Animal viruses are infectious agents that replicate only inside animal cells, including those of humans. They contain either DNA or RNA as their genetic material and depend entirely on the host cell’s metabolic machinery for replication. For understanding their diversity, behavior, and medical significance, viruses are classified systematically and cultivated using living systems.
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2. Classification of Animal Viruses
Because viruses differ widely in structure and genome type, classification is based on several criteria rather than traditional biological taxonomy.
A. Basis of Classification
The main criteria used for classification of animal viruses include:
| Criterion | Example/Description |
| Type of nucleic acid | DNA or RNA |
| Nature of nucleic acid | Single-stranded (ss) or double-stranded (ds) |
| Sense (polarity) | + sense or − sense (for ssRNA viruses) |
| Number of strands | Segmented or non-segmented genome |
| Capsid symmetry | Icosahedral, Helical, or Complex |
| Presence/absence of envelope | Enveloped or Naked virus |
| Host range | Mammalian, avian, reptilian, etc. |
| Site of replication in host cell | Nucleus or cytoplasm |
| Mode of transmission | Airborne, vector-borne, sexual, etc. |
3. Hierarchical Classification System
The International Committee on Taxonomy of Viruses (ICTV) provides the official framework for viral classification.
It recognizes the following taxonomic levels:
Realm → Kingdom → Phylum → Class → Order → Family → Genus → Species
Example:
Order: Mononegavirales
Family: Rhabdoviridae
Genus: Lyssavirus
Species: Rabies virus
4. Classification Based on Type of Genome
Viruses are broadly divided into DNA and RNA viruses, and further into enveloped and non-enveloped types.
A. DNA Animal Viruses
| Type | Family | Example | Disease Caused |
| Double-stranded DNA (dsDNA) | Adenoviridae | Adenovirus | Respiratory infections |
| Herpesviridae | Herpes simplex virus, Varicella-zoster virus | Cold sores, Chickenpox | |
| Poxviridae | Variola virus, Vaccinia virus | Smallpox, Cowpox | |
| Papillomaviridae | Human papillomavirus (HPV) | Warts, Cervical cancer | |
| Hepadnaviridae | Hepatitis B virus | Hepatitis B | |
| Single-stranded DNA (ssDNA) | Parvoviridae | Parvovirus B19 | Erythema infectiosum (fifth disease) |
B. RNA Animal Viruses
| Type | Family | Example | Disease Caused |
| Double-stranded RNA (dsRNA) | Reoviridae | Rotavirus | Infantile diarrhea |
| Single-stranded RNA (+ sense) | Picornaviridae | Poliovirus, Rhinovirus | Polio, Common cold |
| Togaviridae | Rubella virus | German measles | |
| Flaviviridae | Dengue virus, Zika virus, Yellow fever virus | Dengue, Zika, Yellow fever | |
| Coronaviridae | SARS-CoV, MERS-CoV, SARS-CoV-2 | COVID-19 | |
| Single-stranded RNA (− sense) | Orthomyxoviridae | Influenza virus | Influenza (Flu) |
| Paramyxoviridae | Measles, Mumps virus | Measles, Mumps | |
| Rhabdoviridae | Rabies virus | Rabies | |
| Filoviridae | Ebola virus | Hemorrhagic fever | |
| Single-stranded RNA (diploid) | Retroviridae | HIV (Human Immunodeficiency Virus) | AIDS |
C. Other Special Groups
| Type | Example | Feature |
| Arboviruses | Dengue, Zika, Chikungunya | Arthropod-borne |
| Oncogenic viruses | Epstein–Barr virus, HPV | Cause cancer |
| Enteric viruses | Poliovirus, Rotavirus | Transmitted via fecal–oral route |
5. The Baltimore Classification System
Proposed by David Baltimore (1971), this system classifies viruses based on the relationship between their genome type and mRNA production.
| Group | Genome Type | Example |
| I | dsDNA | Adenovirus, Herpesvirus |
| II | ssDNA | Parvovirus |
| III | dsRNA | Reovirus |
| IV | (+) ssRNA | Poliovirus, Dengue virus |
| V | (−) ssRNA | Influenza, Rabies virus |
| VI | ssRNA (RT; retrovirus) | HIV |
| VII | dsDNA (RT) | Hepatitis B virus |
6. Cultivation of Animal Viruses
Since viruses require living host cells for replication, they cannot be grown on artificial media (like bacteria).
They are cultivated using living systems that provide host cells and metabolic support.
A. Purpose of Cultivation
- Isolation and identification of viruses from clinical samples
- Study of viral replication, morphology, and pathogenesis
- Preparation of vaccines
- Research on antiviral agents
- Serological and diagnostic testing
B. Methods of Cultivation
1. Animal Inoculation
- One of the earliest and classical methods.
- Involves inoculating virus samples into susceptible live animals (e.g., mice, rabbits, guinea pigs, monkeys).
- Observations include symptoms, lesions, or death of the animal.
Advantages:
- Demonstrates infectivity and pathogenicity.
- Used for rabies virus and encephalitis viruses.
Disadvantages:
- Expensive, ethical concerns, and variable results.
2. Embryonated (Fertilized) Egg Inoculation
- Introduced by Goodpasture (1931).
- Fertilized chicken eggs (7–12 days old) are used.
- The virus is injected into specific parts of the egg.
| Site of Inoculation | Example of Virus |
| Chorioallantoic membrane (CAM) | Poxviruses, Herpesvirus |
| Allantoic cavity | Influenza, Newcastle disease virus |
| Amniotic cavity | Influenza virus |
| Yolk sac | Rickettsiae, some viruses (e.g., Herpesvirus) |
Advantages:
- Economical, sterile environment, easy to maintain.
- Used extensively for vaccine production (e.g., Influenza vaccine).
3. Cell Culture Technique
The most widely used and reliable method for virus cultivation today.
It involves growing animal or human cells in controlled laboratory conditions.
Types of Cell Cultures:
| Type | Source | Lifespan | Example/Use |
| Primary cell culture | Normal tissues (kidney, fibroblast) | Short (few divisions) | Used for rabies virus |
| Diploid cell strain | Human embryonic tissues | Moderate (≈50 generations) | Used for measles, polio |
| Continuous cell line | Cancer cells (HeLa, Vero) | Immortal | Used for large-scale viral production, diagnostics |
Detection of Virus Growth in Cell Culture
Viral infection produces characteristic Cytopathic Effects (CPEs):
| Type of CPE | Description | Example |
| Cell rounding/detachment | Cells shrink, lose adherence | Poliovirus |
| Syncytium formation | Fusion of cells → multinucleated giant cells | Measles, RSV |
| Inclusion bodies | Accumulation of viral material in cytoplasm/nucleus | Negri bodies (Rabies), CMV inclusions |
| Hemadsorption | RBCs adhere to infected cells | Influenza virus |
| Cell lysis | Destruction of host cells | Enteroviruses |
C. Other Advanced Methods
- Organ Culture: Small pieces of animal tissue (e.g., tracheal ring) maintained in vitro.
Used for respiratory viruses. - Molecular Cloning Systems: Recombinant DNA and plasmid-based replication of viral genes.
- Transgenic Animal Models: Mice genetically engineered to express viral receptors (for HIV, COVID-19 studies).
7. Preservation of Animal Viruses
For research and vaccine production, viruses are preserved by:
- Lyophilization (freeze-drying)
- Deep freezing (−70°C to −196°C)
- Cryopreservation in liquid nitrogen
Table
| Feature | DNA Viruses | RNA Viruses |
| Genome | DNA (ss or ds) | RNA (ss or ds) |
| Site of replication | Nucleus (usually) | Cytoplasm (usually) |
| Examples | Herpesvirus, Poxvirus | Influenza, Rabies, HIV |
| Cultivation | Embryonated eggs, cell culture | Same methods |
| Diagnostic marker | Intranuclear inclusion bodies | Cytoplasmic inclusion bodies |
