9.1 Orthomyxoviruses and paramyxoviruses
4 min read•august 1, 2024
RNA viruses are sneaky shape-shifters. Orthomyxoviruses and paramyxoviruses, two key families, cause widespread infections like flu and . They differ in genome structure and replication sites, but both use surface proteins to invade cells.
These viruses spread easily, causing seasonal outbreaks and occasional pandemics. Vaccines are our main defense, but viral mutations pose ongoing challenges. Public health measures and global surveillance are crucial to stay ahead of these evolving threats.
Orthomyxoviruses vs Paramyxoviruses
Structural and Genomic Differences
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- Orthomyxoviruses and paramyxoviruses possess enveloped, negative-sense RNA genomes
- Orthomyxoviruses feature segmented genomes
- Paramyxoviruses contain non-segmented genomes
- Viral envelopes of both families display glycoprotein spikes
- Orthomyxoviruses exhibit hemagglutinin (HA) and neuraminidase (NA) ()
- Paramyxoviruses present fusion (F) and attachment proteins (HN, H, or G) (measles virus)
- Replication sites differ between families
- Orthomyxoviruses replicate in the host cell nucleus
- Paramyxoviruses replicate entirely in the host cell cytoplasm
Replication and Pathogenesis Mechanisms
- Orthomyxoviruses employ cap-snatching from host mRNAs during replication
- Paramyxoviruses utilize a stuttering mechanism for mRNA editing
- Both families use surface glycoproteins for host cell attachment and entry
- Fusion mechanisms vary between the two families
- Pathogenesis targets different body systems
- Orthomyxoviruses primarily affect the respiratory tract ()
- Paramyxoviruses cause respiratory, systemic, or neurological infections (measles, mumps)
Epidemiology of Common Viral Infections
Influenza and Measles Characteristics
- Influenza (orthomyxovirus) occurs in seasonal epidemics and occasional pandemics
- Rapid global spread results from high transmissibility
- Antigenic changes contribute to virus evolution
- Measles (paramyxovirus) demonstrates high contagiousness
- Severe complications arise in undernourished children and immunocompromised individuals
- Outbreaks often occur in densely populated areas (schools, refugee camps)
Mumps and Respiratory Syncytial Virus (RSV) Features
- Mumps (paramyxovirus) typically affects salivary glands
- Can cause orchitis, oophoritis, and meningitis
- Outbreaks frequently occur in close-contact settings (dormitories, sports teams)
- RSV (paramyxovirus) leads lower respiratory tract infections in young children
- Potential for severe bronchiolitis and pneumonia in infants
- Seasonal patterns vary by geographic location
Epidemiological Patterns and Clinical Manifestations
- Incubation periods vary among viruses
- Influenza: 1-4 days
- Measles: 7-14 days
- Mumps: 16-18 days
- RSV: 2-8 days
- Modes of transmission differ
- Respiratory droplets (all four viruses)
- Direct contact with infected secretions (RSV)
- Duration of infectivity impacts control strategies
- Influenza: 1 day before symptoms to 5-7 days after onset
- Measles: 4 days before rash to 4 days after rash onset
- Clinical manifestations range from mild to severe
- Influenza and RSV: mild respiratory symptoms to pneumonia
- Measles: characteristic rash, fever, cough
- Mumps: swollen salivary glands, fever, headache
Prevention and Control Strategies
Vaccination and Antiviral Treatments
- Vaccination serves as primary prevention strategy
- Annual influenza vaccines target predicted strains
- Combination MMR (measles, mumps, rubella) vaccine provides long-term immunity
- RSV vaccines under development for various age groups
- Antiviral drugs play dual roles in treatment and prophylaxis
- Neuraminidase inhibitors (oseltamivir) for influenza
- No specific for measles, mumps, or RSV
- Passive immunization with prevents RSV in high-risk infants (palivizumab)
Public Health Measures and Surveillance
- Implement isolation of infected individuals to prevent transmission
- Conduct contact tracing to identify potential cases
- Promote hygiene practices (hand washing, respiratory etiquette)
- Utilize global surveillance networks
- WHO Global Influenza Surveillance and Response System monitors virus evolution
- Guides vaccine development and strain selection
- Aim for herd immunity through vaccination
- Particularly important for measles and mumps prevention
- Requires high vaccination coverage (95% for measles)
Challenges in Prevention and Control
- Viral mutation rates challenge vaccine effectiveness
- Influenza requires annual vaccine updates
- Potential for vaccine mismatch with circulating strains
- Vaccine hesitancy impacts population-level protection
- Misinformation and distrust in medical authorities
- Resurgence of measles outbreaks in areas with low vaccination rates
- Emergence of new viral strains poses ongoing threats
- Zoonotic spillover events (influenza from avian or swine sources)
- Need for rapid response and vaccine development capabilities
Antigenic Drift and Shift in Influenza
Mechanisms of Antigenic Change
- involves gradual accumulation of point mutations
- Affects hemagglutinin (HA) and neuraminidase (NA) genes
- Results in seasonal influenza epidemics
- Necessitates annual vaccine updates
- Antigenic shift occurs through genetic reassortment
- Different influenza A subtypes exchange genome segments
- Produces novel strains with pandemic potential
- Facilitated by segmented genome structure
Pandemic Potential and Surveillance
- Pandemic influenza strains arise from antigenic shift events
- Introduce viral subtypes with little human population immunity
- Historical examples: 1918 Spanish flu (H1N1), 2009 swine flu (H1N1pdm09)
- Animal reservoirs play crucial roles in new strain emergence
- Birds and swine serve as mixing vessels for influenza viruses
- Interspecies transmission leads to novel human-infecting strains
- Global surveillance monitors antigenic changes
- Predicts vaccine effectiveness for upcoming seasons
- Identifies potential pandemic threats
- Informs public health preparedness and response strategies
Implications for Vaccine Development and Control
- Rapid evolution challenges vaccine development timelines
- 6-8 month lead time for current egg-based production
- Efforts to develop universal influenza vaccines targeting conserved viral regions
- Antiviral drug efficacy affected by viral mutations
- Emergence of resistant strains necessitates new drug development
- Combination therapies may reduce resistance development
- Continuous research and adaptation of control strategies required
- Improved surveillance techniques (next-generation sequencing)
- Development of platform technologies for rapid vaccine production
- Enhancement of global coordination for pandemic preparedness
Key Terms to Review (17)
Adaptive immunity: Adaptive immunity is a specialized immune response that develops over time, allowing the body to recognize and remember specific pathogens for more effective defense upon subsequent exposures. It involves the activation of lymphocytes, specifically T and B cells, which work together to target and eliminate invading viruses and other pathogens, leading to long-lasting immunity. This tailored response is critical in managing infections from various viruses and adapting to their evolving nature.
Antigenic drift: Antigenic drift refers to the gradual accumulation of mutations in the genes encoding viral surface proteins, leading to changes in the antigenic properties of a virus. This process allows viruses, especially RNA viruses, to evade the host immune response, making it harder for the immune system to recognize and respond to the virus over time.
Antivirals: Antivirals are medications designed to treat viral infections by inhibiting the development or replication of viruses within the host. They work by targeting specific stages of the viral life cycle, which helps reduce the severity and duration of infections. These drugs play a crucial role in managing illnesses caused by certain viruses, particularly in high-risk populations, and are vital for controlling outbreaks and protecting public health.
Enveloped virus: An enveloped virus is a type of virus that has a lipid membrane surrounding its protein coat, or capsid. This envelope is derived from the host cell's membrane as the virus buds off, and it plays a crucial role in the virus's ability to infect host cells. The envelope contains viral glycoproteins that are essential for attachment to and entry into host cells, making enveloped viruses generally more sensitive to environmental conditions than non-enveloped viruses.
Fusion protein: A fusion protein is a type of protein that is created by joining two or more different proteins or protein domains together, often through genetic engineering. This process can enhance the properties of the proteins involved, allowing for functions such as improved stability, solubility, or targeting to specific cellular locations. In the context of certain viruses, like orthomyxoviruses and paramyxoviruses, fusion proteins play a critical role in the viral entry process by facilitating the fusion of the viral envelope with the host cell membrane.
Hemagglutination: Hemagglutination is the clumping of red blood cells due to the interaction with specific viruses or other agents that possess hemagglutinin, a type of glycoprotein. This process is crucial in understanding how certain viruses, particularly orthomyxoviruses and paramyxoviruses, attach to and infect host cells by binding to sialic acid residues on the surface of red blood cells.
Inactivated vaccine: An inactivated vaccine is a type of vaccine that is made from viruses or bacteria that have been killed or inactivated, meaning they can no longer cause disease but still stimulate an immune response. These vaccines often require multiple doses to ensure adequate immunity and are used to protect against various infectious diseases.
Influenza: Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. This disease is significant in virology due to its classification, transmission patterns, pandemic potential, and vaccine challenges.
Influenza virus: The influenza virus is an RNA virus that causes the highly contagious respiratory illness known as influenza or the flu. It belongs to the Orthomyxoviridae family and is characterized by its ability to undergo frequent genetic changes, making it a significant public health concern due to seasonal epidemics and occasional pandemics.
Innate immunity: Innate immunity is the body's first line of defense against pathogens, consisting of physical barriers, immune cells, and chemical signals that provide immediate, non-specific responses to infections. This type of immunity is crucial for early detection and response to invading viruses and other pathogens, laying the groundwork for a more specific adaptive immune response.
Live attenuated vaccine: A live attenuated vaccine contains a weakened form of the pathogen that causes the disease, which stimulates an immune response without causing the disease itself. These vaccines often provide long-lasting immunity and are generally more effective than inactivated vaccines, but they may present challenges regarding stability, storage, and safety in certain populations.
Measles: Measles is a highly contagious viral disease caused by the measles virus, which belongs to the paramyxovirus family. Characterized by fever, cough, runny nose, and a distinctive red rash, measles can lead to serious complications, especially in young children. The virus spreads through respiratory droplets, making it one of the most infectious diseases known.
Monoclonal antibodies: Monoclonal antibodies are laboratory-made molecules that can mimic the immune system's ability to fight off harmful pathogens like viruses. They are produced from a single clone of immune cells and can target specific antigens on pathogens, making them invaluable in diagnostics and treatment, especially for viral infections and cancer therapies.
Mumps virus: The mumps virus is an RNA virus belonging to the Paramyxoviridae family, known for causing mumps, a contagious viral infection characterized by swelling of the parotid glands. This virus is an important member of the paramyxovirus group, which also includes other significant human pathogens, illustrating the diversity and impact of this family on public health.
Nucleoprotein: Nucleoprotein is a complex formed by the association of nucleic acids (DNA or RNA) with proteins, playing a vital role in the structure and function of viruses. In the context of orthomyxoviruses and paramyxoviruses, nucleoproteins are essential for viral replication and the packaging of the viral genome within the host cell. These proteins help stabilize the viral RNA and are crucial in regulating the processes of transcription and translation during viral infection.
Segmented RNA: Segmented RNA refers to a type of viral genome organization where the genetic material is divided into multiple separate RNA segments. This characteristic allows viruses to undergo genetic reassortment, which can lead to increased genetic diversity and the emergence of new viral strains. It is particularly significant in the context of certain viral families, where this feature influences their replication, pathogenesis, and potential for causing epidemics.
Zoonotic transmission: Zoonotic transmission refers to the process through which infectious diseases are transferred from animals to humans. This type of transmission plays a crucial role in understanding the emergence of new viral infections and the patterns of disease spread, highlighting the interconnectedness of human health and animal health.