ZOO 173

Introduction to Parasitology
Dr. Vachel Gay V. Paller | vvpaller@up.edu.ph 22 Feb 2023 | T01

OUTLINE ▪ Outside of cell but within the ground substance or matrix

of tissues and organs
I. Important Terms V. Immunology of parasitic
infections
B. BASED ON MODE OF DEVELOPMENT
II. Classification of A. Effect of parasite on ● Obligate parasite
Parasites host → Needs a host at some stage of their life cycle to complete
B. Immune response of development and to propagate species
A. Based on habitat ● Facultative parasite
host
C. Immune evasion of → May exist in a free-living state or may become parasitic when
B. Based on mode of the need arises
parasite
development ● Incidental parasite
→ Establishes itself in a host where it does not ordinarily lives
III. Hosts and Vectors
● Permanent parasite
A. Hosts → Lives in or on the host for its entire life
● Temporary parasite
B. Vectors → Live in or on the host for a short period of time
● Spurious parasite
IV. Transmission of parasite → A free-living organism that passes through the digestive tract
without infecting the host
I. GENERAL TERMS
III. HOSTS AND VECTORS
● Parasitology
→ Area of biology concerned with the phenomenon of A. TYPES OF HOST
dependence of on living organism on another ● Definitive/Final host
→ Medical Parasitology → Where the parasite attains sexual maturity
▪ Concerned with the animal parasites of humans and their ● Intermediate host
medical significance and importance in human → Harbors the asexual or larval stage of the parasite
communities. ● Paratenic host
● Tropical Medicine → Harbors the parasite in an arrested state
→ Branch of medicine which deals with tropical diseases and ● Incidental host
other special medical problems of tropical regions. → Not necessarily for the parasite’s survival or development
● Symbiosis ● Reservoir host
→ Living together of unlike organisms → Ensure continuity of the parasite’s life cycle and acts as
▪ Commensalism additional source of human infection
− One species benefits without harming or benefiting the A. VECTORS
other
▪ Mutualism ● Vectors
− Two organisms mutually benefit from each other → Responsible for transmitting the parasite from one host to
▪ Parasitism another
One organism lives in or on another for its survival and ● Types of Vectors:
→ Biologic vector
−

at the expense of the host

● Epidemiology ▪ Transmits the parasite only when it has completed
→ Study of patterns of diseases within populations development in the host
→ For parasites, this includes: ▪ e.g. mosquito in transmitting malaria
▪ Host range – what can it infect/infest? → Mechanical vector
▪ Geographic range ▪ Only transports the parasite
▪ Is it a zoonotic agent – can it infect humans? ▪ e.g. flies and cockroaches may carry enteric organisms
▪ Does it have a reservoir (group of vertebrates maintaining and may transfer to food
the parasite)? IV. TRANSMISSION OF PARASITE
▪ Does it have a niche (a small ecosystem which possesses ● Source of infection
all the factors to maintain the survival of the parasite)?
−
− → Human to human
→ Vectors/animals/pets (as carriers)
II. CLASSIFICATION OF PARASITES
→ Environment
A. BASED ON HABITAT ● Mode of transmission
● According to location in host → Contaminated food and water (oral route)
→ Endoparasites → Skin penetration
▪ Living inside the body of host → Arthropods as vectors
▪ Infection → Congenital/transmammary
→ Ectoparasites ● Presence of susceptible host
▪ Living outside the body of the host → Endemicity of parasite depends upon the presence and
▪ Infestation habits of a suitable host
● For parasitic protozoans:
→ Intracellular V. IMMUNOLOGY OF PARASITIC INFECTIONS
▪ Inside membrane of host cells ● Host specificity
▪ Restricted to the size of cell → Determined by the success of the parasite’s interaction with
→ Extracellular the host factors such as:
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 ▪ Structure − WBCs that exert cytotoxic activity on the surface of
▪ Nutrition helminth parasites
▪ Physiology
▪ Immunity
→ Receptors on the surface of some parasites are specific only
for a particular host cell or tissue, and thus are host specific
→ Some require a wide range of hosts and thus considered
generalist
A. EFFECT OF PARASITE ON HOST
● Mechanical
→ e.g. pressure from an enlarging cyst or obstruction of blood
vessels.
● Invasion and destruction of host cells by the parasite
● Inflammatory reaction to the parasite or parasite products
● Competition for host nutrients
B. IMMUNE RESPONSE OF HOST
● Immune response
→ Host protective mechanism through which the parasites are → Cell mediated response
killed and eliminated ▪ Uses T-cells
→ Ability of host to withstand infection may be due to: ▪ Cytotoxic T cells kill invading parasites
▪ Physico-chemical barriers
− e.g. intact skin
▪ Innate/natural resistance
− e.g. Plasmodium vivax interacts with the receptor for
Duffy factors in the human blood for successful RBC
invasion (Duffy is absent among West Africans, thus
they are resistant against P. vivax malaria)
▪ Specific immunity acquired from previous infection

TYPES OF HOST RESPONSES

● Non-specific Immunity
→ Macrophage endocytosis (common for bacteria and small
protozoa)
→ Inflammation
▪ Acute
− Edema and increase in leukocytes
▪ Subacute
− Monocytes and lymphocytes present, with fibrocytes
binding parasite with collagen
▪ Chronic
− Plasma cells present and form a granuloma
→ Hyperplasia B. IMMUNE EVASION OF PARASITE
▪ Parasite causes host to produce more cells
▪ e.g. liver fluke stimulating enlargement of bile duct ● Antigenic variation
→ Neoplasia (cancer) → Change surface glycoproteins regularly
▪ mechanisms are still unknown ● Being poorly antigenic
● Specific/Adaptive Immunity → Do not induce a response or a most mild one
→ Humoral response ● Hide within host cells
▪ Formation of antibodies or immunoglobulin (Ig) by B cells ● Camouflage
▪ IgE fights helminths → Acquisition of host’s molecules on their tegument, disguising
▪ IgM and IgG important against protozoans as ‘self’ and thus not recognized as foreign
▪ Eosinophilia ● Depress host’s immune response
− High level of eosinophils w/c indicates parasitic → Modulate production of T cells
infection → Production of proteases that can degrade antibodies or
▪ Eosinophils interfere with some biological pathways

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