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Chapter 14


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  • 1. Chapter 14: Principles of Disease and Epidemiology
    • Pathology = study of disease
      • Etiology = cause of disease
      • Pathogenesis = manner in which disease develops
      • Structural and functional changes brought about by the disease
      • Final effects the disease has on the body
  • 2. Chapter 14: Principles of Disease and Epidemiology
    • Infection = invasion or colonization of the body by pathogens
    • Disease – occurs when an infection results in any change from a state of health
      • Abnormal state where the body can’t carry out its normal fxs
    • Normal flora or normal microbiota = microbes that have permanent residence without producing disease
      • Transient microbiota – present temporarily and then disappear ex. Streptococcus in lg intestine
  • 3. Chapter 14: Principles of Disease and Epidemiology
    • Microbial antagonism or competitive exclusion = normal microbes preventing overgrowth of pathogens
      • Ex. E. coli produces bacteriocins (proteins that are lethal to closely related bacteria) in the large intestine which inhibit the growth of pathogenic Salmonella and Shigella
  • 4. Chapter 14: Principles of Disease and Epidemiology
    • Symbiosis = relationship between normal microbiota and the host
      • Commensalism = 1 organism benefits while the other is unaffected ex. Normal microbiota
      • Mutualism = both organisms benefit
        • Ex. E. coli in the lg intestine synthesizes vitamin K, B which provides nutrients for you and your body provides nutrients for E. coli in the lg intestine
      • Parasitism = 1 organism benefits at the expense of the other ex. Giardia lamblia
  • 5. Chapter 14: Principles of Disease and Epidemiology
    • Opportunistic pathogens - do not cause disease in their normal habitat in a healthy person
      • Cause disease if they gain entrance to an area that they don’t usually inhabit
        • E. coli in urinary tract
        • Pneumocystis jiroveci in AIDS px – fungal pathogen causes pneumonia
    • Probiotics = live bacterial cultures ingested for beneficial effects
      • Prebiotics are chemicals to promote their growth
  • 6. Koch’s Postulates
    • Demonstrate that a specific microbe is the cause of a specific disease
      • he worked with Bacillus anthracis to prove his postulates
    • The same pathogen must be present in every case of the disease
    • 2. the pathogen must be isolated from the diseased host and grown in pure culture
  • 7. Koch’s Postulates
    • 3. The pathogen from the pure culture must cause disease when inoculated into a healthy, susceptible laboratory animal
    • 4. The pathogen must be isolated from the inoculated animal and be the same pathogen as the original organism
  • 8. Exceptions to Koch’s Postulates
    • A few organisms can’t be grown in laboratory media ex. Treponema – bacteria
    • Some diseases may be caused by a variety of organisms and the specific etiology is not easy to determine from the symptoms ex. Pneumonia
    • Some pathogens may infect a # of organs or tissues and cause very different diseases or sxs ex. Streptococcus pyogenes causes sore throat, scarlet fever, puerperal fever
  • 9. Classifying infectious diseases
    • Symptoms – felt by px, subjective and can’t be seen by a dr. ex. Pain
    • Signs – objective changes that a dr. can observe and measure ex. Fever
    • Syndrome = specific group of sxs or signs accompanying a particular disease
    • Communicable diseases = spread directly or indirectly from 1 host to another ex. Typhoid fever, TB
      • If it is highly communicable esp. thru direct contact = contagious
  • 10. Classifying infectious diseases
    • Non-communicable diseases = caused by microbes like Clostridium tetani which enters the body through a wound
    • Incidence = the # of new cases of a disease during a particular time period
      • It is an indicator of the spread of the disease
    • Prevalence = total # of cases of a disease (old and new) in a certain area and time period
  • 11. Classifying infectious diseases
    • Sporadic diseases occur only occasionally and in an unpredictable fashion ex. Tetanus
    • Endemic diseases are present constantly at a certain level in a particular location
      • Common cold throughout US
      • Lyme disease endemic to some eastern states
  • 12. Classifying infectious diseases
    • Epidemic a sudden increase in morbidity (illness rate) and mortality (death rate) above the usual for a disease = many people in a given area acquire a certain disease in a short period of time
    • Pandemics = worldwide epidemics
    • Acute disease – develops rapidly but lasts for a short period of time ex. Influenza
      • Primary infection = initial acute infection
      • Secondary infection – caused by opportunist after primary infection weakens body defenses
  • 13. Classifying infectious diseases
    • Chronic disease – develops slowly, often the rx is less severe, is continuous or recurrent for long periods of time. Ex TB, leprosy, syphilis
    • Latent disease = pathogen is inactive for long periods of time and then becomes active to produce symptoms
      • Ex. shingles
  • 14. Classifying infectious diseases
    • Herd immunity = when most of the population is immune through vaccination or acquired immunity, few susceptible people so its hard for pathogen to find
    • Local infections – limited to small area of body ex. Boils
    • Systemic or generalized infection – microbes or their products spread throughout the blood or lymphatic system
      • Bacteremia – bacteria in the blood, toxemia – toxins, viremia - viruses
  • 15. Classifying infectious diseases
    • Focal infection = local infection enters blood or lymph and spreads to other parts of body ex. Sinuses, tonsils
    • Sepsis = a toxic, inflammatory condition from spread of bacteria or their toxins from a local infection
      • Septicemia = sepsis from bacterial pathogens in the bloodstream
  • 16. Classifying infectious diseases
    • Sepsis = a toxic, inflammatory condition from spread of bacteria or their toxins from a local infection
      • Septicemia = sepsis from bacterial pathogens in bloodstream
    • Sub-clinical infection – does not cause any noticeable illness
    • Predisposing factors – makes the body more susceptible to a disease
      • Gender, genetics, weather, age etc.
  • 17. Development of disease – follows a certain sequence of events
    • Incubation period – the time between infection and signs and symptoms
    • Prodromal period – mild sxs of the disease
    • Ex. General aches, malaise
    • 3. Period of illness
      • a) Disease is most severe
      • b) Overt signs and sxs
      • c) # of WBCs may increase or decrease
      • d) If body doesn’t overcome the pathogen the px. May die
  • 18. Development of disease
    • 4. Period of decline
      • a) Signs and sxs subside
      • b) Px susceptible to secondary infection
    • 5. Period of convalescence
      • a) Px regains pre-diseased state
      • b) Px can serve as a reservoir of disease and spread infection to others
          • Ex. Typhoid fever, cholera
  • 19. Reservoirs of Infection
    • Reservoirs are sites that are a continual source of the pathogen from which they may be transmitted
    • All pathogens must have 1 or more reservoirs to exist
  • 20. Reservoirs of Infection
    • Human reservoirs
    • a) people harbor and transmit pathogens to others directly or indirectly
    • b) these people may be active carriers and are diseased and obvious transmitters or are healthy carriers and have no sxs
    • c) people are the only known reservoirs for smallpox, gonorrhea, measles, polio, etc.
    • d) smallpox was eradicated once the last known infected person was cured
  • 21. Reservoirs of Infection
    • The reservoir may be different from the source of the pathogen
      • Typhoid Mary was a reservoir of typhoid fever and shed typhoid bacilli in her feces
      • The immediate source would be water or food contaminated with the fecal material – she was a cook early 1900s
  • 22. Reservoirs of Infection
    • Animal reservoirs
      • Diseases that occur in wild and domestic animals that can be transmitted to humans are called zoonoses
      • Transmission is direct contact with infected animals or their hides, fur or feathers, contamination of food or water, insect vectors, or eating infected animal products
  • 23. Reservoirs of Infection
    • Nonliving reservoirs
      • Some organisms are able to survive and multiply in nonliving environments such as soil and water
      • Clostridium that causes tetanus and botulism can survive many years in the soil
      • Hookworms (a helminth) deposit their eggs into the soil
      • Water contaminated by human or animal feces cause GI tract disease (list includes bacteria, viruses, protozoa)
  • 24. Transmission of disease
    • Transmission is the mechanism by which an infectious agent is spread to a susceptible person
    • Contact transmission
      • Direct – person to person by kissing, touching, bites, or sexual intercourse
        • Colds, flu, STD’s
      • Droplet infection – pathogen spreads in saliva or mucus droplets by coughing or sneezing, travel short distance from reservoir
  • 25. Transmission of disease
    • Contact transmission cont.
      • Indirect – involves a nonliving objects = fomite ex. Cup, towel
    • Vehicle transmission
      • Inanimate reservoirs – food, water, blood
      • Airborne transmission – pathogens traveling in droplets or on dust for distances over 1 meter
        • Viruses, bacteria, fungal spores
  • 26. Transmission of disease
    • Vectors = animals that carry pathogens from 1 host to another, arthropods most important
    • Mechanical transmission – carry pathogens passively on their bodies and feet to food
    • Biological transmission – arthropod passes pathogen in a bite or feces which enter wound caused by the bite
  • 27. Nosocomial infections
    • Nosocomial infections are those infections acquired by pxs or hospital staff in a hospital or long term health care facility and account for 20,000 deaths/year in US
    • 3 conditions that compromise a host
      • Broken skin or mucous membranes
      • Suppressed immune system
      • Impaired defense cell activity – drugs, radiation, steroids, etc.
    • Pathogens transmitted by surgical procedures, fomites, medical equipment, air, H2O, people
  • 28. Emerging Infectious Diseases
    • Emerging infectious diseases show an increase in incidence or a potential for increase in the near future ex. West nile virus, SARS, whooping cough
    • Factors that contribute to emerging disease
      • New strains from genetic recombination
      • New serovars ex. Vibrio cholerae O139
      • Use of Abs and pesticides encourage growth
      • Global warming and weather changes may increase the distribution and survival of reservoirs and vectors
      • Diseases may spread to new geographic areas
  • 29. Epidemiology
    • Epidemiology = scientific study of the source and transmission of a disease with in a population
    • Epidemiology has a long hx
      • Hippocrates 460 -377 BC made observations about the link between malaria and yellow fever and swamps (breeding ground for the mosquito vector)
      • Edward Jenner – late 1700’s – observations of cowpox led to his vaccination for smallpox
  • 30. Epidemiology
    • Epidemiology
      • Ignaz Semmelweis – mid 1800s proved that childbed fever resulted from physicians not washing their hands after dissections
      • British epidemiologist John Snow investigated cholera outbreak in London
        • Most of the cholera victims lived in the Broad Street area and used the water from the Broad Street Pump
        • Removing the pump handle ended the cholera epidemic
  • 31. Epidemiology
    • Descriptive epidemiology – collecting all data that describes the occurrence of the disease
      • Info of affected individuals – place, time, age, M/F
      • Snow’s approach was descriptive
      • The approach is looking backwards
  • 32. Epidemiology
    • Analytical epidemiology – look for a common factor among the affected persons that might have preceded the disease
    • Experimental epidemiology tests a hypothesis ex. Effectiveness of a drug
      • 2 groups – 1 to receive the drug and 1 a placebo
      • Compare groups to determine if any difference with drug
  • 33. Center for Disease Control and Prevention (CDC)
    • Branch of US Public Health Service in Atlanta, Georgia – which is the central source of information in the US
    • CDC issues the Morbidity and Mortality Weekly Report (MMWR) at
    • Notifiable diseases – drs and health care workers must report to Public Health Service ex. AIDS, measles, gonorrhea, tetanus, typhoid fever