IMMUNOEⅤ ASION OF PARASITES(寄生虫免疫逃进) Many parasites survive and proliferate in immunologically competent host by of mechanisms l) Intracellular location(eg, toxoplasma弓形虫, leishmania利什曼原虫) 2) Antigenic shedding(e.g, Entamoeba内阿米巴 Toxoplasma号形虫, fasciola片形吸虫, Trichinella旋毛虫) 3) Antigenic variation( Trypanosoma锥虫) 4) Antigenic mimicry( Schistosoma血吸虫),and )Modification of host immune responses. This is caused by inactivation of complement(e. g, Taenia带绦虫), Immune suppression(e.g, Plasmodium疟原虫, leishmania利什 曼原虫, Toxoplasma弓形虫, Wuchereria吴策线虫属,Bngi布鲁线虫属) activation of lymphocytes(eg, Trypanosoma brucei布氏锥虫, Schistosoma species), modified leucocyte function(e:g, Fasciola hepatica肝片吸虫) and immune complex(e.g, Leishmania利什曼原虫 Trypanosoma brucei布氏锥虫, Toxolasma弓形虫, Plasmodium疟原虫) CLINICAL MANIFESTATION(临床表现) Clinical manifestation of parasitic diseases are variable. It may be acute or chronic. However, many of the diseases are chronic in nature. The onset of the disease is slow. In a few parasitic diseases, the onset may be sudden. For example, in ascariasis, pneumonitis develop immediately few days after ingestion of Ascaris $ss eggs. Ingestion of pork infected with the larvae of Trichinella spiralis MEEs, causes gastro-intestinal disturbances within a few hours, simulating food-poisoning Allergic manifestation are important in many a helminthic infections. Schistosoma eggs produce an allergic reaction in the host tissue. Similarly, the adult Brugia and Wuchereria worm in the lymphatics cause frequent attacks of filarial fever, lymphangitis, etc. A variety of localized generalized allergic and neurological manifestations may be caused by inoculation of toxins the skin by arthropods, during the bite DIAGNOSIS The diagnosis of parasitic infections depends upon: 1) Clinical diagnosis, and 2)Laboratory Clinical diagnosis In areas where the disease is endemic, clinical diagnosis may be made by the characteristic clinical manifestations of the disease. However, in some situations even in endemic areas, the clinical diagnosis is hindered by 1)Low prevalence of major clinical signs 2) Late development of clinical signs 3)Lack of specificities of clinical signs 4)Occurrence of asymptomatic carriers In non-endemic areas, clinical diagnosis may even be more difficult Laboratory diagnosis It plays an important role in establishing the specific diagnosis and supplementing the clinical diagnosis of the condition. It depends upon 1)Morphological recognition of parasites in relevant specimens( parasitic diagnosis) 2)Immunological tests(immuno diagnosis) 3)DNA probes or PCR etc
10 IMMUNOEVASION OF PARASITES（寄生虫免疫逃避） Many parasites survive and proliferate in immunologically competent host by of mechanisms. These include. 1) Intracellular location(e.g., Toxoplasma 弓形虫, leishmania 利什曼原虫) 2) Antigenic shedding(e.g., Entamoeba 内阿米巴, Toxoplasma 弓形虫, Fasciola 片形吸虫, Trichinella 旋毛虫) 3) Antigenic variation(Trypanosoma 锥虫) 4) Antigenic mimicry(Schistosoma 血吸虫), and 5) Modification of host immune responses. This is caused by inactivation of complement(e.g., Taenia带绦虫), immune suppression(e.g., Plasmodium疟原虫, leishmania利什 曼原虫, Toxoplasma 弓形虫, Wuchereria 吴策线虫属, Brugi 布鲁线虫属), activation of lymphocytes(e.g., Tryanosoma brucei 布氏锥虫, Schistosoma species), modified leucocyte function(e.g., Fasciola hepatica 肝片吸虫) and immune complex(e.g., Leishmania 利什曼原虫, Trypanasoma brucei 布氏锥虫, Toxplasma 弓形虫, Plasmodium 疟原虫). CLINICAL MANIFESTATION（临床表现） Clinical manifestation of parasitic diseases are variable. It may be acute or chronic. However, many of the diseases are chronic in nature. The onset of the disease is slow. In a few parasitic diseases, the onset may be sudden. For example, in ascariasis, pneumonitis develop immediately few days after ingestion of Ascaris 蛔虫 eggs. Ingestion of pork infected with the larvae of Trichinella spiralis 旋毛虫, causes gastro-intestinal disturbances within a few hours, simulating food-poisoning. Allergic manifestation are important in many a helminthic infections. Schistosoma eggs produce an allergic reaction in the host tissue. Similarly, the adult Brugia and Wuchereria worm in the lymphatics cause frequent attacks of filarial fever, lymphangitis, etc. A variety of localized and generalized allergic and neurological manifestations may be caused by inoculation of toxins into the skin by arthropods, during the bite. DIAGNOSIS The diagnosis of parasitic infections depends upon: 1) Clinical diagnosis, and 2) Laboratory diagnosis. Clinical diagnosis In areas where the disease is endemic, clinical diagnosis may be made by the characteristic clinical manifestations of the disease. However, in some situations even in endemic areas, the clinical diagnosis is hindered by: 1) Low prevalence of major clinical signs 2) Late development of clinical signs 3) Lack of specificities of clinical signs 4) Occurrence of asymptomatic carriers In non-endemic areas, clinical diagnosis may even be more difficult. Laboratory diagnosis It plays an important role in establishing the specific diagnosis and supplementing the clinical diagnosis of the condition. It depends upon 1) Morphological recognition of parasites in relevant specimens(parasitic diagnosis) 2) Immunological tests(immuno diagnosis) 3) DNA probes or PCR etc
4)Other laboratory a)Parasitic diagnosis The definitive diagnosis is made by demonstration of parasites in appropriate clinical specimens. The parasitic diagnosis can be made by a)microscopy, b) ) cultivation,c) animal inoculation(动物接种)andb) xenodiagnosis(异体接种诊断法).The nature of clinical specimen to be collected depends upon the habitat of the parasite ① Luminal parasites(腔道寄生虫) of the gastrointestina, genitourinary and pulmonary tract Infections with these parasites are confirmed by the presence of their characteristic forms in the faeces, urine, sputum and other body secretions Faeces(排泄物/粪便): It is an important clinical specimen for the diagnosis of a) intestinal parasitic infections and b)helminthic infections of the biliary tract in which eggs are discharged ons, the cysts(包囊) and trophozoites(滋养体)of Entamoeba histolytica, giardia intestinalis, can be demonstrated in the faeces The trophozoites are usually excreted in acute infections whereas cysts are found in chronic infections. In helminthic infections the eggs, larvae and adult worms are found in the faeces as follows Non-faecal specimens(EA): These include a) anal swabs( e. g, for the eggs of Enterobius vermicularis),b) genital specimens(e. g, for Trophozoites of trichomonas vaginalis阴道滴虫),c) sputum (e.g, the eggs of Paragonimus westermani), d) urine( e.g, the eggs of Schistosoma haematobium), e)cerebrospinal fluid(e. g, Trypomastigotes Hi Ef of Tiypanasome brucei), f) aspirations and biopsies(e. g, duodenal fluid, sigmoidoscopy aspirates, abscess aspirate, and 2 Blood and tissue parasites The infections caused by these blood and tissue parasites are confirmed by the presence of their morphological stages in the blood, tissue and other specimens. Blood: The blood can be examined by direct smear, concentration, culture and animal inoculation ete Tissue biopsy and aspiration: for example, the larvae of Trichinella spiralis and Taenia soliu can be demonstrated in muscle biopsy b) Immunological diagnosis These methods are particularly useful in the latent or asymptomatic infections as well as in some chronic infections. The immunological tests broadly are of two types, a)skin test, and b) serological test O Skin test Skin test are performed by the intradermal injection of a low volume of sterilized parasitic antigen and noting whether erythema and induration occurs after 30 mins(immediate hypersensitivity)or after 48 hrs(delayed hypersensitivity) 2 Serological test Serological tests important both for mass screening and case detection particularly during the chronic phase of infection. These essentially are based on: a) Antibody detection and b)Antigen detection TREATMENT Treatment of parasitic diseases primarily is based on chemotherapy and in some cases surgery Chemotherapy The chemotherapy is employed for the treatment and prophylaxis of the parasitic infections. Many parasitic diseases can be treated by chemotherapy. Albendazole( pa] Fik 唑), mebendazole(甲苯咪唑) and pyrantal pamoate(噻嘧啶) are some of the commonly used anthelmintics. Praziquantel(吡喹酮) is used against many cestode( tape worm绦虫)and 11
11 4) Other laboratory a) Parasitic diagnosis The definitive diagnosis is made by demonstration of parasites in appropriate clinical specimens. The parasitic diagnosis can be made by a) microscopy, b)cultivation, c) animal inoculation（动物接种） and b) xenodiagnosis(异体接种诊断法). The nature of clinical specimen to be collected depends upon the habitat of the parasite. ① Luminal parasites（腔道寄生虫） of the gastrointestinal, genitourinary and pulmonary tract Infections with these parasites are confirmed by the presence of their characteristic forms in the faeces, urine, sputum and other body secretions. Faeces（排泄物/粪便）: It is an important clinical specimen for the diagnosis of a) intestinal parasitic infections and b) helminthic infections of the biliary tract in which eggs are discharged in the intestine. In protozoal infections, the cysts（包囊） and trophozoites（滋养体） of Entamoeba histolytica, giardia intestinalis, can be demonstrated in the faeces The trophozoites are usually excreted in acute infections whereas cysts are found in chronic infections. In helminthic infections the eggs, larvae and adult worms are found in the faeces as follows: .Non-faecal specimens（标本）: These include a) anal swabs( e.g., for the eggs of Enterobius vermicularis), b) genital specimens(e.g., for Trophozoites of trichomonas vaginalis 阴道滴虫) , c) sputum (e.g., the eggs of Paragonimus westermani), d) urine( e.g., the eggs of Schistosoma haematobium), e) cerebrospinal fluid(e.g., Trypomastigotes 鞭毛体 of Trypanasome brucei), f) aspirations and biopsies(e.g., duodenal fluid, sigmoidoscopy aspirates, abscess aspirate, and biopsies etc). ② Blood and tissue parasites The infections caused by these blood and tissue parasites are confirmed by the presence of their morphological stages in the blood, tissue and other specimens. Blood: The blood can be examined by direct smear, concentration , culture and animal inoculation etc. Tissue biopsy and aspiration: for example, the larvae of Trichinella spiralis and Taenia solium can be demonstrated in muscle biopsy. b) Immunological diagnosis These methods are particularly useful in the latent or asymptomatic infections as well as in some chronic infections. The immunological tests broadly are of two types, a) skin test, and b) serological test. ① Skin test Skin test are performed by the intradermal injection of a low volume of sterilized parasitic antigen and noting whether erythema and induration occurs after 30 mins(immediate hypersensitivity) or after 48 hrs(delayed hypersensitivity). ② Serological test Serological tests important both for mass screening and case detection particularly during the chronic phase of infection. These essentially are based on: a) Antibody detection and b) Antigen detection. TREATMENT Treatment of parasitic diseases primarily is based on chemotherapy and in some cases surgery. Chemotherapy The chemotherapy is employed for the treatment and prophylaxis of the parasitic infections. Many parasitic diseases can be treated by chemotherapy. Albendazole（阿苯达 唑）, mebendazole（甲苯咪唑） and pyrantal pamoate（噻嘧啶） are some of the commonly used anthelmintics. Praziquantel（吡喹酮） is used against many cestode(tape worm 绦虫) and
trematode(fuke吸虫) infections. An ideal agent for use in chemotherapy against a parasitic infection should be 1)Of high therapeutic index 2) Administered orally, preferably in a single dose or divided doses on the same day 3) Stable over a long period expensive 5)Also the parasite should not develop drug resistance Surgical management Management by surgery is indicated in the parasitic diseases for which anti-parasitic drugs are not yet available or if available are not completely effective. It is recommended especially for the hydatid disease, paragonimiasis, etc PREVENTION AND CONTROL Prevention of parasitic disease refers to it interception. Control refers to check the possibilities of dissemination of infection and epidemics, reduce and maintain a low level of parasitic infections prevalent in human population Control methods can be aimed at controlling and eradicating the disease at its reservoir and It can be achieved by 1) Chemotherapy and isolation. It is useful to prevent the spread of infection and eliminate the reservoir and source of infection 2) Immunoprophylaxis(免疫预防) It is carried out by artificial immunization such as vaccines but has not been possible for most of the parasit malaria and a few other infections pilot studies of the vaccine has been carried in the field 3) Control of infection in animal reservoir for zoonotic prophylaxis (fj) PARASITOLOGYIN FUTURE Human parasitic diseases caused by protozoans, helminths and arthropod include a wide variety of infectious organisms, a broad spectrum of disease processes, and some of the most fascinating scientific and important public health challenges that we will still face in the 21 st century. Many of these diseases represent very well adapted host/parasite/vector relationships that imply a degree of co-evolution of these"partners in disease" that stretches the imagination. This is often exemplified by chronic infections that in a high percentage of hosts, cause little morbidity, and life-cycles that are exquisitely timed and organized for optimal, but regulated, transmission of the pathogen. A few of these diseases are more fulminant, and some clearly lead to more disease in recently infected populations of hosts than in long-standing situations of medium-to-high prevalence. Work with these parasites and their hosts and vectors spans the gamut from very basic scientific research, to extremely practical implementation of eradication pro critical to remember that a sound scientific understanding of the organism the vector and the host, and how they interact biologically, spatially, and temporally, is always the foundation upon which effective control, elimination, or eradication programs are based. Until the proper understanding of the situation is in hand, and the appropriate tools are assembled, sheer determination is simply usually not enough to effectively dominate these well-established and spreading diseases. These seemingly all-encompassing characteristics make Parasitology facing scientists, clinicians, and public health officials as we move into the 21st century
12 trematode(fluke 吸虫) infections. An ideal agent for use in chemotherapy against a parasitic infection should be: 1) Of high therapeutic index 2) Administered orally, preferably in a single dose or divided doses on the same day 3) Stable over a long period 4) Inexpensive, and 5) Also the parasite should not develop drug resistance. Surgical management Management by surgery is indicated in the parasitic diseases for which anti-parasitic drugs are not yet available or if available are not completely effective. It is recommended especially for the hydatid disease, paragonimiasis, etc. PREVENTION AND CONTROL Prevention of parasitic disease refers to it interception. Control refers to check the possibilities of dissemination of infection and epidemics, reduce and maintain a low level of parasitic infections prevalent in human population. Control methods can be aimed at controlling and eradicating the disease at its reservoir and source. It can be achieved by 1) Chemotherapy and isolation. It is useful to prevent the spread of infection and eliminate the reservoir and source of infection. 2) Immunoprophylaxis(免疫预防). It is carried out by artificial immunization such as vaccines but has not been possible for most of the parasitic diseases. In falciparum malaria and a few other infections pilot studies of the vaccine has been carried in the field. 3) Control of infection in animal reservoir for zoonotic prophylaxis（预防）. PARASITOLOGY IN FUTURE Human parasitic diseases caused by protozoans, helminths and arthropod include a wide variety of infectious organisms, a broad spectrum of disease processes, and some of the most fascinating scientific and important public health challenges that we will still face in the 21st century. Many of these diseases represent very well adapted host/parasite/vector relationships that imply a degree of co-evolution of these "partners in disease" that stretches the imagination. This is often exemplified by chronic infections that, in a high percentage of hosts, cause little morbidity, and life-cycles that are exquisitely timed and organized for optimal, but regulated, transmission of the pathogen. A few of these diseases are more fulminant, and some clearly lead to more disease in recently infected populations of hosts than in long-standing situations of medium-to-high prevalence. Work with these parasites and their hosts and vectors spans the gamut from very basic scientific research, to extremely practical implementation of eradication programs. It is, however, critical to remember that a sound scientific understanding of the organism, the vector and the host, and how they interact biologically, spatially, and temporally, is always the foundation upon which effective control, elimination, or eradication programs are based. Until the proper understanding of the situation is in hand, and the appropriate tools are assembled, sheer determination is simply usually not enough to effectively dominate these well-established and spreading diseases. These seemingly all-encompassing characteristics make Parasitology facing scientists, clinicians, and public health officials as we move into the 21st century
Section protozoa(原虫) I INTRODUCTION Protozoa are usually defined as singly celled animals, they belonging to the animal kingdom, subkingdom protozoa. Structurally, protozoa resemble single metazoan(多细胞动物)cl;a protozoan cell has a full complement of cellular organelles(细胞器)ie. nucleus(细胞核) mitochondria(线粒体), endoplasmic reticulum(内质网), Golgi apparatus(高尔基体)etc together with endoplasm and ectoplasm. Functionally, each protozoan cell is equivalent to a whole metazoan animal; the single cell relatively has complex metabolic activities such as digestion, reproduction, respiration, excretion, etc At least 45,000 species of protozoan have been described to date, many of which are parasitic Parasitic protozoa still kill, mutilate, and debilitate more people in the word than any other group of disease organisms. Because of this, studies on protozoa occupy a prominent place in parasitology. MORPHOLOGY Protozoa, that is, the whole body consists of a singular cell. Like a cell in the tissue of metazoa., a protozoan cell is composed of plasma membrane(原生质膜), cytoplasm(细胞质) and nucleus Plasma membrane: The membrane appears three layered in electron micrographs because (electron dense)protein layer. The trilaminar unit membrane support by a sheet of contractile fibrils, which enable the cell to change its shape and help in locomotion protection nutrition Cytoplasm: The cytoplasm matrix consists of very small granules and filaments suspended in al low-density medium with physical properties of a colloid. In some species, the cytoplasm is divisible into two portions: ectoplasm(外质) and endoplasm(内质) 1)ectoplasm: The ectoplasm is the outer transparent layer with function of protection comotion and sensation. It is often in the gel state 2)endoplasm: The endoplasm is the inner granular layer containing vacuoles and organelles. It is in the sol state of the colloid, and it bears the nucleus, mitochondria, Golgi bodies and so on. These can only be visualized by electron microscopy. The endoplasm helps in nutrition and reproduction 3Organelles( !H #E 2 ): There are several membrane organelles characteristic of eukaryotes (真核细胞), such as endoplasmic reticulum, mitochondria various membrane- bound vesicles,and Golgi bodies, are usually found in protozoa. Mitochondria that bear the enzymes of oxidative phosphorylation and tricarboxylic acid cycle often have tubular rather than lamellar cristae in Protozoa have several locomotory organelles(运动细胞器) flagella(鞭毛,cili(纤毛) and pseudopodia (BlAE) Flagella are long delicate thread like filaments; flagella composed a central axoneme and an out sheath that is a continuation of the cell membrane. a flagellum is capable of a variety of movements, which may be fast or slow, forward, backward, lateral,or spiral. Pseudopodia are temporary organelles found in Sarciodina(F E )(and other organisms) that cause the organism to move and aid it in capturing food. They do not occur in all sarciodines
14 SectionⅡ PROTOZOA (原虫) Ⅰ. INTRODUCTION Protozoa are usually defined as singly celled animals, they belonging to the animal kingdom, subkingdom Protozoa. Structurally, protozoa resemble single metazoan (多细胞动物) cells; a protozoan cell has a full complement of cellular organelles (细胞器), i.e. nucleus (细胞核), mitochondria (线粒体) , endoplasmic reticulum (内质网), Golgi apparatus (高尔基体)etc. together with endoplasm and ectoplasm. Functionally, each protozoan cell is equivalent to a whole metazoan animal; the single cell relatively has complex metabolic activities such as digestion, reproduction, respiration, excretion, etc. At least 45,000 species of protozoan have been described to date, many of which are parasitic. Parasitic protozoa still kill, mutilate, and debilitate more people in the word than any other group of disease organisms. Because of this, studies on protozoa occupy a prominent place in parasitology. MORPHOLOGY Protozoa, that is, the whole body consists of a singular cell. Like a cell in the tissue of metazoa, a protozoan cell is composed of plasma membrane (原生质膜), cytoplasm (细胞质) and nucleus. Plasma membrane: The membrane appears three layered in electron micrographs because the central lipid portion looks light or clear (electron lucent) and is enclosed by the darker (electron dense) protein layer. The trilaminar unit membrane support by a sheet of contractile fibrils, which enable the cell to change its shape and help in locomotion protection & nutrition. Cytoplasm: The cytoplasm matrix consists of very small granules and filaments suspended in al low-density medium with physical properties of a colloid. In some species, the cytoplasm is divisible into two portions: ectoplasm (外质) and endoplasm (内质). 1) ectoplasm: The ectoplasm is the outer transparent layer with function of protection, locomotion and sensation. It is often in the gel state. 2) endoplasm: The endoplasm is the inner granular layer containing vacuoles and organelles. It is in the sol state of the colloid, and it bears the nucleus, mitochondria, Golgi bodies, and so on. These can only be visualized by electron microscopy. The endoplasm helps in nutrition and reproduction. 3) Organelles(细胞器): There are several membrane organelles characteristic of eukaryotes (真核细胞), such as endoplasmic reticulum, mitochondria, various membrane-bound vescles, and Golgi bodies, are usually found in protozoa. Mitochondria that bear the enzymes of oxidative phosphorylation and tricarboxylic acid cycle often have tubular rather than lamellar cristae in protozoa, althougher they may be absent. Protozoa have several locomotory organelles (运动细胞器): flagella (鞭毛), cilia (纤毛), and pseudopodia (伪足). Flagella are long delicate thread like filaments; flagella composed a central axoneme and an out sheath that is a continuation of the cell membrane. A flagellum is capable of a variety of movements, which may be fast or slow, forward, backward, lateral, or spiral. Pseudopodia are temporary organelles found in Sarciodina (肉足类) (and other organisms) that cause the organism to move and aid it in capturing food. They do not occur in all sarciodines
Cilla are structurally similar to flagella, they are fine needle like filaments covering the entire surface of the body Some species of protozoa have rudimentary digestive organs such as cytostome and vtopharvnx Nuclei: it is the vital structure of a cell. It is present in the endoplasm. A membrane known as nuclear membrane surrounds nucleus externally. In all protozoa excepting Balantidium coli, the nucleus is vesicular Nucleus contains a karyosome(kf)and chromatin granules The karyosome is found inside the nucleus either at the centre or at the periphery. The protozoan karyosome belonging to the Phylum Apicomplex (JiAlI)(e. g: malaria parasites (锥虫)an itic amoebae do not contain any DNA. Chromatin granules giving the appearance of condensation on linin threads line the nucleus membrane internally Only a single nucleus is present in most of the protozoa but the ciliates have two nuclei, small nucleus(micro-nucleus) and a large nucleus (macro-nucleus). They are homogeneous in composition. In certain protozoa such as trypanosomes, a non-nuclear DNA-containing body alled kinetoplast(动基体) )is also present in addition to the nucler Trophozoite(2FA): It is the reproductive stage of the most protozoa(e.g,intestinal flagellate, amoebae, and ciliates). It is active feeding stage of the parasite and this stage is associated with the pathogenesis of the disease Cyst(E*): It is the resistant form of the protozoa with a protective membrane or thickened wall. It is produced during unfavorable circumstances. The protective wall of cyst enables the parasite to survive outside the host in an environment under adverse circumstance for a variable period ranging from a few days to years The cyst is resting stage of the parasite. Replication usually does not occur in this stage However, multiplication may occur in the cysts of some species(e.g, Entamoeba histolytica), where the nucleus divides to produce asexually. The cysts formed sexually are called oocysts TYPE OF LIFE CYCLE The life cycle of protozoa may be simple or complex. According to the characteristics of transmission, they can be divided into three types Person to Person transfer(人际传播型 In this type, no intermediate host is needed. The transmission is only from person to person For example, the life cycle intestinal amoebae, flagellates and ciliates are simple and completed in a single host only. The parasites replicate only by asexual method of reproduction mostly in the trophozoite stage. Sometimes, they multply in cystic stage(e.g. Entamoeba, Giardia Under unfavourable circumstance, the actively growing trophozoites secrete a resistant cyst wall and are transformed into cysts. This process is called encystation (EX*). When the condition becomes favourable the cyst wall are lysed to liberate trophozoites. This process is called excystation(BR Both the processes of encystation and excystation take place in a single host. The parasites are transformed from one host to another host after encystation only Circulation transfer(循环传播型) Some species of the parasitic protozoa, like Toxoplasma gondii(刚地弓形虫) it needs more than one vertebrate to finish its life cycle, in which cat is the definitive host, man and others are
15 Cilla are structurally similar to flagella, they are fine needle like filaments covering the entire surface of the body. Some species of protozoa have rudimentary digestive organs such as cytostome and cytopharynx. Nuclei: it is the vital structure of a cell. It is present in the endoplasm. A membrane known as nuclear membrane surrounds nucleus externally. In all protozoa excepting Balantidium coli, the nucleus is vesicular. Nucleus contains a karyosome (核仁) and chromatin granules. The karyosome is found inside the nucleus either at the centre or at the periphery. The protozoan karyosomes belonging to the Phylum Apicomplex (顶复门) (e.g.: malaria parasites) contain DNA; in contrast, the karyosomes of trypanosomes (锥虫) and parasitic amoebae do not contain any DNA. Chromatin granules giving the appearance of condensation on linin threads line the nucleus membrane internally. Only a single nucleus is present in most of the protozoa but the ciliates have two nuclei, a small nucleus (micro-nucleus) and a large nucleus (macro-nucleus). They are homogeneous in composition. In certain protozoa such as trypanosomes, a non-nuclear DNA-containing body called kinetoplast (动基体)is also present in addition to the nucleus. Trophozoite(滋养体): It is the reproductive stage of the most protozoa (e.g., intestinal flagellate, amoebae, and ciliates). It is active feeding stage of the parasite and this stage is associated with the pathogenesis of the disease. Cyst(包囊): It is the resistant form of the protozoa with a protective membrane or thickened wall. It is produced during unfavorable circumstances. The protective wall of cyst enables the parasite to survive outside the host in an environment under adverse circumstance for a variable period ranging from a few days to years. The cyst is resting stage of the parasite. Replication usually does not occur in this stage. However, multiplication may occur in the cysts of some species (e.g., Entamoeba histolytica), where the nucleus divides to produce asexually. The cysts formed sexually are called oocysts. TYPE OF LIFE CYCLE The life cycle of protozoa may be simple or complex. According to the characteristics of transmission, they can be divided into three types Person to Person transfer (人际传播型) In this type, no intermediate host is needed. The transmission is only from person to person. For example, the life cycle intestinal amoebae, flagellates and ciliates are simple and completed in a single host only. The parasites replicate only by asexual method of reproduction mostly in the trophozoite stage. Sometimes, they multply in cystic stage (e.g. Entamoeba, Giardia) Under unfavourable circumstance, the actively growing trophozoites secrete a resistant cyst wall and are transformed into cysts. This process is called encystation (成囊). When the condition becomes favourable the cyst wall are lysed to liberate trophozoites. This process is called excystation (脱囊). Both the processes of encystation and excystation take place in a single host. The parasites are transformed from one host to another host after encystation only. Circulation transfer(循环传播型) Some species of the parasitic protozoa, like Toxoplasma gondii (刚地弓形虫), it needs more than one vertebrate to finish its life cycle, in which cat is the definitive host, man and others are