Parasitism and Disease Ecology

Quick Quiz: Test Your Learning

Question: What is a disadvantage of being an endoparasite compared to being an ectoparasite?

  • a) Feeding is more difficult for endoparasites

  • b) Endoparasites have greater vulnerability to hosts’ immune system

  • c) Endoparasites have greater exposure to predators and the environment

  • d) Host is more likely to have defense mechanisms against endoparasites

Lecture Outline

  1. Simple models of host-pathogen dynamics

  2. Zoonotic diseases

  3. Controlling disease spread

Host-Pathogen Dynamics

Simple Models

  • Models similar to the Lotka-Volterra Predator-Prey model can be adapted for host-pathogen dynamics with two key modifications:

    • Parasites or disease do not always lead to the removal of hosts from the population.

    • Hosts may develop immunity that protects them from reinfection.

  • Susceptible-Infected-Resistant (SIR) Model

    • Simplest model for infection and disease transmission incorporating immunity:

    • S: Number of susceptible individuals

    • I: Number of infected individuals

    • R: Number of resistant individuals (those who have recovered and gained immunity)

    • Initially, 100% of individuals are susceptible:

    • Some become infected (I)

    • Some infected individuals recover and develop immunity (R)

    • This model helps evaluate the rate of pathogen spread and predicting potential epidemics or decline of a parasite/disease.

Disease Dynamics in the SIR Model

  • Graphical Representation:

    Disease Dynamics Graph

    • X-axis: Time

    • Y-axis: Percentage of Population

  • Key Components:

    • Susceptible individuals (S)

    • Infected individuals (I)

    • Recovered individuals (R)

Mathematical Formulation of the SIR Model

  • Definitions:

    • S: Number of susceptible individuals

    • I: Number of infected individuals

    • R: Number of resistant individuals

    • β: Rate of infection (transmission coefficient)

    • m: Rate of recovery or death

Key Equations:

  1. Rate of infection:

    • ext{Infection Rate} = S imes I imes eta

  2. Rate of recovery:

    • extRecoveryRate=Iimesmext{Recovery Rate} = I imes m

  3. Change in infected individuals over time:

    • rac{dI}{dt} = (S imes I imes eta) - (m imes I)

  4. Disease establishes and spreads if rac{dI}{dt} > 0

    • This indicates that I is increasing over time.

Reproductive Ratio (R0)

  • R0: Reproductive ratio of infection

    • Defined as the rate of new infections divided by the rate of recoveries:

    • R_0 = rac{S imes I imes eta}{I imes m} = rac{S imes eta}{m}

  • Implications:

    • If R_0 > 1, the infection continues to spread.

    • If R_0 < 1, the infection fails to establish.

Activity: SIR Model Reproductive Ratio

  • Participants will work through a worksheet titled "The Susceptible-Infected-Resistant Disease Model: Reproductive Ratio".

  • Time allocated: 8-10 minutes for independent or group work (up to 4 persons).

  • Review as a class afterward.

Activity Review Questions

  1. Question 1 - Considering variables S, I, β, and m, how do they impact R0? Check options related to their impact on R0.

  2. Question 2 - Assessment of vaccine program effects on black-footed ferrets.

Pathways of Plague Transmission

Multi-Species Impact Analysis

  • Primary transmission method for prairie dogs in assigned article:

    • a) Pneumonic (via respiratory droplets)

    • b) Consumption of infected tissue

    • c) Vector-borne (via fleas)

    • d) Cannibalism

  • Primary transmission method for black-footed ferrets:

    • Same options as prairie dogs.

Case Study: The Black Death

  • Bubonic Plague: Killed approximately 1/3 of the European population between 1347-1351.

    • Causative agent: Yersinia pestis (bacteria)

    • Transmission pathways include:

    • Human to human (via bodily fluids)

    • Flea to human via bites

    • Rat to human through close contact.

    • Significant hosts of Yersinia pestis include rats and their associated fleas.

Black Death Case Study Activity

  • Participants will work through a worksheet titled "Black Death Case Study" with group options.

Review Questions from the Activity

  1. Assess whether disease is likely to spread from different groups of visitors based on given questions.

  2. Analyze infection rates for various groups (virulent vs. less virulent strains).

Zoonotic Diseases Overview

  • Definition: Zoonotic diseases are those that spill over from animal species to humans.

  • More than 50% of human diseases originate in wild or domesticated animals.

  • Examples of zoonotic diseases:

    • Influenza virus: Birds and pigs

    • West Nile virus: Birds (via mosquitos)

    • HIV: Primates

    • Lyme disease: Deer and mice (via ticks)

    • Dengue fever: Apes (via mosquitos)

    • Ebola: Bats

    • MERS: Camels (from bats)

    • SARS: Civet cats (from bats)

    • SARS-CoV-2: Unknown host (from bats)

Human Impact on Zoonotic Diseases

  • Factors increasing the risk of new zoonotic diseases include:

    • Habitat fragmentation

    • Deforestation

    • Biodiversity loss

    • Intensive agriculture and livestock farming

    • Uncontrolled urbanization

    • Pollution

    • Bushmeat hunting and trading

  • Climate Change Implications:

    • Stresses animal populations, increasing their susceptibility.

    • Contributes to habitat degradation.

    • Expands the range of disease vectors (like mosquitos and ticks).

Controlling Disease Spread

  • Strategies for controlling disease spread include:

    • Keeping the density of susceptible individuals below threshold (ST):

    • Reduce the number of susceptible individuals

      • Culling domestic animals

      • Vaccinating individuals

    • Increasing the threshold density (ST) through:

      • Improvement of recovery rates through early detection and treatment

    • Lowering transmission rate (β):

      • Quarantine sick individuals

      • Use of protective measures (e.g. wearing masks)

Case Studies on Disease Vaccination Impact

  • Measles Example:

    • Correlation between vaccination rates and incidence of measles in Romania.

    • As vaccination rates increased, the number of measles cases dramatically decreased.

      Measles Vaccination Impact

  • Rinderpest (cattle plague) eradication:

    • Declared eradicated worldwide in 2011.

Summary and Questions

  • Topics Covered:

    • Simple models of host-pathogen dynamics

    • Zoonotic diseases

    • Strategies for controlling disease spread

  • Any further questions on these topics?