Epidemiology

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Epidemiology
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Epidemiology

• Occurrence, distribution and control of disease
  in populations
• Worldwide infectious diseases account for 30 of
  annual deaths
• Emerging diseases SARS

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Virulent vs. Chronic Pathogens

• Several pathogens cannot survive outside host
• Virulent kills host immediately not allowing for
  effective spread
• Chronic lives in balance with host causing
  minimal harm
• Some live outside of host ex. Clostridium
  inhabits soil and accidental transmission can
  lead to botulism or tetanus
• Epidemiologist studies spread of disease to
  identify origin and mode of transmission

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• Prevalence the proportion of diseased
  individuals in a population in a given time
  period
• Incidence number of cases of an individual
  disease in a population in a given time period
• Epidemic unusually high numbers of people
  affected in a population at the same time
• Pandemic worldwide
• Endemic constantly present at low incidence in a
  population

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• Outbreak number of cases observed in a short
  time period, in an area previously experiencing
  only sporadic cases
• Sub-clinical infection diseased individuals that
  do not show symptoms or mild symptoms (carriers)
  
• Carriers can be identified Ex. TB test
• Carriers can be original source of spread Ex.
  Typhoid Mary
• Mortality incidence of death in a population
• Morbidity incidence of disease in a population
  (fatal non fatal) provide more accurate
  health statistics of a population

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5 Stages of Disease Progression

• Infection organism multiplies in host
• Incubation period time between infection and
  appearance of symptoms
• Ex. Influenza days AIDS years
• Length of incubation period dependent on
  inoculum size, virulence, host resistance
• Acute period disease at peak with full blown
  symptoms
• Decline period symptoms subside
• Convalescent period regain strength and return
  to normal (often times resulting in immunity)

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Reservoirs

• Inanimate or animate sites where infectious
  agents remain viable
• Saprophytic pathogens that live on dead or
  decaying matter
• Pathogens that live in a single host can be
  eradicated
• Zoonosis primarily occurring in animals
  accidental transmission to humans
• Control of infectious agent in animal host is
  required

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Transmission patterns

• Transmission can be tracked by correlating
  factors such as geography, season, age group etc.
  
• Ex. Malaria transmitted by a mosquito vector
  found in tropics
• Respiratory pathogens tend to be airborne
• Intestinal pathogens spread though food and water
  
• Host-to-host transmission direct transmission to
  a susceptible host
• Ex. infectious droplets of cold or influenza
  exchange of bodily fluids
• Indirect host-to-host transmission facilitated
  by agents
• Ex. vectors (living agents)
• Fomites (non-living agents)

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Epidemics

• Common source epidemic arises as a result of
  contamination of a common source such as food or
  water. Ex. E.coli outbreak
• Pathogen leaves body in fecal material---contamina
  tes food /water---enters intestinal tract of
  recipient during ingestion

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Coevolution

• Immunity gradually develops against a pathogen
  leading to a balance/equilibrium between host and
  pathogen
• Failure to reach equilibrium can lead to
  extinction of both host and pathogen
• Evolutionary success of a pathogen based on
  ability to maintain balance with host, rather
  than ability to destroy host
• Host and pathogen affect each others evolution
  coevolution, sometimes (host-host transmission)
  leading to decreased virulence of pathogen over
  time
• No pressure to reduce virulence in vector borne
  illnesses, in fact increase in virulence may
  sometimes be apparent ex. virulent E.coli strains
  

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Herd Immunity

• Resistance of a group to infection due to high
  proportion of immunity
• Ex. If 70 of population immunized against polio
  for protection
• Highly infectious diseases such as influenza 95
  immunity

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AIDS pandemic

• Viral disease of immune system
• First reported cases in 1981
• 70 million infected worldwide
• Initial studies suggested an unusually high
  prevalence among homosexual men and intravenous
  drug abusers suggesting agent was transmitted
  via blood
• HIV not transmitted by casual contact,
  contaminated food or respiratory route, instead
  via bodily fluids i.e. blood and semen
• Increased probability of transmission in high
  risk groups
• Engage in exchange of bodily fluids
• Multiple partners (needle-sharing sexual
  activity)
• Incidence in blood transfusion recipients almost
  entirely eliminated due to rigorous screening
  techniques
• Africa incidence of women higher than men-
  therefore sexual orientation is not the primary
  determining factor

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Nosocomial Infections

• Hospital acquired infections
• Infectious diseases spread rapidly in hospitals
  due to
• Patients have weakened resistance
• Patients serve as reservoirs
• Cross-infection likely due to several patients in
  one ward/room
• Patients moved between wards
• Routine tests/procedures such as injections,
  biopsys
• Surgical procedures
• Certain drugs (steroids-to reduce inflammation)
• Use of antibiotics creates resistant strains

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SARS

• Severe acute respiratory syndrome emerging
  zoonotic disease
• Appeared in November 2002 in Guandong, China
• By 2003 virus spread to 32 countries due to
  global travel
• SsRNA Coronavirus of animal origin SARS-CoV
• Causes severe respiratory illness

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Public Health Measures

• I. Control directed against the reservoir
• II. Control against transmission
• III. Immunization
• IV. Quarantine
• V. Surveillance
• VI. Eradication

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Public Health Measures (Contd.)

• I. Control directed against the reservoir
• Destruction of infected reservoir
• domestic vs. wild animals
• Insects DDT effective against mosquitoes
• Humans challenging ex. AIDS

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Public Health Measures (Contd.)

• II. Control against transmission
• Food/water borne pathogens can be eliminated by
  preventing or controlling contamination
• Food protection laws and water purification
  systems have effectively controlled transmission
• Respiratory pathogens chemical disinfection of
  air has been unsuccessful
• Face masks, filtration systems

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Public Health Measures (Contd.)

• III. Immunization
• Smallpox, diphtheria, tetanus, pertussis, mumps,
  polio, rubella etc.
• 100 immunization is not necessary (varies with
  virulence and living conditions of population)
• Childhood vaccines do not offer lifetime immunity
  boosters

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Public Health Measures (Contd.)

• IV. Quarantine
• Restricting the movement of an infected
  individual to prevent spread (time limit) not
  as strict as isolation
• Six diseases require quarantine smallpox,
  cholera, plague, yellow fever, typhoid fever and
  relapsing fever

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Public Health Measures (Contd.)

• V. Surveillance
• Observation, recognition and reporting of
  diseases in a timely manner as they occur
• Centers for Disease Control (CDC)
• See Table 25.5

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Public Health Measures (Contd.)

• VI. Eradication
• Smallpox eradicated in 1980
• Targets polio, leprosy, syphilis, rabies

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Global Health

• WHO classified world into 6 regions for
  collecting and reporting purposes
• Africa (infectious disease)
• Americas (cancer cardiovascular disease)
• Eastern Mediterranean
• Europe
• Southeast Asia
• Western Pacific

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Emerging Diseases

• Diseases that suddenly become prevalent (new or
  reemerging)
• Ex. SARS, West Nile, Plague, Dengue, Cholera
• Factors contributing to emergence include
• Travel
• Human demographics and behavior
• Technological advances
• Transportation
• Changes in land use patterns Ex. Reforestation,
  Lyme disease)
• Microbial adaptation and change Ex. HIV, plasmids
  etc.
• Breakdown of public health measures Ex.
  Contaminated water supplies inadequate
  vaccination programs

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Biological Warfare/Weapons

• Use of biological agents to incapacitate or kill
  a military or civilian population in an act or
  war or terrorism
• Characteristics of weapons
• Easy to produce and deliver
• Safe for use by offensive party
• Able to kill or incapacitate several individuals
  
• Ex. B.anthracis, C.botulinum (toxin disseminated
  in water supplies), Hemorrhagic fever viruses
  etc, smallpox (aerosolized)
• Table 25.10

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