Chapter 14 epidemiology.cj

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

1. 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

1. Incubation period – the time between infection and signs and symptoms

2. Prodromal period – mild sxs of the disease Ex. General aches, malaise3. 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 reservoirsa) people harbor and transmit pathogens to others directly or indirectlyb) 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

http://www.cdc.gov/

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Chapter 14 epidemiology.cj