micr 454l emerging and re-emerging infectious diseases lecture 2: epidemiology (tortora et al.,...
TRANSCRIPT
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>.