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The Anatomy of an Epidemic: A Rational Approach to Understanding, Preventing and Combating Infectious Diseases Stephen Weber, MD, MS Assistant Professor.

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Presentation on theme: "The Anatomy of an Epidemic: A Rational Approach to Understanding, Preventing and Combating Infectious Diseases Stephen Weber, MD, MS Assistant Professor."— Presentation transcript:

1 The Anatomy of an Epidemic: A Rational Approach to Understanding, Preventing and Combating Infectious Diseases Stephen Weber, MD, MS Assistant Professor Section of Infectious Diseases Hospital Epidemiologist Director, Infection Control Program University of Chicago Hospitals

2 Overview 1. Introduction 2. Modeling and the Anatomy of Epidemics 3. Preventing and Controlling Epidemics 4. Epidemics and Luck

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4 Smallpox Smallpox SARS SARS Anthrax Anthrax Monkeypox Monkeypox Mumps Mumps Antibiotic-resistant Acinetobacter Antibiotic-resistant Acinetobacter Community- associated MRSA Community- associated MRSA Supertoxigenic Clostridium difficile Supertoxigenic Clostridium difficile Avian influenza Avian influenza Bordatella pertussis Bordatella pertussis Measles Measles West Nile Virus West Nile Virus Highly-resistant Pseudomonas aeruginosa Highly-resistant Pseudomonas aeruginosa

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6 Defining an epidemic 1. An outbreak of a contagious disease that spreads rapidly and widely. 2. An increased frequency of infection above the normal or usual level

7 Epidemic Surveillance World Health Organization (WHO) Centers for Disease Control and Prevention Illinois Department of Public Health Chicago Department of Public Health UCH Infection Control Program Individual Clinicians

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9 Modeling and the Anatomy of Epidemics

10 Modeling Measles Keeling, et al. Proc R Soc Lond. 2002

11 Modeling Malaria McKenzie and Samba, et al. Am J Trop Med Hyg dX/dt = A B Y (N - X) - r X dY/dt = A C X (M - Y) - m Y dX/dt = A B Y (N - X) - r X dY/dt = A C X (M - Y) - m Y

12 R 0 = 1 Progression of an Epidemic R 0 = 2 R 0 = 3 Basic reproductive number (R 0 ) Basic reproductive number (R 0 ) Expected number of secondary cases on the introduction of one infected individual in a susceptible population Expected number of secondary cases on the introduction of one infected individual in a susceptible population R 0 > 1 Epidemic disease R 0 = 1 Endemic disease R 0 < 1 Disease dies out

13 Generation # R …

14 Basic Reproductive Numbers SARS in general population: 0.49 SARS in general population: 0.49 SARS (hospital transmission): 2.6 SARS (hospital transmission): 2.6 Smallpox in a vulnerable population: Smallpox in a vulnerable population: Measles (pre-vaccine): Measles (pre-vaccine): Measles in Belgian schools (1996): Measles in Belgian schools (1996): pandemic influenza: pandemic influenza: Influenza on a commercial airliner: 10.4 Influenza on a commercial airliner: 10.4 Liao, et al. Risk Anal. 2005; Chowell, et al. Emerg Inf Dis. 2004; Mossong, et al. Epidemiol Infect. 2005; Meltzer, et al. Emerg Inf Dis

15 R 0 = p x k x d p = transmissibility k = contacts d = duration of contagiousness

16 Transmissibility (p) 1. Quantity of pathogen released 2. Mechanism of dissemination 3. Inherent infectiousness of the pathogen R 0 = p x k x d

17 Quantity of pathogen released Varies with state of disease Varies with state of disease Early chickenpox Early chickenpox Herpes simplex Herpes simplex Cattarhal phase of viral infections Cattarhal phase of viral infections Varies with activity Varies with activity Coughing vs. sneezing vs. talking Coughing vs. sneezing vs. talking R 0 = p x k x d

18 Mechanism of dissemination Respiratory Respiratory Influenza, tuberculosis Influenza, tuberculosis Contact Contact Seasonal viruses Seasonal viruses Antibiotic-resistant bacteria Antibiotic-resistant bacteria Fecal-oral Fecal-oral Salmonella, shigella, hepatitis A Salmonella, shigella, hepatitis A Blood and body fluid Blood and body fluid HIV, Hepatitis B and C HIV, Hepatitis B and C R 0 = p x k x d

19 Respiratory dissemination DropletDroplet nuclei Pathogen BacteriaTB Size ≥ 5µ < 5µ Distance < 3 feet? Persistence 1 hr. Destination Upper airwaysAlveoli R 0 = p x k x d

20 Inherent infectiousness R 0 = p x k x d E. coli infecting bladder epithelium Biological differences between organisms Biological differences between organisms Adhesions, proteinases Adhesions, proteinases Variation in host response Variation in host response Expressed as the minimal infectious dose Expressed as the minimal infectious dose

21 Contacts Number of contacts Number of contacts May be facilitated by environmental factors May be facilitated by environmental factors Intensity of contacts Intensity of contacts R 0 = p x k x d

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23 Duration of Contagiousness (d) Assuming a constant frequency of contacts and an unchanging degree of transmissibility, the longer the period of time that a patient is contagious the more likely he/she is to transmit the pathogen. Assuming a constant frequency of contacts and an unchanging degree of transmissibility, the longer the period of time that a patient is contagious the more likely he/she is to transmit the pathogen. For some infections, the period of contagiousness may not always be associated with symptoms of illness. For some infections, the period of contagiousness may not always be associated with symptoms of illness. R 0 = p x k x d

24 Duration of Contagiousness (d) The Ebola paradox The Ebola paradox Rapid mortality reduces period of contagiousness Rapid mortality reduces period of contagiousness R 0 = p x k x d

25 Preventing and Controlling Epidemics

26 Childbed fever: Vienna, 1847 Robert A. Thom (1966)

27 Cholera: London 1854

28 R 0 = p x k x d Interventions to prevent the spread of epidemics target transmissibility (p), contacts (k) or duration of contagiousness (d). Modeling and Infection Control

29 Limiting transmissibility (p) Reduce the quantity of pathogen released Reduce the quantity of pathogen released Symptom control Symptom control Anti-tussives Anti-tussives Barrier precautions Barrier precautions Masks for patients Masks for patients

30 Limiting transmissibility Act on the mechanism of dissemination Act on the mechanism of dissemination Environmental controls Environmental controls Reduce inherent infectiousness Reduce inherent infectiousness Difficult to reduce, but possible to increase Difficult to reduce, but possible to increase Overall, 63% of VRE (+) patient rooms are contaminated Sheets: 40% Bedside Tables: 20% Bed rails: 26% Blood pressure cuffs: 14%

31 Preventing Contact

32 Quarantine and Isolation “une quarantaine de jours (a period of forty days)” SMTWRFS Exposed Symptoms Begin Contagious Quarantine Isolation

33 Social Controls Restriction on public events and gatherings Restriction on public events and gatherings Travel limitations Travel limitations Building quarantines Building quarantines Import/Export controls Import/Export controls

34 Reducing duration of contagiousness Antimicrobial therapy Antimicrobial therapy Influenza control Influenza control Anti-HIV therapy Anti-HIV therapy Enhanced case recognition Enhanced case recognition Syndromic surveillance Syndromic surveillance Limit contacts Limit contacts

35 Ebola revisited 0 Period of infectivity Death 12 Days of illness Ebola: Natural History 3

36 Ebola revisited 0 Death 12 Days of illness 33 4 Traditional funeral practices Period of infectivity Ebola: Current Practice 3

37 Ebola revisited 0 Death 12 Days of illness Period of infectivity 33 4 ICU Support Ebola: USA 3

38 Epidemics and Luck

39 Epidemic Misfortune Epidemics do not conform to the predictions of deterministic models. Stochastic phenomena prevail. Epidemics do not conform to the predictions of deterministic models. Stochastic phenomena prevail. Monkeypox: Co-transport of Ghanan giant rat with prairie dogs Monkeypox: Co-transport of Ghanan giant rat with prairie dogs West Nile Virus: Survival of carrier mosquito through transatlantic flight West Nile Virus: Survival of carrier mosquito through transatlantic flight SARS: Co-mixing of viruses between humans, fowl and civets SARS: Co-mixing of viruses between humans, fowl and civets HIV: Single African ancestral event HIV: Single African ancestral event

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42 Improving the Odds Understanding the role of chance in epidemics permits the deployment of manageable strategies to prevent spread. Understanding the role of chance in epidemics permits the deployment of manageable strategies to prevent spread. Improved performance of day to day practices may be more important than an elaborate emergency response system. Improved performance of day to day practices may be more important than an elaborate emergency response system.

43 Conclusions 1. Epidemics are driven by a relatively understandable interplay of pathogens, infected and susceptible hosts. 2. Understanding the mathematical as well as the biological underpinnings of epidemics is critical to prevention and control. 3. Sometimes, it really is better to be lucky than to be good.


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