MEDICAL PARASITOLOGY

医学寄生虫学 ( Medical parasitology) Medical parasitology ， the study of animality pathogen 动物性病原体 harming human health, is one of basic science of preventive medicine and clinical medicine. It studies the pathogenic parasites 寄生虫 and their relationships to the hosts 宿主 and environment. it consists of medical protozoology 医学原虫 学, medical helminthology 医学蠕虫学 and medical entomology 医学节肢动物学.

A large mass of Ascaris lumbricoides that was passed from the intestinal tract. The ruler at the bottom of the image is 4 cm (about 1.5 inches) in length.

S.japonicum:

WHO 重点防治的寄生虫病种类 malaria ( 疟疾 ) schistosomiasis （血吸虫病） filariasis ( 丝虫病 ) leishmaniasis ( 利什曼病 ) trypanosomiasis （锥虫病）

Malaria <30 万 Schistosomiasis 约 70 万 (03 年 100 万 ) Filariasis 近期被消灭 Leishmaniasis 300~400 例 / 年 Hookworm disease 2 亿多 (3930 万 ) 流行概况（国内）

我国寄生虫病流行情况 1. 约 5 亿人感染土源性寄生虫 (80%) ， 下降了约 60% （与全国第一次普查） 2. 食源性寄生虫病上升、并向城镇扩 散，上升了 75% （与第一次普查） 3. 囊虫病在中、西、北部地区流行 4. 弓形虫病呈扩大传播趋势 5. 包虫病造成畜牧业巨大经济损失

一、寄生现象 (Parasitism) ( 一 ) 共生 （ symbiosis ） Any organism that spends a portion or all of its life intimately associated with another living organism of a different species is known as symbiont 共生者, and the relationship is designated as symbiosis. 根据两种生物间的利害关系，大致可分 为共栖、互利共生和寄生三种现象。

1. 共栖（ commensalism) Commensalism 共栖现象 denotes an association that is beneficial to one partner and the neither benefited nor damaged to the other. （人 —— 结肠内阿米巴）

2. 互利共生（ mutualism) Mutualism 互利共生 is seen when such associations are beneficial to both organisms. （白蚁 —— 鞭毛原虫）

3. 寄生（ parasitism ） Parasitism 寄生现象 is a symbiotic relationship in which one animal, the host 宿主, is to degree injured through the activities of the other, the parasite 寄生 虫.Parasite usually is the smaller of the two organisms. （蛔虫 —— 人）

Parasite Obligate parasite –An organism that cannot survive in any other manner is called an obligate parasite( 专性寄生虫 ). Facultative parasite –Facultative parasite( 兼性寄生虫 ) is an organism that may exist in a free-living state or as a commensal and that, if opportunity presents itself, may become parasitic. –It is implicit in this term that the organism does not of necessity have to be a parasite at any stage of its existence.

Temporary parasite –Temporary parasite are obligatory parasites at one or more stages of their life cycles but free living at others. Endoparasites and ectoparasites –Parasites living within the host may be described as endoparasites, whereas those that are found on the surface of the body are called ectoparasites.

opportunistic parasite ： ( 机会致病性寄生虫 ) 在宿主免疫功能正常时处于非致 病的隐性感染状态，当宿主免疫功 能受损时，可出现异常增殖和致病 力增强使人致病的寄生虫。（弓形 虫）

（二） Adaptations to Parasitism （ 寄生虫对寄生生活的适应） Morphologic adaptations –Apicomplexa and Microspora protozoa have no locomotor organelles. –A digestive tract, is reduced in the trematodes and is absent in the cestodes. –The reproductive system is very highly developed in the trematodes and cestodes. –Specialized attachment organs have been developed by the flatworms. –Many of parasites are much larger than their free-living relatives.

Biochemical changes Develop specialized mechanisms for effecting entrance into the body or tissue. Successfully overcome the immune response of the host –Locate in relatively protected sites –Changes in the parasite surface antigenic structure –Modify the host immune response by products of parasite metabolism Increase reproductive capacity

（三 )host Definitive host （终宿主） –The host in which sexual reproduction occurs is called the definitive host. Intermediate host （中间宿主） –The specie in which larval develop is called intermediated host.

Reservoir host （保虫宿主 / 储蓄宿主） – Reservoir host is an animal which harbors parasites in nature and acts as a source of infection. Paratenic host （转续宿主） – A host which acts as a transporting agent for the parasite and in which the parasite does not undergo any development is called paratenic host.

二、 life cycle （ 生活史 ） The parasitic life cycle is the obligatory of parasites for their growth, development and multiplication that may be accomplished in their host (or hosts) or environment. 直接型生活史：不需要中间宿主 间接型生活史：需要中间宿主

Migration and localization (pathogenicity treatment and diagnosis) Stool Urine Mouth Bronchial Contact Secretion Vector Vector (Pathogenicity (diagnosis) and Prophylaxis) (Epidemiology and Prophylaxis) Stage in environment (vector) Stage in man Infective stage Stage left man

Infective stage( 感染期 ) –A stage of parasite which can enter the human body and develop continuously there.

Carrier –Carrier is a person who carries a certain parasite without displaying any sign or symptom of diseases acting the source of infection

Effects of the Parasite on the Host Parasite infection and parasitic disease. Overt symptoms of infection may depend on the number of worms present, the nutritional status of the host, or both. Injury to the host may be brought about in many ways. –Interference with the vital processes of the host through the action of secretions, excretions, or other products of the parasite. –Invasion and destruction of host tissue. –Deprive essential substances of the host.

Effects of the Host on the parasite The genetic constitution of the host. –Absence of the Duffy blood ~ P. vivax –Sickle cell trait ~ P. falciparum The diet or nutritional status of the host. –A high-protein diet ~ intestinal protozoa –Carbohydrate in the diet ~ tapeworms –Nutritional status of the host

Innate immunity Acquired immunity –Can be demonstrated in many parasitic disease –Absolute immunity to reinfection is rarely following following protozoal infections and probably never with helminth infection. Just resists to hyperinfection. –No long-lasting immunity maintains while the parasites are still in the body. –Premuniton in malaria Concomitant immunity in schistosomiasis

Epidemiology Source of infection patient, carrier and reservoir host Transmission route water, food, soil, air, insect vector and so on Susceptible population People except those will special background are all susceptible to parasitic infection.

Epidemiological factors Natural factors –Climate, geography, species of parasites and vectors Socio-economic factors –Social system, economic situation, life condition and mode, hygienic condition Individual factors –Sex, age, profession, ethnic group

Control Destruction of source of infection Interruption of transmission Protection of susceptible population

Prevalence of Parasitic Infections Malaria Infected, 400 to 490 million Annual deaths, 2.2 to 2.5 million

Schistosomiasis Infected, 150 million Annual deaths, 500,000 to 1 million

Lymphatic filariasis 128 million infected

Leishmaniasis Infected, 1.2 million

Hookworm –1.3 billion infected Amebiasis –Infected, 1% world population –Annual deaths, 40,000 to 110,000 Ascariasis –1.3 billion infected –Annual deaths (intestinal obstruction), 1550

Important Groups of Human Parasites Helminthes Nematode Trematode Cestode Protozoa Amoeba Flagellate Sporidium Ciliate Arthropoda Mosquito Fly Tick Mite Bug Flea Lice

Nematode The intestinal nematodes –Ascaris –Hook worm –Pinworm –Whip worm The blood- and tissue dwelling nematodes –The filaria –Trichinella

Infectivename Pathogenesis stageroutePrevention mode Life cyclePathogenicname Pathogenesis stagelocalizationTreatment migration Diagnosis Diagnosticstage-morphologyDiagnosis stagemethods stage outside name Epidemiology (vector, other host) name Prevention Learning method