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Zoonotic and Vector-Borne Diseases

Learning Objectives

  • Understand the link between environmental change and infectious disease.

  • Describe how diseases are transmitted from animal reservoirs to humans.

  • Define vector-borne and zoonotic diseases with examples.

  • Compare three human diseases transmitted by arthropod vectors.

  • Discuss control methods for vector-borne and zoonotic diseases.

Introduction

Zoonotic and vector-borne diseases (ZVDs) cause significant global illness and death, particularly in vulnerable populations, due to emerging and reemerging infections like malaria and dengue.

Key Terminology

  • Zoonosis: An infection transmissible under natural conditions from vertebrate animals to humans.

  • Vector: A living carrier (e.g., insect) that transports an infectious agent.

  • Vector-Borne Infection: An infectious disease transmitted to humans via the bite of blood-feeding arthropods, where the pathogen multiplies or develops within the vector.

Examples of Zoonotic Diseases

  • Vector-Borne: Malaria, dengue fever, leishmaniasis, Lyme disease.

  • Direct/Indirect Zoonoses (non-vector-borne): Anthrax (bacterial), rabies (viral), tularemia (bacterial).

Malaria

  • Causative agent: Plasmodium parasites (P. falciparum most severe), transmitted by infected female Anopheles mosquitoes.

  • Impact: Major global public health issue, primarily in tropical regions, especially Sub-Saharan Africa.

  • Symptoms: Cyclical fever, chills, flu-like symptoms; severe cases lead to anemia, neurological complications, acute kidney failure, and death.

  • Control: Insecticide-treated bed nets (ITNs), indoor residual spraying (IRS), environmental management, prompt diagnosis and artemisinin-based combination therapy (ACTs), preventive medications.

Leishmaniasis

  • Transmission: Via infected female phlebotomine sand flies.

  • Forms: Endemic in tropics/subtropics, concerns due to climate change:

    • Cutaneous Leishmaniasis (CL): Skin sores/ulcers, often leaving scars.

    • Mucocutaneous Leishmaniasis (MCL): Destructive ulcers of nose, mouth, throat.

    • Visceral Leishmaniasis (VL) (Kala-azar): Most severe, affects internal organs (spleen, liver, bone marrow), can be fatal without treatment.

Plague

  • Causative agent: Bacterium Yersinia pestis, transmitted to humans primarily via bites of infected fleas from rodents.

  • Prognosis: Historically high fatality rates if untreated; prompt antibiotic treatment significantly reduces mortality.

  • Forms:

    • Bubonic Plague: Most common, from flea bite, characterized by swollen, painful lymph nodes (buboes), fever.

    • Pneumonic Plague: Lungs infected, highly contagious person-to-person via airborne droplets, rapidly fatal if untreated.

    • Septicemic Plague: Bacteria multiply in blood, leading to organ failure.

Viral Hemorrhagic Fevers (VHFs)

  • Description: Diverse group of severe, life-threatening illnesses caused by various RNA viruses (e.g., Arenaviridae, Filoviridae). Often zoonotic, with rodents or arthropods as reservoirs/vectors.

  • Symptoms: Fever, fatigue, muscle aches, bleeding abnormalities, organ damage, shock.

  • Examples:

    • Ebola Virus Disease (EVD): Transmitted by direct contact with body fluids, characterized by high fever, severe headache, hemorrhage, high fatality.

    • Hantavirus Pulmonary Syndrome (HPS): Transmitted via inhalation of rodent aerosols, affects lungs, acute respiratory distress.

    • Crimean-Congo Hemorrhagic Fever (CCHF): Tick-borne Nairovirus, sudden fever, muscle aches, widespread bleeding.

Arboviral Diseases

  • Description: Viral diseases transmitted by arthropod vectors, replicating in both host and vector.

  • Symptoms: Range from mild fevers/rashes to severe neurological impacts (meningitis, encephalitis) or hemorrhagic manifestations.

  • Examples:

    • West Nile Virus (WNV): Transmitted by mosquitoes (Culex species), primarily infecting birds. Most human infections asymptomatic; severe cases (neuroinvasive disease) in older adults.

    • Dengue Fever: Caused by one of four dengue virus serotypes, transmitted by Aedes aegypti and Aedes albopictus mosquitoes. Known as "breakbone fever" due to severe muscle and joint pain. Can progress to severe dengue (plasma leakage, severe bleeding, organ impairment), which can be fatal.

Control Strategies

  • Integrated Vector Management (IVM): WHO-endorsed comprehensive strategy:

    1. Environmental control: Source reduction (eliminating breeding sites), habitat modification, sentinel animal monitoring.

    2. Chemical control: Insecticides (larvicides, adulticides).

    3. Biological control: Natural predators (e.g., fish) or parasites.

    4. Public health education: Awareness of prevention (repellents, protective clothing, screening), early symptom reporting.

    5. Personal protective measures: Repellents, bed nets (insecticide-treated), protective clothing.

    6. Surveillance and early warning systems: Monitoring vector populations, disease incidence, environmental factors.

    7. Vaccination: Available for some diseases (e.g., Yellow Fever, Japanese Encephalitis, Dengue) and under development for others.

    8. Drug treatments: Prompt access to effective treatments (e.g., antimalarials, antibiotics for plague, supportive care for VHFs).

Conclusion

Zoonotic and vector-borne diseases pose complex global public health challenges, exacerbated by driving factors such as climate change, urbanization, human migration, deforestation, and antimicrobial resistance. Continued vigilance, robust surveillance, interdisciplinary "One Health" approaches, and innovative strategies are crucial for effective management.