza? Tumpey 2002 study says yes. Viral resistance has been shown. We won't know until it happens. 12.15 Lessons Learned from the SARS Outbreak of 2003 Within 6 weeks of its discovery: SARS CoV infected thousands of people in 16 countries around the world. Spread quickly by air travel. Identification of pandemic strains and rapid response to contain outbreaks are paramount.