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.

1. EPIDEMIOLOGY OF COMMUNICABLE DISEASES Dr. Dalia El- Shafei Assistant professor, Community Medicine Department, Zagazig University

2. Learning Objectives : 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. Epidemiologic triad Disease is the result of forces within a dynamic system called an “ epidemiologic triad ” which consisting of:

4. Aim of studying epidemiology of communicable diseases 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. INFECTIOUS CYCLE or CHAIN

6. AGENT

7. Factors affecting Agent infectivity

8. Source & 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 )”.

9. Human Reservoir 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.

10. Animals reservoir Diseases that can be transmitted under natural conditions from vertebrate animals to humans are called “Zoonosis” (e.g. rabies, yellow fever, plague, anthrax, brucellosis). F amous animal reservoirs: cat , dog, horse, cattle, poultry and rodents.

11. Reservoir in non-living things Soil can also act as reservoir of infection (e.g. soil may harbor agents that cause tetanus & anthrax ). Water. Food: meat, milk, vegetables.

12. Portal of exit Path by which an agent leaves its human or animal source host

13. Modes of transmission

14. Portal of Entry Route the agent uses to get into the new host. In general, portal of entry is similar to portal of exit

15. Host factors Intrinsic factors that influence an individual's exposure, susceptibility, or response to a causative agent

16. Immunity Factors affecting host immunity

17. Herd immunity (immunity of the community ) Describes the immunity level that is present in a population group. P rovides an immunological barrier to the spread of diseases in the community. M ay 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. Types of immunity

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 1 st days after labor. Naturally Acquired Immunity

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 .

21. Vaccine preparation

22. Route of Vaccination

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. 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. 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. D isadvantages: Cost: expensive compared to expected potential benefit” Side effects of drugs Development of drug resistant strains Temporary protection. Artificial acquired passive immunity

26. Incubation period “IP” The time interval between contact with an agent and the 1 st clinical evidence of the disease . It depends on :

27. The IP varies individually according to: Defense mechanism “Ability to react against agent invasion in the body”.

28. Importance of IP :

29. Clinical picture & Complication Differs from disease to another & from one case to another.

30. S pread of Communicable diseases

31. Epidemics

32. Types of Epidemic

33. Epidemic investigation S et 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.

34. Steps of epidemic investigation

35. 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.

36. 2. Establish the existence of an epidemic: Comparing current level (incidence) with past level of the disease in that locality & population

37. 3. Description of the epidemic as regard TPP: - P lotting the cases by time of onset (Epidemic curve). - P lotting the cases by location (Spot map). - Collect data on the age, sex, etc. of the cases.

38. 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.

39. Reservoir Mode of transmission Exposed Host Epidemiology of Infectious Diseases Case Carrier Zoonosis Man Animal Soil Exit Exit Exit Droplet Food Arthropod Contact Vertical Immunity & Susceptibility

40. PREVENTION OF COMMUNICABLE DISEASES

41. Environmental sanitation 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 :

42. Health education & Health promotion 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 : C an be achieved by fulfilling requirements of health: Physical, mental and social health Prenatal, natal and post natal care

43. Specific prevention S pecific protection of man against causative agents of infectious diseases by: Immunization & Chemoprophylaxis

44. International preventive measures

45. Secondary prevention

46. Control measures to be taken for existing infectious disease with the following objectives :

47. Control of human reservoir 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

48. 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

49. 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

50. 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

51. 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.

52. 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.

53. Tertiary prevention

54. Tertiary Prevention "Disability Limitation & Rehabilitation"