Text book of Human Parasitology Edited by Wu Zhongdao(吴忠道)、 Zheng Xiaoyin(郑小英) chizogor Section I INTRodUCTION TO PARASITOLOGY
Text Book of Human Parasitology Edited by Wu Zhongdao(吴忠道)、Zheng Xiaoyin(郑小英) Section I INTRODUCTION TO PARASITOLOGY
Parasitology(寄生虫学), the study of parasites(寄生虫) and their relationships to their hosts(TE=E), is one of the most fascinating areas of the biology. While it is entirely proper to classify many bacteria(细菌) and fungi(真菌) and all viruses as parasites, parasitology has traditionally been limited to parasitic protozoa(原虫), helminthes(蠕虫), and arthropods(节 肢动物), as well as those species of arthropods that serve as vectors(媒介) for parasites. It follows, then, that parasitology encompasses elements of protozoology (Rs), helminthology (蠕虫学), and medical arthropodology(医学节肢动物学) Human parasitology, an important part of parasitology, study the medical parasites including their morphology(形态学, life cycle(生活史), the relationship with host and environment.The objectives are to study the way or the measurement of parasitic diseases control IMPORTANCE OF PARASITOLOGY Why do students need to learn the course now? In part time, parasitic infections or parasitic diseases were the most common diseases in the world. Therefore, parasitology played important ole on the medicine and public health, none neglect the important of parasitology. with the nearly 杀虫剂),an antiparasitic agents, it was for a time widely believed that the infectious diseases would for all practical purposes disappear from the clinical scene. Someone has asked the question, why do medical students still need to learn parasitology? Befor let ep of parasitic diseases in th world. According to the who(世界卫生组织)2001 year report parasitic diseases is still an important human diseases. In the world, 210 million people reside in the endemic areas of alaria(E#), 10 million cases with malaria occur every year; 20 million infected individuals was estimated in the world So TDR/WHO has proclaimed that 10 major unconquered human tropical diseases(热带病), African trypanosomiasis(非洲锥虫), Dengue(登革 热), Leishmaniasis(利什曼病), Malaria(疟疾), Schistosomiasis(血吸虫病), Tuberculosis (结核病), Chagas disease(夏格病又称美洲锥虫病), Leprosy(麻风), Lymphatic filariasis (淋巴丝虫病), Onchocerciasis(盘尾丝虫病). Among them7 diseases are parasitic in the traditional sense. In addition, DDT and other insecticides(s fill) not only have failed to eliminate the vectors of malaria, schistosomiasis, and other parasitic diseases but have themselves brought on problems too well-known to require mention here. The development of resistance to the synthetic antimalarials has been an ominous occurrence in recent years. The increased mobility of large segments of the population, and popularity of the tropics and subtropics as vacation areas, exposes them to a largely undiminished threat of parasitic infection, and the speed of transportation ensures that many return to their native shores before their infections become patent For these reasons it remains necessary that all physicians have some familiarity with the parasitic diseases. no matter ho Modifications of the environment maybe have brought about major increases in parasitic diseases, flooding of vast areas has resulted in new habitats for the snail hosts of schistosomiasis Global warming is suggested as a possible reason for the eventual spread of diseases now seen primarily in the tropics to more temperate climes. An important development of recent years has been the appearance of the human immunodeficiency virus(HIV) and its sequel, the immunodeficiency syndrome(AIDS), which results in greatly increased prevalence and severity of a number of parasitic, viral, and bacterial diseases. As immunosuppression becomes more
1 Parasitology（寄生虫学）, the study of parasites（寄生虫） and their relationships to their hosts（宿主）, is one of the most fascinating areas of the biology. While it is entirely proper to classify many bacteria（细菌） and fungi（真菌） and all viruses as parasites, parasitology has traditionally been limited to parasitic protozoa（原虫）, helminthes（蠕虫）, and arthropods（节 肢动物）, as well as those species of arthropods that serve as vectors（媒介）for parasites. It follows, then, that parasitology encompasses elements of protozoology（原虫学）, helminthology （蠕虫学）, and medical arthropodology（医学节肢动物学）. Human parasitology, an important part of parasitology, study the medical parasites including their morphology(形态学), life cycle（生活史）, the relationship with host and environment. The objectives are to study the way or the measurement of parasitic diseases control. IMPORTANCE OF PARASITOLOGY Why do students need to learn the course now? In part time, parasitic infections or parasitic diseases were the most common diseases in the world. Therefore, parasitology played important role on the medicine and public health, none neglect the important of parasitology. With the nearly simultaneous development of antibiotic drugs, synthetic pesticides（杀虫剂）, and various antiparasitic agents, it was for a time widely believed that the infectious diseases would for all practical purposes disappear from the clinical scene. Someone has asked the question, why do medical students still need to learn parasitology? Before answer the question, let me review the epidemic situation of parasitic diseases in the world. According to the WHO（世界卫生组织） 2001 year report, parasitic diseases is still an important human diseases. In the world, 210 million people reside in the endemic areas of malaria（疟疾）, 10 million cases with malaria occur every year; 20 million infected individuals was estimated in the world. So TDR/WHO has proclaimed that 10 major unconquered human tropical diseases（热带病）, African trypanosomiasis（非洲锥虫）, Dengue（登革 热）, Leishmaniasis（利什曼病）, Malaria（疟疾）, Schistosomiasis（血吸虫病）, Tuberculosis （结核病）, Chagas disease（夏格病又称美洲锥虫病）, Leprosy（麻风）, Lymphatic filariasis （淋巴丝虫病）, Onchocerciasis（盘尾丝虫病）. Among them 7 diseases are parasitic in the traditional sense. In addition, DDT and other insecticides（杀虫剂） not only have failed to eliminate the vectors of malaria, schistosomiasis, and other parasitic diseases but have themselves brought on problems too well-known to require mention here. The development of resistance to the synthetic antimalarials has been an ominous occurrence in recent years. The increased mobility of large segments of the population, and popularity of the tropics and subtropics as vacation areas, exposes them to a largely undiminished threat of parasitic infection, and the speed of transportation ensures that many return to their native shores before their infections become patent. For these reasons it remains necessary that all physicians have some familiarity with the parasitic diseases, no matter how “exotic”. Modifications of the environment maybe have brought about major increases in parasitic diseases, flooding of vast areas has resulted in new habitats for the snail hosts of schistosomiasis. Global warming is suggested as a possible reason for the eventual spread of diseases now seen primarily in the tropics to more temperate climes. An important development of recent years has been the appearance of the human immunodeficiency virus(HIV) and its sequel, the acquired immunodeficiency syndrome(AIDS), which results in greatly increased prevalence and severity of a number of parasitic, viral, and bacterial diseases. As immunosuppression becomes more
widespread, not simply because of AIDS, but also as necessitated by organ transplantation, the result of cancer chemotherapy, or the indiscriminate release of toxic chemicals and carcinogens into environment, heretofore unknown or extremely rare infections are being reported from human These are the reason why the course on Human or Medical Parasitology has been keeping Table I-I Current disease portfolio( from WHO report 2001) Disease burden Deaths DALYs*(thousands) (thousands) tegory Male Female Total Male Female 1,585 1,013 trypanosomiasis Leishmaniasis 1,810 1.067 Malaria 40,2131923720,9761,080 Chagas disease Leprosy 2223333 *DALY: disability- adjusted life years((失能调整生命年) In our country, various parasites have long been recognized as one of the important endemic diseases for many years. In the early 1950s the estimated number of people suffered from schistosomiasis totaled cases 10 million, and that from malaria and filariasis, 30 million each Since the founding of the People's Republic, the Chinese government has paid great attention to investigation and control of parasites, with particular emphasis on the five major ones,i.e schistosomiasis, malaria, filariasis, hookworm diseases and kala azar. Through 50 years'endeavor outstanding achievements have obtained Nevertheless, schisto somiasis is still prevalence in lake-marsh and mountain regions along Changjiang river; at the present, falciparum malaria(恶性疟疾) has not been under effective control in several southern provinces due to the emergence of multi-drug resistar ecological characteristics of the vector mosquito, Anopheles dirus(大劣按蚊) population migration. Besides, a nationwide survey conducted in 1988-1992 disclosed a striking number of parasite-infected population, and a high proportion of polyparasitism(多虫寄生)as well, e.g., overall prevalence of parasites was 59.67%(62.632-0 339%), and more than 700 million cases of infected individuals was estimated in China Although soil-transmitted (EEt) parasites infection have been reduced significantly with improvement of food-transmitted frFt) parasite infection, such as infection of clonorchis, have been become a new public health problem for dietary habits So it is considered that parasitic infection /or parasitic diseases are still one of the important problems in public health in our country. As a candidate for doctor, to learn some knowledge of parasitology is necessary GENERAL CONSIDERATION Medical(human) Parasitology consists of medical protozoology, medical helminthology
2 widespread, not simply because of AIDS, but also as necessitated by organ transplantation, the result of cancer chemotherapy, or the indiscriminate release of toxic chemicals and carcinogens into environment, heretofore unknown or extremely rare infections are being reported from human. These are the reason why the course on Human or Medical Parasitology has been keeping TableⅠ-Ⅰ-1Current disease portfolio(from WHO report 2001) TDR disease category Disease burden DALYs* (thousands) Deaths (thousands) Total Male Female Total Male Female African trypanosomiasis 1 1,585 1,013 572 50 32 18 Dengue 1 433 286 147 12 8 4 Leishmaniasis 1 1,810 1,067 744 41 23 18 Malaria 2 40,213 19,237 20,976 1,080 522 558 Schistosomiasis 2 1,713 1,037 676 11 8 3 Tuberculosis 2 35,792 21,829 13,962 1,660 1,048 613 Chagas disease 3 680 360 320 21 12 9 Leprosy 3 141 76 65 2 2 1 Lymphatic filariasis 3 5,549 4,245 1,304 0 0 0 Onchocerciasis 3 951 549 402 0 0 0 * DALY：disability-adjusted life years(失能调整生命年) In our country, various parasites have long been recognized as one of the important endemic diseases for many years. In the early 1950s, the estimated number of people suffered from schistosomiasis totaled cases 10 million, and that from malaria and filariasis, 30 million each. Since the founding of the People’s Republic , the Chinese government has paid great attention to investigation and control of parasites, with particular emphasis on the five major ones, i.e. schistosomiasis, malaria, filariasis, hookworm diseases and kala azar. Through 50 years’ endeavor, outstanding achievements have obtained. Nevertheless, schistosomiasis is still prevalence in lake-marsh and mountain regions along Changjiang River; at the present, falciparum malaria（恶性疟疾） has not been under effective control in several southern provinces due to the emergence of multi-drug resistant strain and ecological characteristics of the vector mosquito, Anopheles dirus（大劣按蚊）, as well as population migration. Besides, a nationwide survey conducted in 1988-1992 disclosed a striking number of parasite-infected population, and a high proportion of polyparasitism（多虫寄生） as well, e.g., overall prevalence of parasites was 59.67%(62.632±0.339%), and more than 700 million cases of infected individuals was estimated in China Although soil-transmitted（土源性） parasites infection have been reduced significantly with improvement of living conditions, food-transmitted（食源性）parasite infection, such as infection of clonorchis, have been become a new public health problem for dietary habits. So it is considered that parasitic infection /or parasitic diseases are still one of the important problems in public health in our country. As a candidate for doctor, to learn some knowledge of parasitology is necessary. GENERAL CONSIDERATION Medical (human) Parasitology consists of medical protozoology, medical helminthology
and medical arthropodology Symbiosis(#+E) Symbiosis means"living together of both members of species. Any organism that spends a portion or all its life intimately associated with another living organism of a different species is known as a symbiont( or symbiote ), and the relationship is designated as symbiosis. The term symbiosis, as used here, does not imply mutual or unilateral physiologic dependency; rather, it is used in its original sense(living together )without any reference to benefit "or"damage"to the symbionts. There are at least three categories of symbiosis whose are commonly recognized: commensalisms, mutualism and parasitism Commensalism(#tf) It was from Latin for"eating at same table" denotes an association which is beneficial to one partner and at least not disadvantageous to the other The two partners can survive independently. Mutualism(互利共生) Mutualism is an association in which the mutual ist and the host depend on each other physiologically. It is seen where such associations are beneficial to both orga Parasitism(RHE) Parasitism is onother type of symbiotic relationship between two organisms: a parasite(寄生虫), usually the smaller of the two, and a host(宿主), upon which the parasite is physiologically dependent. The relationship may be permanent, as in the case of worms found in the vertebrate intestine as with female mosquitoes. some leeches, and ticks, which feed intermittently on host blood. In other words, it is a symbiotic relationship in which one animal, the host, is to some degree injured through the activities of the Parasite(REs) Its biological definition is an animal or plant which lives in or upon another organism(technically called its host)and draws its nutriment directly from it By this definition all infectious agents, viruses, bacteria, fungi, protozoa, and helminths are parasites, but traditionally protozoa, helnimths and medical arthropod(节肢动物), so called parasites,are tudied in medical or human Parasitology. Therefore,, the textbooks of parasitology today deal only with protozoa, helminthes and some arthropod The parasites broadly are of two types: endoparasite(体内寄生虫) and Ectoparasite(体外 REs). The parasite which lives within the host is called the endoparasite(e. g, leishmania) Invasion by the parasite is called infection. Usually, the endoparasites cause most human diseases The endoparasites include 3 types, such as obligate parasite, facultative parasite and accidental parasite. Obligatove parasites(专性寄生虫) are physiologically dependent upon their hosts and usually cannot survive if kept isolated from them(eg, Toxoplasma gondi 5 s). Facultative parasites(兼性寄生虫), on the other hand, are essentially free-living organisms that are capable of becoming parasitic if placed in a situation conductive to such a mode. An example of facultative parasite is Strongyloides stercoralis(粪类圆线虫). Accidental parasites(偶然寄生 sh), the parasite which attacks an unusual host. Ectoparasite, the parasite which lives on the outer surface or in the superficial tissues of the host(e.g, lice). The infection by these parasites is called infestation(侵扰) Host Host is defined as an organism which harbours the parasite and provides the nourishment and shelter. These hosts, in comparison to their parasites are relatively larger in size The hosts may be of the following types: definitive host, intermediate host, reservoir host and paratenic host etc. 1)Definitive host(AHE E) The hosts which harbour the adult parasites(e. g, taenia
3 and medical arthropodology. Symbiosis（共生） Symbiosis means “living together of both members of species. Any organism that spends a portion or all its life intimately associated with another living organism of a different species is known as a symbiont( or symbiote), and the relationship is designated as symbiosis. The term symbiosis, as used here, does not imply mutual or unilateral physiologic dependency;rather, it is used in its original sense(living together)without any reference to “benefit” or “damage” to the symbionts. There are at least three categories of symbiosis whose are commonly recognized: commensalisms, mutualism and parasitism. Commensalism（共栖） It was from Latin for “eating at same table”, denotes an association which is beneficial to one partner and at least not disadvantageous to the other. The two partners can survive independently. Mutualism（互利共生） Mutualism is an association in which the mutualist and the host depend on each other physiologically. It is seen where such associations are beneficial to both organisms(partners). Parasitism（寄生） Parasitism is onother type of symbiotic relationship between two organisms: a parasite（寄生虫）, usually the smaller of the two, and a host（宿主）, upon which the parasite is physiologically dependent. The relationship may be permanent ,as in the case of tapeworms found in the vertebrate intestine, or temporary, as with female mosquitoes, some leeches, and ticks, which feed intermittently on host blood. In other words, it is a symbiotic relationship in which one animal, the host, is to some degree injured through the activities of the other animal, the parasite. Parasite（寄生虫） Its biological definition is an animal or plant which lives in or upon another organism(technically called its host) and draws its nutriment directly from it. By this definition all infectious agents, viruses, bacteria, fungi, protozoa, and helminths are parasites, but traditionally protozoa , helnimths and medical arthropod（节肢动物）, so called parasites, are studied in medical or human Parasitology. Therefore,, the textbooks of parasitology today deal only with protozoa, helminthes and some arthropod. The parasites broadly are of two types: endoparasite（体内寄生虫） and Ectoparasite（体外 寄生虫）. The parasite which lives within the host is called the endoparasite(e.g, leishmania). Invasion by the parasite is called infection. Usually, the endoparasites cause most human diseases. The endoparasites include 3 types, such as obligate parasite, facultative parasite and accidental parasite.Obligatove parasites(专性寄生虫) are physiologically dependent upon their hosts and usually cannot survive if kept isolated from them(e.g., Toxoplasma gondii 弓形虫). Facultative parasites(兼性寄生虫)，on the other hand, are essentially free-living organisms that are capable of becoming parasitic if placed in a situation conductive to such a mode. An example of a facultative parasite is Strongyloides stercoralis（粪类圆线虫）. Accidental parasites（偶然寄生 虫）, the parasite which attacks an unusual host. Ectoparasite, the parasite which lives on the outer surface or in the superficial tissues of the host(e.g., lice). The infection by these parasites is called infestation(侵扰). Host Host is defined as an organism which harbours the parasite and provides the nourishment and shelter. These hosts, in comparison to their parasites are relatively larger in size. The hosts may be of the following types: definitive host, intermediate host, reservoir host and paratenic host etc. 1) Definitive host（终宿主） The hosts which harbour the adult parasites(e.g., Taenia
saginata causing intestinal taeniasis), most highly developed form of the parasite(e.g Trypanosoma cruci causing African sleeping sickness) or where the parasite replicates sexually(e. g, Paragonimus westermani) are called the definitive hosts. The definitive hosts may be human or non-human living things 2) Intermediate host(中间终主) The hosts which harbour the larval stages of parasite development or the asexual forms of the parasite are called intermediate host. Some times two different hosts may be required to complete different larval stages. These are known as the first and second intermediate hosts respectively(e. g snails t] are the first intermediate hosts and fresh water fish are the second intermediate hosts for Clonorchis sinensi AfRs s 3) Reservoir host(保虫宿主) The animal which harbours the parasites and serves as an important source of infection to other susceptible hosts are known as reservoir host(e.g,water buffalo is the reservoir host for schistosomiasis血吸虫病) 4) Paratenic host or transport host(转续宿主) The larva of some parasites can invade a non-normal host, but can not develop, and only keep the larva stage. If the larva enter a normal definitive host, it can continue to develop into adult worm. The non-normal host is called paratenic lost or transport host. It functions as a transport or carrier host TAXONOMY(分类学) ature(命名法) Nature”1758), each parasite has two names: a Genus(属名) and a Species name(种名). These names are derived either from (1)Greek or Lati n wor (2) Names of their discoverers (3) Geographical area where found (4)Hosts in which parasites are found, or (5)Habitat of the parasite The correct scientific name of the parasite consists of the genus and species to which belong, the name of the designator and the year in which it was discovered(e.g, Angiostrongylus cantonensis管圆线虫Chen,1935) Dougherty,1946) The animal parasites of human and most vertebrates are contained in five or more major ubdivisions or phyla (IT) Phylum Sarcomastigophora(肉鞭毛虫门). This phylum is divided into two subphyla:the Mastigophora or flagellates(鞭毛虫纲), and the Sarcodina or amebae(肉足纲) Phylum Apicomplexa(JiAl]) Members of this phylum are tissue parasites. Apicomplexa have a complex life cycle with alternating sexual and asexual generations. Phylum Microspore(微孢子门) Members of the Microspora are minute intracellular parasites of many kinds of vertebrates and invertebrates, and they differ significantly in structure from the Apicomplexa. Microsporidia rarely cause diseases in immunocompetent persons, but many do so with greater frequency in immunosuppressed persons Phylum Ciliophora(纤毛虫纲) The ciliates include a variety of free- living and symbiotic species. The only ciliate parasite of human is Balantidium coli, found in the intestinal tract. Although rare, it is important, as it may produce severe intestinal symptoms Phylum Platyhelminthes(扁形动物门) The Platyhelminthes, or flatworms,are multicellular animals characterized by a flat bilaterally symmetric body. Most flatworms are
4 saginata causing intestinal taeniasis), most highly developed form of the parasite(e.g., Trypanosoma cruzi causing African sleeping sickness) or where the parasite replicates sexually(e.g., Paragonimus westermani) are called the definitive hosts. The definitive hosts may be human or non-human living things. 2) Intermediate host（中间终主） The hosts which harbour the larval stages of parasite development or the asexual forms of the parasite are called intermediate host. Some times two different hosts may be required to complete different larval stages. These are known as the first and second intermediate hosts respectively(e.g., snails 钉螺 are the first intermediate hosts and fresh water fish are the second intermediate hosts for Clonorchis sinensi 肝吸虫 s). 3) Reservoir host（保虫宿主） The animal which harbours the parasites and serves as an important source of infection to other susceptible hosts are known as reservoir host(e.g., water buffalo is the reservoir host for schistosomiasis 血吸虫病). 4) Paratenic host or transport host（转续宿主） The larva of some parasites can invade a non-normal host, but can not develop, and only keep the larva stage. If the larva enter a normal definitive host, it can continue to develop into adult worm. The non-normal host is called paratenic host or transport host. It functions as a transport or carrier host. TAXONOMY（分类学） According to the biomial nomenclature（命名法） as suggested by Linnaeus(“Systema Nature” 1758),each parasite has two names: a Genus(属名) and a Species name（种名）. These names are derived either from ⑴ Greek or Latin words ⑵ Names of their discoverers ⑶ Geographical area where found ⑷ Hosts in which parasites are found, or ⑸ Habitat of the parasite The correct scientific name of the parasite consists of the genus and species to which it belong, the name of the designator and the year in which it was discovered(e.g., Angiostrongylus cantonensis 管圆线虫(Chen, 1935) Dougherty, 1946). The animal parasites of human and most vertebrates are contained in five or more major subdivisions or phyla（门）. Phylum Sarcomastigophora(肉鞭毛虫门). This phylum is divided into two subphyla: the Mastigophora or flagellates(鞭毛虫纲), and the Sarcodina or amebae（肉足纲）. Phylum Apicomplexa(顶复门) Members of this phylum are tissue parasites..Apicomplexa have a complex life cycle with alternating sexual and asexual generations. Phylum Microspora(微孢子门) Members of the Microspora are minute intracellular parasites of many kinds of vertebrates and invertebrates, and they differ significantly in structure from the Apicomplexa. Microsporidia rarely cause diseases in immunocompetent persons, but many do so with greater frequency in immunosuppressed persons. Phylum Ciliophora(纤毛虫纲) The ciliates include a variety of free-living and symbiotic species. The only ciliate parasite of human is Balantidium coli, found in the intestinal tract. Although rare, it is important, as it may produce severe intestinal symptoms. Phylum Platyhelminthes( 扁 形 动物 门) The Platyhelminthes, or flatworms, are multicellular animals characterized by a flat, bilaterally symmetric body. Most flatworms are
hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes. The classes Trematoda and Cestoda contain parasitic forms Phylum Aschelminthes(蛔线虫门) The nematodes, or roundworm s. are elongate lindrical worms, frequently attenuated at both ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free-livinge. g, Caenorhabditis elegans), a large number of species parasitize humans, animals, and plants. Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species Phylum Acanthocephala(棘头虫门) The thorny- headed worms are all endoparasite organisms. While thorny-headed worms are widely distributed among wild and domestic animal only three genera have been reported in human beings including Macracanthorhynchus hirudinaceusy(猪巨吻棘头虫 Phylum Arthropoda(节肢动物门) The phylum is subdivided into a number if classes. many of which are of medical importance. The classes main include the Class arachnida( E=) and Class Insecta(EE s!). The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites. Certain ticks and mites many transmit diseases Insects have three pairs of legs and a body divided into three distinct parts: Insects head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs etc MORPHOLOGY(形态学) The protozoa are small, unicellular organisms which are morphologically and fu complete. A single cell carries out all the functions such as digestion, respiration, excretion The helminths are larger organisms. A particular function such as reproduction, digestion or excretion is performed by a group of special cells Arthropods are segmented and bilaterally symmetrical (XfFk BJ) animals with a body enclosed in a stiff, chitinous(甲壳质) covering or exoskeleton(外骨骼) and bearing paired jointed appendages. The digestive system is well developed. Sexes are separate LIFE CYCLE(生活史) The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stage of the parasite pleted in a single host such as man. Change of host is required only to propagate the parasite in the community(eg, E histolytica溶组织阿米巴, Trichuris trichiura H s, etc). Some of the parasites require two different hosts to complete their various stage of development(e.g, Schistosoma japonicum日本血吸虫etc). In a complex life cle many parasites require two different hosts, one definitive host and one intermediate host to complete their life cycle(e. g, Schistosoma species require man as definitive host and snail as intermediate hosts). Few of the parasite require two different intermediate hosts apart from a single definitive host(e.g, Paragonimus westerman卫氏并殖吸虫 requires snails as the first intermediate host and fresh water fish and crabs as the second intermediate host, apart from man and the fish eating mammals as the definitive host
5 hermaphroditic, having both male and female reproductive organs in the same individual. The sexes are separate in the schistosomes. The classes Trematoda and Cestoda contain parasitic forms only. Phylum Aschelminthes(蛔线虫门) The nematodes, or roundworms, are elongate, cylindrical worms, frequently attenuated at both ends. The sexes are separate, the male frequently being considerably smaller than the female. A well-developed digestive tract is present. While most nematodes are free-living(e.g., Caenorhabditis elegans), a large number of species parasitize humans, animals, and plants. Intermediate hosts are necessary for the larval development of some forms. Parasites of humans include intestinal and tissue-inhabiting species. Phylum Acanthocephala(棘头虫门) The thorny-headed worms are all endoparasite organisms. While thorny-headed worms are widely distributed among wild and domestic animal, only three genera have been reported in human beings including Macracanthorhynchus hirudinaceus(猪巨吻棘头虫). Phylum Arthropoda(节肢动物门) The phylum is subdivided into a number if classes, many of which are of medical importance. The classes main include the Class Arachnida(蛛形纲) and Class Insecta(昆虫纲). The Arachnida, or spiderlike animals, possess a body divided into two parts, the cephalothorax and the abdomen. Adults have four pairs of legs. Included in this class are the scorpions, the spiders, and the ticks and mites. Certain ticks and mites many transmit diseases. Insects have three pairs of legs and a body divided into three distinct parts: Insects head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs etc. MORPHOLOGY（形态学） The protozoa are small, unicellular organisms which are morphologically and functionally complete. A single cell carries out all the functions such as digestion, respiration, excretion, reproduction, etc. The helminths are larger organisms. A particular function such as reproduction, digestion or excretion is performed by a group of special cells. Arthropods are segmented and bilaterally symmetrical（对称的）animals with a body enclosed in a stiff, chitinous（甲壳质） covering or exoskeleton（外骨骼） and bearing paired, jointed appendages. The digestive system is well developed. Sexes are separate. LIFE CYCLE（生活史） The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stage of the parasite are completed in a single host such as man. Change of host is required only to propagate the parasite in the community (e.g., E.histolytica 溶组织阿米巴, Trichuris trichiura 鞭虫, etc). Some of the parasites require two different hosts to complete their various stage of development (e.g., Schistosoma japonicum 日本血吸虫, etc). In a complex life cycle many parasites require two different hosts, one definitive host and one intermediate host to complete their life cycle(e.g., Schistosoma species require man as definitive host and snail as intermediate hosts). Few of the parasite require two different intermediate hosts apart from a single definitive host(e.g., Paragonimus westermani 卫氏并殖吸虫 requires snails as the first intermediate host and fresh water fish and crabs as the second intermediate host, apart from man and the fish eating mammals as the definitive host.)
TRANSMISSION OF PARASITES It depend upon: Source or reservoir of infection, and Mode of transmission Source ofinfection(传染源 1)Humans Humans is the source or reservoir in a majority of parasitic infections(e. g taeniasis, amoebiasis, etc). The condition in which the infection is transmitted from one infected man to another man is called anthroponosis(人类传染病) 2)Animal In many of the parasitic diseases, animals act as the source of infection. The condition where infection is transmitted from animals to humans is called zoonoses人兽共患病 (eg, hydatid disease包虫病) Mode of transmission(传播方式) Transmission of infection fiom one host to another; cause by a certain form of the parasite is known as the infective stage. The infective stage of various parasites many be transmitted fiom one host to another in the following ways 1)Oral route Ingestion of food, water and vegetable: The infection is transmitted orally by ingestion of food, water or vegetables contaminated by the faeces that contain the infective stages of the parasite. This mode of transmission is referred to as faecal-oral route(e.g, cysts of Giardia intestinalis贾第虫 and Entamoeba histolytica溶组织阿米巴; ova of Ascaris lumbricoides蛔虫, Trichuris trichura鞭虫 and Enterobius vermicularis蛲虫) ngestion of raw or undercooked meat: The infection is transmitted orally also by ingestion of raw or undercooked meat harbouring the infective stage of the parasite(e. g, pork containing cysticercus cellulosae猪囊尾蚴, the larval stage of Taenia solium猪带绦虫) Ingestion of raw or uncooked fish and crab: Infection is transmitted by ingestion of raw or infective stage of the parasite(e. g, crab or cray fish containing the metacercariae of Paragonimus esterman卫氏并殖吸虫i, fish harbouring the metacercariae囊蚴 of Clonorchis sinensis华支睾 吸虫/肝吸虫,etc) Ingestion of raw or under cooked water plants: Infection can be transmitted bt eating raw or under cooked water plants harbouring the infective form of the parasite(e.g, water chest nuts, etc containing metacercariae of Fasciolopsis buski布氏姜片虫 and Fasciola hepatica肝片形吸虫) 2) Penetration of the skin and mucous membrane The infection is transmitted by A)Penetration of the intact skin by filariformlarvae2i#h)of hookworm, Sreongyloides stercoralis on coming in contact with faecally polluted soil, and B) Piercing the skin by cercariae of Schistosoma japonicum日本血吸虫, S mansoni曼氏 血吸虫andS. haematobium埃及血吸虫 on coming in contact with infected water. 3)Inoculation by an arthropod vector The infection also can be transmitted by A) ne blood by Anopheles( 疟原虫) B) Inoculation into the skin by mosquitoes( vectors for Wuchereria bancrofti斑氏丝虫 Brugia malayi马来丝虫 Trichomonas is transmitted by sexual contact. Frequently, Entamoeba also is transmitted by sexual contact among homosexuals(同性恋) HOST-PARASITE EXISTENCE Establishment of the parasite in its host is referred to as an infection The outcome of the infection is highly variable. It may be(a) sub-clinical latent infection,(b) clinical disease or(c) carrier(携带者) 6
6 TRANSMISSION OF PARASITES It depend upon: Source or reservoir of infection, and Mode of transmission. Source of infection（传染源） 1) Humans Humans is the source or reservoir in a majority of parasitic infections(e.g., taeniasis, amoebiasis, etc). The condition in which the infection is transmitted from one infected man to another man is called anthroponoses(人类传染病). 2) Animal In many of the parasitic diseases, animals act as the source of infection. The condition where infection is transmitted from animals to humans is called zoonoses 人兽共患病 (e.g., hydatid disease 包虫病). Mode of transmission（传播方式） Transmission of infection from one host to another, cause by a certain form of the parasite is known as the infective stage. The infective stage of various parasites many be transmitted from one host to another in the following ways. 1) Oral route Ingestion of food, water and vegetable: The infection is transmitted orally by ingestion of food, water or vegetables contaminated by the faeces that contain the infective stages of the parasite. This mode of transmission is referred to as faecal-oral route(e.g., cysts of Giardia intestinalis 贾第虫 and Entamoeba histolytica 溶组织阿米巴； ova of Ascaris lumbricoides 蛔虫, Trichuris trichura 鞭虫 and Enterobius vermicularis 蛲虫). Ingestion of raw or undercooked meat: The infection is transmitted orally also by ingestion of raw or undercooked meat harbouring the infective stage of the parasite(e.g., pork containing cysticercus cellulosae 猪囊尾蚴, the larval stage of Taenia solium 猪带绦虫). Ingestion of raw or uncooked fish and crab: Infection is transmitted by ingestion of raw or under cooked fish and crab containing the infective stage of the parasite(e.g., crab containing the infective stage of the parasite(e.g., crab or cray fish containing the metacercariae of Paragonimus westerman卫氏并殖吸虫i, fish harbouring the metacercariae 囊蚴of Clonorchis sinensis华支睾 吸虫/肝吸虫, etc). Ingestion of raw or under cooked water plants: Infection can be transmitted bt eating raw or under cooked water plants harbouring the infective form of the parasite(e.g., water chest nuts, etc., containing metacercariae of Fasciolopsis buski 布氏姜片虫 and Fasciola hepatica 肝片形吸虫). 2) Penetration of the skin and mucous membrane The infection is transmitted by A) Penetration of the intact skin by filariformlarvae（丝状蚴）of hookworm, Sreongyloides stercoralis on coming in contact with faecally polluted soil, and B) Piercing the skin by cercariae of Schistosoma japonicum 日本血吸虫, S. mansoni 曼氏 血吸虫 and S. haematobium 埃及血吸虫 on coming in contact with infected water. 3) Inoculation by an arthropod vector The infection also can be transmitted by A) Inoculation into the blood by Anopheles(vector for Plasmodium 疟原虫). B) Inoculation into the skin by mosquitoes(vectors for Wuchereria bancrofti 斑氏丝虫, Brugia malayi 马来丝虫 etc). 4) Sexual contact Trichomonas is transmitted by sexual contact. Frequently, Entamoeba also is transmitted by sexual contact among homosexuals（同性恋）. HOST-PARASITE EXISTENCE Establishment of the parasite in its host is referred to as an infection. The outcome of the infection is highly variable. It may be(a) sub-clinical latent infection, (b) clinical disease or (c) carrier（携带者）
The disease is the clinical manifestation of the infection which shows the active pre and replication of the parasite causing damage in the host. It may be mild, severe, fulminant(W tE J ), and in some cases may even cause death of the host The person who is infected with the parasite but without any clinical or sub clinical disea is referred to as a carrier PARASITIC ZOONOSES(人兽共患病) These are the infections which are naturally transmitted between the vertebrate animals and man. The condition usually includes those infections in which the proof of strong circumstantial evidence of transmission between the man and animals are documented PATHOGENESIS AND PATHOLOGY(发病机制与病理学 Pathogenesis(发病机制) of the parasitic diseases is a dynamic process and depends on the complex interaction of a variety of host and parasitic factors Host factors The host factor include 1) Nutritional status of the host, whether malnutrition or under nutrition 2) Immune response to parasitic infection 3) Immune status of the host whether there is immuno-suppression or not 4)The presence or absence of the co-existing disease or other physiological conditions such as pregnancy, 5) The age and level of the immunity at the time of infection Parasitic factors The parasitic factors include 1) Site of the attachment of the parasite and the size of the parasite. 2) Number of invading parasites, and 3)Parasite strain(pathogenic or non-pathogenic) and the growth, development and multiplication of parasites inside the human body and their metabolic products The parasites can cause disease in man in various ways as follows: trauma by adult worm larva, and egg(e. g, hookworm cause oozing of the blood at the site of attachment); Invasion and destruction of host cell( Plasmodium and Toxoplasma are obligate intracellular parasites of man they produce several enzymes which cause digestion and necrosis of host cells); Inflammatory reaction(many of the parasite induce inflammatory reactions in the host leading to the formation of various pathological lesions); Toxin( parasites like bacteria also produce toxins but they appear to have a minimal role in the pathogenesis of the disease processes; Allergic manifestation( many of the metabolic and excretory products of the parasites absorbed in the circulation, produce a variety of immunological and allergic manifestations in the sensitized hosts). Various pathogenic mechanisms in parasitic diseases are summarized in the below table I-1-2 The parasitic infections usually are designate by generic names of the parasites en asis or-osis.(e.g, Schistosoma infection is called as schistosomiasis) Table 1-1-2 Pathogenic Mechanisms in Parasitic Diseases Mechanism Parasitic diseases Trauma Strongyloidiasis, enterobiasis, taeniasis, clonorchiasis, schistosomiasis and hookworm infection
7 The disease is the clinical manifestation of the infection which shows the active presence and replication of the parasite causing damage in the host. It may be mild, severe, fulminant（爆发 性的）,and in some cases may even cause death of the host. The person who is infected with the parasite but without any clinical or sub clinical diseases is referred to as a carrier. PARASITIC ZOONOSES(人兽共患病) These are the infections which are naturally transmitted between the vertebrate animals and man. The condition usually includes those infections in which the proof of strong circumstantial evidence of transmission between the man and animals are documented. PATHOGENESIS AND PATHOLOGY（发病机制与病理学） Pathogenesis（发病机制） of the parasitic diseases is a dynamic process and depends on the complex interaction of a variety of host and parasitic factors. Host factors The host factor include: 1) Nutritional status of the host, whether malnutrition or under nutrition. 2) Immune response to parasitic infection 3) Immune status of the host whether there is immuno-suppression or not. 4) The presence or absence of the co-existing disease or other physiological conditions such as pregnancy, and 5) The age and level of the immunity at the time of infection. Parasitic factors The parasitic factors include: 1) Site of the attachment of the parasite and the size of the parasite. 2) Number of invading parasites, and 3) Parasite strain(pathogenic or non-pathogenic) and the growth, development and multiplication of parasites inside the human body and their metabolic products. The parasites can cause disease in man in various ways as follows: trauma by adult worm, larva, and egg(e.g., hookworm cause oozing of the blood at the site of attachment); Invasion and destruction of host cell(Plasmodium and Toxoplasma are obligate intracellular parasites of man , they produce several enzymes which cause digestion and necrosis of host cells); Inflammatory reaction(many of the parasite induce inflammatory reactions in the host leading to the formation of various pathological lesions); Toxin(parasites like bacteria also produce toxins but they appear to have a minimal role in the pathogenesis of the disease processes; Allergic manifestation(many of the metabolic and excretory products of the parasites absorbed in the circulation, produce a variety of immunological and allergic manifestations in the sensitized hosts). Various pathogenic mechanisms in parasitic diseases are summarized in the below table Ⅰ-Ⅰ-2. The parasitic infections usually are designate by generic names of the parasites ending with -iasis or –osis. (e.g., Schistosoma infection is called as schistosomiasis) TableⅠ-Ⅰ-2 Pathogenic Mechanisms in Parasitic Diseases Mechanism Parasitic diseases Trauma Strongyloidiasis, enterobiasis, taeniasis,clonorchiasis, schistosomiasis and hookworm infection
Invasion and destruction of host cell Malaria, leishmaniasis, trypanosomiasis, Inflammation Trichnellosis, lymphatic filariasis, paragonimiasis, Amoebiasis, Chaga's disease and sleeping sickness Alergic manifestation Schistosomiasis, hydatid disease HOST IMMUNITY The host resistance or immunity in parasitic infections refers to the resistance offered by the host towards the injury caused by the parasites and their products. It may be classified into: a Innate, and b)Acquired immunity Innate immunity(先天性免疫) t is the inherited but non- immune type of the host defence against a parasite,eg, Haemoglobin- s thalassaemia(地中海贫血)andg! mucose-6- phosphate dehydrogenase(葡萄糖6-磷酸脱氢酶,G6PD) deficient erythrocytes(红细胞)are resistant against Plasmodium falciparum恶性疟原虫; persons with Duffy- negative genes(Dufy 阴性基因) are resistant to malaria,etc. Acquired immunity(获得性免疫) t may be a)Non- specific b)Specif. 1)Non-specific or acquired immunity It confers protective immunity against many elminthic infections. It is developed during exposure of persons to antigenically unrelated micro-organisms, microbial extracts or some sythetic products. Non-specific immunity has been shown to supplement the specific acquired immunity. These appear to be mediated by macrophages or their active products and also by interferons 2)Specific acquired immunity It is mediated by both humoral and cell mediated Immunities a) Humoral immunity It is mediated through the production of specific antibodies These antibodies are serum proteins and gamma globulins in nature. These antibodies may be protective or non-protective. The antibodies may offer protection in following ways O The antibodies prevent the parasites from attaching and penetrating the host cells by binding the specific sites on the surface of parasites the The antibodies neutralize parasite toxins and inactivate parasite enzymes by binding with determinants of parasitic antigens 8 The secretory IgA antibodies found in various body secretions prevent attachment of some protozoal parasites in the gut wall epithelium In a few parasitic infections(e.g, trypanosomiasis), the parasites are killed by lysis of antibody-coated cells mediated by the complement, and 6 The antibody dependent cell mediated cytotoxicity ADCC) helps in the killing of a few helminths coated by specific antibodies. It is an important mechanism by which the parasites are killed. This is mediated mainly by the lymphocytes and to some extent by neutrophils, eosinophils and macrophages. The antibodies are mainly of IgG, IgE and bind specifically to the parasites b)Cell mediated immunity (CMI) It is mediated through T cell which are cytotoxic. The CMI offers protection against many parasitic infections in following ways O Cytotoxic T lymphocytes alter the osmotic permeability of parasitic cells causing
8 Invasion and destruction of host cell Malaria, leishmaniasis,trypanosomiasis, toxoplasmosis and amoebiasis Inflammation Trichnellosis, lymphatic filariasis, paragonimiasis, Amoebiasis Toxin Amoebiasis, Chaga’s disease and sleeping sickness Allergic manifestation Schistosomiasis, hydatid disease HOST IMMUNITY The host resistance or immunity in parasitic infections refers to the resistance offered by the host towards the injury caused by the parasites and their products. It may be classified into: a) Innate, and b) Acquired immunity. Innate immunity（先天性免疫） It is the inherited but non-immune type of the host defence against a parasite., e.g., Haemoglobin-S thalasaemia（地中海贫血） and glucose- 6- phosphate dehydrogenase （葡萄糖 6-磷酸脱氢酶，G6PD）deficient erythrocytes（红细胞） are resistant against Plasmodium falciparum 恶性疟原虫; persons with Duffy-negative genes（Duffy 阴性基因）are resistant to malaria, etc. Acquired immunity（获得性免疫） It may be a) Non-specific or acquired immunity, or b)Specific acquired immunity. 1) Non-specific or acquired immunity It confers protective immunity against many protozoal and helminthic infections. It is developed during exposure of persons to antigenically unrelated micro-organisms, microbial extracts or some sythetic products. Non-specific immunity has been shown to supplement the specific acquired immunity. These appear to be mediated by macrophages or their active products and also by interferons. 2) Specific acquired immunity It is mediated by both humoral and cell mediated immunities. a) Humoral immunity It is mediated through the production of specific antibodies. These antibodies are serum proteins and gamma globulins in nature. These antibodies may be protective or non-protective. The antibodies may offer protection in following ways: ① The antibodies prevent the parasites from attaching and penetrating the host cells by binding the specific sites on the surface of parasites. ② The antibodies neutralize parasite toxins and inactivate parasite enzymes by binding with the determinants of parasitic antigens. ③ The secretory IgA antibodies found in various body secretions prevent attachment of some protozoal parasites in the gut wall epithelium. ④ In a few parasitic infections(e.g., trypanosomiasis), the parasites are killed by lysis of antibody-coated cells mediated by the complement, and ⑤ The antibody dependent cell mediated cytotoxicity(ADCC) helps in the killing of a few helminths coated by specific antibodies. It is an important mechanism by which the parasites are killed. This is mediated mainly by the lymphocytes and to some extent by neutrophils, eosinophils and macrophages. The antibodies are mainly of IgG, IgE and bind specifically to the parasites. b) Cell mediated immunity(CMI) It is mediated through T cell which are cytotoxic. The CMI offers protection against many parasitic infections in following ways. ① Cytotoxic T lymphocytes alter the osmotic permeability of parasitic cells causing
swelling and disruption of cells there by lysing the cells(e. g, Plasmodium falciparum infection in (2 Activated macrophages kill in various ways such as producing enzymes or activated substances(e. g, Toxoplasma, nania. the schistosomule of Schistosoma species or by producing hydrogen peroxide( Leishmania species etc) 3 Natural killer cells These cells appear to be helpful in the initial host resistance against many parasitic infections IMMUNE RESPONES ox The immune response of man against parasitic infections are variable. It m Protective, or b)Harmful to host Protective immune response 1)Sterilizing immunity The sterilizing or complete immunity is associated with the clinical cure, complete elimination of the parasite from the host and life long resistance against ubsequent infection. It occurs rarely in humans. It occurs only in the cutaneous leishmaniasis 2)Incomplete immunity It is associate with the clinical recovery from the disease and the development of immunity to specific challenge with the parasite. The parasites always persist in the host, even though relatively at a low level. This incomplete immunity also known as premunition(带虫免疫) typically is found in many protozoal infections(e.g, malaria,as concomitant immunity(伴随免疫” typically is found in helminthic infection( schistosomiasis 3)Absence of an effective immunity It is seen after complete clinical cure from tryp Harmful immune responses In this condition, immune regulatory system shows a negative effect by inhibiting protective immune responses and instead produces harmful effects in the host. It is manifested development of hypersensitivity reactions This is of four reaction: Type I, Il, Ill, IV hypersensitivity ① Anaphylactic reaction(过敏反应) It is type I hypersensitivity reaction. It is involved in the pathogenesis of tropical pulmonary eosinophilia, Loefflers pneumonia, swimmers'itch and anaphylactic reaction of ruptured hydatid cyst inside the body. The skin manifestations of tl anaphylactic reactions characteristically are seen during the phase of invasion of the skin by the arvae of of hookworm(钩虫), Strongyloid(类圆线虫), Schistosoma and other parasites ② Cytoxic(细胞毒) t is type ll reaction. t is responsible for a) anaemia in malaria, b) chronic myocarditis(心肌炎) and megacolon(巨结肠) in Chagas' disease,c) quinine(奎 宁) induced massive haemolysis(溶血) and haemoglobinuria(血蛋白尿) in malaria Immune complex mediated(免疫复合物介导) This is type Ill reaction and responsible for development of glomerulonephritis(肾小球肾炎) seen in malaria. It is also responsible for immune complex mediated nephritis in leishmaniasis, trichinosis and ④ Delayed hypersensitive(迟发性变态反应) This is type IV reacti responsible for development of pathological lesions in a) Schistosoma species infections, b)local lymphatic inflammation in filariasis, c)inflammation of muscle tissue around Trichinella and d) survival and proliferation of parasites
9 swelling and disruption of cells there by lysing the cells(e.g., Plasmodium falciparum infection in man). ② Activated macrophages kill parasites in various ways such as producing enzymes or activated substances(e.g., Toxoplasma, Leishmania, the schistosomule of Schistosoma species or by producing hydrogen peroxide(Leishmania species etc). ③ Natural killer cells These cells appear to be helpful in the initial host resistance against many parasitic infections. IMMUNE RESPONES The immune response of man against parasitic infections are variable. It may be: a) Protective, or b) Harmful to host. Protective immune response 1) Sterilizing immunity The sterilizing or complete immunity is associated with the clinical cure, complete elimination of the parasite from the host and life long resistance against subsequent infection. It occurs rarely in humans. It occurs only in the cutaneous leishmaniasis. 2) Incomplete immunity It is associate with the clinical recovery from the disease and the development of immunity to specific challenge with the parasite. The parasites always persist in the host, even though relatively at a low level. This incomplete immunity also known as “premunition(带虫免疫)” typically is found in many protozoal infections(e.g., malaria), as “concomitant immunity(伴随免疫)” typically is found in helminthic infection(schistosomiasis). 3) Absence of an effective immunity It is seen after complete clinical cure from infections(e.g.,amoebiasis, visceral leishmaniasis, American trypanosomiasis). Harmful immune responses In this condition, immune regulatory system shows a negative effect by inhibiting protective immune responses and instead produces harmful effects in the host. It is manifested development of hypersensitivity reactions. This is of four reaction: Type I, II, III, IV hypersensitivity. ① Anaphylactic reaction(过敏反应) It is type I hypersensitivity reaction. It is involved in the pathogenesis of tropical pulmonary eosinophilia, Loefflers pneumonia, swimmers’ itch and anaphylactic reaction of ruptured hydatid cyst inside the body. The skin manifestations of the anaphylactic reactions characteristically are seen during the phase of invasion of the skin by the larvae of of hookworm（钩虫）, Strongyloide（类圆线虫）, Schistosoma and other parasites. ② Cytoxic（细胞毒） It is type II reaction. It is responsible for a) anaemia in malaria, b) chronic myocarditis（心肌炎） and megacolon（巨结肠） in Chagas’ disease ， c) quinine（奎 宁） induced massive haemolysis（溶血） and haemoglobinuria（血蛋白尿） in malaria . ③ Immune complex mediated（免疫复合物介导） This is type III reaction and is responsible for development of glomerulonephritis（肾小球肾炎） seen in malaria. It is also responsible for immune complex mediated nephritis in leishmaniasis, trichinosis and schistosomiasis. ④ Delayed hypersensitivity（迟发性变态反应） This is type IV reaction and is responsible for development of pathological lesions in a) Schistosoma species infections, b) local lymphatic inflammation in filariasis, c) inflammation of muscle tissue around Trichinella and d) survival and proliferation of parasites