Epidemiology of Communicable Diseases PDF

Epidemiology of Communicable Diseases By Dr Asmaa Fawzy Ali Abdelnaby Lecturer of Public Health, Faculty of Medicine, Cairo University  By the end of this chapter the stude...

Epidemiology of Communicable Diseases By Dr Asmaa Fawzy Ali Abdelnaby Lecturer of Public Health, Faculty of Medicine, Cairo University  By the end of this chapter the students should be able to: ❖ Define measures of disease frequency ❖ Outline measures of association between risk factors and disease. ❖ Define the common terms used in the field of epidemiology ❖ Differentiate types of carrier and spectrum of case presentation. ❖ Identify the components of the infection cycle ❖ Understand types of immune reaction. ❖ Outline epidemiological causes of infections. ❖ Outline disease prevention and control.  Epidemiology is defined as is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in defined population. It is a cornerstone of public health, and shapes Definitions: policy decisions and evidence-based practice by identifying risk factors for disease and targets for preventive healthcare. Greek word come from: Epi = upon, Demos = people, Ology = science, Epidemiology = the science which deals with what falls upon people. It discusses the Distribution, Dynamics, and Determinants of disease in the community.  Disease etiology: is the study of all factors that may be involved in the development of a disease, including: and the way in which the the susceptibility of the the nature of the disease patient's body is invaded by patient, agent, the agent disease ecology: from οικία = house in Greek, is the science that deals with the mutual relation between man and his environment. Infection: is invasion of the body by pathogenic or potentially pathogenic organisms. whether this invasion cause clinical picture of the disease (cases) or not (apparently healthy individual which could be subclinical infection, latent infection, or carrier). Sequelae of infection is Number of determined by: Pathogenicity invading organisms: and virulence Agent factors: dose of infection. General defense mechanism of Specific Host factors: the body, including immunity. natural barriers of infection. Epidemiological triad  Infection cycle Infection cycle: is the method describe how we get infection. It consists of 3 factors: 1. Source of infection, 2. mode of transmission 3. and host reaction  Source of infection: could be human in most of diseases in form of a case as in common cold and COVID-19 or a carrier as in amoeba stool passer , or an animal as brucellosis and rabies in zoonotic infections, or insects as in Malaria, or soil as Tetanus and anthrax. case: are infected people that show manifestations of disease. They are infectious for varied period of time, according to the 1- Source nature of disease, availability and use of specific therapy. of infection carrier: is apparently healthy individual who is infected and harbors pathogenic organisms as foci of infection in different parts of his body, without showing manifestations of disease, but he can spread infection e.g. diphtheria, enterica and poliomyelitis. Zoonoses (single is zoonosis): diseases that primarily infect, and spread between animals, but may be transmitted to man when exposed to infection. Strictly zoonotic diseases: reservoirs of infection are animals only, no man to man infection e.g. brucellosis and Q fever. Both animals and man are the reservoirs, Infection may be animal to man, or man to man. e.g. Monkey Pox (monkeys and man) and Salmonellosis and salmonella food poisoning (man and some animal species as cattle, swine, poultry and rats).  Types of carriers: 1- Incubatory Carrier: the case in some diseases in the incubation period. Organisms are found in body discharges and fluids, and so can spread infection. As in chicken pox and measles children can infect before fever and rash, hepatitis A,B and C viruses, HIV virus. 2- Convalescent Carrier: a certain percent of convalescents (recovered cases) for varied period of time (weeks, months, years or lifelong). As in chronic salmonellosis, poliomyelitis and shigellosis. 3- Contact Carrier: contacts of cases or carriers of some infectious diseases may be infected, but not diseased, and can spread infection in around two weeks. COVID-19, Hepatitis A, B and C Viruses. 4- Healthy Carrier: inhabitants of endemic areas of infectious diseases may get infected from polluted environment e.g. water supply and food. If not become diseased, infection is eliminated in around two weeks, except for hepatitis B virus infection where healthy carriers are infectious for years.  There are 6 main modes of transmissions: 2- Modes of 1- Airborne transmission in which transition occur via inhalation. it includes either droplet infection or airborne transmissions infections; example common cold, COVID-1, Measles, streptococcal tonsilitis Meningococcal meningitis, pneumonia, and respiratory TB.  Ingestion transmission: it occurs when we swallow contaminated food or water or milk. It is also called feco- oral infection Examples food poisoning, cholera, salmonella, 2- Modes of amoeba and E.coli. transmissions  2- Modes of transmissions 3- Arthropod transmission: either mechanical transmission by carrying organisms on the insect body (flies and cockroaches) or in its excreta (flies), or biological transmission in which insect play a role in developing of the organism inside its body and transmit it to human by insect bite as Malria, Zika virus, Dengue fever transmitted by Mosquitos. Or plague transmitted by ticks.  4- Contact transmission: organisms enter through intact or non-intact Skin or Mucous Membranes. This mode of transmission includes all sexually transmitted diseases e.g. Syphilis, Gonorrhea, HBV, HPV, Herpes virus and HIV. Or 2- Modes of direct contact with contaminated wounds or abscesses as pyogenic infections, eye infections and scabies. Contact transmissions could be with Animal bites as Rabies by canines or Tetanus in cats, or injury in contaminated soil with Tetanus spores (in all dust) or gas gangrene.  5- special type of contact infection is called injection infection or parenteral infection: in which skin is injured with contaminated sharp: either needle stick, razors, scissors, 2- Modes of or contaminated broken glass as HBV, HCV and HIV. Another transmission can occur via contaminated blood transmissions transmission or contaminated organ transmission.  6- Vertical transmission: occurs from mother to her fetus via placenta in pregnancy (TORCH: Toxoplasmosis, Rubella, HCV, HIV), or through birth canal during delivery: HIV, Herpes simplex, and eye infections. 2- Modes of Transmission can occur during breastfeeding as in HIV. transmissions  There are diseases with single mode Incubation Period: It is the period of transmission as droplet in between invasion of the body by Influenza and ingestion as in cholera, pathogenic organisms and or have more than one mode of appearance of symptoms of a transmission as poliomyelitis: disease. It may be very short (few Airborne and ingestion, chicken pox hours) as in staphylococcal food airborne and contact, TB: airborne, poisoning May be years in HIV. ingestion and contact.  There are natural barriers of infection including intact skin and mucous membranes, and inner body defenses are from phagocytes that destroy invading organism. 3- Host Host immunity is its ability to react with organism and/or get red of it. Human immunity are two main types: Naturally acquired immunity: passive via placenta and reaction breast milk to fetus and newborn or active through infection with organism (either with full clinical picture or subclinical infection) and formulate antibodies against organism or cell mediated immunity. Artificially acquired immunity: Passive via transmitting ready formed immunoglobulin (sero-prohylaxis) or active through vaccination; in which weak or killed or part of pathogen is introduced into human body to formulate antibodies against organism or cell mediated immunity.  Vaccines are the antigenic (immunogenic) preparations Vaccine efficacy is measured used for active immunization. by its ability to STOP disease They stimulate immune transmission and prevent system of the body to induce disease. immunity. Vaccination:  Health care providers: should be immunized against: Meningococcal meningitis Influenza (Hib influenza bacteria vaccine and Annual influenza vaccine) Measles vaccine if not immunized before or has no immune titer. Tuberculosis (BCG for nonreactors) HBV Diphtheria and tetanus vaccine (Td) Others, according to the circumstances.  Post-exposure immunization: in which we abrupt infection through stimulating immune system by giving vaccine but after exposure not before exposure. Vaccine is protective in shorter period than disease incubation period as in Rabies and Measles. Or vaccine work as a booster to stimulate immune system as in diphtheria and tetanus, for those actively immunized before, a booster dose of toxoid given after exposure to infection induces rapid protective antitoxin response  Seroprophylaxis: in which we abrupt infection through stimulating immune system by giving ready formed antibody after exposure. It is valuable mean of specific prevention of some- infectious diseases, through giving the at-risk preparations of ready-formed (preformed) antibodies. Seroprophylaxis is needed for rapid, but temporary specific protection of susceptible. It is either human preparation (less allergic) or animal preparation. Example of seroprphylaxis in HAV infections in elderly to avoid fulminant hepatitis and severe form of disease.  Endemic spread: A disease constantly present in the community, due to maintenance of infection by existing ecological factors (host, agent and environment). e.g HCV in Egypt. Spread of infection shows: Sporadic cases that appears here and there, unrelated. Carriers of infection in some diseases. Spread of Epidemic spread: Increased number of cases significantly more than the usual pattern of Infectious spread of the disease. Epidemic is characterized Diseases by the fact that all the cases are interrelated i.e. have certain features in common regarding; Time i.e. All cases appear within a certain period of time Place: All cases are reported within the same community or area Epidemic spread could be to small group of people (outbreak) or across single governorate or country (epidemic) or in between countries (pandemic). Prevention Disease prevention is stopping infection from occurrence. This could happen in 3 ways: and control to General prevention, communicable specific prevention, diseases: and international measures.  General prevention is through Prevention health promotion; and control to safe sufficient nutrition and physical exercise, communicable sanitary environment with clean water supply, good housing conditions and health education to people. diseases:  Specific preventions: via immunization either with vaccine Prevention or seroprohylaxis and control to or giving drugs that avoid infection development in the body as chemoprophylaxis e.g. Penicillin (long acting) to prevent communicable rheumatic fever. Tetracycline: oral capsules or tablets, to prevent Cholera and Plague. INH: oral isonicotinic acid hydrazide, for diseases: prevention of Tuberculosis. Rifampin: oral, to prevent meningococcal meningitis.  Disease control: it is preventing infection from destroying body more or causing complications and deaths. It can be done Prevention through: Case-finding: detection and diagnosis of cases. and control to Management of cases, and preventing complications and communicable sequelae of disease. Measures for contacts, and protecting susceptible and at- diseases: risk groups who may be exposed to infection.