Viruses of animals and plants

Viruses are acellular, ultramicroscopic infectious entities that occupy a unique position between living and non-living matter. They are obligate intracellular parasites, meaning they can multiply only within living host cells by utilizing the host’s metabolic machinery.

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They infect all forms of life — animals, plants, bacteria (bacteriophages), fungi, algae, and even other viruses (virophages). Despite their small size, viruses cause enormous economic and health impacts, from plant yield losses to human and animal pandemics.

A. Plant Viruses

1. Nature and Structure

Most plant viruses possess single-stranded RNA (ssRNA) as their genetic material, though a few contain DNA.
They are usually non-enveloped and appear in rod-shaped, filamentous, or spherical forms.

• Genome: Single molecule of RNA or DNA (never both).
• Capsid: Protein coat made of repeating subunits (capsomeres).
• No organelles, no ribosomes, hence complete dependence on host cell enzymes for replication.

Example:
Tobacco Mosaic Virus (TMV) — first virus ever discovered — consists of a single RNA strand encapsulated by thousands of identical protein subunits arranged helically.

2. Replication Cycle

1. Entry: Plant viruses enter cells through mechanical wounds, insect feeding, or grafting (they cannot penetrate the rigid cell wall unaided).
2. Uncoating: Capsid disassembles, releasing nucleic acid into the cytoplasm.
3. Replication and Protein Synthesis: Viral genome uses host ribosomes and enzymes to synthesize viral proteins and replicate its nucleic acid.
4. Assembly: New nucleic acid and capsid proteins spontaneously assemble into virions.
5. Movement: Viruses spread cell-to-cell via plasmodesmata and systemically through phloem.

3. Transmission of Plant Viruses

4. Pathogenic Effects on Plants

• Mosaic or mottling of leaves (irregular light and dark patches).
• Leaf curling, chlorosis (yellowing), necrosis (dead spots).
• Stunted growth and reduced yield.
• Malformed fruits or flowers.

Example Diseases:

• Tobacco mosaic (TMV)
• Banana bunchy top (BBTV)
• Tomato leaf curl (TLCV)
• Rice tungro disease

5. Control and Management

Because there are no curative treatments, control relies mainly on prevention and resistance:

• Use of virus-free seeds or propagules.
• Vector control using insecticides or biological methods.
• Rogueing — removal of infected plants.
• Cross-protection (infection with mild strain gives protection).
• Breeding genetically resistant cultivars.
• Meristem culture to produce virus-free plants.

B. Animal Viruses

1. Nature and Structure

Animal viruses are more diverse than plant viruses. They may contain either DNA or RNA, be single- or double-stranded, and may be enveloped or naked.

• DNA viruses replicate mostly in the nucleus (e.g., Herpesvirus).
• RNA viruses replicate in the cytoplasm (e.g., Influenza virus).
• The envelope, derived from host cell membrane, contains viral glycoproteins responsible for attachment to host receptors.

Examples:

• Herpes simplex virus — DNA virus, causes cold sores.
• Influenza virus — RNA virus, causes respiratory infections.
• Rabies virus — RNA virus, affects nervous tissue.
• Coronavirus — RNA virus, causes respiratory disease including COVID-19.

2. Replication Cycle

1. Attachment (Adsorption): Virus binds to specific receptors on host cell membrane.
2. Penetration: Entry via endocytosis or fusion with cell membrane.
3. Uncoating: Viral capsid disassembles, releasing genome into cytoplasm.
4. Replication and Synthesis:
   • DNA viruses → replicate in nucleus.
   • RNA viruses → replicate in cytoplasm (using RNA-dependent RNA polymerase).
5. Assembly: New virions form from synthesized components.
6. Release:
   • Non-enveloped viruses → released by cell lysis.
   • Enveloped viruses → bud through the membrane, acquiring their envelope.

3. Transmission of Animal Viruses

4. Pathogenic Mechanisms

Animal viruses cause disease by:

• Cell lysis: Destruction of host cells (e.g., poliovirus).
• Cell transformation: Inducing cancerous changes (e.g., HPV, EBV).
• Latency: Virus remains dormant and reactivates later (e.g., herpesviruses).
• Immunopathology: Immune system damage caused by viral infection (e.g., HIV).

5. Examples of Major Animal Viral Diseases

6. Control and Prevention of Animal Viral Diseases

• Vaccination: The most effective method (rabies, polio, influenza, FMD).
• Quarantine and culling of infected animals.
• Sanitation and hygiene to reduce spread.
• Vector control (mosquito eradication programs).
• Antiviral therapy: Limited but useful (e.g., acyclovir, AZT).
• Biosecurity measures in farms and labs.

7. Comparative Overview

Economic and Epidemiological Importance

• Plant viruses threaten global food security by reducing yield and quality of crops like tomato, potato, and rice.
• Animal viruses have public health importance due to zoonotic transmission — viruses crossing from animals to humans (e.g., SARS-CoV-2, avian flu, Nipah virus).
• Viral epidemics and pandemics cause major economic burdens on agriculture and healthcare systems.