MID EPIDEMIOLOGY.ppt

7/29/2019 MID EPIDEMIOLOGY.ppt

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EPIDEMIOLOGY


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EPIDEMIOLOGY

The science that deals with when and

where diseases occur. How they are transmitted in the human

population.


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EPIDEMIOLOGY

Modern epidemiology began in 1854

when John Snow reasoned that cholerawas transmitted by contaminated water.

He questioned some victims of a choleraepidemic in London and found that mostof them obtained water from the BroadStreet well.


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EPIDEMIOLOGY

He determined that the incidence of

cholera was higher among people whodrank from the London section of theThames River than among the populationusing cleaner water upstream.


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EPIDEMIOLOGY

Etiology

Identifies other possible factors and

patterns concerning the persons affected.Age, gender, occupation, personal habits,history of immunization, presence of anyother diseases, and the common history ofaffected individuals.

Most concerned with the rate of a disease.


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RATE OF DISEASE IN A

POPULATION

Attach ratethe proportional numberof cases developing in a populationexposed to the infectious agent. Forexample, if 100 people at a party atechicken that was contaiminated withSalmonella, and 10 people cam down with

symptoms of disease, then the attackrate was 10%.


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POPULATION

MORBIDITY RATE - is calculated as

the number of cases of an illness in agiven time period divided by thepopulation at risk. Contagious diseaseshave a high morbidity rate because each

infected individual may transmit theinfection to several others.


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POPULATION

MORTALITY RATE- reflects the

percentage of the population that diesfrom the disease.


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OCCURRENCE OF DISEASE

INCIDENCE measures the number ofnew cases over a certain time period, ascompared with the general healthypopulation.

PREVALENCE total number of existingcases with respect to the entire

population usually represented by apercentage of the population.


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OCCURRENCE OF DISEASE

SPORADIC when occasional cases arereported at irregular intervals.

ENDEMIC disease that exhibits a relativelysteady frequency over a long period of time ina particular geographic locale.

EPIDEMIC when prevalence of a disease isincreasing beyond what is expected.

PANDEMIC epidemic across continents


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PATTERNS OF DISEASE

OCCURRENCE


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EPIDERMIOLOGIC STUDIES

RESVOIRS OF INFECTION

PORTAL OF EXIT TRANSMISSION

PORTALS OF ENTRY

DISEASE


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OVERVIEW OF DISEASE


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RESERVOIRS OF INFECTION

The reservoirs of infection is importantbecause it affects the extent anddistribution of a disease.

Recognizing the reservoir can helpprotect a population from disease,because measures can then be

instituted to prevent the people fromcoming into contact with the source.


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HUMAN RESERVOIRS

SYMPTOMATICshow signs andsymptoms of the disease.

ASMYPTOMATIC CARRIERdo notshow signs or symptoms of the disease.


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ANIMAL RESERVOIRS

Both wild and domestic animals are livingreservoirs of microorganisms that cancause human disease.

Diseases that occur primarily in wild anddomestic animals and can betransmitted to humans are calledzoonose.

Examples include rabies, Rocky Mountainspotted fever.


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ENVIRONMENTAL RESERVOIRS

The two major nonliving reservoirs ofinfectious disease are soil and water.

Soil harbors such pathogens as fungi, andClostridium botulinum.

Water that has been contaminated by the

feces of humans and other animals is areservoir for microorganism which areresponsible for gastrointestinal diseases.


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PORTALS OF EXIT

Microorganism must leave one host inorder to be transmitted to another.

Respiratory, saliva

Skin

Fecal exit

Urogenital tract Removal of blood


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TRANSMISSION OF DISEASE

The causative agents of disease can betransmitted from the reservoir to a

susceptible host by 4 routes.

1. ContactDirect

2. A common vehicleIndirect

3.

Airborne route4. Vectors


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CONTACT TRANSMISSION

DIRECT CONTACTreservoir to host

INDIRECT CONTACTreservoir to vehicle

to host. Vehicle inanimate material, food, water,

biological products, fomites

DROPLETreservoir to air (short distance)to host


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TRANSMISSION

Common vehicletransmission refers to thetransmission of diseaseagents by a commoninanimate reservoir (food,

drugs, blood) to anindividual.

Airborne transmissionrefers to the spread ofagents by droplet nuclei ordust at a distance of morethan 1 meter from thereservoir to host.


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VECTORS

A vector, animalsthat carry

pathogens fromone host toanother.

Biological: Bite or feces Mechanical:Transported

on feet


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PORTALS OF ENTRY

To cause disease, not only must apathogen be transmitted from its

reservoir to a new host, it must alsocolonize a surface of or enter the new

host.


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PORTALS OF ENTRY

Mucous membranes

Skin

Gastrointestinal tract Respiratory tract

Urogenital tract


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PORTALS OF EXIT


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MUCUS MEMBRANES

Mucus membranes are present inrespiratory, gastrointestinal,genitourinary tracts, and theconjunctiva of the eye. The

respiratory tract is the easiest andmost frequently traveled portal of

entry for infectious microbes.Examples include: cold, pneumonia,influenza, measles, and smallpox.


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MUCUS MEMBRANES

Microorganisms can gain access to thegastrointestinal tract in food and water.Most microbes that enter the body are

destroyed by HCl and enzymes. Thosethat survive can cause disease.Examples include: hepatitis A,

poliomyelitis, typhoid fever, and cholera.


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MUCUS MEMBRANES

An important pathogen capable ofpenetrating the mucous membranes of

genitourinary tract is Trponemapallidum, the causative agent of syphilis.


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SKIN

Some microbes gain access to the bodythrough the openings in the skin, thehair follicles and sweat gland ducts.

Examples include some fungi andhookworms.


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PARENTERAL ROUTES

Parenteral routes are the result ofpenetration or injury to the surface

epithelial tissue and connective tissue.Punctures, injections, bites, cuts,

surgery call all establish parenteral

routes.


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INFECTIOUS DOSE (ID)

minimum number of microbes required forinfection to proceed

microbes with small IDs have greater

virulence 1 rickettsial cell in Q fever

10 bacteria in TB, giardiasis

109 bacteria in cholera

Lack of ID will not result in infection

ID50 dose that infects 50% of those exposed


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LETHAL DOSE

LD50 lethal dose that 50% of thoseexposed


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PATHOGEN PENETRATION

Once pathogens gain entry to a host, almost allof them have some means of attaching

themselves to host tissue. The attachment

between pathogen and host takes place bymeans of surface molecules on the pathogencalled adhesins or ligands that bind

specifically to complementary surface

receptors on the cells of certain host tissues.Once attached, the pathogen is ready to

invade a sterile body compartment.


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MECHANISMS OF ADHESION


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VIRULENCE ENHANCING ENZYMES

Leukocidinsdestroy WBCs by degradinglysosomes in WBC. Ex. Streptococci

Hemolysinslyse RBCs. Ex. Streptococci,

Staphylococci, Costridium Coagulasesclot blood. Ex. Staphylococcus Kinasesdissolve clots therefore bacteria

can travel throughout body and prevents

isolation of bacteria. Ex. streptokinase,staphylokinase. Hyaluronidasedissolve matrix of connective

tissue. Ex. collagenase from Clostridium


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TOXINS

Exotoxins proteins secreted by gram-positive and gram negative bacteria.

Endotoxins lipid A of LPS of gram-negative bacteria.


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EXOTOXINS

Characteristics:

1. usually from Gram + organisms.

2. Proteins3. Secreted from the bacteria whilethe bacteria is alive.

4. Generally destroy host cells orinhibit metabolic functions.


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ENDOTOXINS

Characteristics:1. Usually from Gram organisms.

2. Lipopolysaccharide which is part of

bacterial cell wall. It is released after thebacteria dies.

3. Illicit side effects such as fever,

weakness, and shock.4. Example: Salmonella typhi--typhoid fever


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EXOTOXINS

Examples:

1. Cytotoxinskills host cells. Ex.diphtheria.

2. Neurotoxinsinterfere with nervefunctions. Ex. Botulism.

3. Eneterotoxinsaffect cells of GItract.

Ex. Staphylococcifood poisoning.


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HERD IMMUNITY

A phenomenon that occurs when a criticalconcentration of immune hosts prevent

the spread of an infectious agent.


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NOSOCOMIAL

INFECTIONS


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NOSOCOMIAL INFECTIONS

Nosocomial infections are hospitalacquired infections.

Not surprising because of the highdensity of susceptible people.

In the US about 5-6% of patientsadmitted to the hospital develop a

nosocomial infection.


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Nosocomial infections aredefined as hospital-acquired infections Infections may range

from mild to fatal

Numerous factorsdetermine whichorganisms and agents areresponsible

Length of time ofexposure

Manner of exposure

Virulence and number oforganisms

State of host defenses


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Commonly implicated organism include Enterococcus species

Part of normal intestinal flora

Escherichia coliand other species in familyEnterobacteriaceae Part of normal intestinal flora

Pseudomonas species Common cause of nosocomial pneumonia and urinary

tract and burn infections

Staphylococcus aureus Survives in environment for prolonged periods

Easily transmissible to fomites

Other Staphylococcus species Often part of normal skin flora



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Reservoirs of infectious agents in

hospitals

Other patients Patients can harbor infectious agents and dischargeinto environment

Hospital environment Certain bacteria do not require many nutrients and

can survive long periods on surfaces Many of these organism are antibiotic resistant

Due to continual exposure to antibiotics



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Health care workers

Outbreaks can sometimes be traced to

hospital worker Often as a result of improper handwashing or

sterile technique

Patients own normal flora

Invasive treatments often introduce surfaceflora to interior regions of body


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Transmission of infectious agentsin hospitals Medical devices

Devices routinely breach first-linebarriers

Catheterization, mechanicalrespirators, and inadequatelysterilized instruments

Healthcare personnel

Handwashing between patientseffective against spread of disease

Airborne Airflow is regulated to specific

parts of hospital Keeps certain areas contained


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Preventing nosocomial infections Most important step is to recognize their

occurrence and establish policies to

prevent their development Infection Control Committee

Committee often chaired by hospitalepidemiologist trained in hospital infection control

Infection control practitioner Active surveillance of types and numbers of

infections occurring in the hospital setting

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