# Lecture 12: RNA Viruses ## Unit 2.8: Microbiology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the classification and replication strategies of RNA viruses 2. Explain Orthomyxoviridae (influenza) and Paramyxoviridae 3. Describe Picornaviridae, Caliciviridae, and Reoviridae 4. Explain Flaviviridae and Togaviridae (arboviruses) 5. Describe Rhabdoviridae (rabies) and Filoviridae 6. Explain Retroviridae (HIV) and Coronaviridae --- ## Lecture Outline ### I. RNA Virus Overview RNA viruses constitute a remarkably diverse group of pathogens unified by their use of ribonucleic acid as genetic material, a feature that confers distinctive characteristics distinguishing them fundamentally from DNA viruses. The classification of RNA viruses begins with the polarity of their single-stranded genomes: positive-sense (+) ssRNA can be directly translated by host ribosomes upon entry into the cell, functioning essentially as messenger RNA, while negative-sense (-) ssRNA requires transcription by viral RNA-dependent RNA polymerase before translation can occur. This fundamental difference determines the early steps of viral replication and the necessity for virions to carry their own polymerase. Some RNA viruses possess segmented genomes divided into multiple RNA molecules, while others have a single continuous genome; this distinction has profound implications for viral evolution and pandemic potential through reassortment. The most consequential feature of RNA virus biology is their extraordinarily high mutation rate, resulting from the lack of proofreading activity in RNA-dependent RNA polymerases. While DNA polymerases possess 3' to 5' exonuclease activity that corrects misincorporated nucleotides, RNA polymerases lack this function, resulting in error rates of approximately 1 mutation per 10,000 nucleotides copied compared to 1 per 10 billion for DNA polymerases. This high error rate generates tremendous genetic diversity within viral populations, creating viral quasispecies that facilitate adaptation to new hosts, immune evasion, and development of drug resistance. The rapid evolution of RNA viruses explains the need for annual influenza vaccination, the emergence of drug-resistant HIV variants, and the successive waves of SARS-CoV-2 variants during the COVID-19 pandemic. The replication of RNA viruses occurs predominantly in the cytoplasm, reflecting the absence of nuclear machinery for RNA synthesis and the presence of viral-encoded RNA-dependent RNA polymerases. Notable exceptions include influenza viruses, which require nuclear localization for mRNA capping and splicing, and retroviruses, which must access the nucleus for integration of proviral DNA into host chromosomes. The presence of an envelope, acquired by budding through host cell membranes, distinguishes certain families and impacts stability and transmission; enveloped RNA viruses like influenza and coronaviruses are generally more susceptible to environmental inactivation than non-enveloped viruses like picornaviruses and reoviruses, which can survive longer outside the host. The clinical importance of RNA viruses spans virtually every organ system and includes some of the most significant infectious disease threats facing humanity. Respiratory RNA viruses including influenza, respiratory syncytial virus, and coronaviruses cause enormous global morbidity and mortality, particularly among elderly and immunocompromised populations. Gastrointestinal RNA viruses, notably norovirus and rotavirus, are leading causes of acute gastroenteritis worldwide. Hemorrhagic fever viruses including Ebola, dengue, and yellow fever cause dramatic and often fatal disease with significant epidemic potential. Neurotropic viruses such as rabies and poliovirus target the nervous system with devastating consequences. Chronic infections established by HIV and hepatitis C virus persist for life without treatment, representing major global health challenges with effective but not curative therapies available. Panel A: RNA virus genome classification diagram - four main categories arranged as flowchart: positive-sense ssRNA (directly translated, examples: Picorna, Flavi, Corona), negative-sense ssRNA (requires RdRp first, examples: Orthomyxo, Paramyxo), dsRNA (segmented, example: Reovirus), and retroviruses (+ssRNA with reverse transcriptase), with replication strategy arrows for each. Panel B: Mutation rate comparison bar graph - horizontal bars showing error rates for DNA polymerase (1 in 10 billion), RNA-dependent RNA polymerase (1 in 10,000), with clinical consequences arrows pointing to antigenic drift, drug resistance emergence, and new variant formation. Panel C: Replication site diagram - cell with nucleus and cytoplasm showing most RNA viruses replicating in cytoplasm (viral RdRp machinery illustrated), influenza needing nuclear access (mRNA processing), and retrovirus integrating in nucleus (provirus DNA shown in chromosome). Panel D: Clinical manifestation overview - human body silhouette with RNA virus disease categories: respiratory (influenza, RSV, coronavirus in lungs), GI (norovirus, rotavirus in intestines), hemorrhagic (Ebola, dengue with bleeding icon), neurotropic (rabies, polio in brain/spinal cord), and chronic (HIV, HCV with persistence symbol). --- ### II. Orthomyxoviridae (Influenza) Influenza viruses represent the prototype members of the Orthomyxoviridae family, characterized by their segmented negative-sense RNA genome and their surface glycoproteins that determine host range, virulence, and immune recognition. The viral genome consists of eight separate RNA segments encoding the major structural and functional proteins, a segmented arrangement that enables genetic reassortment when two different influenza strains co-infect the same cell. The lipid envelope acquired from host cell membranes is studded with two critical surface glycoproteins: hemagglutinin (HA), which mediates attachment to sialic acid receptors on host cells, and neuraminidase (NA), which cleaves sialic acid to release newly formed virions and prevent viral aggregation. Influenza A viruses are classified by HA and NA subtypes (H1-18, N1-11), with the familiar H1N1, H3N2, and H5N1 designations reflecting these combinations. The two mechanisms of influenza antigenic variation have profoundly different clinical and epidemiological implications. Antigenic drift results from the accumulation of point mutations in HA and NA genes during routine viral replication, gradually altering epitopes recognized by antibodies from previous infections or vaccinations; this ongoing drift necessitates annual reformulation of influenza vaccines to match circulating strains and causes seasonal epidemics when population immunity to current strains has waned. Antigenic shift represents a dramatic and abrupt change in viral surface proteins through reassortment, when two different influenza A viruses co-infect a single cell and exchange genome segments, potentially creating novel combinations against which the population has no pre-existing immunity. Antigenic shift has caused all influenza pandemics, including the 1918 H1N1, 1957 H2N2, 1968 H3N2, and 2009 H1N1 pandemics, and requires the segmented genome structure that is unique to influenza among respiratory viruses. Clinical influenza presents with characteristic sudden onset of systemic symptoms distinguishing it from the common cold. Following an incubation period of 1-4 days, patients develop abrupt high fever, severe myalgias often described as body aches, headache, and prostration, accompanied by respiratory symptoms including non-productive cough and sore throat. The systemic manifestations reflect cytokine release rather than direct viral dissemination and typically resolve within a week in uncomplicated cases. Complications include primary influenza viral pneumonia, secondary bacterial pneumonia (classically with Staphylococcus aureus or Streptococcus pneumoniae), myocarditis, and rarely encephalitis. High-risk groups for severe disease include adults over 65 years, young children, pregnant women, and those with chronic medical conditions including lung disease, heart disease, diabetes, and immunosuppression. Influenza prevention and treatment strategies include both vaccines and antiviral medications. Annual influenza vaccination remains the cornerstone of prevention, with vaccines reformulated yearly based on surveillance data predicting circulating strains; available formulations include inactivated injectable vaccines, live attenuated nasal spray vaccines (for healthy individuals 2-49 years), and recombinant vaccines. Antiviral treatment with neuraminidase inhibitors (oseltamivir oral, zanamivir inhaled) or the newer cap-dependent endonuclease inhibitor baloxavir provides benefit when initiated within 48 hours of symptom onset, reducing symptom duration and potentially preventing complications. Treatment is particularly valuable in hospitalized patients and those at high risk for complications. Antiviral resistance surveillance is ongoing, with different subtypes and strains showing variable susceptibility patterns. Panel A: Influenza virus structure - spherical virion with lipid envelope showing HA spikes (labeled "attachment to sialic acid, antibody target") and NA mushroom shapes (labeled "release, drug target"), internal showing 8 RNA segments in ribonucleoprotein complexes, and M2 ion channel. Panel B: Antigenic drift versus shift comparison - drift shown as gradual accumulation of point mutations over seasons (small changes in HA shape, epidemic arrows), shift shown as dramatic reassortment event in pig co-infected with human and avian strains (genome segment exchange producing novel virus, pandemic arrow). Panel C: Clinical presentation timeline - day 0 exposure, incubation 1-4 days, sudden onset symptoms (fever spike, severe myalgia, headache, cough), resolution over 1 week, and complication branch showing viral pneumonia, secondary bacterial pneumonia, and rare myocarditis/encephalitis. Panel D: Prevention and treatment algorithm - vaccination branch (annual, formulations listed, timing in fall), antiviral branch (oseltamivir, zanamivir, baloxavir with mechanism icons), treatment window emphasized (<48 hours), and high-risk groups for prioritization listed (elderly, young children, pregnant, comorbidities). --- ### III. Paramyxoviridae The Paramyxoviridae family includes several clinically important respiratory viruses distinguished from influenza by their non-segmented negative-sense RNA genome, precluding the reassortment events that drive influenza pandemics. These enveloped viruses possess surface glycoproteins including the fusion (F) protein that mediates viral entry and cell-to-cell spread, producing characteristic syncytia, multinucleated giant cells formed when infected cells fuse with neighboring uninfected cells. The family encompasses diverse pathogens including measles virus, mumps virus, respiratory syncytial virus, parainfluenza viruses, and more recently emerged pathogens including Nipah and Hendra viruses. The non-segmented genome means these viruses evolve more slowly than influenza, contributing to the long-term effectiveness of measles and mumps vaccines developed decades ago. Measles (rubeola) represents one of the most contagious infectious diseases known, with a basic reproduction number (R0) of 12-18 meaning each infected individual infects 12-18 susceptible contacts on average. Transmission occurs through respiratory droplets and aerosolized particles that remain infectious in enclosed spaces for hours. The characteristic clinical progression begins with a prodrome of the "3 C's" - cough, coryza (runny nose), and conjunctivitis - accompanied by high fever. Koplik spots, pathognomonic white or bluish-white papules on the buccal mucosa opposite the molars, appear during the prodrome and provide early diagnostic confirmation before rash onset. The maculopapular rash begins at the hairline and behind the ears, spreading cephalocaudally to involve the trunk and extremities over several days. Complications include pneumonia (the most common cause of measles death), acute encephalitis, and subacute sclerosing panencephalitis (SSPE), a rare but invariably fatal late complication occurring years after initial infection. Respiratory syncytial virus (RSV) stands as the most common cause of bronchiolitis and pneumonia in infants and young children, while also causing significant disease in elderly and immunocompromised adults. The virus derives its name from the characteristic syncytia formed in cell culture and infected tissues. Primary infection typically occurs by age 2 and does not confer complete protection, with reinfections common throughout life, though subsequent infections are generally milder. In infants, RSV bronchiolitis presents with wheezing, tachypnea, and respiratory distress, sometimes requiring hospitalization and mechanical ventilation. Prevention strategies have advanced substantially, with palivizumab (monthly monoclonal antibody injections for high-risk infants) largely supplemented by nirsevimab (single-dose extended half-life monoclonal antibody) and new vaccines approved for elderly adults and pregnant women to provide passive protection to newborns. Parainfluenza viruses (PIV types 1-4) are leading causes of croup, the syndrome of laryngotracheobronchitis presenting with the characteristic "barking" or "seal-like" cough, inspiratory stridor, and hoarseness. Croup primarily affects children 6 months to 3 years of age, with PIV-1 and PIV-2 causing most cases in distinctive seasonal patterns. The inflammation and edema of the subglottic airway produce the classic "steeple sign" on anteroposterior neck radiograph, showing narrowing of the subglottic trachea. Most cases are mild and self-limited, managed with humidified air and observation, but moderate to severe cases benefit from systemic corticosteroids (dexamethasone) and nebulized epinephrine for immediate symptomatic relief. Mumps virus causes parotitis (painful swelling of the parotid salivary glands) and can produce complications including orchitis (testicular inflammation, rarely causing sterility), meningitis, and pancreatitis. Both measles and mumps are prevented by the MMR (measles-mumps-rubella) vaccine. Panel A: Paramyxoviridae structure and syncytia - virus particle with non-segmented RNA genome, envelope with HN/H attachment protein and F fusion protein, and microscopy image showing multinucleated syncytial giant cell formed by fusion of infected and uninfected cells. Panel B: Measles clinical progression - timeline showing prodrome (3 C's: cough, coryza, conjunctivitis), Koplik spots on buccal mucosa (photograph with characteristic white spots), rash progression (starting at hairline, spreading cephalocaudally with day markers), and complications branch (pneumonia, encephalitis, SSPE years later). Panel C: RSV bronchiolitis presentation - infant with respiratory distress (nasal flaring, intercostal retractions), chest X-ray showing hyperinflation, age susceptibility graph peaking at 2-6 months, and prevention options (palivizumab monthly, nirsevimab single-dose, maternal vaccination for passive protection). Panel D: Croup anatomy and presentation - normal versus croup airway cross-section (subglottic edema narrowing), AP neck X-ray with steeple sign, clinical features (barking cough audio wave, stridor, hoarseness), and treatment ladder (supportive care, dexamethasone, nebulized epinephrine for severe cases). --- ### IV. Picornaviridae The Picornaviridae family comprises small (pico-), non-enveloped RNA viruses with positive-sense genomes that can be directly translated upon entry into host cells. The absence of an envelope confers environmental stability, enabling transmission through fecal-oral routes and survival on fomites. Important genera include Enterovirus (poliovirus, coxsackieviruses, echoviruses, numbered enteroviruses), Rhinovirus (common cold), and Hepatovirus (hepatitis A). Enteroviruses demonstrate remarkable acid stability, surviving passage through the stomach to replicate in intestinal epithelium and spread to other tissues, while rhinoviruses are acid-labile and replicate in the upper respiratory tract. The tremendous antigenic diversity within this family, with over 100 rhinovirus serotypes alone, has precluded development of effective vaccines for most members except poliovirus and hepatitis A. Poliovirus historically caused devastating paralytic disease before the development of effective vaccines. Transmission occurs via the fecal-oral route, with the virus replicating initially in pharyngeal and intestinal lymphoid tissue before potential viremic spread to the central nervous system. Paralytic poliomyelitis occurs in less than 1% of infections when the virus reaches and destroys anterior horn motor neurons of the spinal cord, causing acute flaccid paralysis without sensory involvement. The two vaccine types have different characteristics: the inactivated poliovirus vaccine (IPV, Salk) contains killed virus, requires injection, and produces systemic but limited mucosal immunity; the oral poliovirus vaccine (OPV, Sabin) contains live attenuated virus, produces excellent mucosal immunity interrupting transmission, but carries rare risk of vaccine-associated paralytic poliomyelitis and circulating vaccine-derived poliovirus. Global eradication efforts have reduced wild poliovirus to two remaining endemic countries (Afghanistan and Pakistan), though vaccine-derived strains continue to cause outbreaks. Coxsackieviruses and other enteroviruses cause diverse clinical syndromes reflecting their tropism for multiple tissue types. Coxsackie A viruses are best known for causing hand-foot-and-mouth disease (HFMD), presenting with vesicular lesions on the palms, soles, and oral mucosa, predominantly in young children; they also cause herpangina, characterized by vesicles and ulcers on the posterior pharynx. Coxsackie B viruses have tropism for heart, pleura, and pancreas, causing viral myocarditis and pericarditis, epidemic pleurodynia (Bornholm disease), and occasionally pancreatitis - a memory aid is "B for Body" referring to thoracic and abdominal organs. Echoviruses cause aseptic meningitis, a self-limited meningitis syndrome with lymphocytic cerebrospinal fluid pleocytosis. Enterovirus D68 has emerged as a significant pathogen causing severe respiratory illness and acute flaccid myelitis resembling poliomyelitis. Rhinoviruses and hepatitis A virus represent additional clinically important picornaviruses with distinct presentations. Rhinoviruses are the most common cause of the common cold, with over 100 serotypes explaining the repeated susceptibility to infection throughout life. The virus replicates optimally at 33 degrees Celsius, explaining its predilection for the cooler temperatures of the nasal passages. Hepatitis A virus causes acute viral hepatitis transmitted through the fecal-oral route, typically from contaminated food or water. Unlike hepatitis B and C, hepatitis A does not cause chronic infection; most cases resolve completely, though rare fulminant hepatic failure can occur, particularly in adults and those with underlying liver disease. An effective vaccine is available and is recommended for travelers to endemic areas, individuals with chronic liver disease, and in outbreak settings. Panel A: Picornaviridae characteristics - small non-enveloped icosahedral virion with (+) ssRNA genome, stability comparison (enteroviruses acid-stable surviving stomach, rhinoviruses acid-labile staying in upper airway), and direct translation diagram showing viral RNA functioning as mRNA immediately upon uncoating. Panel B: Poliovirus pathway and eradication - fecal-oral transmission, replication in gut lymphoid tissue, rare CNS spread to anterior horn cells, paralysis mechanism (motor neuron destruction, flaccid paralysis), vaccine comparison (IPV injection with systemic immunity, OPV oral with mucosal immunity but VAPP risk), and world map showing remaining endemic countries. Panel C: Enterovirus syndrome gallery - hand-foot-mouth disease (vesicles on palms, soles, oral mucosa), herpangina (posterior pharyngeal ulcers), Coxsackie B diseases ("B for Body" - myocarditis with heart, pleurodynia with chest, pancreatitis), aseptic meningitis (lymphocytic CSF), and EV-D68 (severe respiratory, acute flaccid myelitis). Panel D: Rhinovirus and HAV contrast - rhinovirus (common cold, >100 serotypes explaining repeat infections, 33C temperature preference in cool nasal passages) versus hepatitis A (fecal-oral transmission, acute self-limited hepatitis, no chronicity, vaccine-preventable), with clinical course timelines for each. --- ### V. Caliciviridae and Reoviridae Norovirus, the leading cause of acute viral gastroenteritis worldwide, belongs to the Caliciviridae family and demonstrates remarkable transmissibility through multiple routes. This non-enveloped virus with positive-sense RNA genome spreads via the fecal-oral route, contaminated food and water, fomites, and even aerosolized vomitus, with an extremely low infectious dose of only 18-1000 viral particles. The virus is highly resistant to environmental conditions, surviving on surfaces for weeks and resisting many common disinfectants; bleach-based products are required for effective environmental decontamination. Outbreaks characteristically occur in closed or semi-closed settings including cruise ships, nursing homes, hospitals, schools, and military barracks, where person-to-person spread and environmental contamination perpetuate transmission. The clinical presentation of norovirus gastroenteritis features the sudden onset of profuse vomiting, which is often the predominant symptom, accompanied by watery non-bloody diarrhea, abdominal cramps, and sometimes low-grade fever. The incubation period is brief, typically 12-48 hours, and symptoms resolve within 24-72 hours in most cases, though viral shedding continues for days to weeks after symptom resolution. The self-limited nature of illness means treatment is primarily supportive, focusing on oral or intravenous rehydration depending on severity. Diagnosis in outbreak settings is typically clinical or confirmed by PCR of stool specimens; the rapid evolution of norovirus creates changing strains that periodically escape population immunity and cause increased disease burden. No vaccine is currently available, though several are in development. Rotavirus, a member of the Reoviridae family, was historically the most common cause of severe dehydrating gastroenteritis in infants and young children before vaccine introduction. This non-enveloped virus possesses a distinctive double-stranded RNA genome consisting of 11 segments, visible by electron microscopy as a wheel-like structure (rota- = wheel). Rotavirus gastroenteritis typically presents in children between 6 months and 2 years of age with several days of vomiting followed by profuse watery diarrhea and fever, with total duration of 3-8 days. The severity relates to both direct viral damage to intestinal epithelium and an enterotoxin-like effect of the NSP4 protein. Before widespread vaccination, rotavirus caused approximately 500,000 deaths annually, predominantly in developing countries where dehydration management was limited. The introduction of rotavirus vaccines has dramatically reduced the global burden of severe rotavirus gastroenteritis. Two live attenuated oral vaccines are widely used: the monovalent human rotavirus vaccine (Rotarix) and the pentavalent human-bovine reassortant vaccine (RotaTeq). Both are administered in early infancy starting at 2 months of age, with the vaccination series completed by 8 months to avoid a potential increased risk of intussusception observed with later vaccination. Post-licensure surveillance confirmed a small increased risk of intussusception following vaccination, estimated at 1-5 additional cases per 100,000 vaccinated infants, which is far outweighed by the benefits of preventing severe rotavirus disease. In countries with high vaccine coverage, rotavirus hospitalizations have decreased by 80-90%, representing one of the most successful vaccine introductions in recent decades. Panel A: Norovirus outbreak dynamics - transmission routes (fecal-oral, contaminated food, fomites, aerosolized vomitus), low infectious dose emphasized (18-1000 particles), outbreak settings (cruise ship, nursing home, hospital floor plan), environmental persistence (surviving on surfaces for weeks), and decontamination requirement (bleach-based products). Panel B: Norovirus clinical presentation - timeline showing brief incubation (12-48 hours), sudden onset with prominent vomiting, watery diarrhea and cramping, resolution by 24-72 hours, but continued viral shedding beyond symptom resolution (curve showing shedding for weeks), and supportive treatment focus. Panel C: Rotavirus structure and pathogenesis - electron microscopy showing wheel-like appearance, 11 dsRNA segment diagram, intestinal epithelium damage mechanism, NSP4 enterotoxin effect, age susceptibility graph peaking at 6-24 months, and pre-vaccine mortality burden (500,000 annual deaths). Panel D: Rotavirus vaccine impact - vaccine administration timeline (starting 2 months, completing by 8 months), intussusception risk discussion (1-5 per 100,000 vs benefit), before/after hospitalization graphs showing 80-90% reduction, and global mortality decline with vaccine implementation mapped. --- ### VI. Flaviviridae The Flaviviridae family encompasses medically important viruses including the mosquito-borne flaviviruses (dengue, yellow fever, Zika, West Nile, Japanese encephalitis) and hepatitis C virus, all sharing a positive-sense RNA genome within an enveloped virion. The arthropod-borne members of this family cause diseases ranging from self-limited febrile illness to hemorrhagic fever and fatal encephalitis, with geographic distribution determined by the range of their mosquito vectors. The name derives from yellow fever (flavus = yellow in Latin), the prototype member. Hepatitis C virus, despite sharing family classification, transmits through blood exposure rather than arthropod vectors and causes chronic hepatitis with potential progression to cirrhosis and hepatocellular carcinoma. Dengue virus causes the most prevalent arthropod-borne viral disease globally, with an estimated 400 million infections annually across tropical and subtropical regions. The virus is transmitted by Aedes aegypti and Aedes albopictus mosquitoes and exists as four distinct serotypes (DENV-1 through DENV-4). Primary infection typically causes dengue fever, characterized by high fever, severe headache, retro-orbital pain, myalgias and arthralgias (the historical term "breakbone fever" reflects the intensity of musculoskeletal symptoms), and sometimes a maculopapular rash. The critical concern with dengue is the phenomenon of antibody-dependent enhancement (ADE), where antibodies from prior infection with one serotype may enhance uptake and replication of a different serotype during subsequent infection, increasing risk for severe dengue (dengue hemorrhagic fever and dengue shock syndrome) with plasma leakage, bleeding, and potential vascular collapse. Additional mosquito-borne flaviviruses cause significant human disease with varying clinical presentations. Yellow fever, endemic in tropical Africa and South America, causes hemorrhagic fever with hepatic failure manifesting as jaundice, hence the name; a highly effective live attenuated vaccine is available and required for travel to endemic areas. Zika virus gained global attention during the 2015-2016 epidemic when vertical transmission was associated with congenital Zika syndrome including microcephaly and other brain abnormalities, as well as Guillain-Barre syndrome in adults. West Nile virus has become established in North America since 1999, causing a spectrum from asymptomatic infection to neuroinvasive disease with encephalitis or meningitis, particularly affecting elderly individuals. Japanese encephalitis is the leading cause of viral encephalitis in Asia, preventable with available vaccines. Hepatitis C virus represents a distinct clinical paradigm within the Flaviviridae, causing chronic infection in the majority (75-85%) of those infected, contrasting with the acute, self-limited infections caused by most flaviviruses. Transmission occurs through blood exposure, historically through transfusions before 1992 screening and currently primarily through injection drug use. Acute infection is usually asymptomatic, meaning most patients present years later with complications of chronic infection. Chronic hepatitis C progresses over decades, with 15-30% developing cirrhosis and an associated risk of hepatocellular carcinoma. The treatment revolution brought by direct-acting antivirals (DAAs) targeting viral proteins has transformed HCV from a difficult-to-treat chronic infection to a curable disease, with combination regimens achieving sustained virologic response (cure) rates exceeding 95% in 8-12 weeks of treatment. Despite treatment advances, no vaccine exists due to the virus's extreme genetic variability. Panel A: Flavivirus arthropod-borne transmission cycle - mosquito vector (Aedes species) feeding on infected human, viral replication, transmission to susceptible human, endemic geographic regions mapped (tropical/subtropical), and spectrum of disease outcomes (febrile illness, hemorrhagic fever, encephalitis) with virus examples for each. Panel B: Dengue pathogenesis and antibody-dependent enhancement - four serotypes illustrated, primary infection causing dengue fever (symptoms listed), secondary infection with different serotype showing ADE mechanism (antibody binding virus enhancing FcR-mediated uptake), increased replication leading to severe dengue (vascular leak, hemorrhage, shock). Panel C: Flavivirus disease gallery - yellow fever (jaundice, hemorrhagic fever, vaccine preventable), Zika (microcephaly in fetus, Guillain-Barre in adults), West Nile (North America map, encephalitis in elderly), Japanese encephalitis (Asia distribution, vaccine available), with clinical images for each. Panel D: Hepatitis C treatment revolution - transmission routes (pre-1992 transfusion, current IVDU), chronic infection rate (75-85%), progression timeline to cirrhosis and HCC, DAA mechanism (targeting NS3/4A protease, NS5A, NS5B polymerase), and cure rate transformation (from <50% with interferon to >95% with DAAs in 8-12 weeks). --- ### VII. Other Arboviruses The Togaviridae family includes the alphaviruses and rubivirus, enveloped viruses with positive-sense RNA genomes that cause distinct clinical syndromes. Alphaviruses transmitted by mosquitoes include the equine encephalitis viruses and chikungunya virus. Eastern equine encephalitis (EEE) is the most severe, with case fatality rates of 30-50% and frequent neurologic sequelae in survivors; it occurs primarily in the eastern United States in swampy areas where the bird-mosquito cycle involves Culiseta melanura mosquitoes with occasional spillover to horses and humans. Western equine encephalitis (WEE) causes less severe disease, while Venezuelan equine encephalitis (VEE) occurs in Central and South America with periodic epidemic spread. Chikungunya virus has spread dramatically since 2004, causing large epidemics characterized by severe polyarthralgia that may persist for months to years, giving rise to its name meaning "that which bends up" in Makonde language. Rubella virus, though a togavirus, transmits via respiratory droplets rather than arthropod vectors and causes German measles, a mild childhood illness of significance primarily for its teratogenic effects during pregnancy. The postnatal disease presents with low-grade fever, posterior auricular and occipital lymphadenopathy (a distinctive finding), and a pink maculopapular rash beginning on the face and spreading downward. The rash and illness are milder than measles (rubeola), hence the historical term "three-day measles." Congenital rubella syndrome (CRS) occurs when maternal infection during the first trimester allows viral passage across the placenta, causing a devastating combination of cataracts, sensorineural deafness, and congenital heart defects (classically patent ductus arteriosus). Prevention through MMR vaccination has dramatically reduced CRS incidence in countries with high vaccination coverage. The Bunyavirales order (formerly Bunyaviridae) includes diverse viruses with segmented negative-sense RNA genomes, many of which cause hemorrhagic fever or encephalitis. Hantaviruses, unlike other bunyaviruses, are transmitted through rodent excreta rather than arthropod vectors. In the Americas, Sin Nombre virus and related species cause hantavirus pulmonary syndrome (HPS), characterized by rapid progression from febrile illness to severe pulmonary edema with high mortality. In Europe and Asia, hantaviruses cause hemorrhagic fever with renal syndrome (HFRS), featuring renal dysfunction and bleeding. La Crosse virus and other California serogroup viruses cause pediatric encephalitis in the United States. Crimean-Congo hemorrhagic fever, transmitted by Hyalomma ticks, causes severe hemorrhagic disease across a wide geographic range from Eastern Europe through the Middle East to Asia. Rift Valley fever virus represents another bunyavirus of concern, endemic in Africa and the Arabian Peninsula where it causes disease in both livestock and humans. Large outbreaks occur following periods of heavy rainfall that increase mosquito populations, with human infection resulting from mosquito bites or direct contact with infected animal tissues during slaughter. Most human infections are mild febrile illness, but a small percentage develop hemorrhagic fever, encephalitis, or retinitis with potential permanent vision loss. The virus poses bioterrorism concerns and could potentially establish in new geographic regions if introduced. No approved vaccines are currently available for human use, though veterinary vaccines exist for livestock protection. Panel A: Equine encephalitis viruses map and severity - United States map showing EEE (eastern swampy regions), WEE (western), with VEE in Central/South America, transmission cycle (bird-mosquito with horse/human spillover), case fatality comparison bars (EEE highest at 30-50%, WEE lower), and neurologic sequelae notation. Panel B: Chikungunya global spread - world map showing expansion from Africa/Asia since 2004, mosquito vector (Aedes species), clinical presentation (severe polyarthralgia affecting joints, bent posture reflecting name meaning), chronicity timeline showing joint pain persisting months to years, and outbreak magnitude indicators. Panel C: Rubella and congenital rubella syndrome - postnatal disease (mild fever, posterior auricular/occipital lymphadenopathy highlighted, pink rash), versus congenital infection (first trimester infection risk, placental passage), CRS triad (cataracts in eye, hearing loss in ear, heart with PDA), and vaccine prevention through MMR. Panel D: Bunyavirus diversity - hantavirus transmission (rodent excreta, distinct from arthropod-borne), HPS in Americas (pulmonary edema X-ray), HFRS in Europe/Asia (renal and hemorrhagic), La Crosse (pediatric encephalitis), and Crimean-Congo (tick-borne hemorrhagic fever distribution map). --- ### VIII. Rhabdoviridae and Filoviridae Rabies virus exemplifies the Rhabdoviridae family, named for their distinctive bullet-shaped morphology visible by electron microscopy. This enveloped virus with negative-sense RNA genome causes an almost invariably fatal encephalitis that has terrified humans throughout recorded history. Transmission occurs through the saliva of infected mammals, most commonly through bites that inoculate virus into wounds, with the virus then traveling via peripheral nerves to the central nervous system. Reservoir species vary geographically: domestic dogs remain the primary source in developing countries accounting for 99% of human cases globally, while in the United States and other developed countries where dog rabies is controlled, exposure typically comes from bats, raccoons, skunks, or foxes. The incubation period is highly variable, ranging from weeks to months (rarely years), influenced by the distance of the bite from the central nervous system and the viral inoculum. The clinical course of rabies follows a predictable progression once symptoms appear. The prodromal phase features nonspecific symptoms including fever, malaise, and often paresthesias or pain at the bite site reflecting viral replication in dorsal root ganglia. Encephalitic (furious) rabies, the more common form, progresses to hydrophobia (intense spasms of the diaphragm and accessory muscles of inspiration triggered by attempts to swallow water), aerophobia (similar spasms triggered by air drafts), periods of hyperactivity and agitation alternating with lucid intervals, and autonomic instability. Paralytic (dumb) rabies, accounting for approximately 20% of cases, presents with ascending paralysis resembling Guillain-Barre syndrome. Both forms progress to coma and death, with survival extraordinarily rare once clinical symptoms develop despite aggressive intensive care, and the handful of survivors typically have severe neurologic impairment. Post-exposure prophylaxis (PEP) effectively prevents rabies if administered before symptom onset, making prompt evaluation of animal exposures critical. Management begins with immediate thorough wound cleaning with soap and water, reducing viral inoculation substantially. PEP for previously unvaccinated individuals includes both human rabies immunoglobulin (HRIG) infiltrated around the wound to neutralize virus locally, and rabies vaccine administered as four doses on days 0, 3, 7, and 14. Decision to administer PEP depends on the animal species, availability for observation or testing, circumstances of exposure, and geographic rabies epidemiology. Prophylaxis is not needed for intact skin exposures or if the animal tests negative for rabies. Laboratory diagnosis uses direct fluorescent antibody testing of brain tissue (animal) or nuchal skin biopsy (human), with Negri bodies (eosinophilic cytoplasmic inclusions in neurons) representing the classic but insensitive histopathologic finding. The Filoviridae family includes Ebola and Marburg viruses, causes of severe hemorrhagic fever with high case fatality rates. These enveloped viruses with negative-sense RNA genomes have distinctive filamentous morphology visible by electron microscopy. Fruit bats are believed to be natural reservoir hosts, with transmission to humans occurring through contact with infected animals or subsequently through human-to-human transmission via bodily fluids. The 2014-2016 West African Ebola outbreak was the largest in history, with over 28,000 cases and 11,000 deaths, exposing limitations in outbreak response capacity while accelerating development of vaccines and therapeutics. Clinical features include fever, severe headache, myalgia, vomiting and diarrhea, and hemorrhagic manifestations in advanced disease. Treatment is supportive, with monoclonal antibody therapies (including Inmazeb) improving survival when available. The rVSV-ZEBOV vaccine has proven effective for outbreak control through ring vaccination strategies. Panel A: Rabies virus structure and transmission - bullet-shaped virion electron microscopy image, transmission diagram (animal bite inoculating saliva containing virus), neural pathway from peripheral nerve to CNS over weeks to months, and geographic reservoir species (dogs in developing countries, bats/raccoons/skunks in North America). Panel B: Rabies clinical progression - prodrome (fever, bite site paresthesias), encephalitic/furious form (hydrophobia with diaphragm spasm diagram, aerophobia, agitation), paralytic/dumb form (ascending paralysis), and outcome (coma, near-universal death, extremely rare survival with impairment). Panel C: Post-exposure prophylaxis protocol - wound care first (thorough washing with soap), HRIG infiltration at wound site (local virus neutralization), vaccine schedule (days 0, 3, 7, 14), decision algorithm based on animal type and availability for testing, and lab diagnosis (DFA testing, Negri bodies in neurons). Panel D: Filovirus characteristics and outbreaks - filamentous morphology by EM, bat reservoir with spillover to humans, person-to-person transmission via bodily fluids, 2014-2016 West Africa outbreak magnitude (map with case numbers), clinical features (hemorrhagic manifestations), and advances (monoclonal antibodies, rVSV-ZEBOV vaccine for ring vaccination). --- ### IX. Retroviridae (HIV) Human immunodeficiency virus (HIV) possesses unique molecular features among RNA viruses that underlie its pathogenesis and inform treatment strategies. The HIV virion contains two copies of positive-sense RNA genome along with reverse transcriptase, the enzyme that synthesizes DNA from the RNA template, classifying HIV as a retrovirus. Following entry into CD4+ T cells through binding of viral gp120 to CD4 and coreceptors (CCR5 or CXCR4), reverse transcription produces double-stranded DNA that integrates into the host chromosome as provirus, establishing permanent infection of that cell and its progeny. This integration explains why HIV infection cannot be cured by current antiretroviral therapy: even when plasma viral load is suppressed to undetectable levels, latently infected cells harboring integrated provirus persist and can reactivate if treatment is stopped. The clinical course of untreated HIV infection progresses through defined stages over years. Acute HIV infection (acute retroviral syndrome) occurs 2-4 weeks after transmission and manifests as a mononucleosis-like illness with fever, lymphadenopathy, pharyngitis, rash, and myalgias; during this phase, plasma viral load reaches extremely high levels before the immune response develops. Following acute infection, patients enter clinical latency during which they are generally asymptomatic but CD4+ T cells gradually decline at varying rates. Without treatment, progression to AIDS occurs over a median of approximately 10 years. AIDS is defined by CD4 count below 200 cells per microliter or the occurrence of an AIDS-defining condition regardless of CD4 count. AIDS-defining conditions include opportunistic infections (Pneumocystis jirovecii pneumonia, toxoplasmosis, cryptococcal meningitis, disseminated Mycobacterium avium complex, CMV retinitis, and others) and certain malignancies (Kaposi sarcoma, primary CNS lymphoma, invasive cervical cancer). The development of antiretroviral therapy (ART) has transformed HIV from a uniformly fatal diagnosis to a manageable chronic condition. Six drug classes target different steps in the viral life cycle: nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) block reverse transcription; integrase strand transfer inhibitors (INSTIs) prevent proviral integration; protease inhibitors (PIs) block cleavage of viral polyproteins; and entry inhibitors target either CCR5 (maraviroc) or fusion (enfuvirtide). Modern ART regimens combine drugs from multiple classes to suppress viral replication and prevent resistance emergence. Current guidelines recommend INSTI-based regimens as initial therapy due to high efficacy, favorable tolerability, and high barrier to resistance. With effective ART, individuals living with HIV can achieve normal life expectancy. Treatment as prevention has revolutionized HIV epidemic control alongside traditional prevention strategies. When plasma HIV viral load is suppressed to undetectable levels by ART, sexual transmission does not occur, encapsulated in the concept "U=U" (Undetectable = Untransmittable). Pre-exposure prophylaxis (PrEP) with tenofovir-based regimens taken by HIV-negative individuals at high risk provides highly effective protection when taken consistently. Post-exposure prophylaxis (PEP) with 28 days of ART initiated within 72 hours of potential exposure prevents infection following occupational or non-occupational exposures. Despite these advances, barriers to testing, linkage to care, treatment adherence, and PrEP access mean the epidemic continues, with approximately 1.3 million new infections annually. Vaccine development remains challenging due to HIV genetic diversity and immune evasion mechanisms. Panel A: HIV structure and life cycle - virion with gp120/gp41 envelope proteins, RNA genome with reverse transcriptase, CD4 and coreceptor (CCR5/CXCR4) binding for entry, reverse transcription to DNA, integration as provirus in host chromosome, and latent reservoir concept (provirus persisting in resting memory CD4 cells despite ART). Panel B: HIV clinical course without treatment - timeline showing acute infection (mononucleosis-like syndrome, high viral load spike), clinical latency (gradual CD4 decline over years, relatively stable viral load), and AIDS (CD4 <200, opportunistic infections and malignancies), with CD4 count and viral load curves over approximately 10-year median progression. Panel C: Antiretroviral drug classes and targets - life cycle diagram with drug intervention points: entry inhibitors (CCR5 antagonist, fusion inhibitor), NRTIs and NNRTIs (reverse transcriptase), INSTIs (integrase/integration step), and PIs (protease/polyprotein cleavage), with current preferred INSTI-based regimens highlighted. Panel D: Prevention strategies - treatment as prevention (U=U concept with undetectable viral load), PrEP (daily tenofovir-based regimen for high-risk HIV-negative individuals), PEP (28-day ART within 72 hours of exposure), and remaining challenges (testing gaps, adherence barriers, PrEP access, vaccine development difficulties). --- ### X. Coronaviridae Coronaviruses are enveloped viruses possessing the largest genomes among RNA viruses, encoding complex replication machinery and multiple structural and accessory proteins. The family name derives from the crown-like appearance of spike (S) proteins projecting from the viral envelope in electron micrographs (corona = crown in Latin). Human coronaviruses include four species causing common cold (HCoV-229E, OC43, NL63, and HKU1) along with three highly pathogenic species that have emerged this century: SARS-CoV (2002-2003), MERS-CoV (ongoing since 2012), and SARS-CoV-2 (COVID-19 pandemic beginning 2019). All are believed to have originated in bats before spilling over to humans, sometimes through intermediate animal hosts. The emergence of three novel pathogenic coronaviruses in two decades highlights the ongoing pandemic threat from this viral family. SARS-CoV-2, the causative agent of COVID-19, has caused unprecedented global impact since its emergence in late 2019. The virus binds to angiotensin-converting enzyme 2 (ACE2) receptors on host cells, with the spike protein mediating both attachment and membrane fusion for entry. Transmission occurs primarily through respiratory droplets and aerosols, with the virus capable of airborne spread in poorly ventilated indoor spaces. The incubation period ranges from 2-14 days with a median of 5 days. Clinical manifestations span a spectrum from asymptomatic infection through mild upper respiratory symptoms to severe pneumonia with acute respiratory distress syndrome (ARDS), with multi-organ involvement possible including cardiovascular, neurologic, and thromboembolic complications. Risk factors for severe disease include advanced age, obesity, diabetes, cardiovascular disease, chronic lung disease, and immunocompromise. The rapid evolution of SARS-CoV-2 has produced successive variants with altered transmissibility, immune evasion, and disease severity. Major variants of concern have included Alpha (increased transmissibility), Delta (markedly increased transmissibility and severity), and Omicron (extensive spike mutations conferring immune evasion and altered tropism). The emergence of Omicron and its subvariants demonstrated the capacity of the virus to substantially escape immunity from both natural infection and vaccination, though protection against severe disease has remained more durable than protection against infection. The continuing evolution of SARS-CoV-2 suggests this virus will remain endemic, circulating with periodic waves of infection while population immunity from vaccination and natural infection provides partial protection. The COVID-19 pandemic stimulated unprecedented development of vaccines and therapeutics. Messenger RNA vaccines (Pfizer-BioNTech, Moderna) encoding the spike protein represented a novel vaccine platform achieving regulatory authorization with remarkable speed, demonstrating high efficacy against symptomatic infection and especially severe disease. Viral vector vaccines (Johnson & Johnson, AstraZeneca) and protein subunit vaccines provided additional options. Antiviral therapeutics include remdesivir (nucleoside analog inhibiting RNA-dependent RNA polymerase), nirmatrelvir-ritonavir (Paxlovid, protease inhibitor combination), and molnupiravir (nucleoside analog inducing lethal mutagenesis). Immunomodulatory therapy with dexamethasone reduces mortality in patients requiring oxygen supplementation. Monoclonal antibodies targeting spike protein provided early treatment options but have been limited by variant evolution causing treatment resistance. Management strategies continue to evolve as the virus and population immunity change. Panel A: Coronavirus structure and diversity - spherical enveloped virion with spike proteins creating crown appearance, large (+) ssRNA genome emphasized (largest among RNA viruses), human coronavirus spectrum (common cold species vs emerging pathogenic SARS-CoV, MERS-CoV, SARS-CoV-2), and bat reservoir with spillover events timeline. Panel B: SARS-CoV-2 pathogenesis and clinical spectrum - spike protein binding ACE2 receptor on respiratory epithelium, transmission routes (respiratory droplets, aerosols in poorly ventilated spaces), clinical spectrum from asymptomatic through mild respiratory to severe ARDS and multi-organ involvement, and risk factors for severe disease listed. Panel C: Variant evolution - spike protein with key mutation sites, variant timeline (Alpha, Delta, Omicron progression), characteristics comparison (transmissibility, immune evasion, severity), and implications for vaccines and therapeutics (protection against severe disease more durable than against infection). Panel D: COVID-19 countermeasures - vaccine platforms (mRNA mechanism diagram, viral vector, protein subunit), therapeutic categories (remdesivir mechanism as RdRp inhibitor, Paxlovid as protease inhibitor, dexamethasone for hypoxic patients), and monoclonal antibody limitation (variant evolution causing resistance). --- ## Summary - RNA viruses have high mutation rates due to lack of polymerase proofreading, enabling rapid evolution, antigenic drift, drug resistance, and emergence of new variants - Influenza has a segmented genome enabling reassortment (antigenic shift) causing pandemics; neuraminidase inhibitors and annual vaccines are key interventions - Paramyxoviruses include measles (3 C's, Koplik spots, highly contagious), RSV (infant bronchiolitis, new vaccines/antibodies), and parainfluenza (croup) - Picornaviruses include poliovirus (near eradication), coxsackieviruses (HFMD, myocarditis), rhinovirus (common cold), and hepatitis A (acute hepatitis, vaccine) - Norovirus causes highly contagious gastroenteritis outbreaks; rotavirus causes severe infant diarrhea but vaccination has dramatically reduced burden - Dengue has 4 serotypes with antibody-dependent enhancement risk in secondary infection; HCV causes chronic hepatitis but is now curable with DAAs - Rabies is nearly 100% fatal once symptomatic; post-exposure prophylaxis with wound care, HRIG, and vaccine is effective if given before symptoms - HIV integrates as provirus, depletes CD4 cells leading to AIDS; ART suppresses but cannot cure; U=U and PrEP are prevention pillars - SARS-CoV-2 caused the COVID-19 pandemic; mRNA vaccines, antivirals (Paxlovid, remdesivir), and dexamethasone are key interventions --- ## Key Terms | Term | Definition | |------|------------| | Antigenic drift | Gradual accumulation of mutations in viral surface proteins causing seasonal epidemics | | Antigenic shift | Major abrupt change in viral surface proteins through genetic reassortment causing pandemics | | Reverse transcriptase | Retroviral enzyme that synthesizes DNA from RNA template | | Provirus | Viral DNA integrated into host chromosome, establishing permanent latent infection | | Arbovirus | Arthropod-borne virus transmitted by mosquitoes, ticks, or other arthropods | | Antibody-dependent enhancement | Phenomenon where antibodies from prior infection enhance uptake and replication of related virus | | Syncytia | Multinucleated giant cells formed by fusion of infected cells, characteristic of paramyxoviruses | | Direct-acting antiviral | Drug directly targeting viral proteins rather than host factors | | Post-exposure prophylaxis | Preventive treatment given after potential exposure to an infectious agent | | Spike protein | Coronavirus surface glycoprotein mediating receptor binding and membrane fusion | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*