The life cycle of parasites refers to the whole process that parasites complete the growth, development and reproduction of a generation. There are many kinds of parasites with various life histories, which can be roughly divided into the following two types:

1. The direct type does not need an intermediate host to complete the life history, and the eggs or larvae directly infect people after the external development reaches the infection stage. Such as roundworms, pinworms, whipworms, hookworms, etc.

2. The indirect life history needs an intermediate host, and the larvae develop into an infection period in their bodies, and then infect people through the intermediate host. Such as filariasis, Trichinella spiralis, Schistosoma japonicum, Clonorchis sinensis and Taenia solium.

In epidemiology, worms with direct life cycle are usually called soil-borne worms, while worms with indirect life cycle are called biogenic worms.

Some parasites only reproduce asexually in their life history. Such as amoeba, trichomonas vaginalis, Giardia, Leishmania, etc. Some parasites only have sexual reproduction, such as roundworms, pinworms and filariasis. Some parasites, such as plasmodium, Toxoplasma gondii and trematode, have the above two reproductive modes to complete the development of one generation, that is, the alternation of asexual reproductive generations and sexual reproductive generations, which is called generation alternation. Some parasites live parasitically throughout their life history, such as Taenia solium and Plasmodium. Some live parasitically only at certain developmental stages, such as hookworms. Some parasites only need one host, such as roundworms and pinworms; Some need two or more hosts, such as Fasciola brucei and Paragonimus westermani.

Parasites need not only suitable hosts, but also suitable external environmental conditions to complete their life cycle. The whole life history process of parasites actually includes the ways and means of invading the host during the infection stage, the ways of migrating in the host or reaching the parasitic parts, the normal parasitic parts, the ways of leaving the host body, and the types of the final host (and the pest protection host), the intermediate host or the required transmission medium. Therefore, mastering the law of parasite life history is the necessary basic knowledge to understand the pathogenicity of parasites and the diagnosis, epidemic and prevention of parasitic diseases.

Second, the category of parasites and hosts.

(1) Species of parasites

According to the relationship between parasites and hosts, parasites can be divided into:

1. Life history and parasitic life at various stages, such as filariasis; Or a certain stage of life history must be parasitic, such as hookworm, whose larvae live independently in the soil, but after developing into filamentous larvae, they must invade the host parasitic life to continue to develop into adults.

2. facultative parasites can live by themselves. For example, Angiostrongylus faecalis (adult) can live autonomously in the intestine or soil of the host.

3. Accidental parasites (Accidental parasites) are parasites that accidentally enter abnormal hosts, such as some fly maggots entering human intestines.

4. Endoparasites and ectoparasites, such as worms or protozoa parasitic in intestines, tissues or cells; The latter, such as mosquitoes, sandflies, fleas, lice, ticks and so on. When sucking blood, they come into contact with the host surface, and most of them leave after eating.

5. Permanent parasites and temporary parasites, such as ascaris lumbricoides, must live a parasitic life in the adult stage; The latter, such as mosquitoes, fleas and ticks, temporarily invade the host when sucking blood.

6. Opportunistic parasites, such as Toxoplasma gondii and Cryptosporidium carinii. , usually in a state of recessive infection in the host, but when the immune function of the host is affected, abnormal proliferation can occur and efforts can be made to enhance it.

(2) the type of host

Parasites complete the life history process, some need only one host, others need two hosts. Hosts parasitized by parasites at different developmental stages include:

1. Intermediate hosts refer to the larvae of parasites or hosts that are parasitized during asexual reproduction. If there are more than two intermediate hosts, they can be divided into the first and second intermediate hosts according to the parasitic situation. For example, some kinds of freshwater snails and freshwater fish are Clonorchis sinensis and the first and second intermediate hosts respectively.

2. Determine the host refers to the adult or parasitic host in sexual reproduction stage. For example, human beings are the ultimate hosts of schistosomiasis.

3. Storage host (also called storage host) Some adults or protozoa can be parasitic on both humans and some vertebrates at a certain stage of development, and can be transmitted to people under certain conditions. In epidemiology, these animals are called pest protection hosts or storage hosts. For example, adult Schistosoma japonicum can be parasitic on humans and cattle, and cattle is the protective host of Schistosoma japonicum pests.

4. Parasitic hosts (transport hosts) The larvae of some parasites invade abnormal hosts and cannot develop into adults. It remains a larva for a long time, and then it can continue to develop into an adult after it has a chance to re-enter the normal final host. This abnormal host is called a parasitic host. For example, Paragonimus westermani larvae can not develop into adults when they enter the abnormal host wild boar, and can remain in the state of larvae for a long time. If a dog swallows wild pork containing this larva, the larva can develop into an adult in the dog. Wild boar is the persistent host of this insect.

Third, the classification of parasites

The purpose of parasite classification is to understand insect species and reflect the genetic relationship between various parasites, trace the clues of the evolution of various parasites, comprehensively and accurately understand various insect groups and insect species, and understand the relationship between parasites and humans.

According to the animal classification system, among invertebrates in the animal kingdom, human parasites belong to the phylum Platycladus, Nematoda and Echinoderm. It is related to the phylum Arthropoda, the phylum Carniflagellata, the phylum Api complex and the phylum ciliata. The scientific name of the parasite is named after the animal, followed by the surname and the year of naming (the year when the paper was officially published). Scientific names are written in Latin or Latin culture. Such as entamoeba histolytica (1903); Plasmodium falciparum [Welch, 1987] Saudine, 1902] refers to Saudine (1902), and this scientific name has been confirmed.

Four. Nutrition and metabolism of parasites

1. The nutritional species of parasites can vary according to the species of insects and the nutritional methods and sources at different stages of their life history. Because parasites live in different organs and tissues of the host, their nutrients include tissues, cells and noncellular substances of the host, such as plasma, lymph, body fluids and undigested, semi-digested or digested substances in the digestive tract of the host. These substances consist of water, inorganic salts, carbohydrates, fats and vitamins. If the parasite has a well-developed digestive tract, it contains various enzymes from the worm and the host. These enzymes are beneficial to the digestion of nutrients and help parasites invade tissues or migrate in the host. However, tapeworm lacks digestive tract, and its nutrients are mainly absorbed by membrane. Some protozoa, such as ciliates in the colonic sac, have endocytosis and endocytophagy, and amoeba has pseudopoda, which can swallow nutrients and form food vacuoles, so protozoa can also be digested and absorbed in the body. Many protozoa can absorb nutrients through the surface membrane without forming food vacuoles. The absorption of nutrients is carried out through the plasma membrane of any part of the parasite, which can be regarded as a "fence" selective to solutes.

The absorption of oxygen by parasites is that oxygen dissolves in the cortex, the inner wall of digestive tract or other parts in contact with oxygen and enters the worm. In protozoa, mainly through the cell membrane; Some parasites can also use some substances as carriers, such as hemoglobin and iron porphyrin compounds, to spread oxygen to various parts of the worm. Oxygen ingested by parasites is used to oxidize and decompose nutrients and release energy. Many internal parasites are in an environment of low oxygen partial pressure or even hypoxia at a certain period in their life history, and their adaptability to low oxygen partial pressure environmental conditions has been strengthened to varying degrees. Such as improving the efficiency of oxygen transport in parasites, using oxygen more economically through various forms, and overcoming the difficulties caused by insufficient oxygen supply.

2. Metabolism of parasites The metabolism of parasites can be divided into energy metabolism and synthetic metabolism. The main source of energy is sugar. Glucose metabolism can be roughly divided into homolactic fermentation and carbon dioxide fixation. The former is found in blood tissue parasites and the latter in intestinal parasites. Parasites constantly produce energy during anaerobic glycolysis, and its typical end product is lactic acid. But many parasites may get energy from protein metabolism when they can't get carbohydrate nutrition.

The rapid propagation of parasitic protozoa in the body and the spawning or larva of worms need a lot of protein, and its anabolism is vigorous. The amino acids needed to synthesize protein come from the decomposition of free amino groups in protein or food; As for the base of nucleic acid, pyrimidine itself should be synthesized by purine, such as protozoa and nematodes in blood. Lipids mainly come from parasitic environment, and some of them may be self-synthesized. For example, Plasmodium Knowles can synthesize phospholipids by its own fermentation. It is known that nematodes can oxidize fatty acids stored in their intestinal cells as an energy source.

First, the basic link of parasitic disease epidemic

(1) source of infection

The source of infection of human parasitic diseases refers to people and animals parasitized by human parasites, including patients, carriers and savings hosts (domestic animals, domestic animals and wild animals). As a source of infection, it exists in the body and can be excreted at a certain development stage of the parasite's life history, and can continue to develop in the outside world or in another host. For example, carriers or patients infected with various worms excrete eggs from feces; Entamoeba histolytica can excrete cysts; Eggs or cysts are infectious when they are discharged, or develop into infection period (infection period) in a suitable external environment. Infection period refers to the developmental stage when parasites invade the host and can continue to develop or reproduce.

Transmission route

Refers to the process in which parasites spread from the source of infection to susceptible hosts. The common transmission routes of human parasites are as follows:

Soil intestinal parasites live in the ground soil during the infection period. For example, ascaris eggs and trichuris eggs develop into infectious eggs in soil polluted by feces; The eggs of hookworm and strongyloides faecalis develop into infected larvae in the soil. Human infection is related to contact with soil.

Many kinds of parasites in water can reach human body through fresh water. For example, the water may contain cysts of Entamoeba and Giardia in infected period, eggs of Taenia solium, some infectious nematodes, cercariae of Schistosoma japonicum and metacercariae of Fasciola brucei.

The food is mainly vegetables and fish. Because the vast rural areas use fresh manure to fertilize, vegetables often become the main way of parasite transmission. Such as infected ascaris eggs, trichuris eggs, infected larvae of Taenia solium eggs and hookworm, and primary cysts, can be transmitted by eating unwashed or undercooked vegetables, while Trichinella spiralis and Taenia solium can be transmitted by eating raw or undercooked pork. Some freshwater fish can spread Clonorchis sinensis and so on.

Arthropod vectors, many medical arthropods can be used as vectors of various parasites. For example, mosquitoes are plasmodium and filariasis, sandflies are Leishmania, and fleas are the vectors of hymenolepis.

Direct transmission from human body; Direct contact between people can directly spread some parasites. For example, trichomonas vaginalis can be transmitted through sexual intercourse, and sarcoidosis can be transmitted through direct contact with patients' skin.

There are mainly the following ways and modes of infection of human parasites:

Many parasitic oral infections can enter the human body through food, drinking water, contaminated fingers, toys or other media, which is the most common infection mode. Such as ascaris lumbricoides, trichuris, pinworm, Clonorchis sinensis, cysticercus cellulosae, etc.

Some parasites infected through the skin actively invade the human body through the skin during the infection period, such as hookworm filariasis in the soil, schistosomiasis cercaria in the water, and scabies and demodex that directly invade the skin. Some parasites enter the human body through the skin through the stings of blood-sucking arthropods. For example, mosquitoes transmit plasmodium, filariasis and sandflies transmit Leishmania.

Some self-infected parasites can cause repeated infection of the host. For example, the eggs of Hymenolepis brevis can hatch into hexagamete larvae in the small intestine, and the larvae can develop into adults in the small intestine. Taenia solium is parasitic in the small intestine, and its pregnant node falls back into the stomach due to vomiting and is digested. After reaching the small intestine from the stomach, the eggs hatch to the intestinal wall and reach all parts of the body with blood circulation, causing self-infection of cysticercosis.

Retrograde infected pinworm lays eggs around human anus, eggs can hatch near anus, and larvae enter intestinal parasitic parts through anus and develop into adults.

Placental infection Some parasites can infect the fetus through the placenta with the maternal blood, such as Toxoplasma gondii, plasmodium, hookworm larvae and so on.

In addition, some parasites can spread through the respiratory tract, such as pneumocystis carinii; Such as trichomonas vaginalis passing through vagina; For example, plasmodium enters the human body through blood transfusion.

(3) Susceptible population

Susceptibility refers to people who lack immunity to parasites. When people are infected with parasites, they usually have acquired immunity, but most of them are insect-borne immunity. When the parasite disappears from the human body, the immunity gradually declines and fades. Therefore, when there is a chance of infection, it is easy to get infected with this parasite. People in non-endemic areas or local areas where malaria has been eliminated become susceptible because of lack of specific immunity after entering malaria areas. Susceptibility is also related to age, and children's immunity is lower than that of adults.

Second, the factors affecting the prevalence of parasitic diseases

natural causes

Including temperature, humidity, rainfall, light and other climatic factors, as well as geographical environment and biological population. External climatic factors affect the growth and development of parasites, such as warm and humid environment is conducive to the development of eggs and larvae in the soil; Climate affects the reproductive activities and reproduction of intermediate hosts or vector arthropods, and also affects the development and growth of parasites in their bodies. If the temperature is lower than 15~ 16 degrees Celsius or higher than 37.5 degrees Celsius, plasmodium can't develop in mosquitoes. The warm and humid climate is not only conducive to the growth and reproduction of mosquitoes, but also suitable for mosquito blood-sucking activities, which increases the chances of spreading malaria and filariasis. Temperature affects the invasion of parasites. For example, the infectivity of Schistosoma cercariae to human body is related to temperature. Geographical environment is closely related to the growth and development of intermediate hosts and the reproduction and habitat of vector arthropods, which can indirectly affect the prevalence of parasitic diseases. Soil properties directly affect the development of soil-borne worm eggs or larvae.

biotic factor

The development of life history is an indirect parasite, and the existence of intermediate hosts or arthropods is a necessary condition for the prevalence of these parasitic diseases. For example, schistosomiasis is prevalent in the south of the Yangtze River in China, which is consistent with the geographical distribution of snails; The prevalence of filariasis and malaria is consistent with the geographical distribution and active season of mosquito hosts or vectors.

(3) Social factors

Including social system, economic situation, scientific level, culture and education, medical and health care, epidemic prevention and health care, and people's production methods and living habits. The influence of these factors on the prevalence of parasitic diseases has been paid more and more attention. The natural and biological factors in a region are relatively stable in a certain period of time, while the social factors are often variable, especially with the changes of political and economic conditions, which can affect the natural and biological factors to a certain extent. The backwardness of economy and culture is inevitably accompanied by backward production methods and lifestyles, as well as bad hygiene habits and sanitary environment. Therefore, it is inevitable that many parasitic diseases will spread widely and seriously endanger human health. Therefore, social factors are very important for the prevalence of parasitic diseases.

Third, the epidemic characteristics of parasitic diseases

(1) location

The prevalence and distribution of parasitic diseases are often obviously endemic. It is mainly related to the following factors: climatic conditions, such as the prevalence and wide distribution of most parasitic diseases in warm and humid places; Related to the geographical distribution of intermediate hosts or vector arthropods. For example, the endemic area of trematode is closely related to the distribution of intermediate hosts, kala-azar is prevalent in the north of the Yangtze River, and it is also closely related to the distribution of sandfly, a vector insect. It has something to do with people's living habits. For example, Taenia solium and Taenia solium are prevalent in areas where raw or undercooked pigs and beef are eaten, while Clonorchis sinensis is prevalent in areas where raw or undercooked fish are used to eating. It is related to the mode of production, such as hookworm disease, which is usually prevalent in dryland crop areas where human feces are used for fertilization.

(2) Seasonality

The prevalence of parasitic diseases often has obvious seasonality. Parasites that need arthropods as hosts or vectors in their life history, and the epidemic season of such parasitic diseases is consistent with the seasonal fluctuation of related arthropods, such as the epidemic season of Plasmodium vivax is consistent with the active season of Anopheles sinensis or Anopheles anthropophagus; For another example, the relationship between human leishmaniasis and sandfly activity in China is consistent. Secondly, people's production activities or life activities form seasonal infections. For example, acute schistosomiasis occurs mostly in summer, and people are infected with schistosomiasis because of contact with infected water in farmland production or water activities.

(3) Natural focus

Among human parasitic diseases, some parasitic diseases can spread naturally between vertebrates and humans, which is called parasitic zoonosis. In virgin forests or desert areas, these parasites can always spread among vertebrates. When people accidentally enter this area, they can spread from vertebrates to people through certain channels. This parasitic disease that exists in nature without human participation has obvious natural foci. Such areas are called natural foci. This natural focus of parasitic diseases not only reflects the evolution of parasitic diseases in nature, but also shows the complexity of epidemiology and prevention of some parasitic diseases.

Four, the prevention and control measures of parasitic diseases

The life history of parasites varies from species to species, some of them are very complicated, and the epidemic factors of parasitic diseases are also varied. Therefore, in order to achieve effective control purposes, comprehensive control measures must be formulated on the basis of understanding the life history of various parasites and the epidemic law of parasitic diseases. According to the epidemic links and factors of parasitic diseases, the following measures are taken to prevent the completion of parasitic life history, so as to control and eliminate parasitic diseases.

1. Eliminate the source of infection, treat the carriers and patients through general survey, and check or treat the savings hosts. In addition, the floating population should be monitored to control the input and spread of infectious sources in epidemic areas.

2 cut off the route of transmission, strengthen the management of feces and water sources, improve environmental hygiene and personal hygiene, and control or kill vector animals and intermediate hosts.

3. Protect susceptible people, strengthen collective and individual protection, change bad eating habits, improve production methods and conditions, apply repellent to the skin to prevent the bite of blood-sucking arthropods, and take preventive medication for some parasitic diseases.

In the process of prevention and control of parasitic diseases, prevention and control schemes must be formulated according to the specific conditions of various parasites in various regions. To control and eliminate soil-borne worms and parasites infected by mouth, we should first pay attention to the management of feces and water sources and personal food hygiene. For example, the infection of Clonorchis sinensis and paragonimiasis is caused by raw or undercooked freshwater fish, shrimp, crab and mole respectively; Taenia solium and Trichinella spiralis in pigs and cattle are caused by eating uncooked pork and beef. These helminthes are also called food-borne helminthes. Through "oral transmission and heart-to-heart transmission", people are educated to change bad eating habits, strengthen the inspection of feces and meat, and reduce the chance of transmission. The prevention and control of echinococcosis is mainly based on slaughter hygiene management, domestic dog management and drug deworming. Combined with the actual situation of epidemic areas in China, it will be the most economical and effective prevention and control countermeasure to implement "pollution-free deworming" for sick dogs.

Only by mobilizing the masses and even the whole society to actively participate can we do a good job in the prevention and treatment of parasitic diseases. Therefore, it is necessary to strengthen publicity so that the masses and leaders at all levels can hear and see the harm of parasitic diseases to people's health and economic development, realize that "small insects" are related to the physical quality of the whole Chinese nation and the important significance of preventing parasitic diseases, and make leaders at all levels incorporate the prevention and treatment of parasitic diseases into the goals of local economic development and the construction of two civilizations; Through the propaganda of parasite life history, we will increase the scientific knowledge of preventing parasitic diseases and improve the people's self-care and disease prevention awareness. Only in this way can we carry out mass prevention and treatment, and consolidate and improve the control effect of parasitic diseases.

Verb (abbreviation of verb) Achievements and present situation of parasitic diseases prevention and control in China

After the founding of People's Republic of China (PRC), the prevention and treatment of parasitic diseases in China was put on the agenda. First of all, we have made great efforts to prevent and control the five most popular and harmful parasitic diseases and achieved remarkable results.

In the early 1950s, the annual incidence rate of malaria in China exceeded 30 million, which decreased from 1.990 to 1.750. Of the 992 malaria-endemic counties (cities) in China, 937 counties (cities) have reached the basic elimination standard. Schistosomiasis seriously harms the health of people and livestock, and it is prevalent in the Yangtze River Basin 12 provinces (cities, districts), with the number of patients reaching 1 1.9 million. After decades of prevention and treatment, 1 1 10,000 patients have been cured. By the end of 1992, 259 counties (cities) out of 380 endemic counties (cities) in China had reached the standard of elimination or basic elimination. At the beginning of the People's Republic of China, it was estimated that there were 30.99 million people infected with lymphatic filariasis, and it was prevalent in 864 counties (cities) in 15 provinces (autonomous regions and municipalities). By 1990, 1 provinces, except for 28 counties, all have reached the basic elimination goal. Kala-azar was once prevalent in 665 counties (cities) in the north of the Yangtze River 16 provinces (autonomous regions and municipalities), with 530,000 patients. After the patients were treated and the disease vector sandfly was eliminated, it was completely and effectively controlled at 1958. At present, there are only 70 sporadic cases in more than 30 counties in 6 provinces (autonomous regions and municipalities).