Chapter One: INTRODUCTION

1. Importance of Parasitology
2. General consideration
3. Taxonomy & Morphology
4. Life cycle
5. Transmission of parasites
6. Parasitic zoonoses
7. Pathogenesis and pathology
8. Host immunity
9. Immune respones
10. Immunoevasion of parasites
11. Clinical Manifestation
12. Treatment
13. Prevention and control
14. Parasitology in future

A parasite (寄生虫)  is an organism that lives on or in a host organism (宿主) and gets its food from or at the expense of its host. There are three main classes of parasites that can cause disease in humans: Medical Protozoa(原虫), Medical Helminths（蠕虫） and Ectoparasites/Medical arthropods（体外寄生虫或医学节肢动物）.
Parasitology (寄生虫学), the study of parasitical their relationships to their hosts, is one of the most fascinating areas of 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, helminthes, and arthropods, as well as those species of arthropods that serve as vectors (媒介) for parasites. Therefore, parasitology encompasses elements of protozoology (原虫学), helminthology (蠕虫学), and medical arthropodology (医学节肢动物学).
Medical parasitology, an important part of parasitology, studies the medical parasites including their morphology (形态学), life cycle (生活史), and the relationship with host and environment. The objective is to study the way to control parasitic diseases.
It is intended primarily for medical students and physicians, this book is to serve as a guide both to the clinical diagnosis and treatment and to the laboratory diagnosis of the protozoan and helminthic diseases of medical importance, and to a lesser extent to the arthopods inrelation to disease.

IMPORTANCE OF PARASITOLOGY
Parasitic infections or parasitic diseases were once the most common diseases in the world, which cause a tremendous burden of disease in both the tropics and subtropics as well as in more temperate climates. Of all parasitic diseases, malaria causes the most deaths globally. Malaria kills approximately 1 million people each year, most of them young children in sub-Saharan Africa.. Therefore, parasitology played an important role in the medicine and public health, and none neglected the importance of parasitology.
With the nearly simultaneous development of various new antiparasites agents and synthetic pesticides, it was for a time widely believed that parasitic diseases would for all practical purpose disappear from the clinical scene. That this has not happened is obvious.
According to the WHO (世界卫生组织) report, parasitic disease is still an important human disease. In the world, 210 million people reside in the endemic areas of  malaria (疟疾), 10 million cases with malaria occur every year, and 20 million infected individuals was estimated in the world. So TDR/WHO has proclaimed 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), and Onchocerciasis (盘尾丝虫病). Among them 7 diseases are parasitic in the traditional sense（see TDR website: http://www.who.int/tdr/diseases/default.htm）.
In China, various parasites have been recognized as one of the important endemic diseases for many years. In the early 1950s, an estimated 10 million people suffered from schistosomiasis, and 30 million each sufferd from malaria and filariasis. 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 been obtained. Nevertheless, schistosomiasis is still prevalent in lake-marsh and mountain regions along Changjiang River. At 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. For example, overall prevalence of helminthic infection was 21.74% in Chinese population. Although soil-transmitted (土源性) parasites infection has been reduced significantly with improvement of living conditions, food-transmitted (食源性) parasite infection, such as infection of clonorchis, has become a new public health problem.
Why the parasitic diseases are still endemic in the world now? The main reasons are as follows:
DDT and other insecticides not only have failed to eliminate the vectors of malaria, filariasis, 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.
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 ensure that many will return to their native shores before their infections become patent.
Modifications of the evironment, such as the construction of dam and the highway, may bring about major increases in parasitic diseases, for example schistosomiasis and leishmaniasis.
Global warming is suggested as a possible reason for the eventual spread of parasitic diseases now seen primarily in the tropics to more tempereture climas.
An importment development has been the appearance of the human immunodeficiency virus(HIV) and its sequeal, the acquired immunodeficiency sydrome(AIDS), which results in greatly increased prevalence and severity of a number of parasitic diseases, such as Cryptosporidiosis. In addition to the AIDS pandemic are other immunodeficiency. Important among such as various malignancies and the cytotoxic effects of radiation, prolonged corticosteroid therapy, and the drugs used to prevent organ transplant rejection.
For these reasons it remains necessary that all physicians have some familiarity with the parasitc diseases, no matter how “exotic.”

GENERAL CONSIDERATIONS

Medical (human) Parasitology consists of medical protozoology(医学原虫学), medical helminthology（医学蠕虫学）, and medical arthropodology（医学节肢动物学）. Terms used in parasitology is summaried in following:
Symbiosis (共生) : Symbiosis means “both members of different species living together”. 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. The categories of symbiosis include commensalisms, mutualism and parasitism.
Commensalism(共栖)  The word was from Latin for “eating at same table”, denoting 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, and such an association is beneficial to both organisms (partners).
Parasitism(寄生)Parasitism is another 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, leeches, and ticks, which feed intermittently on host blood.  In other words, it is a symbiotic relationship in which one partner, the host, is to some degree injured through the activities of the other partner, 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 nourishment directly from it. By this definition all infectious agents, viruses, bacteria, fungi, protozoa, and helminths are parasites, but traditionally protozoa, helminthes and medical arthropod (节肢动物) are studied in medical or human Parasitology. Therefore, the textbooks of parasitology today deal only with protozoa, helminthes and some arthropod. Broadly speaking, the parasites are divided into two types: endoparasite (体内寄生虫) and ectoparasite (体外寄生虫). The parasite that 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, obligate parasite, facultative parasite and accidental parasite.
Obligatory 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) are the parasites which attack an unusual host. Ectoparasites are the parasites that live 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（宿主） Host is defined as an organism that harbours the parasite and provides the nourishment and shelter. These hosts, in comparison to their parasites, are relatively larger in size. The hosts have the following types: definitive host, intermediate host, reservoir host and paratenic host.
Definitive host (终宿主) The hosts which harbour the adult parasites (e.g., Taenia saginata causing intestinal taeniasis) and most highly developed form of the parasite (e.g., Trypanosoma cruzi causing African sleeping sickness) or in which the parasite replicates sexually (e.g., Paragonimus westermani) are called the definitive hosts. The definitive hosts may be human or non-human living things.
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. Sometimes 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).
Reservoir host (保虫宿主) The animal which harbours the parasites and serves as an important source of infection to other susceptible hosts is known as reservoir host (e.g., water buffalo is the reservoir host for schistosomiasis血吸虫病).
Paratenic host or transport host (转续宿主)  The larva of some parasites can invade a non-normal host, but can not grow, and only lives in the larva stage. If the larva enters a normal definitive host, it can continue to grow into an 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) and MORPHOLOGY (形态学)
According to the binomial nomenclature (命名法) 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 from: ① Greek or Latin words; ② Names of their discoverers; ③ Geographical area where found; ④ Hosts in which parasites are found; ⑤ Habitat of the parasite.
The correct scientific name of the parasite consists of the genus and species to which it belongs, the name of the designator and the year in which it was discovered (e.g., Angiostrongylus cantonensis广州管圆线虫(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 amoeba(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 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 of classes, many of which are of medical importance. The classes mainly 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 may transmit diseases. Insects have three pairs of legs and the body is divided into three distinct parts: head, thorax, and abdomen. Included in this class are mosquitoes, flies, lice, and bugs.
In generally, human parasites were classified into three classes: Medical Protozoa, Medical Helminths and Ectoparasites/Medical arthropods.
Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature, which are morphologically and functionally complete. A single cell carries out all the functions such as digestion, respiration, excretion, and reproduction. The protozoa that are infectious to humans can be classified into four groups based on their mode of movement:
① Sarcodina(肉足亚门) – the amoeba, e.g., Entamoeba
② Mastigophora（鞭毛纲） – the flagellates, e.g., Giardia, Leishmania
③ Ciliophora（纤毛门） – the ciliates, e.g., Balantidium（小袋虫属）
④ Sporozoa（孢子虫纲） – organisms whose adult stage is not motile e.g., Plasmodium
Helminths are large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. In their adult form, helminths cannot multiply in humans. There are three main groups of helminths (derived from the Greek word for worms) that are human parasites:
① Flatworms (platyhelminths,扁形动物门) – these include the trematodes (flukes) and cestodes (tapeworms).
② Thorny-headed worms (acanthocephalins，棘头虫属) – the adult forms of these worms reside in the gastrointestinal tract. The acanthocephala are thought to be intermediate between the cestodes and nematodes.
③Roundworms (nematodes，线虫纲) – the adult forms of these worms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues. Alternatively, the immature (larval) states can cause disease through their infection of various body tissues.
Some consider the helminths also include the segmented worms (annelids)—the only ones important medically are the leeches. Of note, these organisms are not typically considered parasites.
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.
Some blood-sucking arthropods such as mosquitoes were considered to be the term ectoparasites (because they are dependent on a blood meal from a human host for their survival), this term is generally used more narrowly to refer to organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months). Arthropods are important in causing diseases in their own right, but are even more important as vectors, or transmitters, of many different pathogens that in turn cause tremendous morbidity and mortality.

LIFE CYCLE (生活史)
Life cycle: The period of time that it takes for an organism to live its entire life. Some animals, such as parasite and insects, change their shape several times during their life cycle. The life cycle of a parasite may be simple or complex. In a simple life cycle all the developmental stages of the parasite are completed in a single host such as man. The change of host is required to spread 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 parasites 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 crabs as the second intermediate host, apart from man and the crabs eating mammals as the definitive host.).

TRANSMISSION OF PARASITES（传播）
It depends upon: Source or reservoir of infection, and Mode of transmission.
Source of infection (传染源)  
1) Human: Infected persons are 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).
Modes of transmission (传播方式)
Transmission of infection from one host to another, caused by a certain form of the parasite, is known as the infective stage. The infective stage of various parasites may be transmitted from one host to another in the following ways.
1) Oral route 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溶组织内阿米巴； ova of Ascaris lumbricoides蛔虫, Trichuris trichura鞭虫 and Enterobius vermicularis蛲虫).   
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).
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, fish harbouring the metacercariae 囊蚴of Clonorchis sinensis华支睾吸虫/肝吸虫, etc).
Infection can be transmitted by 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

1. Penetration of the intact skin by filariformlarvae (丝状蚴) of hookworm, Strongyloides stercoralis on coming in contact with faecally polluted soil, and
2. 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
① Inoculation into the blood by Anopheles (vector for Plasmodium疟原虫).
② 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 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 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 ZOONOSIS (人兽共患寄生虫病)
Zoonosis means diseases of animals which are transmissible to men. These are the infections which are naturally transmitted between the vertebrate animals and man. The condition usually includes those infections in which there is strong circumstantial evidence of transmission between the man and animals.

PATHOGENESIS AND PATHOLOGY
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 factors include:

1.  Nutritional status of the host, whether malnutrition or undernutrition,
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 diseases 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-2.
The parasitic infections usually are designated by generic names of the parasites ending with  -iasis or –osis. (e.g., Schistosoma infection is called as schistosomiasis)

TableⅠ-1 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,
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 immunity, and b) Acquired immunity.
Innate immunity (固有免疫)  It is the inherited but non-immune type of the host defence against a parasite. For example, 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.
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 immunityIt confers protective immunity against many protozoan 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.
① Humoral immunity（体液免疫）It is mediated by 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:
a. The antibodies prevent the parasites from attaching and penetrating the host cells by binding the specific sites on the surface of parasites.
b. The antibodies neutralize parasite toxins and inactivate parasite enzymes by binding with the determinants of parasitic antigens.
c. The secretory IgA antibodies found in various body secretions prevent attachment of some protozoan parasites in the gut wall epithelium.
d. In a few parasitic infections (e.g., trypanosomiasis), the parasites are killed by lysis of antibody-coated cells mediated by the complement.
e. 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.
② Cell mediated immunity (CMI，细胞免疫) It is mediated through T cell which are cytotoxic. The CMI offers protection against many parasitic infections in following ways.
a. 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 man).
b. Activated macrophages kill parasites in various ways such as producing enzymes or activated substances (e.g., Toxoplasma, Leishmania, the schistosomule of Schistosoma species) and by producing hydrogen peroxide (Leishmania species etc).
c. Natural killer cells  These cells appear to be helpful in the initial host resistance against many parasitic infections.

IMMUNE RESPONES
The immune responses of man against parasitic infections are variable. They may be protective or harmful to host.
Protective immune responses
1) Sterilizing immunity（消除性免疫）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（非消除性免疫）It is associated 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 found in many protozoal infections (e.g., malaria), is “concomitant immunity (伴随免疫)” typically 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 produces harmful effects in the host such as hypersensitivity reactions.
There are four reactions: 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 (血红蛋白尿) 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.

IMMUNOEVASION OF PARASITES (寄生虫免疫逃避)
Many parasites survive and proliferate in immunologically competent host by the following mechanisms. These include:

1.  Intracellular location（组织学隔离） e.g., Toxoplasma, leishmania.
2. Antigenic shedding（表面抗原脱落） e.g., Entamoeba, Toxoplasma, Fasciola, Trichinella.
3. Antigenic variation （抗原变异） e.g., Trypanosoma
4. Antigenic mimicry （抗原模拟）e.g., Schistosoma, 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, leishmania, Toxoplasma, Wuchereria, Brug), activation of lymphocytes (e.g., Tryanosoma brucei, Schistosoma species), modified leucocyte function (e.g., Fasciola hepatic) and immune complex (e.g., Leishmania, Trypanasoma brucei, Toxplasma, Plasmodium).

CLINICAL MANIFESTATION (临床表现)
Clinical manifestations of parasitic diseases are variable. They may be acute or chronic. 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 develops 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.
Clinical manifestation can be in the form of ①Systemic presentations; ②Skin manifestation; ③ Organ specific presentations.
Systemic presentations  Parasite reactions in the body induce the release of endogenous pyrogens by neutrophils and macrophages thus resulting in pyrexia. Fever can be seen in Malaria, Toxoplasmosis etc; systemic allergic reactions occur when there is leakage from hydatid cysts or when A. lumbricoides reaches lungs during circulation and also in guinea worm infections; Schistosoma eggs produce an allergic reaction in the host tissue.Some parasitic diseases can result in immunosuppression due to impairment of T and B cell response eg. in Filariasis etc.
Skin manifestations  Skin can be involved by the parasite either directly or through a vector. Eg. Cutaneous leishmaniasis. Site of entry in hookworm is affected.
Organ specific manifestations  Parasites are pathogenic to nearly all the organs. Gastrointestinal system bears the maximum burden of parasitic load. E. histilytica, Giardia, Ascaris, Schistosoma commonly attack the gastrointestinal system with resultant dysentery, localized intense pain, intestinal obstruction and perforation. Liver can be attacked by E.histolytica, E.granulosus and Schistosoma leading to abscess, hydatid cyst, granulomas and necrosis resulting in hepatomegaly. Lungs are involved by Toxoplasma, Larval nematodes and also P.falciparum leading to pneumonitis, eosinophilia and even edema; heart can be involved in Toxoplasmosis and Trypanosomiasis causing myocarditis. Muscles are involved by Trichinella, Taenia and Toxoplasma.
There can be painless lymphadenopathy with splenomegaly and anaemia in leishmaniasis, filariasis, malaria and toxoplasmosis. Toxins do not play an important role in pathogenesis of parasitic infestations. Only a few parasitic infections give rise to high mortality but many produce a high degree of morbidity. ie., chronic ill health.

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; ④ Other laboratory tests.
1) 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 d) xenodiagnosis (病媒接种诊断法). 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 lamblia 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: The larvae of Trichinella spiralis and Taenia solium can be demonstrated in muscle biopsy.
2) Immunological diagnosis These methods are particularly useful in the latent or asymptomatic infections as well as in some chronic infections. The immunological tests broadly have two types, a) skin test, and b) serological test.
① Skin test  Skin tests are performed by the intradermal injection of a low volume of sterilized parasitic antigen, noting whether erythema and induration occurs after 30 minutes (immediate hypersensitivity) or after 48 hours (delayed hypersensitivity).
② Serological test Serological tests are important for both mass screening and case detection particularly during the chronic phase of infection. These are essentially based on: a) Antibody detection and b) Antigen detection.

TREATMENT
The treatment of parasitic diseases is primarily 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 anti-helmintics. Praziquantel (吡喹酮) is used against many cestode (tape worm绦虫) and trematode (fluke吸虫) infections.
Artemether(蒿甲醚) and Artesunate(青蒿琥酯) are the active component of the Chinese herb qinghao (Artemesia annua). Extracts of qinghao have been used in traditional Chinese herbal medicine to treat febrile illness. Studies have shown artemether and Artesunate to be effective for treating malaria and schistosomiasis.
An ideal agent used 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. Inducing no drug resistance.

Surgical management  Surgical management 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 or paragonimiasis.

PREVENTION AND CONTROL
Prevention of parasitic diseases refers to interception. Control means to check the possibilities of dissemination of infection and epidemics and maintain a low level of parasitic infections in human population.
Control methods aim at controlling and eradicating the disease at its reservoir and source. The methods includes:

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).

In China, the government has been attaching importance to the control of parasitic diseases. Since early 1950s, control of parasitic diseases, with emphasis on malaria, schistosomiasis, filariasis, leishmaniasis and hookworm disease etc., has become a national goal and after many years of effort, outstanding progress in the control of these diseases have been made with remarkable declines in morbidity and mortality. The critical control strategies include mass Chemotherapy, vector control and modifications of the evironment. Health education and changes of living behaves are considered to be an important strategies for parasitic diseases control.
In USA, there is a special scientific research and prevent institutions, named The Division of Parasitic Diseases(DPD). DPD’s mission is to prevent and control parasitic diseases in the United States and throughout the world. The DPD home website is http://www.cdc.gov/.

PARASITOLOGY IN THE FUTURE
Human parasitic diseases caused by protozoans, helminths and arthropod involve 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 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-cycle 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 diseases in recently infected populations of hosts than in long-standing situations of medium-to-high prevalence. Working 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 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. Unless the proper understanding of the situation is in hand, and the appropriate tools are assembled, sheer determination is simply not enough to effectively dominate these well-established and spreading diseases.
Most available antiparasitic drugs are safe, cheap and highly effective against a broad spectrum of parasites. However, the alarming increase in the number of parasite species that are resistant to these drugs, the issue of residues in the food chain and the lack of new drugs stimulate development of alternative control methods in which vaccines would have a central role. Parasite vaccines are still rare, but there are encouraging signs that their number will increase in the next decade. The modern paradigm is that an understanding of parasite genes will lead to the identification of useful antigens, which can then be produced in recombinant systems developed as a result of the huge investment in biotechnology. However, we should also continue to devote efforts to basic research on the host-parasite interface.
These all-encompassing characteristics make Parasitology face scientists, clinicians, and public health officials as we move into the 21st century.

Reference reading materials

If you haven't heard of leprosy, lymphatic filariasis, onchocerciasis, schistosomiasis, soil-transmitted helminths, and trachoma, you aren't alone. Most of these diseases have little name recognition in industrialized countries as well as some developing countries, but together they cause severe disability in the world's poorest countries, resulting in billions of dollars of lost productivity. Recognizing that multiple voices calling for the world's attention are more effective than singular, disjointed efforts, the separate disease programs aimed at controlling or eliminating these diseases have come together to form the Neglected Tropical Disease Coalition (NTDC). The NTDC provides a unique opportunity for collaborative advocacy activities in industrialized countries while member organizations also explore coordination, or integration of disease treatment and prevention activities in endemic countries. Open to all interested parties, the NTDC is composed of individual disease alliances, international agencies, corporate partners, academic institutions, faith-based groups, and non-governmental organizations.
Both the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have recently identified these diseases as 'targets of opportunity' in the effort to improve global health. All six of these diseases are being targeted for control or elimination due to remarkable scientific breakthroughs and unprecedented corporate philanthropy. Several of these diseases, river blindness in the Americas, trachoma in Morocco, and leprosy worldwide already represent near success. Treating these 'targets of opportunity' will produce more vigorous economies and result in a better quality of life for some of the world's poorest communities.
(from http://www.neglectedtropicaldiseases.org/)

Reference reading articls

Batchelor T, Gherardin T. Prevention of malaria in travellers.Aust Fam Physician. 2007 May;36(5):316-20.
Knox DP, Redmond DL. Parasite vaccines - recent progress and problems associated with their development.Parasitology. 2006;133 Suppl:S1-8
O'Garra A, Vieira P. T(H)1 cells control themselves by producing interleukin-10.Nat Rev Immunol. 2007 Jun;7(6):425-8.
Utzinger J, Xiao S, N'Goran EK, Bergquist R, Tanner M. The potential of artemether for the control of schistosomiasis.Int J Parasitol. 2001 Dec;31(14):1549-62.
Parasitology Online http://www.msu.edu/course/zol/316/
Microbiology and Immunology On-line http://pathmicro.med.sc.edu/book/parasit-sta.htm
Parasitology online http://www.emedicine.com/ped/PARASITOLOGY.htm
The world of Parasites http://martin.parasitology.mcgill.ca/JIMSPAGE/WORLDOF.HTM