《傳染病學》雙語教學講義

1、傳染病學雙語教學講義INTRODUCTIONOF COMMUNICABLE DISEASESIntroduction of Communicable DiseasesRUAN BingCommunicable diseases are caused by various pathogens including microorganisms (e.g. prion, virus, chlamydia, rickettsia, bacteria, fungus, spirochete) and parasites (e.g. protozoa and helminth). They may be

2、transmitted to others as infection and spread in the population as epidemic, therefore they are the diseases with high morbidity. Many of them may be prevented and effectively treated.In China, great change has taken in prevention and treatment of infectious diseases. Before 1949, many pestilences w

3、ere rampant, such as plague, smallpox, malaria, schistosomiasis and kala-azar. After the founding of the Peoples Republic, we have carried out the principle of prevention first, launched with vast efforts the patriotic health campaigns, strengthened our work on infectious diseases prevention and bui

4、lding of health infrastructure. As a result, health situation in both town and country has taken on a new look, and people health has been improved greatly. Now many classical infectious diseases have been controlled and eradicated, such as smallpox, poliomyelitis, plague, cholera, and measles, but

5、some infectious diseases are still popular, such as viral hepatitis, HFRS (hemorrhagic fever with renal syndrome), infectious diarrhea, and so on. Meanwhile, some re-emerging infectious diseases such as syphilis and tuberculosis are re-appeared, and emerging infectious diseases such as AIDS (acquire

6、d immunodeficiency syndrome), SARS (severe acute respiratory syndrome, namely infectious atypical pneumonia), and infections caused by resistant mutants are arisen.Section 1 Infection and immunity1.1 Conception of infectionInfection refers to interaction between pathogen and host. Under normal circu

7、mstances, human being lives in delicate dynamic balance with the microorganisms of internal and external environments. When this balance is upset, organisms may penetrate a sequence of host defensive barriers (e.g. skin, mucous membrane), proliferate and disseminate in the host to cause disease. It

8、s called “infection”.To understand the interaction between microorganism and host, the difference between infection and infectious disease must be appreciated. Infection implies only that a specific microorganism gains entry into the host. Infectious disease is the clinical or subclinical biochemica

9、l, physiologic, metabolic and immunologic manifestation of this interaction.Hospital infection (nosocomial infection) is the infection acquired in the hospital with an incidence of about 5%10% of the hospitalized patients. Most of the hospital infections are iatrogenic infection. Nosocomial infectio

10、n is an important new problem of the hospital.The infection process is depended upon two factors: the pathogenic organisms and human reactions, and both of them may be influenced by the environmental factors. Pathogenic organism is the chief factor of infection. There are constant pathogenic (e.g. s

11、higella, salmonella) and conditional pathogenic (e.g. E. coli, pseudomonas and fungi). The conditional pathogenic microorganism also causes infection called “opportunistic infection”. Human reaction is the decision factor of the infection, such as cellular immunity and humoral immunity.1.2 Manifesta

12、tion of infectious process (Infection spectrum)Infectious process starts when microorganisms enter the human host through various ways. Whether the microorganism is eliminated or colonized in the host, it is mainly dependent on the pathogenicity of the pathogen and the immune function of the host.1.

13、2.1 Pathogen is eliminated or eradicated by host immunityAfter the microorganism enters the host, it can be wiped out by non-specific immunity, and eliminated or eradicated by specific immunity the host has got before the infectious process. The specific immunity includes active and passive ones.1.2

14、.2 Covert infection隱性感染Its also called “subclinical infection”, “inapparent infection”, or “silent infection”. This state refers to that the microorganism only make the host producing specific immune response not having histomorphological damage or even having mild feature after it enters the host.

15、Clinically, there is no any symptoms and signs, even no any biochemical changes. Its only discovered by immunological examinations. Covert infection is the most common manifestation in some infectious diseases (e.g. poliomyelitis and epidemic encephalitis B). Most hosts get the specific active immun

16、ity, and the pathogens can be eradicated after the covert infection ends. But some hosts change to carrier state and the microorganisms exist in the host continuously. It is called “healthy carrier” (e.g. typhoid fever, bacillary dysentery, viral hepatitis B).1.2.3 Overt infection顯性感染Its also called

17、 “clinical infection”, “apparent infection”. When the microorganism enters the host, it not only induces the immune responses of the host but also produces histomorphological組織形態(tài)上的 damage and pathological changes through the role of the microorganism or the allergic reaction of the host. In some inf

18、ectious diseases such as measles and varicella (chickenpox), most infected hosts manifested as overt infection. After overt infection, the pathogens can be eliminated and the hosts acquired consolidated immunity. However, in some infectious diseases (e.g. bacillary dysentery), the immunity is tempor

19、ary. A small amount of overt infection may change to carrier state. Its called “convalescent carrier”.1.2.4 Carrier state病原攜帶狀態(tài)It divides into virus carrier and bacteria carrier according to different pathogens. It also divides into convalescent carrier, healthy carrier and incubatory carrier. There

20、 are acute and chronic carriers according to that whether the carrier state is within 3 months or more. All carrier states have a common characteristic that is no clinical manifestation but the microorganism can be excreted during the state. Therefore, in many infectious diseases (e.g. typhoid fever

21、), the carrier state is one of the main sources of infection. Not all of the infectious diseases have this state. In some diseases (e.g. measles and influenza), carrier state is quite rare.1.2.5 Latent infection潛伏性感染W(wǎng)hen the microorganism infected the host and localized in some area of the host, it

22、can latently for a long time because the hosts immunity is strong enough to locate the pathogen but cannot wipes it out. When the hosts immunity decreased, overt infection can occurs. Latent infection is common in some diseases, such as herpes simplex, herpes zoster, malaria, tuberculosis, and so on

23、. Generally, the microorganism cannot be excreted during latent infection, which is a different point from carrier state. Not every infectious disease has latent infection.1.3 The role of pathogen in the infectious processAfter the pathogen invades the host, whether it can induce the disease is depe

24、ndent on the pathogenicity of the pathogen and the immune function of the host. The pathogenicity of the pathogen is related to the following factors.1.3.1 Invasiveness侵襲力It refers to its ability to invade the host and spread within the host. Some pathogens (e.g. leptospira, filariform larva and anc

25、ylostoma) can invade the host directly. Some bacteria (e.g. vibrio cholera) need to adhere to the surface of the intestinal mucous membrane first and then produce enterotoxin. Some bacterial surface ingredients (e.g. Vi antigen of salmonella typhi) have the abilities to inhibit phagocytosis, which c

26、an promote the spread of the pathogen.1.3.2 Virulence毒力It includes the toxins and various enzymes. The exotoxin of corynebacterium diphtheria or clostridium tetani can reach the remote parts from the site where the pathogen multiply through the bloodstream to induce pathological changes. The enterot

27、oxin of vibrio cholera can induce the functional changes of absorption and excretion of the intestinal mucous membrane locally. Entamoeba histolytica invades the intestinal wall deeply by excreting various enzymes to induce the necrosis of the tissue.1.3.3 Amount of the pathogenThe number of invadin

28、g pathogen is positive relation with the pathogenicity in the same infectious disease. The least number of the pathogen to induce disease differ greatly in different diseases. For example, S. typhi is 10000 and Sh. dysenteriae is only 10.1.3.4 Variability變化性Pathogens can produce variation due to env

29、ironmental, drug and hereditary factors. Generally speaking, under the environment of artificial culture, the pathogenicity of the pathogen (e.g. BCG) will decrease. The repeated spread between the hosts may increase the pathogenicity of some pathogen (e.g. pneumonic plague). The antigenic variation

30、 of the pathogen will make the pathogen escape form the specific immunity of the host and continue induce the disease. Influenza is a good example with antigenic drift and antigenic shift.1.4 The role of individual immunity in the infectious processThe individual immunity will be beneficial to prote

31、ct against the invasion of pathogen and belong to protect immunity. There are non-specific immunity and specific immunity.1.4.1 Non-specific immunity (natural immunity)Anatomic barriers can be in external (skin and mucous membrane) and internal (lymph nodes). Phagocytosis can be in blood (neutrocyte

32、) and tissues (macrophage). Humoral effects include complement, lysosome, properdin, interferon, and so on.1.4.2 Specific immunity (acquired immunity, adaptive immunity, or postinfection immunity)This immunity is specific and active. There are two categories: (a) cell-mediated immunity: T-cell may p

33、lays an important role on stop infection caused by intracellular organisms (e.g. fungi, virus, mycobacteria and protozoa). T-lymphocytes destroy antigen in several ways including antigen specific cellular cytotoxicity (ASCC), spontaneous cell-mediated cytotoxicity (SCMC) and antibody dependent cell-

34、mediated cytotoxicity (ADCC). (b) humoral immunity: B-cell responds to antigenic stimuli by producing immunoglobulin which consists of five distinct groups of glycoprotein: IgG, IgM, IgA, IgD, and IgE.Section 2 Epidemic process and influenced factors of infectious diseases2.1 Essential conditions of

35、 epidemic process with infectious diseases (three basic links)2.1.1 Sources of infection傳染源Sources of infection refers to the person or animal who is able to offer the place for the microorganisms to establish itself and proliferate in vivo, as well as excrete pathogen outside the body. They include

36、 patients (e.g. measles), the persons of covert infection (e.g. poliomyelitis, epidemic cerebrospinal meningitis), pathogen carriers (e.g. typhoid fever, bacillary dysentery), and infected animals (which can spread the pathogens of rabies, plague, leptospirosis, tsutsugamushi disease).2.1.2 Routes o

37、f transmissionThe pathways which pathogen transfers from its source of infection to reach susceptible host are termed “route of transmission”. (A) Respiratory duct transmission: air, droplet nuclei, dust particle (e.g. measles, diphtheria, tuberculosis).(B) Digestive duct transmission: water, food a

38、nd fly (e.g. typhoid fever, shigellosis and cholera).(C) Contagious transmission: direct contagion with water (e.g. leptospirosis, schistosomiasis) and soil (e.g. bacterial spore of tetanus and anthrax, larva of ancylostomiasis or hookworm disease, worm egg of ascariasis); daily-life contagion by ha

39、nd, tool and toy (e.g. bacillary dysentery).(D) Arthropod-bone transmission: blood-sucking arthropods infected with pathogens (e.g. mosquito-malaria, lice-typhus, flea-plague, sandfly-kala-azar, mite-scrub typhus, tick-lyme disease).(E) Blood or body fluid transmission: blood, blood products and bod

40、y fluid (e.g. viral hepatitis B, viral hepatitis C, AIDS).(F) Mother to infant transmission: vertical transmission makes congenital infection that is liable to chronic (e.g. viral hepatitis B, AIDS).Host: there are definitive host (final host), intermediate host and reservoir host.2.1.3 Susceptibili

41、ty of the populationThe person who is deficient in specific immunity to some infectious diseases is termed “susceptible person”. Among particular population, crowd susceptibility depends on the ratio of susceptible persons. When the ratio reaches to a critical level, spread of disease is easy to occ

42、ur if there have been the source of infection and suitable routes of transmission. After epidemic, some infectious diseases which have a strong postinfection immunity (e.g. measles) dont outbreak for another time until several years later when the ratio of susceptible persons increases repeatedly to

43、 a critical level. This phenomenon is called “periodicity of an epidemic”. Under the popularized artificial active immunization to some specific diseases (e.g. smallpox, poliomyelitis, measles), the epidemic may be prevented when the crowd susceptibility will be reduced at very low level.2.2 Influen

44、ced factors of epidemic process2.2.1 Natural factorsIn natural environment, various factors including geography, climate and ecology make a notable impact on the occurrence and development of epidemic process. Regional and seasonal distribution of infectious diseases is closely related to natural co

45、ndition. For example, epidemic of kala-azar黑熱病 is confined to northern area, while schistosomiasis is southern. Besides, the occurrence of encephalitis B is limited to the summer and autumn and epidemic outbreak of leptospirosis is related to flood. Some natural ecologic environment is suitable to t

46、he transmission of infectious diseases among wild animals. This area is called “focus of infection”. The human race may be infected when they get into these areas. This disease is called “zoonosis動物傳染病”, such as plague, leptospirosis, human avian influenza, and so on.2.2.2 Social factorsThe factors,

47、 including social background, economical condition, cultural level and anti-epidemic measures, have decisive influence on epidemic process of infectious diseases. For example, the improvement of drinking water sanitation and feces treatment results in control over schistosomiasis, cholera and ancylo

48、stomiasis respectively.Section 3 The characteristics of infectious diseases3.1 Basic characteristics 3.1.1 Pathogen病原體Infectious disease is caused by a specific pathogen such as microorganism or parasite. In history, many infectious diseases were first understood by their clinical manifestation and

49、epidemiologic feature, and then the pathogen was found. Until recently, some pathogens of infectious diseases are not completely known.3.1.2 Infectivity傳染性Infectivity is the main distinction between the infectious diseases and other infection. For example, both otogenic meningitis and epidemic cereb

50、rospinal meningitis manifested clinically as purulent meningitis. The former has no infectivity and need not to be isolated, while the later has infectivity and must be isolated. Infectivity means that the pathogen can be excreted to contaminate the surrounding. The infective duration of infectious

51、patients is called “infective period”. Every infectious disease has considerably a stable infective period, which can be used as a rule to isolate the patient.3.1.3 Epidemiologic featureUnder the influence of natural and social factors, the infectious process manifests various characteristics. It ca

52、n be exotic and the endemic. The former refers to the diseases (e.g. cholera and yellow fever), which primarily do not exist in China, are transmitted from foreign countries. The later refers to the diseases continuously occur in certain areas under certain special natural and social condition. It a

53、lso can be divided into sporadic, epidemic and pandemic. Sporadic occurrence refers to that the incidence of an infectious disease in an area in recent years is within general level. When the incidence is remarkably higher than general level, it is called “epidemic”. When the epidemic of an infectio

54、ns disease is beyond a country or a continent, we call it “pandemic”. It is called “epidemic outbreak” when incidence of an infectious disease concentrate on a short period of time. The distribution of the incidence of the infectious diseases in time (seasonal distribution), in space (regional distr

55、ibution) and in different populations (age, sex and occupation) is also an epidemiologic feature.3.1.4 Postinfection immunity感染后免疫The host can produce specific protective immunity which is directed to the pathogen or its products (e.g. toxin) after the host was infected by the pathogens. Protective

56、immunity can be discovered by the detection of sera antibodies including antitoxin or neutralizing antibody. Postinfection immunity belongs to active immunity. The immunity through antibody transfer belongs to passive immunity. Postinfection immunity can transfer to newborns through the placenta. Th

57、e lasting time of postinfection immunity varies with different infectious diseases. Generally, the postinfection immunity to viral diseases (e.g. measles, poliomyelitis, epidemic encephalitis) can last for long time and usually for all the life. But there are some exceptions, such as influenza. The

58、postinfection immunity to diseases caused by bacteria, spirocheta or protozoa is relatively short and only lasts several months to several years, such as bacillary dysentery, leptospirosis or amebiasis. The exceptions also can be seen, such as typhoid fever. The protective immunity to helminthiasis

59、(e.g. schistosomiasis, ancylostomiasis or ascariasis) will be lost after helminth eliminating, but can sustain with infection. This phenomenon is called “concomitant immunity”. The host of helminth is easily re-infected.3.2 Clinical feature3.2.1 The phase in the development of clinical courseThere are usually four phases in the occurrence, development and recovery of acute