Lecture 7: Principles of disease and epidemiologyEdith Porter, M.D.

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Case study (Video) Pathology, infection, and disease Normal microbiota Etiology of infectious diseases

▪ Koch’s postulates and exceptions Classifying infectious diseases Patterns of disease Spread of infections Nosocomial infections Emerging infectious diseases Epidemiology

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Pathology Scientific study of disease

Etiology Study of the cause of a disease

Pathogenesis Development of disease

Infection Colonization of the body by microbes that are not

routinely present at this site Disease

A change from state of health Abnormal state in which the body is not function

normally

Pathos = SufferingLogos = Science

HIV pos. AIDSTuberculin pos.

Tuberculosis

Pathogen Causes disease in a healthy adult Expresses special virulence or pathogenicity

factors Is not part of normal flora

Opportunist Does not cause disease under normal

conditions Causes disease at local or systemic

breaches of host defense Often part of normal flora

Cooperation Different microbes together cause disease

Body is sterile in utero Colonized within hours after birth

Lactobacilli Staphylococci

Outnumber body cells by at least 10– fold Normal flora does not cause disease under

normal conditions Transient microbiota

present for a relatively short period of time (days, weeks, or months)

Resident microbiota Normal microbiota permanently colonizing the host

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Low numbers of Candida albicans can be found everywhere

Nose Stomach Intestine

Trichomonas vaginalis is considered a pathogen Trichomonas hominis is normal flora

in large intestine

Competes with potential pathogens for nutrients

Directly inhibits potential pathogens▪ Lactobacilli: lactic acid, low pH▪ Bacteriocins

Produces some vitamins (K, B)

Normal microbiota

Candidiasisafter antibiotic treatment

Clostridium difficile diarrhea after antibiotic therapy

http://www.health-res.com/EX/07-28-04/37FF1.jpeg

Pseudomembranous enterocolitis caused by C. difficile

Administration of viable bacteria to the benefit of human health Lactobacilli, Streptococci,

Bifidobacteria Withstand HCl, bile

salts Adhere to host

intestinal mucosa Produce useful

enzymes or physiological end products

Restore normal microbiota

Prophylactic application Lactobacilli to

prevent development of antibiotic associated diarrhea

Therapeutic applications Supplementary

therapy in chronic UTI with E. coli

Robert Koch established the “Golden Rule” to positively identify a microorganism as the cause of an infectious disease

1. The same pathogen must be present in every case of disease and not in the healthy one.

2. The pathogen must be isolated from the diseased host and grown in pure culture.

3. The pathogen from the pure culture must cause same disease when it is inoculated into a healthy susceptible host.

4. The same pathogen must be isolated from the inoculated host in pure culture.

Microorganism cannot be grown in the laboratory in/on artificial culture media Utilization of animals or eggs for

propagationOne disease can be caused by

multiple microorganisms E. g. nephritis

One microorganism can cause multiple disease conditions 15

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http://www.path.cam.ac.uk/Abnormal/TB_Tuberculosis/TB_Tuberculosis/SN_Spine/A_TB_TB_SN_02.jpg

Microorganism cannot be grown in the laboratory in/on artificial culture media Utilization of animals or eggs for propagation

One disease can be caused by multiple microorganisms E. g. nephritis

One microorganism can cause multiple disease conditions M. tuberculosis can affect skin, lungs, bones etc.

No host other than humans E.g. HIV

More than one microorganisms cause one infection Polymicrobial infections such as abscess caused by

anaerobic bacteria 17

Symptom A change in body function that is felt by a

patient as a result of disease Sign

A change in a body that can be measured or observed as a result of disease

Syndrome A specific group of signs and symptoms that

accompany a disease

Often used interchangeable

Communicable disease A disease that is spread from

one host to another▪ Example: Tuberculosis

Contagious disease A disease that is easily spread

from one host to another▪ Example: Chicken pox

Noncommunicable disease A disease that is not

transmitted from one host to another▪ Example: Tetanus

Incidence Number of people in a

population who develop a disease during a particular time period

Includes new cases Prevalence

Number of people in a population having a specific disease at a given time

Includes old a new cases

Sporadic disease Disease that occurs

occasionally in a population

Endemic disease Disease constantly

present in a population Epidemic disease

Disease acquired by many hosts in a given area in a short time

Pandemic disease Worldwide epidemic

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Acute disease Symptoms develop

rapidly Chronic disease

Disease develops slowly Subacute disease

Symptoms between acute and chronic

Latent disease Disease with a period of

no symptoms when the infectious agent is inactive

Herd immunity Many immune people are

present in a population preventing the spread of a disease

Herd Immunity

Often contagious during incubation and prodormal period!!

Local infection Pathogens limited to a small area of the

bodySystemic infection

An infection spread through the body (via blood or lymphatic system)

Focal infection Systemic infection that began as a local

infection

Bacteremia Bacteria in the blood

Septicemia Spread of bacteria

through the blood with organ manifestation

Toxemia Toxins in the blood

Fungemia Fungi in the blood

Viremia Viruses in the blood

Common cause for bacteremia:

Coagulase negative staphylococci colonizing i.v.

catheter

Factors that make the body more susceptible to disease Primary and secondary infection

▪ Acute infection that causes the initial illness and predisposes to a secondary, often opportunistic infection

Male versus female Genetic background Climate and weather Nutrition Lifestyle Occupation Pre-existing illness

Example: Influenza and Haemophilus

influenzae

Continual sources of the disease organisms Humans — AIDS, gonorrhea

Carriers may have inapparent infections or latent diseases

Animals — Rabies, Lyme disease Some zoonoses may be transmitted to humans

Nonliving — Botulism, tetanus Soil

Tick

Borrelia burgdorferi

Lyme Disease:Skin manifestation

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Direct Requires close

association between infected and susceptible host

Includes fecal-oral Indirect

Spread by fomites Droplets

Transmission via airborne droplets

Inanimate reservoir Food Water

Vectors Arthropods

▪ Fleas : plague▪ Ticks: Lyme disease▪ Mosquitoes: malaria

Mechanical Biological : some part of the

development of the microbe takes place in the vector

Acquired during a hospital stay Source is hospital 5-15 % of all hospital patients

affected

Diseases that are new, increasing in incidence, or showing a potential to increase in the near future

Appearance of new strains by genetic recombination E. coli O157:H7 Avian influenzavirus H5N1

Evolution of new serovars V. cholerae O139

Inappropriate use of antibiotics and pesticides Antibiotic resistant strains Multidrug resistant M.

tuberculosis

Global warming and weather changes Hantavirus pulmonary

syndrome Spread of known

diseases by modern transportation Cholera

Ecological Disaster Coccidioidomycosis after

Northridge earthquake Failures in public

health Missed immunizations

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.”

Discipline that find answers to When? Where? How transmitted?

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.

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.

John Snow Cholera outbreaks in London 1848-1849

Ignaz Semmelweis Childbed fever (puerperal sepsis) 1846 - 1848

Florence Nightingale Epidemic typhus 1858

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

Mandatoryhand washingintroduced

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

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 (or descriptive) 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