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Epidemiology of communicable diseases

Disease is the result of forces within a dynamic system called an “epidemiological triad” which consisting of: agent, host and environment.

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Epidemiology of communicable diseases

  1. 1. Dr. Dalia El-Shafei Assistant professor, Community Medicine Department, Zagazig University
  2. 2. 1) To recognize the infectious cycle (epidemiological triad) 2) To understand the chain of infection. 3) To understand the natural history of the disease
  3. 3. • Disease is the result of forces within a dynamic system called an “epidemiologic triad” which consisting of:
  4. 4. • To put preventive & control measures, limit their spread in the community and limit their complications in cases Communicable diseases: are diseases transmitted from one case to another. There is a cycle for transmission of infection from one host to another.
  5. 5. Biological agent Parasites Protozoa “Amoebae” Metazoa “Ascaris” Fungi “Tinea” Bacteria“TB” Rickettsia “Typhus” Viruses “AIDs”
  6. 6. Natural characteristic of the agent • Production, Metabolism, Motility, Production of toxin. Characteristic of agent related to infection in human • Infectivity: Ability of an organism to invade & infect a host. • Pathogenicity: Ability to cause disease. • Virulence: Ability to cause serious complications /death. • Antigenicity: Ability to stimulate host production of antibody
  7. 7. Source of infection Immediate person or object from which agent passes to host. Reservoir Natural habitat, in which an agent lives, grows & multiplies Human reservoir Animal reservoir Non-living things reservoir Source of infection may be a part of the reservoir or not “food poisoning (reservoir is the carrier who contaminate the food, while source of infection is the food itself)”.
  8. 8. Cases: individuals who are suffering from the disease. Typical: showing typical manifestation of the disease & their level of severity may range from being mild to moderate to severe Atypical: does not show the typical manifestation of diseases or Subclinical / in apparent: can pass undiagnosed. Carriers: a person who is harboring the organism (allows its multiplication inside his body) without showing signs & symptoms. And is capable of transmitting the disease to other person. Carriers “the most dangerous source of infection” Subclinical or in apparent infection Atypical cases Typical cases.
  9. 9. Typesofcarriers Timing of transmitting agent Incubatory Convalescent Contact Duration Temporary Chronic Portal of exit Respiratory Fecal Urinary
  10. 10. • Diseases that can be transmitted under natural conditions from vertebrate animals to humans are called “Zoonosis” (e.g. rabies, yellow fever, plague, anthrax, brucellosis). • Famous animal reservoirs: cat, dog, horse, cattle, poultry and rodents.
  11. 11. • Soil can also act as reservoir of infection (e.g. soil may harbor agents that cause tetanus & anthrax). • Water. • Food: meat, milk, vegetables.
  12. 12. Path by which an agent leaves its human or animal source host Respiratory tract e.g. influenza virus Genitourinary tract e.g. sexually transmittal diseases Alimentary tract e.g. hepatitis A virus (HAV) Blood e.g. hepatitis B virus (HBV) In-utro transmission e.g. rubella, CMV
  13. 13. • Directly: droplets “kissing, sneezing, cough, spitting” (short distance transmission). • Indirectly: droplet nuclei or contaminated articles. Droplet or air borne • Directly: water, hands, food • Indirectly: flies, cockroach, dust Food borne • Intrauterine infection as German measles, malaria, syphilis, AIDS. Mother to infant during pregnancy • Direct contact with case or infected animal. • Indirect: clothes, fomites, dust, water pool. Infection by contaminated syringes occurs through skin penetration. Contact method “Skin & mucous membrane” • Biological transmission: development of new stages of organism occur inside vector “malaria”. • Mechanical: vector just carry organism from case to new host “typhoid”. Arthropod borne
  14. 14. • Route the agent uses to get into the new host. • In general, portal of entry is similar to portal of exit Respiratory tract Ingestion Dermal Blood borne Mucous membranes
  15. 15. Intrinsic factors that influence an individual's exposure, susceptibility, or response to a causative agent Host Immunity Virulence of Organisms
  16. 16. Age Sex Pregnancy Nutrition Trauma, stress, fatigue Occupation Environment Education & culture Socio- economic standard Vaccination Factors affecting host immunity
  17. 17. Herd immunity (immunity of the community) Describes the immunity level that is present in a population group. Provides an immunological barrier to the spread of diseases in the community. May be acquired after frequent mass vaccinations The higher the herd immunity the higher the power to defense of an epidemic occurrence in the community.
  18. 18. Types of immunity Non- specific • Mechanisms that prevent entrance of agents to the body • Intact skin • Cough, sneezing reflexes • Gastric juices • Normal bacterial flora • Secretions: saliva, tears. Specific • Cell-mediated immunity: Delayed hypersensitivity & regulation of anti-body production • Humeral immunity: Circulating antibodies inactivate specific antigens or organisms & prevent replication in body. Antibodies are disease-specific. • Naturally acquired “Active or Passive” • Artificially acquired “Active or Passive”
  19. 19. Active Passive (maternal immunity) After infection, the body forms antibodies against that infection. - Solid immunity: measles - Short time immunity: influenza. Antibodies from mothers during pregnancy are transmitted to infant. It depends on diseases the mother had exposed to before pregnancy to form antibodies. No cellular immunity is transmitted because of placental barriers. Antibodies also are present in colostrum breast milk secreted on the 1st days after labor. Naturally Acquired Immunity
  20. 20. Artificial acquired active immunity • Immunity is induced by immunization by vaccine which stimulates the body to form immunoglobulin. • Duration of immunity depends on the type of vaccine. Live vaccine (small pox) & live attenuated vaccine (MMR) • Give lifelong immunity. Killed vaccine • Contains killed agents but still retain their antigenicity as typhoid, • Give short time or low level of immunity.
  21. 21. Whole organism “polio & measles” Capsule of organism “meningitis” Surface antigen of virus “HBV” Toxin secreted by organism “diphtheria & tetanus” Vaccine preparation
  22. 22. Intradermal “BCG” Oral “Polio” Subcutaneous or Intramuscular “MMR” Intranasal “Influenza” Route of Vaccination
  23. 23. • Spoiled vaccine: change in its optimal temperature (most vaccines need cold chain in transportation & storage) • Vaccination during the first 6 months of life: maternal immunity is still present & interferes with action of vaccine. Causes of failure of Active Immunization
  24. 24. Seroprophylaxis: Ready-made “Ig” given for rapid protection after exposure to infection for prevention or attenuation of disease severity. The body has no role in immunity. It usually gives short duration of immunity. Ig or Antitoxins: • Animal origin as tetanus, diphtheria antitoxins • Human origin: from plasma of actively immunized persons as rabies serum. Artificial acquired passive immunity
  25. 25. Chemoprophylaxis: Administration of antimicrobial drug before exposure or just after exposure to prevent occurrence of disease (not for treatment). May be the basic preventive measure: Penicillin for rheumatic fever, tetracyclines for cholera, INH for TB, rifampicin for meningitis. Disadvantages: • Cost: expensive compared to expected potential benefit” • Side effects of drugs • Development of drug resistant strains • Temporary protection. Artificial acquired passive immunity
  26. 26. • The time interval between contact with an agent and the 1st clinical evidence of the disease. • It depends on: Portal of entry (defense mechanism). Ability of multiplication (infectivity). Number of agents. Level of antibody in the host.
  27. 27. The IP varies individually according to: Defense mechanism “Ability to react against agent invasion in the body”. External defense mechanism • Cough & Sneeze reflex • Vomiting & Gastric acid • Tears • Skin Internal defense mechanism • Cellular & humeral immunity. • If external defense mechanism can’t eliminate agent; internal defense mechanism continue process by: • Inflammation • Isolation by fibrocytes • Macrophage phagocytosis • Antibody reaction
  28. 28. Importance of IP: • Tracing the source of infection & contact • Period of surveillance • Immunization • Identification of point source or propagated epidemics • Prognosis e.g. in rabies & tetanus, the shorter the IP is, the worse the prognosis of disease.
  29. 29. Differs from disease to another & from one case to another.
  30. 30. Sporadic cases Outbreak Epidemic Endemic Pandemic Epizootic
  31. 31. Sporadic cases • Infected cases have no common source of infection. • Spread all over far away areas not related to each other. Outbreak • Spread of infection in confined place as school or camp. • Usually there is common source for infection, if controlled the incidence of infection decreases. Epidemic • Widespread of infection, unexpected increase in No. of already present infection in certain area in short period. • Or it is introduction of new infection to the country never shown that disease before as SARS, Avian flu
  32. 32. Endemic • Constant presence of disease within a given geographic area or population group. Pandemic • Spread of infection all over the world or in many countries e.g. avian flu, Aids. Epizootic • Spread of Zoonotic disease in large scale among animals e.g. cow mad disease.
  33. 33. Common source epidemic • All cases diagnosed are infected from a common source at certain point of time. • The resultant cases all develop with one IP of disease e.g. epidemic of food poisoning. Propagated source epidemic • Cases are exposed to more than one source of infection. • The epidemic usually shows a gradual rise & falls over a longer period of time e.g. epidemic of HAV.
  34. 34. Set of procedures used to identify the cause responsible for the disease, the people affected, the circumstances & mode of spread of the disease, and other relevant factors involved in propagating the epidemic Control disease spread. Especially important if the epidemic: It is a challenging task for health workers.
  35. 35. • Verify the diagnosis or causes. • Establish the existence of an epidemic. • Description of epidemic as regard time, place & person “TPP”. • Develop hypotheses to explain the occurrence of the epidemic. • Test the hypothesis. • Identification of susceptible population. • Management of the epidemic. • Formulation of report, communicate findings & recommendation.
  36. 36. 1. Verify the diagnosis or causes: -Careful analysis of the initial reports. - Confirm diagnosis by performing clinical & lab studies. -Putting the criteria for case definition.
  37. 37. 2. Establish the existence of an epidemic: Comparing current level (incidence) with past level of the disease in that locality & population
  38. 38. 3. Description of the epidemic as regard TPP: - Plotting the cases by time of onset (Epidemic curve). - Plotting the cases by location (Spot map). - Collect data on the age, sex, etc. of the cases.
  39. 39. 4. Develop hypotheses to explain occurrence of the epidemic. 5. Test the hypothesis 6. Identification of susceptible population. 7. Management of the epidemic. 8. Formulation of the report & communicate findings & recommendation to higher levels in the health system, community leaders & other local stakeholders.
  40. 40. Reservoir Mode of transmission Exposed Host Case Carrier Zoonosis Man Animal Soil Exit Exit Exit Droplet Food Arthropod  Contact Vertical Immunity & Susceptibility
  41. 41. 1ry“Healthpromotion& specificprotection” General Environmental sanitation. Health education. Health promotion. Specific Immunization. Chemoprophylaxis. International preventive measures Regulations Quarantine measures. Imported goods Means of transportation
  42. 42. The environment would be free of: Vehicles of infection: polluted air, water, milk, food, soil Vector of disease: infection transmitting arthropods Rodents(including rats): potential reservoir of many infections Infected animal reservoir (Stray dogs and cats) Components of sanitary environment: Proper town, village or district planning & design Good housing with suitable ventilation Sanitary collection & disposal of waste Food & milk sanitation Eradication or control of insects, rodents, stray dogs & cats
  43. 43. Health education of the public: • Health awareness • Proper KAP related to health, with special consideration of life style, habits and behavior Health promotion of the public: Can be achieved by fulfilling requirements of health: Physical, mental and social health Prenatal, natal and post natal care
  44. 44. Specific protection of man against causative agents of infectious diseases by: Immunization & Chemoprophylaxis
  45. 45. International regulations: to prevent transmission of “Quarantinable diseases” in between countries. “Cholera, yellow fever & plague” Quarantine measures: certain animals coming from endemic or infected areas “Monkey for yellow fever, Cattle for rift valley fever” Imported goods: For raw wool, shaving brushes. Authorized disinfection certificate is needed Means of transportation: derating certificate for ships in plague & disinfection of planes coming from yellow fever endemic areas
  46. 46. 2ry“Earlydiagnosis& prompttreatment” Control of human reservoir Control of cases: Case finding. Notification Isolation Disinfection Concurrent Terminal Treatment Release Control of carriers Control of contacts Enlistment Examination Surveillance / Segregation / Isolation Immunization / chemoprophylaxis Control of animal reservoir Community control measures Drastic measures Surveillance Eradication Elimination
  47. 47. Control measures to be taken for existing infectious disease with the following objectives: Case finding: detection & diagnosis of cases. Management of cases & preventing complications. Measures for contacts & protecting susceptible. Preventing or minimizing spread of disease.
  48. 48. A- Control of cases: 1- Case finding. 2- Notification: Cases of definite or suspected diagnosis must be notified to local health office. Value of notification: To take preventive & control measures To help tracing sources and channels of infection in outbreaks To collect significant statistical data
  49. 49. 3- Isolation: at home, hospital or special place Value of isolation: To stop activity and movement of case in the community To protect the case from risk of secondary infection 4- Disinfection: process of destroying pathogenic organisms outside the body  Concurrent Terminal 5- Treatment: kill the infectious agent when it still in the reservoir, i.e. before it disseminated. To reduce the communicability of disease, cut short the duration of illness & prevent the development of 2ry cases. 6- Release: Patient discharge or the formal ending of inpatient care
  50. 50. B- Control of carriers: Carriers may be difficult to control. It is important to do pre-employment & periodic medical examination of certain occupational groups e.g food handlers, medical personnel and personnel serving children
  51. 51. C- Control measures for contacts:  Enlistment: a special list of contacts  Examination: for case finding  Surveillance, segregation or isolation: Surveillance: contacts are put under supervision for IP of disease meanwhile, they can perform their activities Segregation: contacts are excluded from school or work but not isolated e.g measles, enterica & diphtheria Isolation: contacts of following diseases are isolated: - Cholera in non-endemic areas - Pneumonic plague & anthrax  Immunization or chemoprophylaxis
  52. 52. Control of animal reservoir:  Eradication of animal reservoir if applicable.  Control of farm & pet animals to prevent or minimize animal-animal or animal man transmission of infection through sanitary raising, feeding and veterinary care.
  53. 53. Community control measures: Epidemiological study & investigation to trace sources & channels of infection. Drastic control measures to be taken if necessary e.g closing schools Surveillance Eradication of infectious disease: is getting rid of causative organism & consequently of disease: no reported cases & no reservoir of infection. Elimination of disease: means that existing endemic infectious disease is so controlled to reach the level of no reported cases while causative agent is not necessarily eliminated.
  54. 54. Reduce or eliminate long-term impairments & disabilities. Minimize suffering. Optimize function. Prevent further deterioration. Help complicated cases to cope with their handicap.

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