Epidemiology and cycle of microbial diseases


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Epidemiology and cycle of microbial diseases

  1. 1. Epidemiology and cycle of Microbial Diseases Dr. Chhaya SawantShri C. B. Patel Research centre, Mumbai, India
  2. 2. Epidemiology means? The science that evaluates patterns of disease occurrence, frequency, determinants, distribution, and control of health and disease in a defined human population.Epidemiology leads to the identification of causal and preventive factors in human disease.
  3. 3. Epidemiology Defined• ―The study of epidemics‖• Greek roots epi = upon (as in ―epidermis‖) demos = the people (as in demography) ology = ―to speak of‖, ―to study‖• Modern definitions of epidemiology refer to – distributions in populations (statistical) – determinants (pathophysiology, biology, chemistry, psychology) – control of health problems (biological, social, economic, political, administrativ e, legal)
  4. 4. Epidemiology• Also referred to as “Medical Ecology”. Why? - Study of a disease in its natural environment. Study of the clinical aspects and ecological aspects of a given disease are important for the public health measures to control the diseases to be effective
  5. 5. Epidemiology• Also referred to as “Medical Ecology”. Why? - Study of a disease in its natural environment. Study of the clinical aspects and ecological aspects of a given disease are important for the public health measures to control the diseases to be effective
  6. 6. The Black Plague• 1330’s, outbreak of plague in China.• Spread to Italy and the rest of Europe.• 25 million people died in just under fiveyears between 1347 and 1352 – 1/3 of thepopulation of Europe.Influenza – 1918 Killed 50 to 100 million people worldwide, in a single year.
  7. 7. The First Indications of Person-to-Person Spread of an Infectious DiseaseEdward Jenner (1749-1823) - developed a vaccine against smallpox using cow pox (160 yearsbefore virus was identified)In 1773 Charles White, an English surgeon and obstetrician, published his ―Treatise on theManagement of Pregnant and Lying-In Women‖ with Puerperal fever.Puerperal fever is an acute febrile condition that can follow childbirth and is caused bystreptococcal infection of the uterus and/or adjacent regions.In 1795 Alexander Gordon, a Scottish obstetrician, published his ―Treatise on theEpidemic Puerperal Fever of Aberdeen‖ which demonstrated for the first time thecontagiousness of the disease.In 1843 Oliver Wendell Holmes, a noted physician and anatomist in the United States, published apaper entitled ―On the Contagiousness of Puerperal Fever‖.Hungarian physician Ignaz Phillip Semmelweis (1847 and 1849 ), the first person to realize that a pathogen could be transmitted from one person to another.The pioneer of antisepsis in obstetrics – Saviors of Mothers. 32 years before causal agent wasdiscoveredJohn Snow (1813-1858) described the association between dirty water and cholera (44 years beforevibrio was identified
  8. 8. Brief History of Epidemiology Classical Nutritional Epidemiology–James Lind (1716-1794) • conducted an experiment which showed that scurvy could be treated and prevented with limes, lemons, and oranges – ascorbic acid was discovered 175 years later–Joseph Goldberger (1874-1927) • identified that pellagra was not infectious but nutritional in origin and could be prevented by increasing the amount of animal products in the diet and substituting oatmeal for corn grits – niacin was discovered 10 years later
  9. 9. Objectives of Epidemiology in EmergenciesTo identify the priority health problems in the affectedcommunityTo determine the extent of disease existing within acommunityTo identify the causes of disease and possible riskfactorsTo determine the priority health interventionsTo determine the extent of damage and capacity oflocal infrastructureTo monitor health trends of the communityTo evaluate the impact of health programs
  10. 10. Role of an Epidemiologist• Traces the spread of a disease in a population - To identify its origin - Mode of Transmission• With the help of Data Obtained from - Clinical studies - Disease reporting surveys - Insurance questionnaires - interviews with patients This will help him define common factors that constitute a disease.
  11. 11. Important Terms• Attack rate : The proportional number of cases developing in the population that was exposed to the infectious agent• Communicable disease : An infectious disease that can be transmitted from one host to another• Non-communicable disease : A disease that is not transmitted from one host to another• Herd immunity : A phenomenon that occurs when a critical concentration of immune hosts prevents the spread of an infectious agent• Incidence : The number of new cases of a disease in a population at risk during a specified period of time• Index case : The first identified case of a disease in an outbreak or epidemic• Outbreak : A cluster of cases occurring during a brief time interval and affecting a specific population; an outbreak may herald the onset of an epidemic• Portal of entry : Surface or orifice through which a disease-causing agent enters the body• Portal of exit : Surface or orifice from which a disease-causing agent exits and disseminates• Prevalence : The total number of cases in a given population at risk at any point in time• Reservoir : The natural habitat of a disease-causing organism
  12. 12. Infectious disease: the unique factorHistorical approach :Infectious diseases can be spread from human to human(or animal to human)Modern Approach :Alcohol and drug abuse, cancer, mental conditions, acts ofviolence and exposure to lead paint.
  13. 13. Cycle of Microbial Disease Person to person Reservoir transmissionPortal of exit Susceptible Host Portal of entry Agent Mode of transmission
  14. 14. Chain of transmission ReservoirHuman Person with symptomatic illness Carriers: Asymptomatic Incubating Convalescent ChronicAnimal: zoonosisEnvironmental: soil, plant, water
  15. 15. Chain of transmission Portal of exitHuman/animal Respiratory tract Genito-Urinary tract Faeces Saliva Skin (exanthema, cuts, needles, blood-sucking arthropods) Conjunctival secretions PlacentaEnvironmental Cooling towers
  16. 16. Chain of transmission Mode of TransmissionDirect Direct contact Secretions, Blood, Faeces/urine Droplet spreadIndirect Food/water Aerosol Animal vectors Fomites Medical devices and treatments
  17. 17. Transmission• Transmission of a disease from its reservoir to the next susceptible host.• Through contact, ingestion of food or water, or via a living agent such as an insect is called horizontal transmission.• Vertical transmission, the transfer of a pathogen from a pregnant woman to the fetus, or from a mother to her infant during childbirth. e.g. Congenital syphilis, Group B streptococci, can infect the newborn as it passes through the birth canal
  18. 18. Chain of transmission Portal of entryHuman/animal Respiratory tract Genito-Urinary tract Faeces Saliva Skin(exanthema, cuts, needles, blood-sucking arthropods) Conjunctival secretions Placenta
  19. 19. Frequency and distribution based Classification• Sporadic level: occasional cases occurring at irregular intervals• Endemic level: persistent occurrence with a low to moderate level• Hyper Endemic level: persistently high level of occurrence• Epidemic : occurrence clearly in excess of the expected level for a given time period• Pandemic: epidemic spread over several countries or continents, affecting a large number of people
  20. 20. A Graph Illustrating Three EpidemicsThe solid blue line indicates the expected number of endemic cases.The connected red dots indicate the actual number of cases.Epidemics (marked by brackets) are sharp increases in the number of cases of adisease above that which is normally expected (solid line).
  21. 21. Disease Progression• Infection• Incubation period• Prodromal stage• Period of illness• Convalescence
  22. 22. Factors that Influence the Epidemiology of Disease• The Dose• The Incubation Period• Population Characteristics - Immunity to the pathogen. - General health. - Age. - Gender - Religious and cultural practices. - Genetic background.
  23. 23. Measuring Frequency• Three important statistical measures of disease frequency – morbidity rate – prevalence rate – mortality rate
  24. 24. Morbidity rate : Measures the number of individuals that become ill due to a specific disease within a susceptible population during a specific time interval. Morbidity rate = Number of new cases during a specific time Total number of individuals in population• E.g. 700 new cases of influenza per 100,000 individuals is 0.7%. Prevalence Rate The total number of individuals infected in a population at any one time no matter when the disease began
  25. 25. • Mortality rate Number of deaths from a disease per number of cases of the disease• Mortality rate = Number of deaths due to given disease size of total population with diseaseIf 500 people in a town of 100,000 become infected with HIV and 100die, the mortality rate is…
  26. 26. Recognition of an Infectious Disease in a Population1. Generation of morbidity data from case reports2. Collection of mortality data from death certificates3. Investigation of actual cases4. Collection of data from reported epidemics5. Field investigation of epidemics6. Review of laboratory results: surveys of a population for antibodies against the agent and specific microbial serotypes, skin tests, cultures, stool analyses, etc.7. Population surveys using valid statistical sampling to determine who has the disease8. Use of animal and vector disease data9. Collection of information on the use of specific biologics— antibiotics, antitoxins, vaccines, and other prophylactic measures10. Use of demographic data on population characteristics such as human movements during a specific time of the year11. Use of remote sensing and geographic information systems
  27. 27. • Recognition of an Epidemic• Two types of epidemics – common source epidemic – propagated epidemic• Common source epidemic• Reaches a peak within a short period of time -1 to 2 weeks)• A moderately rapid decline in the number of infected patients• A single common contaminated source - food (food poisoning) or water (Legionnaires’ disease).• Propagated epidemic (Host to Host)• A relatively slow and prolonged rise and then a gradual decline in the number of individuals infected.• A single infected individual into a susceptible population.• E.g. Increase in mumps or chickenpox cases, Spread of HIV infection.
  28. 28. Seasonal Cycles of diseases• The season of the year in which the epidemic occurs may also be significant.• Respiratory diseases including Influenza, Respiratory Syncytial Virus infections, and the common cold are more easily transmitted in crowded indoor conditions during the winter.• Conversely, vector- and food-borne diseases are more often transmitted in warm weather when people are more likely to be exposed to mosquitoes and ticks, or eating picnic food that has not been stored properly.
  29. 29. Seasonal Cycles of diseases Seasonal Occurrence of Seasonal Occurrence ofRespiratory Infections Caused by Gastrointestinal Diseases Respiratory Syncytial Virus
  30. 30. Herd Immunity.Resistance of a population toinfection and to spread of aninfectious organism because ofthe immunity of a largepercentage of the populationThe kinetics of the spread of aninfectious disease and the effectof increasing the number ofimmune individuals in thepopulation in limiting the disease.On day 1, a single infectedindividual enters the population.The incubation period is 1 day,and recovery occurs in 2 days.The number of susceptibleindividuals is the total populationon day 1.
  31. 31. Diagrammatic representation of the spread of an imaginary propagated epidemic.The lower curve represents the number of cases and the upper curve the number ofsusceptible individuals. Notice the coincidence of the peak of the epidemic wavewith the threshold density of susceptible individuals.
  32. 32. The number of infected and recovered are illustrated in the two graphs.
  33. 33. Herd immunity• Resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population• Level can be altered by introduction of new susceptible individuals into population• Level can be altered by changes in pathogen – antigenic shift – major change in antigenic character of pathogen (recombination in birds, pigs totally new antigen types) – antigenic drift – smaller antigenic changes (point-mutational changes)
  34. 34. Virulence and the Mode of TransmissionEvidence suggests correlation between modeof transmission and degree of virulence– direct contact less virulent– vector-borne highly virulent in human host; relatively benign in vector– greater ability to survive outside host more virulent
  35. 35. The host community:• Infection --- immunity --- coexistence• Host-pathogen• co evolution: the case of the rabbit-myxoma virus – Transmitted by mosquitoes – Rabbit mortality: susceptibility of new born rabbits to a moderately virulent strain of the virus• – Virus virulence: infection of lab rabbits with wild virus1950 - 1980: Pre introduction levels of rabbit infestation Introduction of a second virus (RHDV; host-to-host transmission; instant killing) --- co evolution.
  36. 36. Emerging and Reemerging Infectious Diseases and PathogensBy the 1990s, the idea that infectious diseasesno longer posed a serious threat to humanhealth was obsolete.It is now clear that globally, humans willcontinually be faced with both new infectiousdiseases and the reemergence of olderdiseases once thought to be conquered(e.g., tuberculosis, dengue hemorrhagicfever, yellow fever)
  37. 37. Emerging and Reemerging Infectious Diseases and PathogensInfectious Disease Mortality in the United States Decreased Greatly during Most of the Twentieth Century.The insert is an enlargement of the right-hand portion of the graph and shows that thedeath rate from infectious diseases increased between 1980 and 1994.
  38. 38. Systematic epidemiology The increased importance of emerging and reemerging infectious diseases has stimulated the establishment of a field called systematic epidemiologyWhich focuses on the ecological and social factors that influence the development of these diseases.
  39. 39. Factors characteristic of the modern world favoring the development and spread of these microorganisms and their diseases1. Unprecedented worldwide population growth, population shifts (demographics), and urbanization2. Increased international travel3. Increased worldwide transport (commerce), migration, and relocation of animals and food products4. Changes in food processing, handling, and agricultural practices5. Changes in human behavior, technology, and industry6. Human encroachment on wilderness habitats that are reservoirs for insects and animals that harbor infectious agents7. Microbial evolution (e.g., selection pressure) and the development of resistance to antibiotics and other antimicrobial drugs (e.g., penicillin- resistant Streptococcus pneumoniae, methicillin-resistant Staphylococcus aureus, and vancomycin-resistant enterococci)
  40. 40. Factors characteristic of the modern world favoring the development and spread of these microorganisms and their diseases8. Changes in ecology and climate9. Modern medicine (e.g., immunosuppression)10. Inadequacy of public infrastructure and vaccination programs11. Social unrest and civil wars12. The possibility of bioterrorism13. Virulence-enhancing mechanisms of pathogens (e.g., the mobile genetic elements—bacteriophages, plasmids, transposons)
  41. 41. Some Examples of Emerging and Reemerging Infectious Diseases.Although diseases such as HIV are indicated in only one or two significant locations,they are very widespread and a threat in many regions.
  42. 42. Public Health System The public health system comprises of a network of clinical microbiologists, nurses,physicians, epidemiologists, and infection control personnel who supply epidemiologicalinformation to a network of local, state, national, and international organizations.
  43. 43. The Role of the Public Health System• Control of Epidemics (Three Types)• First Type : Directed toward reducing or eliminating the source or reservoir of infection1. Quarantine and isolation of cases and/or carriers2. Destruction of an animal reservoir of infection3. Treatment of sewage to reduce water contamination4. Therapy that reduces or eliminates infectivity of the individual
  44. 44. Control of Epidemics (Three Types)• The second type : designed to break the connection between the source of the infection and susceptible individuals. (general sanitation measures - Transmission)1. Chlorination of water supplies2. Pasteurization of milk3. Supervision and inspection of food and food handlers4. Destruction of vectors by spraying with insecticides
  45. 45. Control of Epidemics (Three Types)• The Third type : reduces the number of susceptible individuals and raises the general level of herd immunity by immunization.1. Passive immunization to give a temporary immunity following exposure to a pathogen or when a disease threatens to take an epidemic form2. Active immunization to protect the individual from the pathogen and the host population from the epidemic The most important is Surveillance – Observation, recognition and reporting of Diseases as they occur.
  46. 46. Precautions to be taken by individuals to prevent travel-related infectious diseases.1. If one is traveling to an area where malaria is endemic, weekly prophylaxis before entering the area and after leaving the area is recommended.2. Travelers should recall the benefits of abstinence or protective sexual practices, especially the use of condoms. Hepatitis B vaccine should be administered if it is indicated.3. Travelers should avoid uncooked food, non bottled water and beverages, and unpasteurized dairy products. Use bottled water for drinking, making ice cubes, and brushing teeth.4. Wash hands with soap and water frequently, especially before each meal.
  47. 47. Precautions to be taken by individuals to prevent travel-related infectious diseases.5. To prevent respiratory infections, avoid excessive outdoor activities in areas of heavy air pollution during hot or humid parts of the day. Consider tuberculin skin testing before and after travel.6. Minimize skin exposure and use repellents to prevent arthropod-borne illnesses (e.g., malaria, dengue, yellow fever, Japanese encephalitis).7. Avoid skin-perforating procedures (e.g., acupuncture, body piercing, tattooing, venipuncture, sharing of razors).8. Do not pet or feed animals, especially dogs and monkeys.9. Avoid swimming or wading in non-chlorinated freshwater.
  48. 48. Nosocomial Infections• Nosocomial diseases - caused by bacteria, most of which are noninvasive and part of the normal microbiota;• Viruses, protozoa, and fungi are rarely involved.• Source of Hospital Infection• Endogenous sources - Patient’s own microbiota.• Exogenous sources are other than the patient’s own microbiota .• In either case the pathogen colonizing the patient may subsequently cause a nosocomial disease.
  49. 49. The Hospital Epidemiologist• The services provided by the hospital epidemiologist should include1. Research in infection control2. Evaluation of disinfectants, rapid test systems, and other products3. Efforts to encourage appropriate legislation related to infection control, particularly at the state level4. Efforts to contain hospital operating costs, especially those related to fixed expenses such as the DRGs (diagnosis related groups)5. Surveillance and comparison of endemic and epidemic infection frequencies6. Direct participation in a variety of hospital activities relating to infection control and maintenance of employee health7. Education of hospital personnel in communicable disease control and disinfection and sterilization procedures8. Establishment and maintenance of a system for identifying, reporting, investigating, and controlling infections and communicable diseases of patients and hospital personnel9. Maintenance of a log of incidents related to infections and communicable diseases10. Monitoring trends in the antimicrobial drug resistance of infectious agents
  50. 50. The Emerging Threat of Bioterrorism• The Centers for Disease Control and Prevention recently defined bioterrorism as ―The intentional or threatened use of viruses, bacteria, fungi, or toxins from living organisms to produce death or disease in humans, animals, and plants.‖• a few kilograms of anthrax can kill as many people as a Hiroshima-size nuclear bomb The goal of bioterrorism is to produce fear in the population with subsequent disruption of society
  51. 51. History of Bioterrorism• Biological warfare (BW) employed as far back as 6th century BC.• Examples of past BW: 14th Century: Mongols catapulted corpses with bubonic plague over walls into Crimea. 15th Century: Pizarro presented native South Americans with smallpox- contaminated clothing. 1940: Japan’s ―Unit 731‖ dropped plague-infected fleas over Manchuria & China. 1984 : Rajneeshee Cult contaminated restaurant salad bars with Salmonella typhimurium. 1995: Aum Shinrikyo cult attempted unsuccessfully to disperse BW agents in aerosol form; sarin gas attack in Tokyo. 2001: Anthrax-contaminated letters to U.S. media and government offices.
  52. 52. • In 1998, the U.S. government launched the first national effort to create a biological weapons defense. The initiatives include• The first ever procurement of specialized vaccines and medicines for a national civilian protection stockpile;• investment of more time and money in genome sequencing, new vaccine research, and new therapeutic research;• development of improved detection and diagnostic systems;• preparation of clinical microbiologists and the clinical microbiology laboratory as members of the ―first responder‖ team, which is to respond in a timely manner to acts of bioterrorism.
  53. 53. Preventions: Internationaltreaties, strategicpreparedness (vaccines), make thisworld a better place for everyone• Awareness• Laboratory Preparedness• Plan in place• Individual & collective protection• Detection & characterization• Emergency response• Measures to Protect the Public’s Health and Safety• Treatment• Safe practices
  54. 54. Future Challenges to Public Health• Emergence of new infectious diseases and re-emergence of old ones.• Inequities in health care.• Threat of large scale terrorist attacks using nuclear, biological and chemical agents.• Chronic diseases – coronary heart disease, obesity, diabetes, cancer.• Continued political instability.• International coordination of public health efforts.• Ethics.• Ignorance.
  55. 55. Types of Epidemiological Studies Undertaken• Descriptive Studies – The Person, The Place, The Time,• Analytical Studies - Cross-Sectional Studies : A cross-sectional study surveys a range of people to determine the prevalence of any of a number of characteristics including disease, risk factors associated with disease, or previous exposure to a disease-causing agent. - Retrospective Studies : A retrospective study is done following a disease outbreak. This type of study compares the actions and events surrounding clinical cases (individuals who developed the disease) against appropriate controls (those who remained healthy). - Prospective Studies : A prospective study is one that looks ahead to see if the risk factors identified by the retrospective study predict a tendency to develop the disease. Cohort groups, which are study groups that have a known exposure to the risk factor, are selected and then followed over time.• Experimental Studies
  56. 56. Investigation of a GI illness outbreakhttp://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf
  57. 57. Investigation of a GI illness outbreak http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf
  58. 58. Investigation of a GI illness outbreakIf you were to administer a questionnaire to the churchsupper participants, what information would you collect?1. What did you eat?2. How much did you eat?3. How long after you ate did you begin to feel sick?4. How long did it last?5. Did anyone at the supper show illness prior to the supper?6. What did you drink?7. How was the food prepared/ stored? http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf
  59. 59. Investigation of a GI illness outbreak
  60. 60. Investigation of a GI illness outbreak http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf
  61. 61. Investigation of a GI illness outbreakWithout having clinical isolates from the infectedindividuals for examination, how might you tentativelyidentify the causative agent?1. Temperature of storage vs organism?2. Food vs organism?3. Homemade vs commercial?4. Symptoms5. Culture ice cream.It the outbreak has already ended, what is the value ofworking up the case?