Parasitism is a specific type of interaction between two organisms that has many features in common with other infectious processes, but host–parasite interactions often operate over a longer timescale than those seen with other pathogens.
PROTOZOA
Introduction
Parasitism is a specific
type of interaction between two organisms that has many features in common with
other infectious processes, but host–parasite interactions often operate over a
longer timescale than those seen with other pathogens. This extended process
results in significant host– parasite interaction at the cellular and organismal
level. It is known, for example, that some parasites alter the behaviour of the
host, while others, such as Giardia
lamblia, induce biochemical change in the host cells at the site of infection (the duodenal epithelium).
Most parasites have a life
cycle that often involves several hosts; this means that survival and transmission
between different hosts requires the parasite to exhibit more than one
physiologically distinct form.
Parasites inhabit a wide
range of habitats within their hosts. Some parasites will inhabit only one site
throughout their life cycle, but many move to various sites within the body.
Such movement may require the formation of motile cellular forms, and it will
produce a significant change in the physiology and morphology of the parasite
as a result of environmental change. Parasites moving from the gut to other tissues,
for example, will encounter higher
levels of oxygen, changes in pH and significant exposure to the host immune
response. When life cycles involve more than one host organism these changes
are greater. The reasons why parasites move to various sites in with the host
is driven by evasion of host immune attack and to aid transmission.
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