Parasitism Notes
Symbiosis and Parasitism
Symbionts are organisms that live in or on other organisms; more than half of Earth’s species are symbionts.
Symbiosis is the living together of unlike organisms, including both negative and positive associations.
Humans are habitats for many other species, some of which are mutualists or parasites.
The Human Body as a Habitat
Different parts of our bodies provide suitable habitat for a wide range of symbionts.
Many of these symbionts are parasites.
Mutualist symbionts, such as gut bacteria, are crucial for digestion and outcompete harmful bacteria; a lack of these can cause diseases.
Humans host nearly 300 species of parasitic worms and over 70 species of protozoa.
Parasites Defined
While some symbionts are mutualists, the majority are parasites.
A parasite consumes the tissues or body fluids of the organism (host) on which it lives.
Pathogens are parasites that cause diseases.
Predators vs. Parasites
Parasites typically harm but do not immediately kill their hosts.
The effects of parasites range from mild to lethal. Examples include:
Fungi causing skin infections.
Bacteria causing diseases like the plague.
Parasite Natural History
Macroparasites: Large species like arthropods and worms.
Microparasites: Microscopic organisms like bacteria.
Parasitoids
Parasitoids are unusual parasites; insect larvae that feed on a single host, eventually killing it.
They eat all or most of the host.
Parasite Specificity and Hyperparasites
Most species are attacked by multiple kinds of parasites.
Even parasites have parasites, known as hyperparasites.
Many parasites are closely adapted to particular host species.
This specialization contributes to the high number of parasite species.
Ectoparasites
Animal ectoparasites live on the surface of their hosts. Examples include:
Athlete’s foot fungus
Fleas, mites, lice, and ticks
Some ectoparasites transmit diseases.
Deer ticks carry the bacteria that causes Lyme disease.
Fleas transmit the plague.
Endoparasites
Many pathogens are endoparasites, living within the host.
Most endoparasites do not eat host tissue but rob the host of nutrients.
Tapeworms attach to the host’s intestinal wall and absorb digested food.
Some live in host’s tissues or cells:
Yersinia pestis, the bacterium that causes the plague.
Mycobacterium tuberculosis, the bacterium that causes tuberculosis.
Plant Parasites
Plants also have endoparasites.
Bacterial pathogens cause soft rot.
Fungi can rot various plant parts from the inside out.
Some bacteria invade vascular tissues, disrupting water and nutrient flow, causing wilting and death.
Ectoparasitism vs. Endoparasitism
Advantages and disadvantages of living in or on a host:
Ectoparasitism:
Advantages: Ease of dispersal and feeding, safe from the host's immune system.
Disadvantages: Vulnerability to natural enemies, exposure to the external environment.
Endoparasitism:
Advantages: Protected from the external environment, safer from natural enemies.
Disadvantages: Vulnerability to the host's immune system, feeding more difficult, dispersal more difficult.
Host Defenses
Hosts actively respond and fight back against parasites.
Evolutionary Arms Race
Host organisms have various defense mechanisms.
Protective outer coverings such as skin and exoskeletons.
The host’s immune system, biochemical defenses, or defensive symbionts kill many parasites that gain entry.
Vertebrate Immune Systems
Vertebrate immune systems have memory cells that recognize microparasites from previous exposures.
Immune system cells engulf and destroy parasites or mark them for later destruction.
T cells are a type of white blood cell involved in these processes.
Plant Defense Systems
Plants have nonspecific immune responses such as antimicrobial and antifungal compounds.
Chemical signals warn nearby cells of an imminent attack.
Lignin deposition creates a physical barrier to an invader’s spread.
Lignin provides rigidity, water resistance, and degradation resistance.
Nonspecific Plant Defenses
Cells damaged by microparasite infection release molecules that stimulate the production of antimicrobial compounds called phytoalexins.
Microparasite infection triggers the deposition of lignin, which provides a physical barrier.
Chemical signals "warn" nearby cells of attack.
Plant Chemical Defenses
Plants use chemical weapons called secondary compounds (~ Toxins).
Phenolics: flavonoids, tannins, lignin, salicylic acid.
Terpenoids: aromatic oils, resins, waxes, steroids, rubber, carotenoids.
Alkaloids: often toxic, e.g., strychnine, nicotine, caffeine, cocaine, capsaicin.
Some animals eat specific plants to treat or prevent parasite infections.
Woolly bear caterpillars switch to poison hemlock when parasitic flies lay eggs on them.
Animal Medicinal Behavior
Chimpanzees infected with nematodes eat plants containing chemicals that kill or paralyze the nematodes.
Examples include eating soil for anti-malarial effects, Cordia flowers for TB, and unripe figs for deworming.
Chimpanzees intuitively know the medicinal value of various plants.
Human Defenses Against Malaria
Focusing on the human-malarial parasite interaction.
Malaria Parasite Life Cycle
Plasmodium, the protozoan that causes malaria, has a complex life cycle involving cyclical infection of humans and female Anopheles mosquitoes.
Only females feed on blood to obtain protein and iron for egg production.
Challenges Faced by Plasmodium
Plasmodium parasite faces two challenges in the human host:
Merozoites multiply in red blood cells (RBCs), which cannot divide, grow, or import nutrients.
Infection causes red blood cells to have an abnormal shape, leading to their destruction in the spleen.
Plasmodium Counter-Defenses
Plasmodium has hundreds of genes that modify red blood cells:
Some genes cause transport proteins to be placed on RBC surfaces to transport nutrients.
Others direct the production of knobs that stick to other cells, preventing them from reaching the spleen; varied proteins on the knobs make it difficult for the immune system to detect.
Parasite-Host Coevolution
Coevolution occurs when populations of two interacting species evolve together, each in response to selection pressure imposed by the other.
Specific adaptations in both parasite and host suggest coevolution.
Rabbit and Myxoma Virus Example
In Australia, 24 European rabbits were introduced in 1859, leading to a population explosion due to lack of natural predators.
The rabbits consumed plant materials, posing a threat to cattle and sheep pasturelands.
Control measures like introducing predators, shooting, poisoning, and building fences were ineffective.
Myxoma Virus Introduction
Myxoma virus was introduced in 1950, causing 99.8% mortality among infected rabbits.
Over time, rabbits evolved resistance to the virus, and the virus evolved to become less lethal.
The lethality of virus samples collected in the wild declined.
Impact of Myxoma
Myxoma has killed hundreds of millions of rabbits, making it the most deadly vertebrate virus known to science.
Recent studies (Kerr et al., 2022) suggest the virus is evolving to spread even more quickly from rabbit to rabbit.
Host-Parasite Population Dynamics
Parasites can reduce survival, growth, or reproduction of their hosts (individual scale).
At the population level, harm by parasites reduces population growth rates.
Parasites can sometimes drive local host populations extinct and reduce their geographic ranges.
Amphipod and Trematode Example
An amphipod (Corophium) in North Atlantic tidal mudflats can be extremely abundant (up to 100,000/).
A trematode parasite can dramatically reduce amphipod populations.
In a 4-month period, attack by trematodes caused the extinction of a Corophium population in the Danish Wadden Sea (Mouritsen et al 1998).
American Chestnut and Chestnut Blight Example
American chestnut (Castanea dentata) was a dominant tree in eastern North America.
A fungal pathogen that causes chestnut blight was introduced from Asia in 1904.
By mid-century, it had wiped out most chestnut populations and greatly reduced the geographic range of this species.
The blight killed between 3 and 4 billion American chestnut trees.
Fungus Impact on Geographic Range
The fungus, Cryphonectria parasitica, is a bark pathogen introduced from Asia in 1904.
Communities
Ecological communities are associations of species that co-occur in the same location and at the same time.
Flour Beetle Experiment
Park (1948) conducted experiments with two flour beetles (Tribolium castaneum and T. confusum) and a protozoan parasite.
Parasite absent: T. castaneum outcompeted T. confusum.
Parasite present: T. confusum “won” because the parasite had no effect on it.
Species Interactions
Changes in Species interactions
Protist parasite
Two flour beetle species
T.Castaneum is a superior competitor
Does well when not infected, but Loses when infected
Park, T. (1948). Ecol.Mono
Predator-Prey Interactions
Parasites can alter the outcome of predator-prey interactions by decreasing the physical condition of infected individuals.
This affects both predators and prey.
Predators may be less able to catch their prey, and prey may be less able to escape predation.
Winter tick is a common parasite of moose, leading to loss of blood and cold stress from hair loss.
Ecosystem Engineers
It Affects the ecosystem engineer species population
Ecosystem engineer species can change the physical characteristics of the environment
Ex: beaver builds a dam
The amphipod Corophium is an ecosystem engineer in tidal mudflats.
Corophium and Mud Islands
The burrows built by Corophium hold the mud together, preventing erosion and forming “mud islands” at low tide.
These mud islands are important habitats for other species (shore birds, etc.).
Impact on species
When trematode parasites drive Corophium to extinction, erosion increases, and the islands disappear.
This environmental change affects other species in the community, impacting the food web (10 large species).
Parasites impact on environment.
the erosion rate increases and the silt content of the mudflats decreases and the island disappears
Climate Change
Climate change is affecting the distribution of diseases.
Increasing water temperatures correlate with increased disease in coral reefs, shellfish, and amphibians.
Mosquitoes and other disease vectors (rodents) are more active and reproduce more in warm conditions.
The risk of malaria, cholera, and the plague may also increase.
Leishmaniasis
Climate Change May Increase the Risk of Leishmaniasis in North America
Caused by protists in the genus Leishmania and spread by sandflies
reservoir species