MICR 454L

Emerging and Re-EmergingInfectious Diseases

Lecture 2: Epidemiology(Tortora et al., Chapter 14 )Dr. 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

Concepts of Epidemiology

Epidemiology is an investigative methodology designed to determine the source and cause of diseases and disorders that produce illness, disability and death in human populations. Why at this time and at this place?

Factors considered include age, sex, race, personal habits, geographic location, seasonal changes, modes of transmission.

Epidemiologists are also concerned with methods for controlling the disease.

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 Descriptive

Collect data about affected individuals, the places and the periods in which disease occurred

Typically retrospective E.g. Snow’s study

Analytical Analyzes a particular disease to determine its probable cause 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

Experimental Begins with a hypothesis Prospective study that usually involves controls Semmelweis’ 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 Hepatitis, viral Malaria Meningococcal disease Pertussis Streptococcal diseases,

invasive Group A Tuberculosis

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 a steady level

Epidemic disease 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

Pandemic disease Worldwide epidemic

Classification of diseases by frequency and distribution

Source and Spread of Major Epidemics

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

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.

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.

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/J/JohnSnow.gif http://www.gazellebookservices.co.uk/Military/originals/Military/19th%20Century/Flore

nce%20Nightingale%20and%20the%20Crimean%20War.jpg <http://www.britannica.com/eb/art-70822>.