Introduction to Parasitology
Parasitism
• Relationship in which one of the
participants, the parasite, either harms its
host or in some sense lives at the expense
of the host.
• Parasite’s location on the hosts:
– Endoparasite
– Ectoparasite
– Obligate parasite
– Facultative parasite
Definitions
• Parasite - one animal deriving its sustenance from another
without making compensation. The uncompensated animal
is the host.
• Parasitology - the science or study of host-parasite
relationships.
• Medical parasitology - study of parasites which infect
humans.
• Host - the partner providing food and/or protection. Some
parasites require more than one host to complete their life
cycle; Or may not require a host during some stage(s).
Definitions, Continued.
Types of Hosts
• Definitive host - the host in which sexual maturity
and reproduction takes place.
• Intermediate host - the host in which the parasite
undergoes essential development.
• Reservoir (carrier) host - the host harboring a
parasite in nature, serving as a source of infection
for other susceptible hosts. Reservoir hosts show
no sign or symptom of disease.
• Paratenic host - an accidental host serving as a
holding place for a parasite.
Parasitism
• Relationship in which one of the
participants, the parasite, either harms its
host or in some sense lives at the expense
of the host.
• Parasite’s location on the hosts:
– Endoparasite
– Ectoparasite
– Obligate parasite
– Facultative parasite
Parasitism
1.Facultative parasite: parasites able to
live both free living and parasite living e.g.
Strongyloides species.
2.Obligate parasite: parasite living
permanently in a host and cannot live
without a host e.g. Trichomonos species.
3.Coprozic (spurious) parasites: foreign,
pass through alimentally canal without
affect.
Parasitism
• May occur differently if encounters species
aside from its original:
– Accidental parasites
– Incidental parasites (Baylisascaris, Toxocara)
• Permanent parasites may reside in a host
for their entire lives
• Temporary parasites
• Intermittent parasite
• Micropredators
Predation vs. Parasitism
• Parasites do not normally kill their main
source of food, their hosts unlike
predators.
• Certain species, like wasps and flies may
kill the final hosts in their final stage of
development, thus called as parasitoids.
• Protolean parasites are species of certain
insects that have parisitic juvenile stages.
Parasite ecology
• Combination of abiotic and biotic factors
(environment and host)
• A host always presents the parasite with
rich and highly regulated supply of
nutrients.
• Vertebrates and invertebrates also have
the ability to regulate temperature.
• Many parasites possess traits as well that
increases their probability to encounter a
host.
Hosts
• Parasites do not normally kill their main
source of food, their hosts unlike
predators.
• Certain species, like wasps and flies may
kill the final hosts in their final stage of
development, thus called as parasitoids.
• Protolean parasites are species of certain
insects that have parisitic juvenile stages.
Infection sites
• Refers virtually to the range of organisms,
from humans to protozoans that a parasite
can invade.
• May migrate within an organism towards
definitive sites of the body.
• Coelozoic are organisms that can invade the
lumen of the intestine and of the hollow
organs.
• Histozoic, parasites that reside in tissues
(filarial nematodes)
Infection sites
• Refers virtually to the range of organisms,
from humans to protozoans that a parasite
can invade.
• May migrate within an organism towards
definitive sites of the body.
• Coelozoic are organisms that can invade the
lumen of the intestine and of the hollow
organs.
• Histozoic, parasites that reside in tissues
(filarial nematodes)
Infection sites
Parasite populations
• In epidemiology, assessing the no. of
parasites may greatly predict the
infection/infestation rate/s.
• May vary among populations:
– Age, sex, economic status of the hosts
– Fitness (reproductive success)
• Incidence-refers to the number of new cases
acquired per unit of population per unit of
time, and is more meaningful for acute, short
events
Parasite populations
• Prevalence-refers to the level of infection
within a certain population.
• Abundance- the total number of parasites
found in a hosts (species per species).
• Aggregated populations of parasites
pertains to the rate of distribution of parasites
(some hosts with parasites, some without).
• Multiple species infections may exist in an
organism that includes numerous populations
of different parasites (coyotes and black bears)
Size of the Parasite
• Macroparasites, do not multiply within a host
or do not participate in the hosts’ life cycle
directly (helminths, arthropods,
acanthocephalans)
– Instead, these parasites may clump or group
together to survive as a population.
• Microparasites are parasites that can be
foun systemically in a host (bacteria,
ricketssia and protozoan)
Adaptation for Transmission
• Most known parasites are to exhaust and
exploit the trophic relationships between the
various host species (since they are
heterotrophic).
• Viviparity (live birth) occurs among
nematodes and monogeneans
• Hermaphroditism is another means of
asexual reproduction that occurs in larval or
sexually immature stages
(polyembryonology)
Adaptation for Transmission
Adaptation for Transmission
• The process of Schizogony among parasitic
protozoa produces multiple daugther cells due
to the numerous times the nucleus divides
prior to cytokinesis.
• Metacestodes, Cysticercus and Hyatid are
juvenile stages that can ‘bud’ or asexually
reproduce even if immature.
• Flukes, on the other hand, exhibit a remarkable
polyembrony in which generations of embryos
develop in a single zygote.
Gross survey of Parasites
• Phylum Microsporidia
• Phylum Retortamonada
• Phylum Axostylata
• Phylum Parabasalia
• Phylum Euglenozoa
• Phylum Chromista
• Phylum Ciliophora
• Kinetoplasta-Trypanosomes: Leishmenia
Parasitology
Divisions of Parasitology:
1. Protozoa
2. Helminthes
a. Roundworms (nematodes)
b. Flatworms – Cestodes (tapeworm)
Trematode (fluke)
• Parasitism:
organism depend upon another for living, one is living at the expense
of the other and harmful, called Parasite, the other organism is
called Host.
