Parasites
External parasites Internal parasites Pathogens Adaptations Case studies

A parasite is an organism which spends at least part of its life cycle on, or inside, another organism (its host) from which it obtains food and protection.

External parasites
External parasites, such as fleas, ticks, lice and mites, live in the hair or fur of warm-blooded hosts, bite through the skin with sharp pincer-like jaws and feed on the nutrients in the blood.

Internal parasites
Internal parasites, such as flatworms (tapeworms and flukes), disease-causing bacteria (e.g. tuberculosis) and roundworms (nematodes), live inside their host, often within the digestive system or bloodstream where food nutrients are plentiful.

Parasite-host food chains therefore start with larger animals!

The number of individuals can be represented as an inverted pyramid.

Increase in numbers
Decrease in size
Pathogens
In a parasitic relationship, the host is usually harmed to some degree. However, it is beneficial to the parasite not to kill the host as then it would need to find another host.
Parasites which cause disease are called pathogens. Many hosts can produce antibodies against the pathogen or its toxin, and develop immunity to further attack (e.g. measles in humans). In these situations, the pathogen must reproduce and spread quickly to ensure the survival of the species. Vaccines can be used to artificially stimulate immunity in hosts without actual infection.

Some hosts are resistant to the parasite. Rabbits unaffected by the mosquito-borne myxomatosis virus are an example of host resistance.

Adaptations
Some parasites are so highly adapted to their internal environment that they cannot survive for long outside their host.

Click for larger image Many larger parasites have suckers or hooks to attach tightly to the host.

The roots of the mistletoe plant grow directly into the transport system of the host tree to provide attachment and directly obtain water and nutrients.
Fleas and mites have flattened bodies which make them more difficult to dislodge even by scratching.

Parasites which live in a host's intestines usually have no digestive system as they are able absorb simple nutrients directly through their outer surface.

Most have very reduced sense organs as they have little need to detect danger in their sheltered habitat.

Many internal parasites are hermaphroditic, which means that they have both ovaries and testes so they can self-fertilise their eggs. In tapeworms, the body segments containing the mature eggs simply break off and are passed out with the host's faeces to be picked up by another host. This is essential as many may not encounter another member of their species with which to mate.

Parasites usually produce millions of eggs because the chance of any one egg reaching another suitable host is extremely low.

The eggs of most parasites are able to survive for a long time in the soil to increase the chance of being picked up by another host.

Parasites often have an intermediate host or vector which means that they can be more widely distributed.

Parasites are easily transferred to humans from household pets such as dogs, cats and birds. Regular worming of pets, and careful washing of hands after handling pets, are essential procedures to prevent parasite transfer.

Case studies

  • Malaria is caused by a sporozoan (single-celled organism) which can live in human blood causing high fever, shivering and fatigue. It is spread to other hosts by mosquitoes which feed on infected persons. The sporozoans breed in the mosquito's gut, migrate to their salivary glands and hence into another host with the next bite!

  • Hydatid tapeworms have a life cycle which involves two hosts.
Click for larger image

The intermediate host (sheep or human) is harmed the most, so it is essential to break the cycle. This can be done by isolating any infected dogs. Also, good personal hygiene and thorough cooking of infected meat kills the parasites and prevents them spreading further.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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Habitats and environments: Macro
Eat and be eaten A: Feeding relationships
Eat and be eaten B: Food chains
Ecosystems and food webs
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Glossary
 
Nutrient Pathogen
Toxin Immunity
Vaccine Antibodies