Human Parasitology
Shandong University
Department of Parasitology
He Shenyi （何深一 ）
MD, Ph.D. Professor
Introduction to Human
Parasitology

Concept of Human Parasitology
 The Scope of Human Parasitology
 Terms of Parasitology
 Parasites’ Harms to Man
 Human Immunity against Parasites
 Characteristics of parasitic diseases
Human parasitology

Human parasitology is the study of those
organisms which parasitise humans. According
to the very broad definition of parasitology,
parasites should include the viruses, bacteria,
fungi, protozoa and metazoa (multi-celled
organisms) which infect their host
species. However, for historical reasons the
first three have been incorporated into the
discipline of Microbiology.
2. The Scope of Human Parasitology
Parasitology claims those protozoa (single celled animals),
helminths (worms) and arthropods whose existence depends
on the availability of host animals It is also possible to argue
about whether certain insects and mites are "temporary
parasites" or "micro-predators", insects as a group belong to
the discipline of Entomology, while ticks and mites are the
concern of Acarology. The insects that are of most interest in
human parasitology are those that are vectors of several
parasitic infections.
• Class Lobosea
Human Parasitology
Medical
• Class Zoomastigophorea
Protozoology
• Class Sporozoa
Medical
Helminthology
Medical
Arthropodology
• Class Ciliophora
• Class Nematoda
• Class Trematoda
• Class Cestoda
• Class Metacanthocephala
•
•
•
•
Class
Class
Class
Class
Insecta
Arachnida
Crustacea
Chilopoda
Class Lobosea
Entamoeba histolytica
Non-pathogenic amoeba
Class Zoomastigophorea
Leishmania sp
Giardia
Trichomonas vaginalis
Class Sporozoa
 Plasmodium
spp
Toxopasma gondii
Cryptosporidium
Pneumocycstis carinii
Class Ciliophora
– Balantidium coli
Class Nematoda

Ascaris
lumbricoides
Trichuris trichiura
Hookworm
Enterobius vermicularis
Filaria
Trichinella spiralis
Nematoda
Class Trematoda
 Clonorchis
sinensis
Fasciolopsis buski
Paragonimus westermani
Schistosoma japonicum
Trematoda
Class Cestoda

Taenia solium
Taenia saginata
Echinococcus granulosus
Cestoda
Class Metacanthocephala
Macracanthorhynchus
hirudinaceus
Class Insecta
Anopheles sinensis
Lice
Sandflies ,
Fleas
Lucilia sericata
Class Arachnida
Ticks; Mites
Sarcopes scabiei
follicle mite
Class Crustacea

Cambaroides
 Potamon
Class Chilopoda
centipede
4. Terms of Parasitology
Symbiosis
(1)Symbiosis （ 共 同 生 活 ） Two
different organisms live together and
interact, in this association one
partner lives in or on another one’s
body. including 3 types:Mutualism,
Commensalism, Parasitism.
Symbiosis
 *(2)Mutualism(互利共生）is
a
permanent association between two
different organisms that life apart is
impossible, two partners benefit each
other, such as termites and flagellates.
 The mutuals are metabolically
dependent on one another; one cannot
survive in the absence of the other.
Symbiosis
(3) Commensalism（片利共生或共栖）is the
association of two different organisms, in
which one partner is benefited while the
other neither benefited nor injured, such as
E. coli and man.
(4) Parasitism（寄生关系）is the association
of two different organisms, in which one
partner is benefited while the other is
injured, such as Ascaris lumbricoides and
man.
Parasite
(5) Parasite（寄生虫） In parasitism,
it is the benefited partner. It is an
animal organism which lives in or on
the host in order to obtain
nourishment and shelter from the
host as well as does harms to the host.
Parasite
endoparasite
ectoparasite
temporary parasite
permanent parasite
obligatory parasite
facultative parasite
accidental parasite
opportunistic parasite
Endoparasite
Ectoparasite
Host
(6) Host（宿主） In parasitism, it supplies
the parasite with nourishment and shelter,
it is the injured partner.
(7) Carrier （ 带 虫 者 ） A person who
harbours parasite has no clinical
symptoms, is an important source of
infection in epidemiology.
(8) Definitive (final) host （ 终 宿 主 ）
harbours adult or sexually reproductive
stage of a parasite.
(9) Intermediate host（中间宿主）
harbours larval or asexually
reprodctive stage of a parasite,
according to priority they are classified
into first intermediate host, second
intermediate host, third intermediate
host.
 (10)
Reservoir hosts（保虫宿主）
are the vertebrate hosts which
harbour the same species of parasite
at same stage as a human host. They
are an important source of infection
in epidemiology.
Zoonosis
(11) Zoonosis（人兽共患病）refers to
animal’s diseases which can be
transmitted to man. (These animals
infected with parasites are called
reservoir hosts.)
 (12)
Paratenic host or transport host
（转续宿主）is an abnormal host in
which some parasitic larvae can
survive but can’t develop into adults.
If the larvae have a chance to enter
their appropriate hosts, they can
continue to develop into adults there.
(13) Larva migrans（幼虫移行症）
means that the larvae living in their
abnormal hosts in which they can
not grow into adults but can wander
everywhere and cause the local and
systemic pathological lesions of the
hosts.
Life cycle
(14) Life cycle（生活史） is the process of a
parasite’s growth, development and
reproduction, which proceeds in one or
more different hosts depending on the
species of parasites.
(15)Infective Stage （ 感 染 期 ） is a stage
when a parasite can invade human body
and live in it .
Life cycle
(16) Infective Route （ 感 染 途 径 ） is the
specific entrance through which the
parasite invades the human body.
(17) Infective Mode（感染方式） means
how the parasite invades human body,
such as the cercariae of the blood fluke
actively penetrate the skin of a swimming
man and the infective ascaris eggs are
swallowed by man.
Life cycle
 (18)
Alternation of Generation:
 In life cycles of some parasites, there
are the regular alternations of sexual
and asexual reproductions , this
phenomenon is called alternation of
generation, such as the life cycle of
Plasmodium vivax.
(19) Mechanical Transmission:
Arthropods play a role of the
transportation of pathogens, which
is not indispensable for the disease
transmission, such as flies carry
typhoid bacilli, ascarid eggs and
amoebic cysts.
(20) Biological Transmission: Pathogens have to
spend a part of their life cycle in the vector
arthropods in which they multiply or develop
into the infective stage and then invade the
human body under the help of the arthropod,
such as Anopheles mosquitoes transmit malaria.
Evolution of Parasitism
endoparasite
ectoparasite
temporary parasite
permanent parasite
obligatory parasite
facultative parasite
accidental parasite
opportunistic parasite
Free living
Commensalism（片利共生）
Symbiosis
Mutualism（互利共生）
Parasitism
carrier
parasite
Host
Patient
Final host Intermediate host
Reservoir host
Zoonosis
Paratenic host
Larva migrans
4. Parasites’ Harms to Man

Mechanical effects of parasites on host tissues
and organs:
migrans.

Depriving
e. g. , biliary ascariasis and larva
nourishment from hosts: e.g.
hookworms suck blood.

Toxic effect: e. g., mosquitoes, spiders and ticks
introduce venom when they insert their mouth parts
into the skin.

Immuno-pathological lesion e.g. schistosoma
liver cirrhosis; when hydatid fluid is released from the
rupture of a hydatid cyst anaphylaxis often results.
5. Human Immunity against Parasites

Its intensity and specificity are usually at a lower level
than those produced by bacteria and viruses. It refers
to Non-sterilizing immunity (Premunition带虫免疫；
Concomitant immunity伴随免疫). The host may be
protected from superinfection重复感染as long as the
parasites remain in the body. This situation is known
as premunition( 带 虫 免 疫 ). This may be of great
importance in endemic areas in limiting the severity
of infection with Plasmodium, Schistosome ,
hookworms and other parasites.
(WHO) - Priority Diseases

1. Schistosomiasis
 2. Malaria
 3. Filariasis
 4. Trypanosomiasis
 5. Leishmaniasis
 6. Leprosy (replaced by HIV/AIDS)
Why were they selected?

Schistosomiasis - 200,000,000 infected
500,000-1,000,000 deaths/year
 Malaria - 500,000,000 infected
2,500,000 deaths/year
 Filariasis - 250,000,000 infected
 Trypanosomiasis - 25,000,000 infected
65,000 deaths/year
 Leishmaniasis - 1,200,000 infected
 Leprosy - 1,300,000 infected
 Approximately 25% of world's population infected by
one of these.
General Characteristics:

1. Chronic diseases
 2. No effective vaccine
 3. No practical chemotherapy
 4. Affect young
 5. Affect underprivileged
 6. Vector-borne
Geographic Distribution
Factors (Endemicity):

1. Presence of a suitable host
 2. Habits of the host
 3. Escape from the host
 4. Favorable conditions outside of host
 5. Economic and social conditions
Presence of Diseases in a
Population (Prevalence):

Factors required:
 1. Source
 infected persons
 carriers
 animals
Presence of Diseases in a
Population (Prevalence):

2. Mode of transmission
 direct
 indirect
 vectors
 3. Susceptible host
 immunity
Three key links of disease transmission
1.Source of infection 2. Route of transmission
excrement
secretion
blood
focus of
infection
food, water, finger
direct or indirect contact
blood transfusion, injection,
intermediate host,
insects sucking blood,
congenital ,
touch soil, water, grass
3. Susceptible people
mouth
skin or wound
mucosa
placenta
Diagnosis of Parasitic
Infections:

1. Clinical diagnosis
 2. Laboratory diagnosis
Treatment of Parasitic
Infections:

1. Medical and surgical
 2. Chemotherapy
 3. Adequate nutrition
Prevention and Control:

1. Reduction in sources
 2. Education
 3. Destruction and/or control of reservoir
hosts and vector