Epidemiology and Nosocomial Infections

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Chapter 15

• Epidemiology and Nosocomial Infections

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Epidemiology

• Science that studies when and where diseases
  occur and how they are transmitted
• Branch of microbiology due to microbes as
  pathogens
• Epidemiologist determines the etiology and
  identifies other important factors and patterns
• pathogen relationships with each other, their
  host and the environment
• age, sex, occupation, personal habits,
  socioeconomic status, immunization status of
  infected individuals
• methods for controlling a disease

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Occurrence of a Disease

• Incidence
• number of people in a population who develop a
  disease during a particular time period
• Only looks at new cases reported during a time
  frame
• Usually expressed as new cases per 100,000 people
  per year
• Prevalence
• number of people in a population who are infected
  at a specified time, regardless of when it first
  appeared
• Looks at both old and new cases
• Sporadic disease
• particular disease which occurs only occasionally

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• Endemic disease
• constantly present in a population
• can be endemic to certain parts of the world
• malaria is endemic in the continent of Africa
• Epidemic
• many people in a given area develop a certain
  disease in a relatively short period of time
• outbreak of influenza on campus
• Pandemic
• epidemic disease that occurs worldwide
• AIDS

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• Many disease occur in cycles
• May be annual or occur over decades
• Flu
• plague
• Herd immunity is an important factor in cycles
• Low levels could lead to reemergence of disease
• Small pox

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Epidemiological Studies

• Collecting frequency data and drawing conclusions
  
• Descriptive
• Physical aspects of existing disease
• affected, who affected, location and time of
  cases
• Analytical
• Establishes cause and effect relationships
• Retrospective or prospective studies
• Experimental
• Designs experiments to test hypothesis
• Often deals with effectiveness of disease
  treatment

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• Reservoirs of infection
• continual source of the disease organisms
• can be living or non-living
• Human reservoirs
• Can be carriers
• may not exhibit signs or symptoms
• may have latent stages of disease
• HIV, genital herpes, papillomavirus

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• Animal reservoirs
• zoonoses
• Animal diseases that can be transmitted to humans
  
• Rabies, Lyme disease, West Nile virus
• Non-living reservoirs
• two major areas are soil and water
• Fungi, Clostridium botulinum, and C. tetani
  reside in the soil
• Vibrio cholerae, Salmonella typhi contaminate
  water
• food can also be sources of diseases
• trichinosis and salmonellosis

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How Microorganisms Enter a Host

• Portals of entry
• 1. Mucus membranes
• Respiratory tract
• easiest and most frequently used
• Common cold, flu, pneumonia, tuberculosis, small
  pox
• Gastrointestinal tract
• Contaminated food, water and fingers
• Most microbes are destroyed by HCL in stomach
• Poliomyelitis, hepatitis, amoebic dysentery,
  cholera
• Genitourinary tract
• Sexually transmitted diseases
• HIV, genital warts, chlamydia, herpes, syphilis,
  gonorrhea

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• 2. Skin
• Unbroken skin is almost an impermeable barrier to
  microbes
• Some fungi live in skin cells
• Most microbes gain entrance through sweat glands
  or hair follicles
• 3. Parenteral route
• Directly deposited under the skin
• Punctures, injections, surgery

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• Most microbes have a preferred portal of entry
• May be a prerequisite for pathogenicity
• Streptococci that are inhaled cause pneumonia if
  ingested generally do not produce the disease
• A few microbes cause illness no matter how they
  enter
• May cause different illness based on portal
• Yersinia pestis
• Bubonic verses pneumonic plague

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Portals of Exit

• Most common - respiratory and gastrointestinal
  tracts
• Tuberculosis, whooping cough, pneumonia, scarlet
  fever, meningococcal meningitis, chicken pox,
  small pox and flu
• Feces may be contaminated with pathogens
  responsible for cholera, typhoid fever,
  shigellosis and amoebic dysentery
• Genitourinary tract
• STDs are transmitted through vagina and penis
• Urine contains pathogens causing typhoid fever
  and brucellosis
• Skin or wound infections
• Drainage from wounds can spread infection
• Biting insects
• yellow fever, plague and malaria
• Contaminated needles and syringes
• AIDS and hepatitis

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• Modes of Transmission
• contact transmission
• direct person to person transmission touching,
  kissing, sexual intercourse
• indirect transmitted through a non-living
  object (fomite) tissue, bedding, toys
• droplet discharged by coughing, laughing,
  talking or sneezing
• vehicle transmission
• waterborne under treated of poorly treated
  sewage
• foodborne food poisoning and tapeworm
• airborne dust particles can transmit spores
• vectors arthropods are the most important
• mechanical passive transport on feet or other
  body parts
• biological requires bite to transmit from host
  to host

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Controlling Disease Transmission

• Isolation
• Infected persons prevented contact with general
  population
• Quarantine
• Separation of healthy carriers from general
  population
• Prevents spread during incubation period
• Immunization
• Increases herd immunity
• of people in a population immune to infection
• Vector control
• Requires identification of vector, its feeding
  and breeding behaviors and its habitat

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Public Health Organizations

• Centers for Disease Control and Prevention
• MMWR Morbidity and Mortality Weekly Report
• Notifiable disease
• About 50 changes based on CDC recommendations
• Morbidity rate is the number of people affected
  by a disease in a given period of time in
  relation to the total population
• Mortality rate is the number of deaths resulting
  from a disease in a population in a given period
  of time in relation to the total population

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• World Health Organization
• Regional agencies in Africa, Mediterranean,
  Europe, Asia, Western Pacific and the Americas
• Works with United Nations on population control,
  food supply management and education
• Collects, analyzes and distributes world health
  data, maintains surveillance on potential
  epidemics, provides training and research
  programs
• Publishes Weekly Epidemiological Record

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• Nosocomial Infections
• acquired by a patient while in the hospital
• results from high number of microbes in
  hospital, some are resistant to antimicrobials,
  compromised host, chain of transmission
• S. aureus, E. coli, P. aeruginosa
• CDC estimates 10 of all patients get some kind
  of infection
• Increased 36 in the last 20 years
• 4th leading cause of death in the US
• 100,000 deaths per year