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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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