7.1 Classification and characteristics of major animal virus families
4 min read•august 1, 2024
Animal viruses come in many shapes and sizes, but they all share one goal: to infect and replicate inside host cells. From tiny parvoviruses to massive poxviruses, these microscopic invaders use clever tricks to sneak into our bodies and hijack our cellular machinery.
Understanding how viruses are classified helps us make sense of their diverse strategies. Whether they use DNA or RNA, envelopes or naked capsids, each viral family has evolved unique ways to spread and cause disease. Let's break down the key players and their sneaky tactics.
Animal Virus Classification
Nucleic Acid-Based Classification
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- Animal viruses classified primarily by nucleic acid type (DNA or RNA) and strand configuration (single-stranded or double-stranded)
- Baltimore classification system categorizes viruses into seven groups based on genome and replication strategy
- Group I: viruses ()
- Group II: viruses ()
- Group III: viruses ()
- Group IV: viruses ()
- Group V: viruses ()
- Group VI: viruses ()
- Group VII: viruses ()
- Viral replication strategies determined by genome nature
- Direct translation for (+)ssRNA viruses
- Complementary mRNA production for (-)ssRNA and dsRNA viruses
- Reverse transcription for retroviruses and hepadnaviruses
Taxonomic Classification System
- International Committee on Taxonomy of Viruses (ICTV) provides standardized classification system
- ICTV incorporates genetic, structural, and biological properties for comprehensive classification
- Hierarchical classification system organizes viruses into families, subfamilies, genera, and species
- Virus families subdivided based on shared characteristics and evolutionary relationships
- Examples of virus families: ( viruses), (, )
Animal Virus Structure
Basic Structural Components
- Viral structures consist of nucleic acid genome, protein capsid, and sometimes lipid envelope
- Capsid symmetry serves as key morphological feature
- (adenoviruses, polioviruses)
- (influenza viruses, measles virus)
- viruses acquire lipid bilayer from host cell membranes
- embedded in envelope crucial for cell entry ( in influenza viruses)
- Specific structural proteins play important roles
- in some enveloped viruses aid in virion assembly and stability (M1 protein in influenza viruses)
Complex Virus Structures
- Complex viruses possess additional structural elements
- Poxviruses have lateral bodies and surface tubules
- Herpesviruses contain tegument layer between capsid and envelope
- Virus size varies greatly among families
- Small non-enveloped viruses: 20-30 nm (parvoviruses)
- Large, complex viruses: 200-400 nm (poxviruses, mimiviruses)
- Unique structural features in certain virus families
- Filoviruses () have filamentous morphology
- Rhabdoviruses () exhibit bullet-shaped structure
Animal Virus Replication
General Replication Stages
- Attachment: virus binds to specific receptors on host cell surface
- Entry: viruses enter cells through various mechanisms (endocytosis, membrane fusion)
- Uncoating: viral genome released into host cell
- Gene expression: viral genes transcribed and translated using host or viral machinery
- Genome replication: viral genomes copied using host or viral enzymes
- Assembly: viral components come together to form new virions
- Release: mature virions exit the cell through budding or cell lysis
Replication Strategies of Different Virus Families
- DNA virus families typically replicate in nucleus
- Herpesviridae use host RNA polymerase II for transcription
- Adenoviridae encode their own DNA-dependent DNA polymerase
- RNA virus families often replicate in cytoplasm
- Flaviviridae use virus-encoded RNA-dependent RNA polymerase
- Orthomyxoviridae replicate in nucleus, unique among RNA viruses
- Retroviruses (Retroviridae) employ unique replication strategy
- Reverse transcription of RNA genome into DNA
- Integration of viral DNA into host genome as provirus
- Some virus families use complex replication strategies
- Hepadnaviridae involve both DNA and RNA intermediates
- Reverse transcription of pregenomic RNA to produce DNA genome
Temporal Regulation of Viral Gene Expression
- Viral gene expression often temporally regulated
- : expressed first, often regulatory proteins
- : involved in genome replication and protein production
- : typically structural proteins for virion assembly
- Examples of temporal regulation:
- Herpesviruses have distinct immediate-early, early, and late gene phases
- Adenoviruses express early genes before DNA replication, late genes after
Animal Virus Pathogenesis
Virus-Host Interactions
- Virus tropism determines ability to infect specific cell types or tissues
- Influenced by receptor specificity (CD4 receptor for )
- Cellular factors affect replication (transcription factors, enzymes)
- Transmission modes vary among virus families
- Respiratory droplets (influenza viruses)
- Bodily fluids (HIV, hepatitis B virus)
- (dengue virus, Zika virus)
- Viral pathogenesis mechanisms include:
- Direct cell lysis ( in motor neurons)
- Immune-mediated damage (liver damage in hepatitis B infection)
- Alteration of host cell function (oncogenic viruses)
Infection Patterns and Disease Outcomes
- Some virus families establish latent infections
- Herpesviridae (herpes simplex virus, varicella-zoster virus)
- Periodic reactivation leads to recurrent symptoms
- Other families cause acute infections with rapid clearance
- Picornaviridae (common cold viruses)
- Short-lived symptoms followed by recovery
- Certain virus families associated with oncogenesis
- Papillomaviridae (human papillomavirus causing cervical cancer)
- Retroviridae (human T-cell lymphotropic virus causing leukemia)
- Mechanisms of viral oncogenesis:
- disrupting tumor suppressor genes
- Expression of viral oncoproteins (E6 and E7 in HPV)
Immune Response and Viral Evasion
- Immune response to viral infections varies among families
- Innate immune responses (type I interferons, natural killer cells)
- Adaptive immune responses (antibodies, cytotoxic T cells)
- Viruses evolve mechanisms to evade or suppress host immunity
- Antigenic drift in influenza viruses
- Immune exhaustion in chronic hepatitis B virus infection
- Examples of immune evasion strategies:
- Herpesviruses encode proteins that interfere with antigen presentation
- HIV targets and depletes CD4+ T cells, compromising immune function
Key Terms to Review (42)
(-)ssRNA: (-)ssRNA refers to negative-sense single-stranded ribonucleic acid, which is a type of viral genome that must be converted into a positive-sense RNA before it can be translated into proteins. This conversion is essential for the replication and functioning of viruses, as negative-sense RNA cannot be directly translated by the host's ribosomes. The understanding of (-)ssRNA is vital for categorizing certain families of viruses that utilize this genetic material and for comprehending their unique replication strategies.
(+)ssRNA: (+)ssRNA, or positive-sense single-stranded RNA, refers to a type of viral RNA that can serve directly as messenger RNA (mRNA) for protein synthesis in host cells. This means that once the virus enters a host cell, its (+)ssRNA can be immediately translated by the host's ribosomes into proteins required for viral replication and assembly. This characteristic is crucial for the classification and understanding of various animal virus families that utilize this RNA structure to effectively hijack host cellular machinery.
AIDS: AIDS, or Acquired Immunodeficiency Syndrome, is a chronic, potentially life-threatening condition caused by the human immunodeficiency virus (HIV), which attacks the body's immune system. This disease significantly impacts virology, as it has shaped historical understandings of viral infections and their classifications, as well as influencing the study of immunopathology by highlighting how viruses can severely compromise immune responses.
Coronaviridae: Coronaviridae is a family of viruses known for causing respiratory illnesses in humans and other animals. These viruses are characterized by their crown-like appearance under an electron microscope, which is due to the spike proteins on their surface. Coronaviridae includes several important pathogens, including those responsible for severe respiratory syndromes, highlighting their relevance in the study of emerging viral diseases.
Dengue virus: Dengue virus is an arthropod-borne virus (arbovirus) that causes dengue fever, a tropical disease characterized by high fever, severe headaches, pain behind the eyes, joint and muscle pain, rash, and mild bleeding. This virus is classified within the Flaviviridae family and is primarily transmitted through the bites of infected Aedes mosquitoes, particularly Aedes aegypti.
DsDNA: dsDNA, or double-stranded DNA, is a type of nucleic acid structure that consists of two complementary strands of DNA wound around each other to form a double helix. This structure is crucial for the storage and transmission of genetic information in many organisms, as well as in various viruses. Understanding dsDNA is important because it provides insight into the genome organization strategies employed by different viruses and their classification within animal virus families.
Dsdna-rt: dsdna-rt refers to double-stranded DNA viruses that replicate via a reverse transcription process. This unique replication strategy allows these viruses to convert their DNA into RNA, which then can be translated into proteins. Understanding dsdna-rt is crucial for recognizing the diversity of viral replication mechanisms and their implications for host interaction and disease pathology.
DsRNA: dsRNA, or double-stranded ribonucleic acid, is a type of genetic material consisting of two complementary strands of RNA that are held together by base pairing. This structure is crucial for the replication and expression of certain viruses, and it plays a significant role in their genome organization and classification within the viral families. Understanding dsRNA helps reveal the complexity of viral life cycles and their interaction with host cells.
Early genes: Early genes are viral genes expressed soon after a virus infects a host cell, playing a crucial role in the initial stages of viral replication. These genes typically encode proteins that facilitate the virus's takeover of the host's cellular machinery, allowing for the replication of viral components and the establishment of infection. They can also interfere with host defenses, enhancing the virus's ability to replicate and spread.
Ebola virus: Ebola virus is a highly pathogenic virus that causes severe hemorrhagic fever in humans and non-human primates, leading to high mortality rates. This virus is significant not only for its deadly effects but also as a zoonotic virus, which means it can be transmitted from animals to humans, creating challenges in understanding its transmission dynamics and developing effective prevention strategies.
Enveloped: In virology, 'enveloped' refers to viruses that possess a lipid membrane derived from the host cell in which they were assembled. This envelope is made up of phospholipids and proteins, including glycoproteins that are crucial for the virus's ability to infect host cells. Enveloped viruses are typically more sensitive to environmental factors like heat, detergents, and desiccation compared to non-enveloped viruses, which influences their stability and transmission routes.
Flaviviridae: Flaviviridae is a family of viruses known for their single-stranded RNA genomes and their association with various arthropod-borne diseases. These viruses are primarily transmitted by mosquitoes and ticks, leading to significant human and animal health issues globally. Understanding Flaviviridae is essential as it links to the broader classifications of animal virus families and highlights emerging viral diseases that pose risks to public health.
Helical symmetry: Helical symmetry refers to a structural arrangement found in certain viruses where the capsid proteins are arranged in a spiral or helical pattern around the viral nucleic acid. This organization allows for the efficient packaging of the viral genome and provides a sturdy protective structure. Helical symmetry is a key feature that influences how viruses are classified and how they interact with host cells.
Hemagglutinin: Hemagglutinin is a glycoprotein found on the surface of certain viruses, particularly influenza viruses, that facilitates the binding of the virus to host cells. This protein is crucial for the virus's ability to enter and infect host cells, playing a significant role in viral classification, mechanisms of entry, and interactions with the host immune system.
Hepadnaviridae: Hepadnaviridae is a family of viruses that includes those known to cause liver infections in humans and other animals, characterized by their partially double-stranded DNA genome. This virus family is significant for its role in human health, particularly in relation to chronic liver diseases, including hepatitis and liver cancer.
Herpesviridae: Herpesviridae is a large family of viruses known as herpesviruses that can infect humans and animals, characterized by their ability to establish lifelong latency in host cells. This family is significant for its diverse members, which include various human pathogens that can cause diseases ranging from mild to severe, and it plays an important role in understanding viral behavior, transmission, and pathogenesis.
HIV: HIV, or Human Immunodeficiency Virus, is a retrovirus that attacks the body's immune system, specifically targeting CD4 cells (T cells), which are crucial for fighting infections. Understanding HIV is essential in virology as it has shaped research, treatment approaches, and public health strategies over the decades, particularly in the context of viral diseases and their transmission.
Icosahedral symmetry: Icosahedral symmetry is a form of molecular symmetry exhibited by certain viruses, characterized by a geometric structure that resembles an icosahedron. This symmetry allows viruses to efficiently enclose their genetic material in a protective shell, optimizing stability and structural integrity. This design is crucial in determining how viruses interact with host cells, influencing both their classification and characteristics.
Immediate-early genes: Immediate-early genes are the first set of viral genes expressed following the infection of a host cell, playing a critical role in establishing the viral lifecycle. They are essential for initiating viral replication, facilitating the expression of subsequent early and late genes, and modulating host cell functions to favor virus propagation. Understanding these genes is key when classifying and studying major animal virus families.
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.
Integration into host genome: Integration into host genome refers to the process by which certain viruses, particularly retroviruses, incorporate their genetic material into the DNA of the host cell. This integration allows the viral genome to be replicated alongside the host's DNA during cell division, leading to the production of new virus particles and potentially altering the host's genetic function.
Late genes: Late genes refer to a category of genes expressed during the later stages of viral infection, primarily responsible for the production of structural proteins and proteins required for virion assembly and release. They play a critical role in the viral life cycle, ensuring that once the virus has hijacked the host's cellular machinery, it can efficiently produce new viral particles for propagation.
Lysogenic cycle: The lysogenic cycle is a method of viral reproduction in which the viral genome integrates into the host cell's DNA, allowing the virus to replicate along with the host cell without immediately causing cell death. This cycle enables the virus to persist in a dormant state, becoming a part of the host's genetic material and can later switch to the lytic cycle, where it actively produces new viruses and destroys the host cell.
Lytic Cycle: The lytic cycle is a viral replication process in which a virus infects a host cell, hijacks the cell's machinery to produce new viral particles, and ultimately leads to the destruction of the host cell. This cycle results in the release of newly formed virions, which can go on to infect additional cells, making it a crucial aspect of viral propagation.
Matrix proteins: Matrix proteins are structural proteins found in various types of viruses, playing a crucial role in the assembly, shape, and stability of the viral particle. They form a layer between the viral envelope and the nucleocapsid, helping to maintain the integrity of the virus and aiding in processes such as budding and viral replication. Understanding matrix proteins is key to grasping how different animal virus families are classified and characterized based on their structural features and replication strategies.
Naked capsid: A naked capsid is a type of virus structure that lacks an envelope, consisting solely of a protein coat or capsid that encases the viral nucleic acid. This structure is more resistant to environmental factors like heat and detergents, allowing naked viruses to survive longer outside a host and making them significant in the context of virus classification and characteristics.
Orthomyxoviridae: Orthomyxoviridae is a family of negative-sense single-stranded RNA viruses known for causing influenza in animals and humans. This virus family is characterized by its segmented genome, which allows for genetic reassortment, contributing to its ability to evolve and generate new strains, impacting disease outbreaks and vaccine development.
Parvoviridae: Parvoviridae is a family of small, non-enveloped viruses that are known for their single-stranded DNA (ssDNA) genome. These viruses primarily infect animals and are characterized by their ability to replicate in rapidly dividing cells, making them particularly pathogenic in young or immunocompromised hosts. Understanding Parvoviridae is crucial for recognizing their role in animal virology and distinguishing their unique features from other virus families.
Pathogenicity: Pathogenicity refers to the ability of a virus to cause disease in a host. It encompasses various factors that determine how a virus interacts with the host's immune system, the severity of the disease caused, and the virus's capacity to spread within a population. Understanding pathogenicity involves examining virus characteristics, classification, and environmental factors influencing disease development.
Poliovirus: Poliovirus is a highly contagious virus that belongs to the family Picornaviridae, known for causing poliomyelitis, a disease that can lead to paralysis and even death. Its ability to spread rapidly from person to person, primarily through the fecal-oral route, highlights its significance in understanding viral transmission dynamics, classification, genome replication, protein synthesis, and structural characteristics of viruses.
Rabies virus: The rabies virus is a deadly virus that causes rabies, a preventable viral infection that affects the central nervous system of mammals, leading to encephalitis and ultimately death if not treated promptly. Its significance lies in its classification as a zoonotic virus, its unique replication mechanism, and its potential applications in research and biotechnology.
Reoviridae: Reoviridae is a family of double-stranded RNA viruses that are known to infect a wide range of hosts, including animals, plants, and fungi. This family is characterized by its unique icosahedral structure, non-enveloped virions, and segmented genome, which allows for genetic reassortment. These features play a significant role in the classification of animal viruses and their characteristics.
Retroviridae: Retroviridae is a family of enveloped RNA viruses known for their unique ability to reverse transcribe their RNA genome into DNA once inside a host cell. This process allows them to integrate into the host's genome, leading to persistent infections and the potential for oncogenesis. Retroviruses are classified based on their genomic organization, replication strategies, and their impact on host organisms, particularly in relation to their transmission and disease causation.
Rhabdoviridae: Rhabdoviridae is a family of viruses known for their distinctive rod-shaped (rhabdo means 'rod' in Greek) morphology and single-stranded RNA genomes. This family includes notable pathogens, such as the rabies virus, which can affect both humans and animals, emphasizing its importance in virology and public health.
SsDNA: ssDNA, or single-stranded DNA, refers to a form of DNA that consists of a single strand rather than the double helix structure seen in most organisms. This unique genome structure allows ssDNA viruses to replicate and express their genes in ways that differ from double-stranded DNA (dsDNA) viruses. ssDNA is crucial for understanding various viral replication mechanisms and plays an important role in classifying certain virus families.
SsRNA-RT: ssRNA-RT refers to single-stranded RNA viruses that replicate through a reverse transcription process. These viruses use their RNA genome to first synthesize complementary DNA (cDNA) before integrating it into the host genome or producing more RNA genomes for new virions. This unique replication strategy sets them apart from other virus families and plays a critical role in their classification and characteristics.
Vector-borne transmission: Vector-borne transmission refers to the spread of viruses and other pathogens through living organisms, typically arthropods like mosquitoes and ticks, that carry the virus from one host to another. This form of transmission is crucial in understanding how certain animal and zoonotic viruses spread and impact human health.
Viral glycoproteins: Viral glycoproteins are protein molecules that have carbohydrate chains attached to them and are located on the surface of viruses. They play critical roles in the virus's life cycle, including facilitating entry into host cells by mediating attachment to cell receptors and contributing to immune evasion by altering their structure to escape recognition by the host's immune system.
Viral tropism: Viral tropism refers to the preference of a virus to infect specific types of cells or tissues in a host organism. This selectivity is influenced by factors such as the presence of specific receptors on host cells, the viral genome, and the interplay between viral proteins and host cellular mechanisms, ultimately determining the pathogenesis of viral infections.
Zika virus: Zika virus is a mosquito-borne flavivirus that primarily spreads through the bite of infected Aedes mosquitoes. This virus gained significant attention due to its association with severe birth defects, particularly microcephaly, and other neurological complications, highlighting the complex interplay of viral diseases and their impact on public health.
Zoonosis: Zoonosis refers to diseases and infections that are naturally transmitted between animals and humans. These conditions can arise from various sources, including viruses, bacteria, parasites, and fungi, and often lead to public health concerns. Understanding zoonosis is crucial as it highlights the interconnectedness of human, animal, and environmental health, especially in relation to different animal virus families and specific virus types like flaviviruses and togaviruses.