Exam 3 – Host Microbe Interactions Epidemiology

Effects of microorganisms on hosts

Stages:

• Contamination – presence of microbes

• Infection – multiplication of microbes on or in 

  • infestation – worms or insects

• Disease – host cannot carry on normally

  • stages – mild, severe but reversible, severe and irreversible

Pathogenicity

• capability to produce disease

  • low – rarely cause disease (or disease produced is mild)

  • high – usually cause disease

  • pathogen – parasite

Virulence

intensity of the disease caused by a pathogen

Attenuation

• the weakening of a pathogen’s virulence

  • natural or through genetic manipulation

  • attenuated viruses are often used for vaccines (mumps, measles, influenza, etc.)

Transposal of virulence

• repeated passage of a pathogen to new hosts (new species)

• no longer virulent to original host

Virulence Factors

• adherence/invasive factors

  • pili

  • slime layer/capsules

• avoidance mechanisms

  • capsules (avoid phagocytosis)

• toxins

  • exotoxins

  • endotoxins

What is normal flora and resident microflora?

• normal flora

  • also called normal microflora or normal microbiota

  • live on or in without causing disease (commensals)

  • 10x more microbes per body than cells in body

    • help compete out bad microbes

• resident microflora

  • always present

How do children get normal flora?

• have no normal flora in utero

• get it from

  • passage through the birth canal

  • contact with other people

  • contact with feeding material

  • nursing

  • contact with instruments

Body parts that are normally microbe free – tissues and organs

• middle and inner ear

• sinuses

• internal eye

• bone marrow

• muscles

• glands

• organs

• circulatory system 

• brain and spinal cord

• ovaries and testes

Body parts that are normally microbe free – body fluids

• blood

• cerebrospinal fluid

• saliva in glands

• urine in kidneys/bladder

Transient microflora

not usually there – "just passing through”

Opportunists microflora

• can cause disease if conditions are right

  • failure of defenses – immunocompromised

  • unusual body locations

  • disturbances of normal flora – microbial antagonisms (where normal flora would usually out-compete it)

Modes of disease transmission

• contact transmission

  • direct contact

  • indirect contact by fomites

  • droplets

• vehicle transmission

  • waterborne

  • airborne, including dust particles

  • foodborne

• vector transmission

  • mechanical

  • biological

Horizontal transmission

person to person – contagious diseases

Vertical transmission

parents to offspring

Fomites

nonliving objects that harbor an infectious agent (tissue, utensil, money, etc.)

Course of disease

• incubation period – no signs or symptoms

• prodromal phrase – (“forerunner”) vague symptoms/starting symptoms that are usually nonspecific– fever, malaise

• invasive phrase – most severe signs and symptoms

• acme – peak of disease – when symptoms distinct to a particular illness occur

• decline phase – declining sygns and symptoms

• convalescence period

Epidemiology

• the study of the following in populations

  • disease frequency

  • disease spread

  • disease control

Etiology

the cause of disease

Incidence

number of cases in a population in a given time period (new cases)

Prevalence

percentage in a population having the disease (new and existing)

Morbidity

number of cases in a population – counted in the Morbidity and Mortality Weekly Report (MMWR)

Mortality

number of deaths in a population – counted in the Morbidity and Mortality Weekly Report (MMWR)

How do incidence and prevalence rates look on a graph?

Diseases in populations – endemic

present coninutally

Diseases in populations – epidemic

higher than normal incidence

Diseases in populations – pandemic

• worldwide epidemic

  • ex. 1918 flu, 2009 H1N1

Diseases in populations – sporadic

random/unpredictable

What does an endemic disease look like on a graph?

What does an epidemic look like on a graph?

Say how these different outbreaks on the graphs – sporadic, endemic, epidemic?

Common source outbreak

contact with contaminated substances

Propagated epidemics

person to person (horizontal transmission) – contagious

Compare how common source and propagated epidemics look on a graph

Reservoirs of infection

sites where organisms can maintain their ability to infect

Human reservoirs

• active or subclinical infections (carriers)

  • chronic 

  • intermittent

• Communicable – infectious (possibly before symptoms)

  • contagious

Animal reservoirs

• zoonoses – diseases from other vertebrates (i.e. anthrax, rabies, hantavirus)

  • can be transmitted by a vector

Nonliving reservoirs

soil and water

Nosocomial infections

• infection acquired in a medical facility

  • by patients and staff

• 10% of patients

• $5 billion/year in hospital costs

• 20,000/year die

  • just know if its common or not (not – 10%)

Methods of controlling disease transmission

• isolation – patient with disease isolated

• quarantine – separation of “healthy” after exposure

• vector control – ex. pesticides, bed nets

• active immunizations (vaccines)

• pathogen eradication – getting rid of organisms – very difficult

Herd immunity

• resistance of a group to infection

  • for polio – 70% need to be immunized

  • For influenza – 90%

• A major reason why all vaccinations are critical for all

Pathogen eradication

• accomplished with smallpox

• in progress for polio (close), leprosy, and Guinea worm (close)

• most feasible with organism with a single host