Host-parasite Population Dynamics

Host-Parasite Population Dynamics

Introduction

  • Pathogens significantly impact human populations.

  • The European conquest of North America led to a 95% mortality rate among the native population due to new diseases.

  • Pathogens remain a major cause of human mortality, including diseases like malaria, tuberculosis, and AIDS.

The Black Death (Bubonic Plague)

  • Also known as the "Great Mortality" and the "Great Pestilence."

  • Caused by the bacteria Yersinia pestis.

  • Key components of Yersinia pestis:

    • PMT1 Plasmid

    • PCD1 Plasmid

    • PPCP1 Plasmid

    • Cell Wall

    • Cell Membrane

    • Slime Envelope

    • Ribsomes

    • Chromosomal DNA

    • Bi-polar Staining

  • The Black Death wiped out 30-50% of Europe's population.

  • Originated in rats and spread to humans via infected fleas.

Historical Pandemics: Death Toll Comparison

  • Black Death (Bubonic Plague) (1347-1351):

    • 56 million deaths

  • COVID-19:

    • 7 million deaths

  • Smallpox (1520):

    • 40-50 million deaths

    • Killed an estimated 90% of Native Americans.

    • In Europe during the 1800s, approximately 400,000 annual deaths.

    • The first vaccine was created to combat smallpox.

  • Spanish Flu (1918-1919):

    • 40-50 million deaths

  • Plague of Justinian (541-542):

    • Death toll is debated, may have hastened the fall of the Roman Empire.

Transmission of Plague

  • Primary vectors:

    • Rodents

    • Fleas

    • Humans

Types of Plague

  • Bubonic:

    • Most common form.

    • Transmitted from fleas/rodents to humans.

  • Septicemic:

    • Secondary to bubonic plague or enters through breaks in the skin.

  • Pneumonic:

    • Most deadly form.

    • Spread human-to-human via air droplets.

Real-World Example

  • An individual in Oregon survived the black death plague after pulling a mouse out of his pet cat's mouth (July 11, 2012).

Modeling Host-Pathogen Population Dynamics

  • Mathematical epidemiology is used to model these dynamics.

Model Components

  1. Host Population Categories:

    • Susceptible individuals (S).

    • Infected individuals (I).

    • Recovered and immune individuals (R).

  2. Tracking Host and Pathogen Genotypes:

    • Host genotype differences in resistance.

    • Pathogen differences in their ability to cause diseases.

  3. Other Factors Influencing Disease:

    • Differences in the likelihood that different ages will become infected.

    • Latent period: time when an individual is infected but cannot spread the disease.

    • Vertical transmission: disease is passed from mother to newborn (e.g., AIDS).

Simple Model for Infected Individuals

  • Change in density of infected individuals over time:

    • \frac{dI}{dt} = \beta SI - mI

    • Where:

      • S = Density of susceptible individuals.

      • I = Density of infected individuals.

      • \beta = Transmission coefficient (how effectively the disease spreads).

      • m = Death and recovery rate.

      • \beta SI = Probability of infected individuals encountering susceptible individuals

  • Disease will establish and spread when \frac{dI}{dt} > 0.

  • To prevent the spread of disease, the density of susceptible individuals must be kept low.

Strategies to Reduce Susceptible Individuals

  • Slaughtering susceptible domestic animals (e.g., during bird flu outbreaks).

  • Mass vaccination programs for humans.

Vaccination Success: Measles in Romania

  • Measles vaccination program demonstrates that lowering the density of susceptible individuals can control disease spread.

Carry Home Message: Parasites' Ecological Roles

  • Parasites play prominent roles in:

    • Food webs.

    • Competitive interactions.

    • Biodiversity patterns.

  • Parasites contribute to structuring ecological communities.

  • Without parasites, nature would be overrun by organisms.

Enslaver Parasites

  • Many parasites "enslave" their hosts.

  • They alter the behavior of their hosts to complete their life cycles.

Neuro-Parasitology

  • An emerging branch of science that studies how parasites tap into the host's brain to manipulate cognitive functions.

  • Parasites hijack the host's ability to make decisions.

  • Examples include parasitic fungi and worms that manipulate their host’s navigational system, often leading to the host's suicide.

Hairworm and Cricket Example

  • Hairworms alter the concentration of three amino acids in the host cricket's brain.

  • Taurine, a neurotransmitter in insects, regulates the sense of thirst.

  • The hairworm may induce the cricket to commit suicide by altering its perception of thirst.

  • Neurotransmitters are endogenous chemicals that allow neurons to communicate, enabling the brain to provide a variety of functions.

Hairworm Life Cycle

  1. Cricket drinks water containing hairworm larva.

  2. Larva enters the cricket’s body and feeds on its tissues, growing into an adult.

  3. The cricket jumps into water and drowns.

  4. The hairworm emerges and mates.

Fungi as Enslaver Parasites

  • Some fungi alter the perching behavior of their fly hosts, facilitating spore dispersal after the fly dies.

  • Examples include Cordyceps fungi infecting ants and other arthropods.

Parasitoid Wasps

  • Larvae of tropical parasitoid wasps attach to a host spider and suck body fluids.

  • The larva induces the spider to build a cocoon web, then kills and eats the spider.

  • The larva spins a cocoon and attaches it to the cocoon web for protection.

  • Example: Hymenoepimecis argyraphaga.

Experiments with Wasp Larvae and Spiders

  • When wasp larvae were removed before cocoon webs were made, spiders made abnormal webs.

  • Some spiders recovered normal web-making ability after several days.

Chemical Manipulation

  • The larva likely injects a fast-acting chemical into the spider to alter behavior.

  • The chemical acts in a dose-dependent manner, interrupting the spider's usual web-building sequence.

Toxoplasma Gondii

  • Rats infected with Toxoplasma gondii (protozoan parasite) do not avoid cats and may even be attracted to them.

  • Toxoplasma makes the smell of cat urine appealing to infected rats.

  • This increases the parasite's transmission to its definitive host—a cat.

  • Female rats are more attracted to infected males, as T. gondii makes infected male rat scent more attractive.

Host Stages

  • Cats are the definitive host (adult stage of parasite).

  • Intermediate hosts include humans, rodents, and birds.

Effects on Humans

  • People with latent toxoplasmosis are more likely to get in traffic accidents.

  • Infected women are more likely to attempt suicide.

  • Infected men find the smell of cat urine more attractive.

  • It may causes Schizophrenia/ Brain cancer/ Bipolar disorder.

Prevalence and Persistence

  • Around one in three people worldwide have antibodies to T. gondii, indicating previous infection.

  • After infection, the parasite forms cysts inside muscle cells and neurons, evading the host’s immune system.

  • These cysts don’t seem to cause immediate trouble, but much remains to be discovered.

From Chemicals to Evolution and Ecosystems

  • Enslaver parasites exert strong selection pressure on host populations; resistance might be expected to evolve.

  • Limited evidence of coevolution, but host enslavement provides evidence of previous evolutionary change.

  • Parasites that use chemicals are well adapted to exploit the body chemistry of their hosts.

Evolutionary Links

  • Evolutionary links between enslaver parasites and their hosts illustrate that ecological interactions affect evolution and vice-versa.

  • Evolutionary changes in favor of either host or parasite lead to concomitant changes in the dynamics of other species.

Jewel Wasp and Cockroach Example

  • The female parasitoid jewel wasp uses its cockroach victim as a nursery for its young.

  • The wasp stings the cockroach twice: first in the thorax to paralyze the front legs, then in the head to inject venom into a specific area of the brain.

  • This impairs the cockroach's ability to initiate movement, creating a "zombie cockroach."

  • The wasp guides the cockroach to her nest, lays an egg on it, and seals them both inside.

  • The wasp larva hatches, feeds on the cockroach for several days, pupates in its abdomen, and emerges as an adult a month later.