Disease Ecology Study Notes

Focus on the interactions between organisms and their significance in disease ecology.
Key points include the dynamics of disease spread, immunity, and ecological relationships.

Exploitation: An interaction between populations that enhances the fitness of one individual while reducing the fitness of the exploited individual.
- Herbivory: Consuming live plant material without killing the plants.
- Predation: Killing and consuming other organisms.
- Manipulation: Inducing disease or manipulating host behavior.
- Parasitism: Living on host tissue and reducing host fitness without usually killing the host.
- Competition: Another form of interaction between species.

Definition: A high percentage of vaccination in a population makes it difficult for infectious diseases to spread due to fewer individuals who can be infected.
Significance: Protects vulnerable populations—such as newborns, the elderly, and those with health issues who cannot be vaccinated.
Limitations: Herd immunity does not protect everyone from all vaccine-preventable diseases. It does not provide high individual protection and should not be viewed as an alternative to vaccination.

When a significant portion of the population is vaccinated, the spread of disease is curtailed.
Emphasizes prompt vaccination to prevent the spread of pathogens like COVID-19 and the emergence of variants, as noted by Dr. Ellen Foxman.

Pathogens: Parasites that lead to disease symptoms in humans, animals, and plants, including some viruses, bacteria, protists (eukaryotic), and fungi.
Not all species within these categories are harmful; many are benign.
Pathogens require specific hosts for infection and proliferation.

Basic Reproduction Number (R₀)

Definition: Average number of new cases of disease caused by one infected host.
Significance for disease dynamics:
- R0 = 1: Disease remains stable.
- R0 > 1: Disease frequency increases (more infections).
- R0 < 1: Disease frequency decreases (fewer infections).
Equation: R₀ = NSBL, where each variable determines pathogen spread:
- NS: Number of susceptible hosts in the population.
- B: Transmission rate of the disease.
- L: Average time an infected host can transmit the disease.

Stability of Disease Spread

The minimum population size for stability is given by the equation:
$NT = rac{1}{BL}$
When enough susceptible hosts are infected, their numbers decline, causing the spread of the pathogen to decrease.
Pathogens often decline before infecting all susceptible hosts, leading to a decline in R0 below 1.

Herd Immunity Threshold: The level of immunity needed in the population to ensure pathogens die out.
Individuals become immune either by recovering from infections or through vaccinations.
Vaccines train the immune system to recognize and combat pathogens without causing disease.

New pathogens can arise from existing ones, whereby natural selection favors mutations that enable pathogens to:
1. Become more contagious (example: SARS-CoV-2).
2. Overcome host immunity (example: Influenza).
3. Become less lethal (example: Myxoma).

Flattening the Curve: A public health approach aimed at slowing the spread of disease to manage healthcare resources effectively.
Visualizing the number of infected individuals needing care over time reveals the importance of spreading cases out to avoid overwhelming healthcare systems.

Example: Plagiorhynchus cylindraceus (a parasitic worm) alters the behavior of its host Armadillidium vulgare (pill bug), which in turn benefits the worm's lifecycle by making the host more susceptible to predation by the starlings (Sturnus vulgaris).
Lifecycle of Plagiorhynchus includes:
1. Adult female lays eggs in the intestines of infected birds.
2. Eggs are excreted and consumed by terrestrial isopods.
3. The larvae develop within the host pill bug before altering its behavior for enhanced predation.

Detailed Behavioral Changes

Infected Armadillidium display:
1. Less time in shelters, increasing visibility to predators.
2. Preference for low-humidity environments, increasing exposure risks.

Mutualism: A symbiotic relationship that benefits both species involved.
Types of Mutualism:
- Facultative Mutualism: Mutual benefit is not essential for survival (e.g., corals and symbiotic algae).
- Obligate Mutualism: One species cannot survive without the other (e.g., ants and Acacia plants).
Mutualism plays a crucial role in biodiversity and ecological stability.

Obligate Mutualism Details

Ants (Pseudomyrmex):
- Characteristics include agility, good vision, independent foraging, and aggressive defense roles.
Acacia Plants:
- Features include enlarged thorns, leaf production year-round, foliar nectaries, and leaflet tips modified into food sources.
Reciprocal Benefits:
- Ants gain shelter and sustenance, while Acacias receive protection from herbivores and competition for light and nutrients.
Performance Metrics:
- Growth and survival rates of Acacia shoots with and without presence of mutualistic ants demonstrate significant ecological advantages.

Plants associated with ants have lower levels of herbivorous pest damage, leading to higher overall health and reproductive success.
The interdependence of species reinforces ecological networks and resilience against diseases and exploitation.