MICR 454L Emerging and Re-Emerging Infectious 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 18 th 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 Mandatory hand washing introduced

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) 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 Plague Malaria 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, Brock Biology of Microorganisms, by Madigan and Martinko, Pearson Prentice Hall, Upper Saddle River, NJ, 11 th ed, nce%20Nightingale%20and%20the%20Crimean%20War.jpg nce%20Nightingale%20and%20the%20Crimean%20War.jpg.