infection and host resistance

7/31/2019 Infection and Host Resistance

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Cont. Definition of Terms Symbiosis Refers to the relationship between the

indigenous flora and the host literally.

Commensalism A form of symbiosis in which oneorganism benefits from another without causing harmto it.

Mutualism It is a form of symbiosis in which bothorganisms benefit from the relationship.


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Cont. Definition of Terms Parasitism It is a form of relationship in which one

organism benefits from another and at the same timecauses harm to the other.

Pathogen It is an organism that invades and causesdamage or injury to the host.


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Cont. Definition of Terms Pathogenicity Refers to the ability of an organism to

cause a disease. An organism that can produce diseasein humans is said to be pathogenic.

Contamination This refers to the presence of organisms outside the body, as well as on water, food

and other biological substances.


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Cont. Definition of Terms Pollution Refers to the presence of undesirable

substances in water, air or soil.


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Kochs Postulates This is an experimental procedure developed by Robert

Koch and his colleagues that proves that certainmicroorganisms cause specific disease. Thesepostulates are as follows:

1. The same organism must be found in all cases of a

given disease and must not be present in healthy animals or humans.


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Kochs Postulates 2. The organism must be isolated and grown in pure

culture from the infected animal.

3. The organisms form the pure culture must reproducethe disease when inoculated into a susceptible animal.


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Kochs Postulates 4. The organism must be isolated in pure culture from

the experimentally infected animal.

Once all of the above conditions are fulfilled, we can nowconclude that the organism is indeed the cause of theparticular disease.


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Kochs Postulates Exceptions of Kochs Postulates:

To fulfill Kochs Postulates, it is necessary to grow

(culture) the pathogen in the laboratory (in vitro) in oron artificial culture media. However, certain pathogens will not grow on artificial media. (Ex. Viruses,rickettsias, chlamydias, leprosy & syphilis)


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Cont...Exception To fulfill Kochs Postulates, it is necessary to infect

laboratory animals with the pathogen being studied.However, many pathogens are species-specific,meaning that they infect only one species of animal.

Certain diseases, called synergistic infections, arecaused not by one particular microorganisms but by the combined effects of two or more differentmicroorganisms. It is very difficult to reproduce suchinfection in the laboratory.


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Cont...Exception Certain pathogens become altered when grown invitro. Some become less pathogenic, while othersbecome non-pathogenic. Thus, they will no longerinfect animals after being cultured on artificial media.


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FACTORS THAT INFLUENCE THEOCCURENCE OF INFECTION


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Portal of EntryMucous membranes Many organisms gain entry intothe host by penetrating mucous membranes lining therespiratory tract, gastrointestinal tract, genitourinary tract, and the conjunctiva of the eyes.

Skin The skin, if intact, is relatively impenetrable by

most organisms.


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Portal of EntrySkin Any break in the skin can allow entry of pathogens into the body. Some, such as hookworms,

can gain entry by penetrating intact skin.

Parenteral route Some microorganisms gain entry tothe body when they are deposited directly into thetissues beneath the skin or into mucous membranes when these are penetrated or injured.


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Virulence of the organismMicroorganisms are equipped with certaincharacteristics that can enhance their virulence. Forinstance, there are some organisms that are equipped with a capsule that enables the organism to evadephagocytosis. Other organisms produce specificenzymes and toxins that enable them to producedisease.


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Number of microbesIn general, if only a small number of microorganismsenter the body, they will most likely be overcome by the hosts defenses and therefore do not producedisease. This is not so if a large number of organismsgain entry. The likelihood of disease increases as thenumber of pathogens increases.


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Defensive powers of the hostThe most important factor that affects development of disease is the defensive powers of the host. All of us areprovided with defense mechanisms that can eitherprevent entry of organisms or limit the growth of organisms. Once there is a breakdown of all of thenatural defense mechanisms of the body, there is nostopping microorganisms from entering the body,multiplying within the body and producing disease tothe host.


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Routes of TransmissionContact transmission

- direct

- indirect Vehicle transmission Vectors


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HOW ORGANISMS PRODUCEDISEASE


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MechanicalOrganisms can produce disease by directly damagingtissues or surfaces.This involves colonization of the epithelial surface andpenetration of epithelial cells and deeper tissues.This can be enhanced by the presence of surfacemolecules, such as those on the pili or fimbriae of some bacteria, which promote adherence of theorganism to susceptible cells.


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Chemical A great number of bacteria produce disease as aconsequence of the chemicals and toxins that they produce. (some produce leukocidins which destroy WBC, some produce coagulase, which coagulatesfibrinogen in blood, forming a blood clot)Most important chemical produced by organisms thatcan lead to disease are the toxins.


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Cont...Chemical 2 Major types of toxins contd. 2. Exotoxins are produced inside some bacteria of their

growth and metabolism & are released into thesurrounding medium; mainly proteins & many of them are enzymes (most are produced by Gram-positive bacteria). Exotoxins are soluble in body fluids& are thus easily diffused into the blood and rapidly transported throughout the body.


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Cont...Chemical 3 principal types of Exotoxins:1. Cytotoxins which kill host cells or affect their

functions2. Neurotoxins which interfere with normal nerve

impulse transmission3. Enterotoxins which affect the cells lining the

gastrointestinal tract.


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Cont...Chemical Exotoxins...Diseases produced by exotoxin-producing bacteria are

often due to the effects of the exotoxin and not by thebacteria themselves. It is the exotoxin that producesthe specific signs and symptoms. Exotoxins are,therefore, disease-specific.


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Major differences betweenendotoxins & exotoxins

Property Exotoxin Endotoxin

Bacterial source Mostly from Gram (+) Gram (-) bacteriaRelation to

microorganismMetabolic product of

growing cellPresent in LPS of outer

membrane of cell wall &released only w/

destruction of cellChemistry Protein or short peptide Lipid portion (lipid A) of

LPS of outer membranePharmacology (effect on

body)Specific for a particular

cell structure or functionin the host

General, such as fever, weaknesses, aches, andshock; all produce the

same effectsHeat stability Unstable; can usually be

destroyed at 60- 80C(except staphylococcal

enterotoxin)

Stable; can withstandautoclaving (121C for one

hour)


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Major differences betweenendotoxins & exotoxins

Property Exotoxin Endotoxin

Toxicity (ability to causedisease)

High Low

Fever No YesImmunology (relation to

antibodies)Can be converted totoxoids to immunize

against toxin; neutralizedby antitoxins

Not easily neturalized by antitoxin; not converted

into toxoids

Lethal dose Small Considerably largerRepresentative diseases Gas gangrene, botulism,

diptheria, tetanus, scarletfever

Typhoid fever, urinary tract infections, and

meningococcalmeningitis


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ImmunologicSome organisms produce disease not as a consequenceof mechanical invasion or toxin production, but as aconsequence of the immune response of the host tothe microorganisms or its products. (Ex. The damageto the liver seen in hepatitis infection is not a directeffect of the hepatitis virus, but the response of theimmune system to the virus; the rashes seen inmeasles are due to the specific immune response of thebody to the measles virus.


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Classification of Infectious DiseasesCommunicable disease a disease that spreads from

one host to another, either directly or indirectly.Contagious disease disease that is easily and rapidly spread from one person to another.Non-communicable disease is one that is not spreadfrom one person to another. These are usually causedby organisms that normally inhabit the body andproduce disease only occasionally, or by organisms thatproduce disease only when introduced into the body.


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Classification of Infectious DiseasesFulminating infection is an infection that results inthe death of the patient over a short period of time.(ex. Meningococcemia)Specific infection is one that is caused by amicroorganism that is known.Non-specific infection infection that can be caused

by several organisms.


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Classification of Infectious DiseasesNosocomial infections hospital-acquired infections.Community acquired infections is an infection thatis acquired in the community during the course of ourlives.


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Occurrence of a DiseaseIncidence of a disease refers to the number of peoplein a population who develop a disease during aparticular period of time. It serves as an indicator of the spread of the disease.Prevalence of a disease refers to the number of people in a population who develop a disease at aspecified time, regardless of when it first appeared.


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Occurrence of a DiseaseSporadic disease a disease that occurs only occasionally

Endemic disease a disease that is constantly presentin the population.Epidemic if many people in a given locality develop acertain disease in a relatively short period of time.

Pandemic if the disease has a worldwide occurrence.


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Occurrence of a DiseaseZoonosis a disease primarily infecting lower animals.

Epizoonosis a disease that occurs as epidemic inlower animals.Bacteremia the presence of bacteria in the blood.Septicemia the presence of actively multiplyingbacteria in the blood.Toxemia involves the presence of toxins in the blood.


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Occurrence of a Disease Viremia refers to the presence of virus in the blood.Pyemia refers to the presence of pus-producingbacteria in the bloodstream.


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Severity or Duration of a Disease Acute disease is one that develops rapidly but lastsfor only a short period of time (6 months or less).Chronic disease is one that develops more slowly andoccurs for long periods of time (more than 6 months).Latent disease is one in which the causative organismremains inactive for a time but can become active andproduce symptoms of disease.


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Extent of Host InvolvementLocal infection is one in which the invadingmicroorganisms are limited to a relatively small are of the body (ex. Boils and abscesses)Systemic or generalized infection is one where theinvading microorganisms or their products are spreadthroughout the body by the blood or lymph.Focal infection a local infection enters a blood orlymphatic vessel & spreads to specific parts of thebody, where they are confined to specific areas of thebody.


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Extent of Host InvolvementPrimary infection is an acute infection that causesthe initial illness.Secondary infection is one which is casued by opportunistic pathogens after a primary infection has weakened the bodys defenses. (common to patients with AIDS)


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Predisposing factors A predisposing factor is one that makes the body moresusceptible to the development of a particular disease.

Example: gender, genetics, geographical location,nutritional status, age, lifestyle, habits, occupation,presence of a pre-existing illness, intake of certaindrugs that can affect the immune system, emotionaldisturbances


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Stages of an Infectious Disease1. Incubation period refers to the time intervalbetween the entry of the microorganism and the firstappearance of the signs & symptoms.

2. Prodromal period is relatively short. It ischaracterized by early, mild symptoms of disease which are generally non-specific. (ex. In measles,prodromal period is fever, cough or colds)


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Stages of an Infectious Disease3. Period of illness corresponds to the period of

maximal invasion; the disease is most acute. Patientmanifests signs and symptoms that characterize thedisease.

Fulminant infection when the disease is not

successfully overcome and patient may die of theinfection in a short period of time.

Carrier state patient does not show signs andsymptoms any longer, but still continues to shed theinfecting microorganisms.


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Stages of an Infectious Disease4. Period of decline corresponds to what is known as

the period of defervescence . During this period, thesigns and symptoms of the patient start to subside. It isduring this period that the patient becomes more vulnerable to secondary infection.

5. Period of convalescence it is during this period whenthe patient regains strength and the body returns to itspre-diseased, normal condition. Recovery occurredduring this period.


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Reservoirs of InfectionReservoirs serve as a continual source of disease-producing microorganisms. These can either be livingor non-living organisms. They provide the organisms with conditions that enable the organisms to surviveand multiply and provide opportunity fortransmission.


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Reservoirs of InfectionHuman body principal living reservoir of humandiseaseCarriers specially serve as important livingreservoirs, do not exhibit signs or symptoms of diseaseunknowingly transmit the organisms to otherindividuals.


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Reservoirs of InfectionTransmission of zoonotic infections to humans

through:1. Direct contact with infected animal2. Direct contact with animal wastes3. Contamination of food and water by animal wastes4. Bite of the infected animal5. Air from contaminated hides, furs or feathers6. Ingestion of infected animal products

7. Insect vectors


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Reservoirs of InfectionNon-living reservoirs can be varied.Two major non-living reservoirs: water & soil.


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Routes of transmissionContact transmission - refers to the spread of apathogenic organism through direct contact (person toperson), indirect contact (causative agent fromreservoir to a susceptible host through non-livingobject-fomite), or droplet transmission (throughdroplet nuclei that travel only short distances usually less than a meter).


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Routes of transmission Vehicle transmission refers to transmission of organisms through media such as food, water, air, milkor biological substances such as blood & body secretions. Vectors are animals that carry organisms from onehost to another.


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HOST RESPONSE TO INFECTION


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Host defense mechanisms

Nonspecific hostdefense mechanisms

Specific host defensesmechanisms

Third line of defenseFirst line of

defense

Second line

of defense


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Non-specific Host Defense

MechanismsThey are general and serve to protect the body againstmany harmful substances.One of the nonspecific host defenses is the innate, orinborn, resistance observed among some species of animals, some races of humans, and some persons whohave a natural resistance to certain diseases.Innate or inherited characteristics make these people

& animal more resistant to some diseases than toothers.


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First line of defenseSkin and mucous membranes as physical barriersCellular and Chemical Factors

Microbial Antagonism


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Second line of defenseTransferrin a glycoprotein synthesized in the liver,has a high affinity for iron. Its normal function is tostore and deliver iron to host cells. It serves as anonspecific host defense mechanism by sequesteringiron and depriving pathogens of this essentialnutrient.


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Second line of defenseFever The resulting increased body temperature isconsidered to be a nonspecific host defensemechanism. Substances that stimulate the productionof fever are called pyrogens or pyrogenic substances .


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Second line of defenseInterferons are small, antiviral proteins produces by virus-infected cells. They are not virus-specific,meaning that they effective against variety of viruses,not just the particular type of virus that stimulatedtheir production. They are species-specific, meaningthat they are effective only in the species of animal thatproduced them.


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Second line of defenseThe Complement System The primary importance isthat activation of the complement system isconsidered a nonspecific host defense mechanism; itassists in the destruction of many different pathogen.


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Second line of defenseMajor consequences of complement activation:Initiation & amplification of inflammation Attraction of phagocytes to the sites where they areneeded (chemotaxis) Activation of leukocytesLysis of bacteria and other foreign cells

Increased phagocytosis by phagocytic cells(opsonization)


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Second line of defense Acute-Phase Proteins they are plasma levels of molecules that increase rapidly in response toinfection, inflammation, and tissue injury. They serveas host defense mechanisms by enchancing resistanceto infection and promoting the repair of damagedtissue.


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Second line of defenseCytokines they are chemical mediators that arereleased from many different types of cells in thehuman body. They enable cells to communicate witheach other. They act as chemical messengers both within the immune system and between the immunesystem and other systems of the body.


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Second line of defenseInflammation body normal response to any localinjury, irritation, microbial invasion, or bacterial toxin.

3 major events in acute inflammation:1. Increase in the diameter of capillaries2. Increased permeability of the capillaries3. Exit of leukocytes from the capillaries and their

accumulation at the site of injury


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Second line of defensePhagocytosis the process by which phagocytessurround and engulf (ingest) foreign material. The twomost important groups of phagocytes in the humanbody are macrophages and neutrophils; they aresometimes called professional phagocytes becausephagocytosis is their major function.

infection and host resistance

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