Parasitism

• Parasitism is a non-mutual symbiotic relationship.
• In parasitism, one species (the parasite) benefits at the expense of another species (the host).
• Parasite is derived from the Greek word "parasitos" meaning "one who eats at the table of another"

Macroparasites

• Macroparasites are organisms visible to the naked eye, for example helminths.

Microparasites

• Microparasites are smaller organisms like protozoa, viruses, and bacteria.

Examples of Parasites

• Plants such as Mistletoe and Cuscuta.
• Animals such as Hookworms.
• Parasites generally do not kill their host.
• Parasites are generally much smaller than their host.
• Parasites often live in or on their host for an extended period.
• Parasites reproduce at a faster rate than their host.
• Parasites increase their own fitness by exploiting hosts for resources like food, water, heat, habitat, and transmission.

Obligate Parasite

• Total dependence on a host to complete its life cycle.

Facultative Parasite

• The parasite does not depend on a host to complete its life cycle.

Direct Parasite

• Only uses one host.

Indirect Parasite

• Uses multiple hosts
• It always has has definitive host and an intermediate host.

Ectoparasites

• Live on the outside of the host, for example: lice, fleas, and some mites.

Endoparasites

• Live inside the host, for example: parasitic worms.
• Endoparasites can exist in two forms:
  • Intercellular parasites: Inhabit spaces on the host's body.
  • Intracellular parasites: Inhabit cells in the host's body.

Mesoparasites

• Live in an intermediate position.
• Some parasites rely on a third organism (carrier or vector) to transmit them to the host.
• Ex: Transmission of malaria, caused by a protozoan of the genus Plasmodium, to humans by the bite of an anopheline mosquito

Epiparasite

• Feeds on another parasite, also called hyperparasitism.
• Ex: A protozoan (the hyperparasite) living in the digestive tract of a flea living on a dog.

Social Parasites

• Take advantage of interactions between members of social organisms (ants, termites, and bumblebees).
  • Phengaris arion: Employs mimicry to parasitize certain species of ants.
  • Bombus bohemicus: Invades the hives of other species of bee and takes over reproduction, with their young raised by host workers.
  • Melipona scutellaris: Eusocial bee where the virgin queens escape killer workers and invade another colony without a queen.

Kleptoparasitism

• Derived from the Greek word "kleptes" meaning "thief".
• Steals food or resources gathered by the host.
• Ex: Brood Parasitism practiced by cowbirds, whydahs, cuckoos, and black-headed ducks, do not build nests and leave their eggs in nests of other species.

Host Defenses

Animal Defenses: Skin

• Made up of layers of dead cells that act as a physical barrier.
• Contains keratin, which makes the skin tough and waterproof, which is needed to survive.
• Secretes sebum, which is toxic to most microorganisms.

Mouth

• Contains saliva that prevents organisms from getting into the body orally.
• Contains lysozyme found in tears and saliva, and breaks down the cell walls of invading microorganisms.

Stomach

• Contains hydrochloric acid and gastric acid (pH 2), which kills most microorganisms.

Eyes

• Lashes prevent invading microorganisms from entering the eye.
• Lashes kill most invading microorganisms.

Immune System

• Has Pattern Recognition Receptors (PRRs) which are intracellular and cellular receptors that activate dendritic cells, which in turn activate lymphocytes (T cells and antibody producing B cells) which recognize parasites.
• In a 2000 study, snail populations from 2 different sources where subjected to digenetic trematode taken from the same lakes, the parasites are more highly effective in infecting their sympatric snails than their allopatric snails.

Red Queen Hypothesis

• Proposed by Leigh Van Valen in 1973.
• States that species are in a constant evolutionary race against each other to survive and reproduce.

Parasite Transmission

• Parasite Transmission is how parasites spread from one host to another.
• It depends on the parasite's life cycle and host interactions.

Direct Transmission

• Parasites transfer from one host to another through direct contact.
• Example: Lice and mites spreading through skin-to-skin contact.

Indirect Transmission

• Parasites require an intermediate host or vector before reaching their definitive host.
  • Example: Malaria is transmitted by mosquitoes carrying Plasmodium species.
  • Sporozoites produce thousands of merozoites, inside the infected liver.

Ingestion (Oral-Fecal Route)

• Parasites enter the body through contaminated food, water, or hands.
  • Example: Giardia lamblia and Entamoeba histolytica spreading through contaminated water.

Vertical Transmission (Congenital Transmission)

• Parasites pass from mother to offspring during pregnancy or childbirth.
  • Example: Toxoplasma gondii can infect a fetus through the placenta.
• Minimum 24 hrs for oocysts shed in feces to become infective for people.

Roles of Parasites in the Ecosystem

Maintaining Species Balance

• Parasites help maintain species diversity by preventing any single species from dominating an ecosystem.
  • Ex: Parasitic wasps regulate herbivore populations, preventing excessive plant consumption.

Driving Evolution and Biodiversity

• Parasites exert selective pressure on hosts, leading to evolutionary adaptations like, immune system improvements and behavioral changes.
  • Example: The "Red Queen Hypothesis" suggests that hosts and parasites are in a constant evolutionary arms race, promoting genetic diversity.

Ecosystem Engineers

• Some parasites modify their hosts' behavior or physical traits, like, modifying ecosystem structure.
  • Example: Parasitic barnacles (Sacculina) sterilize crabs, altering crab population dynamics and their role in the ecosystem.