A parasite relationship refers to a type of symbiotic relationship between two organisms where one organism, the parasite, benefits at the expense of the other, the host. The parasite lives on or inside the host, deriving nutrients, shelter, and other needs, while potentially harming the host. There are several key terms used to describe different types of parasite relationships.
Types of Parasite Relationships
Here are some of the main types of parasitic relationships:
Ectoparasitism
An ectoparasite lives on the surface of its host’s body. Examples include lice, fleas, and ticks. Ectoparasites feed on the host’s blood, skin, secretions, and debris. They can cause irritation, blood loss, and spread disease.
Endoparasitism
An endoparasite lives inside the body of its host, such as in the digestive system, bloodstream, organs, or tissues. Examples include tapeworms, roundworms, and malaria parasites. Endoparasites can sap nutrients, release toxic waste, and damage organs.
Parasitoidism
A parasitoid completely destroys its host, eventually killing it. Wasps are parasitoids, laying eggs on or inside a host insect like a caterpillar. When the eggs hatch, the larvae eat and kill their host.
Brood Parasitism
A brood parasite manipulates another species into raising its young. The cuckoo bird is a brood parasite, laying eggs in the nests of other bird species, which raise the young cuckoos often at the expense of their own chicks.
How Parasites Impact Their Hosts
Parasites can affect their hosts in a variety of ways:
- Feeding on host tissues, blood, etc. causing damage
- Depriving the host of nutrients
- Releasing toxic waste products
- Causing infections and spreading disease
- Triggering allergic reactions
- Impairing host reproduction and growth
- Altering host behavior
- Weakening host immune response
- Causing physical deformation or change
The extent of damage depends on factors like the parasite’s life cycle, virulence, location in/on the host, and number of parasites. Many parasites live in a delicate balance with host defenses.
Examples of Parasite Relationships
Here are some common examples of parasites and their hosts:
| Parasite | Host | Type |
|---|---|---|
| Tapeworm | Humans, cattle, pigs | Endoparasite |
| Tick | Dogs, deer, mice | Ectoparasite |
| Plasmodium (malaria) | Mosquitos, humans | Endoparasite |
| Brood parasitic cuckoo birds | Warblers, finches | Brood parasite |
| Aspergillus fungus | Humans, dogs, parrots | Opportunistic parasite |
| Parasitoid wasps | Caterpillars, flies | Parasitoid |
As the examples show, parasites can infect a wide variety of hosts, from humans to insects to large mammals. The same parasite species may use multiple host species at different points in their life cycle.
How Parasitism Evolves
Parasitism has evolved independently many times throughout the tree of life. There are several evolutionary pathways that can lead to parasitism:
Opportunistic Infection
A microbe or fungus living freely in the environment encounters and infects an injured plant or animal host. If it is able to evade the host’s immune defenses, it may adapt to derive more of its nutrients from the new host.
Prey-Predator Arms Race
A predator and prey species become locked in an evolutionary arms race, with the predator evolving adaptations to better attack and consume the prey. Eventually, the predator may evolve to feed on the prey in a parasitic way while keeping the prey alive.
Loss of Essential Functions
A free-living organism loses certain essential functions like making nutrients. It then evolves to hijack those functions from another organism it associates closely with, becoming dependent on its new host.
Hybridization
Gene flow from a parasite to a closely related free-living species can confer parasitic abilities. The resulting hybrid offspring may become parasitic on the parent species.
Co-evolution
Two species evolve in tight association over time, becoming interdependent. This mutual adaptation can lead one species to become more parasitic.
Defense Mechanisms Against Parasites
Host organisms have evolved a variety of defenses to counteract the effects of parasitic infection:
Physical Defenses
- Skin and membranes act as physical barriers
- Shedding of skin or fur helps dislodge ectoparasites
- Saliva and mucus contain compounds toxic to parasites
- Inflammation and immune cells wall off parasites
Immune Defenses
- Antibodies mark parasites for destruction
- Immune cells like eosinophils and macrophages attack parasites
- Complement proteins punch holes in parasite membranes
- Cytokines stimulate inflammation and immunity against parasites
Behavioral Defenses
- Grooming and scratching removes ectoparasites
- Predator inspection behavior checks for parasitic infections
- Sunning raises body temperature to discourage parasites
- Some animals self-medicate using antiparasitic plants
Host defenses apply strong evolutionary pressure on parasites to evade detection and resistance. This drives ongoing adaptation by both host and parasite.
Key Examples of Parasite Adaptations
Parasites have evolved a range of adaptations to establish infections, evade host defenses, exploit resources, and complete their life cycles. Some examples include:
- Molecular mimicry – Parasite surface proteins resemble host proteins to avoid immune attack
- Antigenic variation – Parasites alter surface proteins to prevent antibody binding
- Encystment – Forming a hard shell for protection from host defenses
- Suppressing host immune cells – Releasing compounds that deactivate macrophages, lymphocytes, etc.
- Manipulating host behavior – Altering host reproductive or feeding behavior to aid transmission
- Exploiting host nutrients – Stealing iron, proteins, lipids from host blood and tissues
Key Facts About Parasitism
To summarize some of the key facts about parasite relationships:
- Parasitism is a form of symbiosis where one organism benefits at the expense of another
- Ectoparasites live on the host’s exterior, endoparasites live inside the host
- Parasites can harm hosts by depleting resources, releasing toxins, causing disease
- Parasitism has evolved independently many times via pathways like opportunistic infection and coevolution
- Hosts defend against parasites using physical barriers, immune cells, behavior, and toxins
- Parasites use adaptations like mimicry, variation, and encystment to exploit hosts
Conclusion
Parasitism is a fascinating type of biological interaction involving close, prolonged contact between species. Parasites display many specialized adaptations to locate, enter, and feed on their hosts without killing them right away. Meanwhile, hosts employ an arsenal of physical and chemical defenses to counteract and control parasitic infections. This evolutionary arms race has generated much of the diversity we see among parasites and hosts today.