Definition

Zoonoses are infectious diseases that are naturally transmitted between vertebrate animals and humans. These diseases can be caused by bacteria, viruses, parasites, or fungi. Transmission can occur through direct contact, indirect contact, vectors, or environmental sources.

Transmission Pathways

Mode of Transmission Example Disease Mechanism
Direct Contact Rabies Bite or scratch from infected animal
Indirect Contact Anthrax Contact with contaminated soil/fomites
Vector-borne Lyme disease Tick bite
Foodborne Salmonellosis Consumption of contaminated food
Airborne Hantavirus Pulmonary Syndrome Inhalation of aerosolized particles

Host Reservoirs

  • Wildlife: Bats (Ebola, SARS-CoV-2), Rodents (Hantavirus)
  • Domestic Animals: Dogs (Rabies), Cattle (Brucellosis)
  • Birds: Poultry (Avian Influenza)
  • Aquatic Animals: Fish (Vibrio infections)

Epidemiology

  • Over 60% of emerging infectious diseases are zoonotic.
  • Zoonoses are responsible for significant global morbidity and mortality.
  • Urbanization, deforestation, and climate change are increasing zoonotic spillover events.

Diagram: Zoonotic Transmission Cycle

Zoonotic Transmission Cycle

Classification

By Pathogen Type

  • Bacterial: Brucellosis, Leptospirosis, Plague
  • Viral: Rabies, Ebola, Influenza, SARS-CoV-2
  • Parasitic: Toxoplasmosis, Trichinellosis
  • Fungal: Ringworm (Dermatophytosis)

By Transmission Mode

  • Direct Zoonoses: Direct transmission (e.g., Rabies)
  • Cyclozoonoses: Require more than one vertebrate host (e.g., Taenia solium)
  • Metazoonoses: Require invertebrate vector (e.g., Malaria)
  • Saprozoonoses: Environmental reservoir involved (e.g., Histoplasmosis)

Mnemonic: “ZOO-VIP” for remembering major zoonotic pathogens

  • Z: Zika Virus
  • O: Orthomyxoviruses (Influenza)
  • O: Ovine bacteria (Brucella)
  • V: Viruses (Rabies, Ebola)
  • I: Insect-borne (Plague, Lyme)
  • P: Parasites (Toxoplasma, Trichinella)

Case Studies

1. COVID-19 (SARS-CoV-2)

  • Origin: Likely from bats, with possible intermediate hosts (pangolins).
  • Transmission: Initially animal-to-human, now sustained human-to-human.
  • Impact: Global pandemic, highlighting the importance of zoonotic surveillance.
  • Reference: Zhou et al., Nature, 2020.

2. Hendra Virus (Australia)

  • Reservoir: Fruit bats (Pteropus spp.)
  • Transmission: From bats to horses, then horses to humans.
  • Symptoms: Severe respiratory and neurological disease.
  • Control: Vaccination of horses, surveillance of bat populations.

3. Nipah Virus (Bangladesh, Malaysia)

  • Reservoir: Fruit bats
  • Transmission: Contaminated date palm sap, direct contact with infected pigs.
  • Symptoms: Encephalitis, high mortality rate.
  • Prevention: Covering sap collection pots, limiting animal contact.

Surprising Facts

  1. Over 75% of new human pathogens discovered in the last three decades originated from animals.
  2. Zoonoses can be transmitted via unconventional routes, such as organ transplantation (e.g., rabies from corneal transplants).
  3. Climate change is facilitating the northward migration of vectors, introducing zoonoses to previously unaffected regions.

One Health Approach

  • Integrates human, animal, and environmental health disciplines.
  • Essential for surveillance, prevention, and control of zoonoses.
  • Encourages cross-sector collaboration (veterinarians, physicians, ecologists).

Future Trends

  • Genomic Surveillance: Real-time monitoring of zoonotic pathogens using sequencing technologies.
  • Predictive Modeling: AI and machine learning to forecast spillover events.
  • Vaccine Development: Universal vaccines targeting multiple zoonotic viruses.
  • Climate Adaptation: Strategies to mitigate vector expansion due to global warming.
  • Urban Wildlife Management: Policies to reduce human-animal interface in cities.

Recent Study: Carlson et al., Nature, 2022: “Climate change is increasing the risk of viral spillover from animals to humans, with projections showing up to 15,000 new cross-species transmission events by 2070.”

Prevention and Control

  • Surveillance: Early detection in animals and humans.
  • Vaccination: For animals (e.g., rabies) and humans (e.g., influenza).
  • Education: Public awareness about safe animal handling.
  • Regulation: Control of wildlife trade and wet markets.
  • Personal Protection: Use of PPE for at-risk workers.

Diagram: One Health Model

One Health Model

References

  • Zhou, P. et al. (2020). “A pneumonia outbreak associated with a new coronavirus of probable bat origin.” Nature, 579:270–273.
  • Carlson, C.J. et al. (2022). “Climate change increases cross-species viral transmission risk.” Nature, 607:555–562.
  • World Health Organization. “Zoonoses.” WHO Fact Sheet

Review Questions

  1. What are the major transmission pathways of zoonoses?
  2. Explain the One Health approach and its relevance to zoonotic disease control.
  3. Discuss the impact of climate change on zoonotic disease emergence.

Did You Know?

The largest living structure on Earth is the Great Barrier Reef, visible from space.