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Introduction to Human Parasitology

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					        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
                                                  hirudinaceus

                  Class Insecta                   Anopheles sinensis
Medical           Class Arachnida                 Sarcoptes scabiei
Entomology        Class Eucrustacea               crab and crayfish
                  Class Chilopoda.                centipede

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III.     Evolution of Parasitism
                                  自由生活
 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


                                  Life     Cycle
       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
       flagellate.
(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
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       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
                                (            is
       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
                         (            is
       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
       lesions.
(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
                                  is
       body and continue to live there. The infective stage of ascarid is the
       embryonate egg.
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(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
       and multiplication.
                                  Encystation
           Trophozoite                                  Cyst
                                  Excystation
(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
       malaria.
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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

excrement                       传播途径
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,
                                congenital ,
                            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.

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posted:10/16/2011
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