Principles of Disease and Epidemiology

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Chapter 14

• Principles of Disease and Epidemiology

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Pathology, Infection, and Disease

• Pathology- scientific study of disease
• Etiology-
• Pathogenesis-
• Infection-
• Disease-

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Normal Microbiota

• AKA normal flora
• Bacteria that colonize the body
• Colonization begins at birth
• Actually more bacterial cells than human body
  cells!
• Found in certain regions of the body
• Skin
• Eyes
• URT
• Mouth
• large intestine
• Urinary/reproductive system

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Normal Microbiota

• Factors that affect distribution/composition of
  normal microbiota on the body
• Nutrients
• Physical factors
• Chemical factors
• Mechanical factors
• Defenses of the host

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Relationships between normal microbiota and the
host

• Microbial antagonism-
• Examples
• Large intestine-
• bacteriocins
• Vagina-

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Relationships between normal microbiota and the
host

• Symbiosis
• Types of symbiosis
• Commensalism
• Mutualism
• Parasitism

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Opportunistic microorganisms

• Sometimes normal microbiota can cause problems if
  conditions change
• Opportunistic pathogens-
• Conditions that promote opportunistic infections
  
• Microbe in wrong location
• Ex.
• Compromised host
• Normal microbe in a new host-

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Kochs Postulates

• Demonstrated the Germ Theory of Disease
• Kochs Postulates
• The same pathogen must be present in every case
  of the disease
• Pathogen must be isolated from the diseased host
  and grown in pure culture
• The pathogen from the pure culture must cause the
  disease when inoculated into a healthy
  susceptible laboratory animal
• Pathogen must be isolated from the inoculated
  animal and shown to be the original organism

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Exceptions to Kochs Postulates

• Useful in most cases, but certain exceptions
• May not be able to culture the bacteria
• Ex. Treponema pallidum, Mycobacterium leprae,
  viral diseases
• Many diseases caused by different bacteria can
  have similar symptoms
• Ex. nephritis
• Some pathogens can cause several different
  diseases
• Ex. Streptococcus pyogenes
• Ethical considerations-

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Classifying Infectious Diseases

• Symptoms-
• Signs-
• Syndrome-
• Communicable disease-
• Ex.
• Contagious disease-
• Ex.
• Non-communicable disease-
• Ex.

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Occurrence of a Disease

• sporadic disease-
• Ex.
• endemic disease-
• Ex.
• epidemic disease-
• Ex.
• pandemic disease-
• Ex.

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Severity or Duration of a Disease

• Acute disease-
• Ex.
• Chronic disease-
• Ex.
• Subacute disease-
• Latent disease-
• Ex.
• Herd immunity-

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Extent of Host Involvement

• Local infection-
• Ex.
• Systemic infection-
• Ex.
• Focal infection-
• Ex. teeth, tonsils, sinuses

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Extent of Host Involvement

• Septicemia-
• Bacteremia-
• Viremia-
• Toxemia-

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Extent of Host Involvement

• Primary infection-
• Secondary infection-
• Subclinical infection-

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Predisposing Factors

• Makes body more susceptible to disease
• Examples
• Gender-
• Genetics/race-
• Climate/weather
• Nutrition
• Fatigue
• Stress
• Preexisting illness

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Development of Disease

• Disease development follows a similar
  pattern/sequence
• Incubation period-
• Prodromal period-
• Period of illness-
• Period of decline-
• Period of convalescence-

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Reservoirs of Infection

• Reservoirs of infection- source of disease
• Human reservoirs-
• Carriers-
• Animal reservoirs-
• Zoonoses-
• Nonliving reservoirs-
• Most common

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Transmission of Disease

• Contact Transmission
• Direct contact transmission
• Ex.
• Indirect contact transmission
• Ex.
• Fomite-
• Droplet transmission-
• Ex.

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Transmission of Disease

• Vehicle Transmission
• Transmission by a medium
• Water
• Contaminated/untreated sewage
• Air
• Droplet nuclei in dust traveling gt1m from
  reservoir to host
• Food
• Incomplete cooking, poor refrigeration, improper
  handling

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Vectors

• Animals that carry pathogens from one host to
  another
• Most are arthropods
• Insects, mites, lice, etc.
• Two ways vectors spread disease
• Mechanical transmission
• Ex.
• Biological transmission
• Ex.

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Nosocomial Infections

• Hospital-acquired
• CDC estimate15 of all hospital patients acquire
  some type pf nosocomial infection
• Rate has increased 36 in the last 20 years
• In USA, 2 million people/year 20,000 die

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Factors that contribute to nosocomial infections

• Microorganisms in the hospital environment
• Compromised host
• Chain of transmission in hospital
• If all of these factors are present, it poses a
  significant risk of infection

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Microorganisms in the Hospital

• Major source of microorganisms in the hospital is
  the normal microbiota of patients and staff
• Opportunistic pathogens
• Compromised host
• Antibiotic resistance is a major concern
• Ex. Pseudomonas aeruginosa MRSA
  (methicillin-resistant Staphylococcus aureus)

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Principal sites of Nosocomial Infections
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Chain of Transmission

• Principal routes of transmission of nosocomial
  infections
• Direct contact
• Hospital staff-patient
• Patient-patient
• Indirect contact
• Fomites
• Exs.
• Ventilation system

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Control of Nosocomial Infections

• According to the CDC, hand-washing is the single
  most important means of preventing infection
• Using aseptic techniques
• Handling contaminated materials carefully
• Disinfecting tubs between patients
• Paying special attention to cleanliness of
  respirators/humidifiers
• Single-use dressings/tubes
• Prescribe antibiotics only when necessary

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Emerging Infectious Diseases

• Diseases that are new
• Diseases that are changing
• Diseases that are increasing in incidence, or
  have the potential to increase
• Local disease that is becoming widespread
• EIDs can be bacterial, viral, fungal, protozoan,
  or helminth

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Factors that Contribute to EIDs

• New strains
• Ex. E. coli O157H7 avian influenza (H5N1)
• New serovar
• Genetic changes/evolution of pathogenic species
• Widespread use of pesticides and antibiotics
• Fosters growth of resistant strains
• Global warming/changes in weather patterns
• Modern transportation
• Natural disasters
• Animal control measures
• Failures in public health measures