Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 19.3 Viruses and Prions Are Formidable Pathogens in Animals and Plants
Viruses and prions cause numerous diseases in animals and plants, significantly affecting health, agriculture, and ecosystems. Understanding these infectious agents helps us manage their impacts and develop preventive measures.
Viral Diseases in Animals
- Viruses damage cells by releasing enzymes from lysosomes, causing infected cells to produce toxins, or triggering harmful immune responses.
- Disease severity depends on the type of infected cells and their regenerative ability:
- Respiratory tract epithelium recovers quickly (e.g., common cold).
- Nerve cells regenerate poorly (e.g., poliovirus damage is often permanent).
- Vaccines stimulate the immune system, preventing infections:
- Effective vaccines eradicated smallpox and significantly reduced polio and measles.
- Antibiotics don’t treat viruses; however, antiviral drugs like acyclovir (herpes) and AZT (HIV) target virus-specific enzymes.
Emerging Viral Diseases
- Emerging viruses appear suddenly, causing epidemics:
- Examples: HIV (1980s), West Nile virus (1999), Ebola virus (1976, 2014–2019), Zika virus (2015), chikungunya virus.
- Viruses emerge through:
- Mutation of existing viruses (especially RNA viruses).
- Spread from isolated populations due to globalization.
- Animal-to-human (zoonotic) transmission.
- Influenza pandemics occur when viral strains evolve to spread efficiently in humans, often through mutation or genome reassortment (e.g., H1N1, H5N1).
Viral Diseases in Plants
- Plant viruses cause major economic losses (over $30 billion/year).
- Symptoms: discolored leaves, stunted growth, damaged fruits/roots.
- Plant viruses spread by:
- Horizontal transmission: through wounds or insect vectors.
- Vertical transmission: from infected parent plants via seeds or vegetative propagation.
- Once inside, viruses spread from cell to cell through plasmodesmata.
Prions: Infectious Proteins
- Prions cause degenerative brain diseases:
- Examples: scrapie (sheep), mad cow disease (cattle), Creutzfeldt-Jakob disease and kuru (humans).
- Prions are misfolded proteins that convert normal proteins into abnormal forms, forming harmful aggregates.
- They act slowly, resist heat, and currently have no cure.
Viral Origins and Evolution
- Viruses may have originated from mobile genetic elements like plasmids or transposons.
- They share genes with hosts, showing evolutionary relationships.
- Giant viruses (mimivirus, pandoravirus) challenge definitions of life and virus classification.
In a Nutshell
Viruses and prions are major pathogens in animals and plants, with wide-ranging impacts on health and agriculture. Emerging viruses often arise via mutation or cross-species transmission. Prions are particularly concerning due to their resilience and long-term neurological effects.