Parasites
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.
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Increase in numbers |
Decrease in size |
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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.
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!
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Hydatid tapeworms have a life cycle which involves two
hosts.
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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.
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