Lecture 2-Arthropod borne Disease Epidemiology

Lecture Overview

  • Lecture 2 focuses on Arthropod-borne Disease Epidemiology.

Learning Objectives

  • After this lecture, you will be able to:

    • Define epidemiology.

    • Identify components of the disease triad.

    • Understand One Health and its benefits to science.

    • Describe components of vectorial capacity.

    • Explain various barriers pathogens must cross within a vector.

    • Differentiate between a pathogen and a vector.

Epidemiology Defined

  • Epidemiology: The study of the origin and causes of diseases in a community (CDC).

  • Key factors determining the presence/absence of diseases:

    • Agent: Organism that causes disease.

    • Host: Organism harboring the disease.

    • Environment: External factors facilitating disease transmission.

    • Time: Temporal factors influencing the spread.

Critical Components of Vector-borne Disease

  • Components that define a vector-borne disease:

    • Vector: Organisms that transmit pathogens.

    • Pathogen: The agent responsible for disease.

    • Disease: The manifestation resulting from the pathogen.

    • Vertebrate Host: Animals that harbor the pathogen.

Japanese Encephalitis Virus: Epidemiological Context

  • Environment: Factors influencing vector habitats include vegetation, cultivation, and human activities.

  • Host: Includes both domestic and wild animals, with humans serving as dead-end hosts.

  • Epidemic Areas: Defined geographical regions where outbreaks occur, particularly in varying climates (northern vs. southern Asia).

Key Disease Definitions

  • Disease: An abnormality with recognizable signs, symptoms, and lab findings, caused by various factors (nutrition, climate, infectious agents).

  • Infectious vs. Non-infectious Diseases: Example of the latter includes cardiovascular disease.

  • Epidemic (Epizootic): A surge in disease cases above expected levels.

  • Endemic (Enzootic): Normal levels of disease sustained locally.

Transmission Cycle of Vector-borne Diseases

  • Sequential steps in the lifecycle of vectors:

    1. Adults attach to hosts for feeding and mating.

    2. Nymphs molt into adults after leaving the second host.

    3. Infected adults feed on humans and dogs, transmitting the pathogen (e.g., B. burgdorferi).

    4. Eggs hatch into larvae, which attach to first hosts.

Example of Pathogens in Vector-borne Diseases

  • Well-known agents include:

    • Yersinia pestis: Causes the plague.

    • Plasmodium spp.: Causes malaria.

    • Trypanosoma cruzi: Causes Chagas disease.

    • West Nile virus: Causes West Nile fever.

Understanding Emerging Diseases

  • An emerging disease is defined by:

    • Recent discovery or increased incidence.

    • Geographic range expansion.

    • Introduction to new host species or populations.

Types of Vector-borne Diseases

  • Viral: Includes Flavivirus (WNV) and Orbivirus (Bluetongue).

  • Bacterial: Includes Bartonella species and anthrax.

  • Protozoal: Includes malaria and Babesia microti.

  • Fungal: Phycomycosis.

  • Parasitic: Includes filarial nematodes.

One Health Concept

  • One Health: Collaborative efforts across various disciplines to ensure optimal health for people, animals, and the environment.

    • Integration of veterinary, human medicine, environmental science, architecture, social science, etc.

Role of Medical Entomologists

  • Vital to understand the role of arthropod species in disease transmission.

  • Focus on vector incrimination to determine responsibility in disease spread.

Koch's Postulates for Vector-borne Diseases

  • Four Criteria to establish a causal relationship between pathogen and disease:

    1. Infected vectors must be linked to all disease cases.

    2. Pathogen must be isolated from the vector.

    3. Non-infected vectors must acquire the pathogen from an infected host.

    4. Infected vectors must transmit the pathogen to a naïve host, causing disease.

Types of Transmission

  • Mechanical Transmission: No developmental change in the vector; pathogens transmitted through bites.

  • Biological Transmission: Involves developmental changes in the pathogen within the vector's body:

    • Anterior-station: Pathogens released during feeding.

    • Posterior-station: Pathogens transmitted via feces.

Types of Biological Transmission from Pathogen Perspective

  1. Propagative: Increase in pathogen quantity without life stage change (e.g., Arboviruses).

  2. Cyclodevelopmental: Pathogen changes life stage without multiplication (e.g., Filarial worms).

  3. Cyclopropagative: Pathogen undergoes changes and multiplies (e.g., Malaria).

Vectorial Capacity

  • Definition: Measure of the number of infective bites delivered from all vectors feeding on a single host in one day.

  • Formula: Model includes factors of feeding, survival, and extrinsic incubation period.

  • Components:

    1. Feeding: How often vectors feed on a host.

    2. Survival Rate: Probability of vector survival.

    3. Extrinsic Incubation Period (EIP): Duration from pathogen ingestion to transmission readiness.

Calculation Formula for Vectorial Capacity

  • VC = [number of vectors feeding per unit time] x [survival probability] x [blood meals post-EIP].

MacDonald’s and Garrett-Jones' Equations for Vectorial Capacity

  • Key factors in calculations include:

    • m: Vector density in relation to host.

    • a: Probability a vector feeds on a host (= host preference index x feeding frequency).

    • b: Vector competence (infectivity of vector).

    • p: Survival probability of the vector.

    • n: Duration of extrinsic incubation period.

Management Strategies for Vector-borne Diseases

  • Effective vector control methods include:

    • Reducing vector populations (m).

    • Decreasing the probability of vector survival (p).

    • Inducing refractoriness to reduce infectivity (b).

  • Considerations about counterintuitive effects of control measures:

    • Managing vector-borne diseases requires understanding nuances in environmental context.

Factors Influencing Vectorial Capacity

  • Extrinsic Factors: Climatic conditions (temperature, rainfall, land use).

  • Intrinsic Factors: Vector genetics, immunity, and interactions that influence transmission dynamics.

Vector Competence

  • Vector Competence: Ability of a vector to acquire and transmit a pathogen.

    • Infection: Capacity of virus to infect midgut cells.

    • Dissemination: Presence and replication ability throughout tissues.

    • Transmission: Ability to transfer virus to host saliva during feeding.

Career Opportunities in Entomology

  • Entomologists are essential members of One Health teams influencing various fields:

    • Public health agencies.

    • Veterinary roles.

    • Military and industry positions.

    • Wildlife management.