Introduction to Human Parasitology
I. Concept of Human Parasitology
Human Parasitology is a subject that researches the biological
features of human parasites, the relationship between the human being
and the parasites, the prevention and treatment of the parasitic diseases.
The importance of parasitology: Six major tropical
diseases to which WHO pays great attention include malaria(疟疾),
schistosomiasis( 血吸 虫 病 ), filariasis(丝 虫 病 ), leishmaniasis( 利 什 曼 病 ),
trypanosomiasis( 锥 虫 病 ) and leprosy( 麻 风 ). Above diseases, except
trypanosomiasis, are prevalent in China and five of them are parasitic diseases
except leprosy. Five major parasitc diseases in China are malaria,
schistosomiasis, filariasis, Leishmaniasis and hookworm disease.
II. The Scope of Human Parasitology
Medical Parasitology consists of Medical Protozology,
Helminthology and Entomology (Arthropodology).
Class Zoomastigophora Leishmania donovani
Medical Class Lobosea Entamoeba histolytica
Protozoology Class Sporozoa Plasmodium vivax
Class Kinetofragminophorea Balantidium coli
Class Nematoda Ascaris lumbricoides
Medical Class Trematoda Clonorchis sinensis
Helminthology Class Cestodea Taenia solium
Class Metacanthocephala Macracanthorhynchus
Class Insecta Anopheles sinensis
Medical Class Arachnida Sarcoptes scabiei
Entomology Class Eucrustacea crab and crayfish
Class Chilopoda. centipede
III. Evolution of Parasitism
endoparasite Free living
ectoparasite Mutualism 互利共生
temporary parasite Symbiosis Commensalism 片利共生
permanent parasite 共同生活 Parasitism 寄生关系
obligatory parasite Carrier
facultative parasite Parasite Host 带虫者
accidental parasite 寄生虫 宿主 Patient
opportunistic parasite 病人
Final host Intermediate host Reservoir host Paratenic host
Biohelminth Zoonosis Larva migrans
infective stage infective route infective mode site of inhabitation
IV. Common Terms of Parasitology
(1) Symbiosis 共同生活） Two different organisms live together and
interact on each other, in this association one partner lives in or on
the other. including 3 types: Mutualism, Commensalism, Parasitism.
(2) Mutualism （互利共生或共生）is a permanent association in which
two different organisms so dependent on each other that life apart is
impossible, from which two partners all benefit, such as termite and
(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
is 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.
(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 and does harms to the host.
(6) Host（宿主）: In parasitism, it is the injured partner which supplies the
parasite with nourishment and shelter.
(7) Carrier（带虫者）: A person who harbors parasites has no any clinical
symptom. He is an important source of infection in epidemiology.
(8) *Definitive (final) host（终宿主）is the host which harbors the adult or
sexually reproductive stage of a parasite. Man is the final host of ascarid.
(9) *Intermediate host 中间宿主） the host which harbors the larval or
asexually reproductive stage of a parasite, according to priority they are
classified into first intermediate host, second intermediate host and so on.
Mosquitoes are the intermediat host of filariae.
(10) *Reservoir host 保虫宿主） the vertebrate animal host which harbors
the same species of parasite at the same stage as a human definitive host
does. It is an animal source of infection in epidemiology. Cats are the
reservoir host of liver fluke.
(11) *Parasitic zoonosis （ 人 兽 共 患 或 动 物 源 性 寄 生 虫 病 ） refers to
animal’s parasitic diseases which can be transmitted to man. (These
animals infected with parasites are called reservoir hosts.) Liver fluke
diseases is a zoonosis.
(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 the adult
stage. 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 parasitic larva lives in an
abnormal host in which they can not grow into the adult stage but can
wander everywhere and cause the local and systemic pathological
(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 various parasites.
(15) *Infective Stage（感染期） a stage when a parasite can invade human
body and continue to live there. The infective stage of ascarid is the
(16) *Infective Route（感染途径）is the specific entrance through
which the parasite invades the human body. Hookworms invade
human body by skin. Man gets infection with ascarid by mouth.
(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.
(18) Geohelminth refers to the helminths which complete their life
cycles not requiring the processes of the development in
intermediate hosts. They have only one host and a simple life cycle,
such as ascarid, hookworm, pinworm and etc.
(19) Biohelminth refers to the helminths which have to undergo the
development in intermediate hosts to complete their life cycles, such
as filaria, liver fluke, pork tapeworm and so on.
(20) Alternation of Generation: In life cycles of some parasites,
there is the regular alternation of sexual and asexual reproductions,
this phenomenon is called alternation of generation, such as it is in
the life cycle of Plasmodium vivax.
(21) *Trophozoite is a living stage of protozoa when they can move,
take food and reproduce. (It is usually the pathogenic stage.)
(22) *Cyst is the resting stage of a protozoa with a protective wall. It is
usually the infective stage. Its functions are protection, transmission
(22) *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.
(23) *Biological Transmission: Pathogens have to spend a part of their
life cycle in the specific arthropods in which they multiply or
develop into the infective stage and then invade the human body
under the help of the arthropods, such as mosquitoes transmit
V. Three key links of parasitic disease transmission
1. Source of infection 传染源 2. Route of transmission 传播途径
3. Susceptible people 易感人群
secretion food, water, finger, mouth
blood direct or indirect contact skin or wound
focus of mucosa
infection blood transfusion, injection, placenta
传染源 intermediate host, 易感人群
insects sucking blood,
touch soil, water, grass
VI. Parasites’ Harms to Man
1. Mechanical effect of parasites on host tissues and organs: e.g.,
biliary ascariasis and larva migrans.
2. Depriving the host of nourishment: e.g., hookworms suck blood.
3. Toxic effect: Arthropods introduce toxin into the host skin,
inducing the host local and systemic reactions, e.g., mosquitoes,
spiders and ticks introduce venom when they insert their mouth
parts into the skin.
4. Immuno-pathological lesion: e.g., schistosoma liver cirrhosis;
when hydatid fluid is released from the rupture of a hydatid cyst
anaphylaxis often results.
VII. Human Immunity against Parasite
The intensity, specificity and lastingness of this immunity are usually at a lower
level than those resulting from bacteria and viruses. We refer to it as Non-sterilizing
immunity (非消除性免疫, including 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 and etc.