MICR 420

Emerging and Re-EmergingInfectious Diseases

Lecture 2: EpidemiologyDr. Nancy McQueen & Dr. Edith Porter

Overview

Concepts of epidemiology The cycle of microbial disease

Reservoirs of infection Transmission Portals of entry Portals of exit

Nosocomial infections Epidemiology of fear

What is Epidemiology? As told by CDC … It seems that one of their scientists, on first arriving at CDC from a

clinical practice, found himself somewhat unsure of what epidemiology was all about, so he sought an answer down the street at Emory University.

The first person he asked was a medical student, who told him that epidemiology was "the worst taught course in medical school." The second, a clinical faculty member, told him epidemiology was "the science of making the obvious obscure." Finally, knowing that statistics are important to epidemiology, he asked a statistician, who told him that epidemiology is "the science of long division" and provided him with a summary equation. Giving up on finding a real answer, he returned to CDC. On the way, however, he decided to try one more time. He stopped a native Atlantan who told him that epidemiology was "the study of skin diseases.”

What is Epidemiology really? Study—Epidemiology is the basic science of public health. It's a

highly quantitative discipline based on principles of statistics and research methodologies.

Distribution—Epidemiologists study the distribution of frequencies and patterns of health events within groups in a population. To do this, they use descriptive epidemiology, which characterizes health events in terms of time, place, and person.

Determinants—Epidemiologists also attempt to search for causes or factors that are associated with increased risk or probability of disease. This type of epidemiology, where we move from questions of "who," "what," "where," and "when" and start trying to answer "how" and "why," is referred to as analytical epidemiology.

What is Epidemiology really? Health-related states—Epidemiology as it is practiced today is

applied to the whole spectrum of health-related events, which includes chronic disease, environmental problems, behavioral problems, and injuries in addition to infectious disease.

Populations—One of the most important distinguishing characteristics of epidemiology is that it deals with groups of people rather than with individual patients.

Control—Epidemiological data steers public health decision making and aids in developing and evaluating interventions to control and prevent health problems. This is the primary function of applied, or field, epidemiology.

History of Epidemiology

Hippocrates in 460 – 377 B.C. Malaria associated with swampy environments

Edward Jenner in late 18th century Cow pox protects from small pox First vaccination

Ignaz Semmelweis Childbed fever (puerperal sepsis)

John Snow Cholera outbreaks in London

Florence Nightingale Epidemic typhus

in mid 19th century

Ignaz Semmelweis and Puerperal Sepsis

Mandatoryhand washingintroduced

John Snow and Cholera Cholera epidemics in London 1846

– 1849 Snow analyzed the death records

and interviewed survivors Created map Most individuals who died of

cholera used water from Broad street pump

Survivors did not drink water but beer instead or used another pump

Identified the Broad street water pump as likely source

After closing this pump number of cholera cases dropped significantly

Florence Nightingale and Epidemic Typhus

Recorded statistics on epidemic typhus in English civilian and military populations

Published a 1000 page report in 1858 Statistically linked disease

and death with poor food and unsanitary conditions

Novel graph: coxcomb chart or polar area diagram chart Fixed angle and variable radii

Resulted in reforms in the British Army

Nightingale became the first female member of the Statistical Society

Investigation Types in Epidemiology Experimental

Epidemiologist is in control of the circumstances at the beginning of the study Begins with a hypothesis Prospective study that usually involves controls Example: Semmelweis’ study; vaccine efficacy trials

Observational Epidemiologist is not in control of the circumstances at the beginning of the study Descriptive

Collect data about affected individuals, the places and the periods in which disease occurred (Who? Where? When?)

Typically retrospective E.g. Snow’s study

Analytical Analyzes a particular disease to determine its probable cause (How? Why?) Case control method – look for factors that might have preceded the disease Cohort method – study of two populations, one having had contact with the disease

agent and the other that has not E.g., Nightingale’s study

Case Reporting

Health care workers are required to report specified diseases to local, state, and national offices

Centers for Disease Control and Prevention (CDC) Branch of the U.S. Public Health Service Collects and analyzes epidemiological information in the U.S. Publishes Morbidity and Mortality Weekly Report (MMWR)

www.cdc.gov

Morbidity: incidence of a specific notifiable disease

Mortality: deaths from notifiable diseases

Morbidity rate: number of people affected/total population in a given time period

Mortality rate: number of deaths from a disease/total population in a given time

Notifiable Diseases

Reported to the CDC To detect reemergence

of disease and be able to respond early

List includes AIDS Anthrax Botulism Cholera Cryptosporidiosis Diptheria Ehrlichiosis

Encephalitis (arboviral) Enterohemorrhagic E. coli Hanta virus Hepatitis, viral Malaria Meningococcal disease Pertussis Streptococcal diseases,

invasive Group A Tuberculosis VISA, VRSA

Frequency and Distribution of Disease Sporadic disease

Disease that occurs occasionally in a population at irregular intervals

Endemic disease Disease continually

present in a population at low frequency and a steady level

Epidemic disease High frequency over short

time Disease acquired by many

hosts in a given area in a short time

Disease with sudden increase of morbidity (illness rate) and mortality (death rate) above the normal level

Common source versus propagated

Pandemic disease Worldwide epidemic

Classification of diseases by frequency and distribution

Difference between Endemic and Epidemic Disease

Microbiology: An Evolving Science © 2009 W. W. Norton & Company, Inc.

Source and Spread of Major Epidemics

Common-source epidemic Disease spread from one

source Little transmission from

infected (e.g., food poisoning)

Propagated epidemic Disease continuously

transmitted from infected Higher numbers, longer-

lasting epidemic (e.g. flu)

Propagated or

Herd Immunity Large fraction of a population is immune to a

given disease, and it is difficult for the disease to spread (proportion immune individuals needed depends upon type of infection)

Cycle of Microbial Disease

Reservoir

Transmission

Portal of entry

Portal of exit

Disease

Reservoirs of Infection Where do you get the infection from? Site in nature where microbes survive (and possibly multiply) Continual sources of infection Humans

Small pox, gonorrhea Active carriers: acutely ill Healthy carriers: no symptoms

inapparent infections or latent diseases Example: Typhoid Mary

Animals Zoonoses Rabies, Lyme disease

Fomite Soil, water Botulism, tetanus

Living reservoirs

Non-living reservoirs

Transmission How do you get the infection? Direct

Requires close association between infected and susceptible host

Person – to – person Horizontal

Fecal-oral Droplet

Vertical Mother – to – child

Animal bites Indirect

Vehicle (fomites, water) Airborne (aerosols) Vector

Epidemiological Vectors Arthropods

Invertebrates with jointed appendages Ticks, flies, mosquitoes, fleas

Mechanical vector Biological vector

Microbe proliferates in arthropod Necessary component in the life cycle of the microbe Sometimes transovarial transmission

Tick

Borrelia burgdorferi

Lyme Disease:Skin manifestation

PlagueMalaria

Trypanosomiasis

Portals of Entry and Exit

Respiratory tract Intestinal tract Urinary tract Genital tract Skin Blood

Mucous membranes

Nosocomial Infections Acquired by compromised hosts during a hospital

stay 5-15% of all hospital patients 50% of all major hospital complications At 2 – 4 million admissions/year estimated 20,000 –

40,000 deaths per year in the US

Body Site Distribution of Nosocomial Infections

Factors Contributing to Nosocomial Infections

Patient population is immune comprised Misuse of antibiotics with development of

antibiotic resistant strains Medical devices as new portals of entry Hospital staff as reservoir

Control Measures against Nosocomial Infections

Appropriate use of antibiotics Safe and effective decontamination and

sterilization Staff hygiene

Hand washing Screening of all hospital patients?

Epidemiology of Fear

Fear with epidemic proportions Example: West Nile Virus (CDC national data)

2006: 5000 infections with 180 deaths 2007: 3600 infections with 121 deaths 2008: 1356 infections with 44 deaths 2009: 663 infections with 30 deaths

Not a public threat!

Take Home Message

Epidemiological classification of disease is based on frequency and distribution as sporadic, endemic, epidemic, and pandemic.

The cycle of microbial disease includes reservoir, transmission, portal of entry and portal of exit.

Transmission may be direct from person to person or indirect through an intermediate agent such as a fomite or a vector.

Nosocomial infections are hospital acquired infections often caused by multidrugresistant microorganisms.

Additional Resources

The Microbial Challenge, by Krasner, ASM Press, Washington DC, 2002.

Brock Biology of Microorganisms, by Madigan and Martinko, Pearson Prentice Hall, Upper Saddle River, NJ, 11th ed, 2006.

Tortora et al., Chapter 14 http://www.cdc.gov/excite/classroom/intro_epi.htm http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/J/JohnSnow.gif http://www.gazellebookservices.co.uk/Military/originals/Military/19th%20Century/

Florence%20Nightingale%20and%20the%20Crimean%20War.jpg <http://www.britannica.com/eb/art-70822>. http://www.cdc.gov/ncphi/disss/nndss/phs/files/

NNDSS_event_code_list_January_2010.pdf