LEC 18 #2 (NOT ON MIDTERM) still in progress

Disease Ecology

Definition of Disease

Disease is identified as a disorder distinguished by a specific group of symptoms tied to a known cause. The causes of diseases can be classified into several categories:

  • Genetic abnormalities

  • Exposure to toxins

  • Interactions with organisms known as pathogens, which include:

    • E. coli bacteria

    • SARS-CoV-2 (virus responsible for COVID-19)

    • Parasites such as Plasmodium, including variants P. vivax, P. falciparum, P. malariae, P. ovale, and P. knowlesi, all of which are known to cause malaria in humans.

Focus of the Lecture

The lecture focuses specifically on infectious diseases, which are caused by pathogens like viruses, bacteria, fungi, etc. In contrast, non-infectious diseases such as asthma and cardiovascular diseases are specifically excluded from this discussion.

Significance of Disease in Ecology

The discussion of disease within the context of ecology is crucial due to the ecological interactions that occur between organisms. The impacts of diseases can be observed on two fronts:

  • Individual fitness: How diseases affect the survival and reproductive success of individual organisms.

  • Population dynamics: The effect of diseases on the population sizes and structures within ecosystems.

Disease Transmission

Types of Transmission

Diseases can be transferred in various ways, classified mainly into two categories:

  1. Direct transmission: Involves the physical transfer of an infectious agent directly between individuals. Examples include:

    • Touching an infected individual

    • Droplet spread from actions like coughing or sneezing, specifically by droplets greater than 5μm

  2. Indirect transmission: Involves indirect means of transferring the infectious agent, which can include:

    • Airborne aerosols smaller than 5μm

    • Fomites, which are inanimate objects that have been contaminated

    • Animal vectors, such as ticks, fleas, and mosquitoes.

Vertical vs. Horizontal Transmission

  • Vertical transmission occurs between individuals of the same generation, such as from a mother to her child before or during birth.

  • Horizontal transmission refers to the spread of disease between individuals across different generations or populations.

  • Examples of diseases transmitted both vertically and horizontally include:

    • Zika virus

    • Herpes simplex virus

    • Chicken pox

    • HIV

Disease Classification by Spread

Diseases can also be classified according to their geographical spread:

  • Endemic: Continuous presence of a disease within a certain area but with relatively low spread, e.g., malaria.

  • Epidemic: A sudden increase in disease cases in a specific region, such as Lyme disease or Ebola virus.

  • Pandemic: A global epidemic that affects a large number of people; for example, swine flu in 2009 and COVID-19.

Disease Fitness and Basic Reproduction Number ($R_0$)

The basic reproduction number, denoted as $R0$, is a crucial metric in disease ecology. It is defined as: R0 = rac{eta N}{
u + d + r}
Where:

  • $R_0$: Basic reproduction number

  • $eta$: Transmission rate (i.e., the probability of infection)

  • $N$: Number of hosts available for infection

  • $
    u$: Virulence, which indicates the disease-induced death rate of the host

  • $d$: Mortality rate (natural death rate of host)

  • $r$: Recovery rate of the host

Interpretation of $R_0$

  • $R_0$ serves as a dimensionless metric indicating the average number of susceptible individuals that will contract the disease in a naive population absent of transmission interventions.

  • It is variable and not a constant; influenced by environmental factors and behaviors. After a disease begins spreading, $R0$ is replaced by the effective reproduction number, $Re$.

  • Examples of $R_0$ values include:.

    • COVID-19: $R_0$ approximately 3-4

    • Seasonal flu: $R_0$ approximately 1.3

Implications of $R_0$

  • Understanding $R_0$ aids in determining:

    • The transmissibility of a disease

    • The challenges associated with containment efforts

  • Importantly, $R_0$ does not directly inform about the virulence (the potential of the pathogen to harm the host). For instance, one individual infected with COVID-19 may infect 3-4 others under unrestricted conditions.

Compartment Models in Disease Dynamics

Compartment models are tools used by epidemiologists to comprehend how to impede the spread of diseases. The compartments are divided into subpopulations including:

  • Susceptible (uninfected)

  • Infected

  • Recovered/Immune

  • Death

The model parameters include:

  • $b$ = non-disease-related birth rate

  • $
    u$ = change in $b$ due to disease

  • $d$ = non-disease-related death rate

  • $eta$ = transmission rate

  • $
    u$ = immunity acquisition

  • $ heta$ = immunity decay

Dynamics within Real Compartment Models

Real-world compartment models incorporate diseases such as SARS and the coronavirus from 2002 to 2004. The formulaic structure for the model depicts:

  • Infected (I)

  • Susceptible (S)

  • Recovered (R)

  • Quarantined (Q)

  • Isolated (J)

  • Parameters addressing transitions between states, including:

    • $ ext{Change parameters like } eta ext{, }
      u ext{, and others for transmission and recovery.}$

Isolation and Quarantine

  • Isolation: A strategy that involves separating sick individuals from those who are healthy to mitigate the spread

  • Quarantine: Involves separating individuals who have been exposed to the illness, regardless of whether they are currently displaying symptoms.

Observations in Disease Ecology

Research has observed phenomena such as:

  • Ant behaviors, specifically, removing infected ants from their colonies to manage disease spread.

  • Fungal influences preventing certain insect populations from burgeoning excessively, termed myco-mortality. This term relates to population density affecting disease dynamics.

Strategies to Slow Disease Spread

To slow the transmission of diseases, various practices are recommended, including:

  • Coughing or sneezing into a tissue and disposing of it properly

  • Avoiding contact with the eyes, nose, or mouth using unwashed hands

  • Engaging in behavioral changes, like practicing good hygiene and vaccination (herd immunity).

Herd Immunity

Herd immunity arises when the immunity rates within a population are sufficiently high to ensure that the effective reproduction number ($Re$) of the pathogen becomes negative, potentially leading to extinction of the pathogen. The herd immunity threshold (H) is defined as the minimum percentage of a population that must be immunized to reach herd immunity. The correlation between $R0$ and $H$ is positive; as $R_0$ increases, the required threshold $H$ also rises.

Challenges with High Transmission Pathogens

Diseases characterized by high transmissibility pose particular challenges for eradication due to:

  • Greater difficulty in vaccine development.

  • The existence of small populations of the pathogen that are hard to identify and eliminate.

  • Accelerated population growth rates of the pathogen, complicating herd immunity achievements.

Disease and Climate

The relationship between climate change and disease dynamics illustrates how some diseases with animal vectors are influenced by warmer climates. Warmer temperatures can lead to increased vector populations and expanded ranges, heightening the risk of disease transmission.

Projected Changes in Tick Habitat

Future projections suggest changes in tick habitat over time leading into future decades (e.g., 2020, 2050, and 2080). Understanding these changes is crucial for assessing disease risk and management strategies in relation to vector-borne diseases.