[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 1 Introduction
PARASITIC LIFE CYCLES
Although parasitic life cycles range from simple
to complex, they all have three common
components—a mode of transmission, a morphologic
form that invades humans, known as
the infective stage, and one (or more) forms that
can be detected via laboratory retrieval methods,
known as the diagnostic stage. Some parasites
require only a definitive host, whereas others also
require one or more intermediate hosts.
A parasitic life cycle consists of two common
phases (Fig. 1-1). One phase involves the route a
parasite follows when in or on the human body.
This information provides an understanding of
the symptomatology and pathology of the parasite.
Insights about the best the method of diagnosis
and selection of appropriate antiparasitic
medication may also be determined. The other
phase, the route a parasite follows independently
of the human body, provides crucial information
pertinent to epidemiology, prevention, and control.
Parasites
come in contact
with human
Parasites emerge
from water, food, soil,
intermediate hosts
Parasites enter
and establish residence
in or on human
[Infective stage(s)]
Route a parasite follows inside or on human body
Route a parasite follows independent of a human
Parasites come
in contact with
soil or water,
intermediate
food hosts
Parasites multiply
and compete with
human for
nutritional needs
Parasites
enter outside
environment
Parasites emerge
from human
(diagnostic stages)
Key
Parasite
Water
Human
Food
FIGURE 1-1 Generic parasite life cycle.
Representative
intern host
Morphologic changes
occur as appropriate