Chapter 14

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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 www.cdc.gov
Notifiable diseases – drs and health care workers must report to Public Health Service ex. AIDS, measles, gonorrhea, tetanus, typhoid fever

Chapter 14

by ELAC microbiology

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Chapter 14 Presentation Transcript