INFECTIOUS DISEASESAETIOLOGY PATHOGENESIS &

CONSEQUENCES

Dr.T.V.Rao MD

in·fec·tiondictionary means

• a. Invasion by and multiplication of pathogenic microorganisms in a bodily part or tissue, which may produce subsequent tissue injury and progress to overt disease through a variety of cellular or toxic mechanisms.

• b. The pathological state resulting from having been infected.

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Definitions• Disease and Infectious Disease

–Disease •Any deviation from a condition of good health and well-being

–Infectious Disease ·A disease condition caused by the presence or growth of infectious microorganisms or parasites

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Definitions• Pathogenicity and Virulence

– Pathogenicity

• The ability of a microbe to cause disease • This term is often used to describe or compare

species– Virulence

• The degree of pathogenicity in a microorganism • This term is often used to describe or compare

strains within a species

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Principles of Infection• Understanding the basic principles

of infection is essential for any health care worker in any field of health care.

1. Disease transmission2. Prevention of disease

transmission

Nature of Microorganisms• Microorganisms (microbes) are small,

living organisms that are not visible to the naked eye.

• Pathogens (germs) are microorganisms that cause disease.

• Non-pathogens are microorganisms that do not cause disease; can be beneficial.

• Infection is the invasion of a host organism's bodily tissues by disease-causing organisms, their multiplication, and the reaction of host tissues to these organisms and the toxins they produce. Infections are caused by microorganisms such as viruses, prions, bacteria, and Viroids, and larger organisms like parasites and fungi.

Infection means

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What is an Infection?A harmful invasion and spread of foreign species, or

pathogen, in a host. – VIRUS

• Small pox, measles, influenza, Ebola – PRIONS

• Cow’s disease– BACTERIA

• Tuberculosis, pneumonia, salmonella, anthrax – FUNGUS

• Athlete’s foot, ring worm– PROTISTS

• Malaria, toxoplasmosis, Algae

COURSE OF INFECTIOUS DISEASEs• Exposure/Invasion of

Host• Incubation -- period of

time between exposure and onset of symptoms -- e.g., interval between HIV infection and development of AIDS can be as long as 10-15 years

• Host reactionDr.T.V.Rao MD 9

Nature of Microorganisms

• Saprophytes • Parasites• Pathogens

Causative Agents effecting humans

Bacteria VirusesFungiProtozoaHelminthsPrions

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• Streptococci (chains)• Staphylococci (clusters)• Diplococci (pairs)• Micrococci (tiny)• Flagellated forms (tails)• Bacilli (rod-shaped)• Vibrios• Spirilla (spiral)• Spirochetes (comma)

Types of Bacteria

• Fungi– A plantlike organism that lives on

dead organic matter.– Yeasts and molds can be pathogenic.– Cause conditions such as ringworm,

athlete’s foot, yeast infections, and thrush.

– Antibiotics do not kill fungi. Antifungal medications are available, but expensive and may cause liver damage.

Types of Microorganisms

Types of Microorganisms

• Protozoa– One-celled animal like organisms often found

in decayed materials and contaminated water.

– Many contain flagella which allow them to move freely.

– Cause diseases such as malaria, trichomonas, and amebic dysentery.

• Viruses– Smallest of all microorganisms – visible only using an

electron microscope.– Cannot reproduce unless they are inside another

living cell.– Spread by contact with blood and other body fluids.– Difficult to destroy. Not affected by antibiotics.– Associated with diseases such as the common cold,

chicken pox, herpes, hepatitis B, measles, warts, polio, influenza, and AIDS.

Types of Microorganisms

Classification of Infections • Primary Infection

Reinfection• Secondary Infection,• Focal infection,• Cross infection, • Nosocomial infections• Iatrogenic infection

Source of Infections• Endogenous

infection•Exogenous infections

Clinical Presentations

• In apparent infection subclinical Infection

• In apparent infection,• Atypical infections• Latent infections

Source of Infection• Humans from patient or carrier• A healthy carrier• Convalescent carrier• Temporary carrier• Contact carrier• Paradoxical carrier

Source of Infections• Animals• Insects vectors• Mechanical vector• Biological vector• Soil and water • Food

MODES OF COMMUNICABLE DISEASE TRANSMISSION

• Direct Transmission

• Indirect Transmission

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Modes of Transmission

• Direct contact• Indirect contact• Droplet

transmission• Hands• Vector-borne• Nosocomial

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Transmission of Pathogens

•Direct contact

•Indirect contacts• Air• Objects

•Vectors

Modes of Transmission

Direct transmission

Indirect transmission

Direct contact Airborne

Droplet spread Vehicle borne

Vector borne

DIRECT TRANSMISSION

• Immediate transfer of the disease agent by direct contact between the infected and the susceptible individuals

• Occurs through such acts as touching, biting, kissing, sexual intercourse, or by direct projection (droplet spread) by coughing or sneezing within a distance of one meter

• Examples of diseases for which transmission is usually direct are AIDS, syphilis, gonorrhea, and the common cold

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INDIRECT TRANSMISSION• May be one of three types: air-borne, vehicle-borne, or

vector-borne• Air-borne transmission -- transmission of microbial

aerosols to a suitable port of entry, usually the respiratory tract– Microbial aerosols are suspensions of dust or droplet

nuclei made up wholly or in part by microorganisms -- may be suspended and infective for long periods of time

– Examples of air-borne diseases include tuberculosis, influenza, Histoplasmosis, and Legionellosis

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INDIRECT TRANSMISSION (cont’d.)• Vehicle-borne transmission -- contaminated

materials or objects (fomites) serve as vehicles, nonliving objects by which communicable agents are transferred to a susceptible host– The agent may or may not have multiplied or

developed on the vehicle– Examples of vehicles include toys,

handkerchiefs, soiled clothes, bedding, food service utensils, and surgical instruments

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INDIRECT TRANSMISSION (cont’d.)

• Also considered vehicles are water, milk, food (e.g., common vehicles), or biological products such as blood, serum, plasma, organs and tissues

• Almost any disease can be transmitted by vehicles, including those for which the primary mode of transmission is direct, such as dysentery and hepatitis

Causing an Infection• Endogenous – disease originates within the body.

Ex: metabolic disorders, congenital abnormalities, tumors.

• Exogenous – disease originates outside the body. Ex: chemical agents, electrical shock, trauma.

• Nosocomial – acquired by an individual in a health care facility (workers to patient).– Many are antibiotic resistant, life-threatening.

• Opportunistic – occur when the body’s defenses are weak. Ex: pneumonia w/AIDs.

Signs & Symptoms of Infection

• Redness• Swelling• Tenderness• Warmth• Drainage• Red streaks leading away from

wound

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Development of Infection: Clinical Signs and Symptoms

• Local signs– Inflammation– Purulent exudate if bacterial infection; serous

exudate if viral– Tissue necrosis– Lymphadenopathy– Respiratory effects

• Systemic signs– Fever, fatigue, headache, nausea

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Generalized Stages of Infection

1. Entry of Pathogen – Portal of Entry

2. Colonization – Usually at the site of entry

3. Incubation Period– Asymptomatic period – Between the initial contact with the microbe

and the appearance of the first symptoms

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Generalized Stages of Infection

4. Prodromal Symptoms– Initial Symptoms

5. Invasive period– Increasing Severity of Symptoms – Fever – Inflammation and Swelling – Tissue Damage – Infection May Spread to Other Sites – Acme (Fastigium)

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Factors predisposing pathogenicity

• Pathogenicity means ability of the microbe to produce disease or tissue injury

• Virulence• May undergo

variation

Virulence and Pathogenicity • Pathogenicity: capacity of microbes to cause

disease• Virulence: degree of pathogenicity of specific

microbe– Based on:

• Invasive qualities• Toxic qualities• Presence of pili or fimbriae for adhesion• Ability to avoid host defenses (mutate)

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Virulence Factors and Toxins• State of the Host

Immune System• Number of

Pathogenic Cells encountered by the Host – Infectious Dose

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Virulence Factors and Toxins

• Enzymatic Virulence Factors– Examples:

• Coagulase (Staphylococcus aureus) • Streptokinase (Streptococcus pyogenes)• Hyaluronidase (Many pathogens)• Collagenase (Many pathogens)• Leukocidin (Many pathogens)• Hemolysin (Many pathogens)

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Virulence Factors and Toxins• Adhesion Factors

– Examples:

• Protein A (Staphylococcus aureus)

• Protein M (Streptococcus pyogenes)

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Virulence Factors

• Virulence factors help bacteria to (1) invade the host, (2) cause disease, and (3) evade host defences. The following are types of virulence factors:

• Adherence Factors: Many pathogenic bacteria colonize mucosal sites by using pili (fimbriae) to adhere to cells.

Virulence Factors

• Invasion Factors: Surface components that allow the bacterium to invade host cells can be encoded on plasmids, but more often are on the chromosome.

• Capsules: Many bacteria are surrounded by capsules that protect them from opsonisation and phagocytosis

Virulence Factors and Toxins• Exotoxins

– A type of bacterial toxin with the following properties:

• May be produced by either gram-positive or gram-negative bacteria

• Is secreted by the bacteria • The action of the exotoxin does not necessarily

require the presence of the bacteria in the host • Most exotoxins are peptide or protein • Most exotoxins are heat sensitive (exception:

enterotoxin of Staphylococcus aureus)Dr.T.V.Rao MD 41

Virulence Factors and Toxins• Exotoxins (cont.)

– Classes of exotoxins: Neurotoxic, cytotoxic, or enterotoxic exotoxins

• Neurotoxins: Interfere with proper synaptic transmissions in neurons

• Cytotoxins: Inhibit specific cellular activities, such as protein synthesis

• Enterotoxins: Interfere with water reabsorption in the large intestine; irritate the lining of the gastrointestinal tract

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Virulence Factors and Toxins

• Endotoxins– A type of bacterial toxin having the following

properties: • Produced only by gram-negative bacteria • Endotoxins are a component of the gram-negative

cell wall • The action of endotoxin requires the presence of

the bacteria in the host. The endotoxin may be released from the cell wall as the cells die and disintegrate

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Virulence Factors and Toxins• Endotoxins (cont.)

• Endotoxin is composed of Lipid A: Part of the lipopolysaccharide layer

• Mode of action: Irritation/inflammation of epithelium, GI irritation, capillary/blood vessel inflammation, hemorrhaging

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Endotoxins• Endotoxins: The

lipopolysaccharide endotoxins on Gram-negative bacteria cause fever, changes in blood pressure, inflammation, lethal shock, and many other toxic events.

Mechanism of Endotoxin activity

Exotoxins• Exotoxins: Exotoxins include several

types of protein toxins and enzymes produced and/or secreted from pathogenic bacteria. Major categories include cytotoxins, neurotoxins, and enterotoxins.

Exotoxins• Exotoxins, unlike the lipopolysaccharide endotoxin,

are protein toxins released from viable bacteria. They form a class of poisons that is among the most potent, per unit weight, of all toxic substances. Most of the higher molecular-sized exotoxin proteins are heat labile; however, numerous low molecular-sized exotoxins are heat-stable peptides. Unlike endotoxin, which is a structural component of all Gram-negative cells, exotoxins are produced by some members of both Gram-positive and Gram-negative genera.

Exotoxins • The functions of these exotoxins for the

bacteria are usually unknown, and the genes for most can be deleted with no noticeable effect on bacterial growth. In contrast to the extensive systemic and immune-system effects of endotoxin on the host, the site of action of most exotoxins is more localized and is confined to particular cell types or cell receptors.

Exotoxins • Tetanus toxin, for example, affects only intern

uncial neurons. In general, exotoxins are excellent antigens that elicit specific antibodies called antitoxins. Not all antibodies to exotoxins are protective, but some react with important binding sites or enzymatic sites on the exotoxin, resulting in complete inhibition of the toxic activity (i.e., neutralization).

Exotoxins• Endotoxin is comprised of toxic

lipopolysaccharide components of the outer membrane of Gram-negative bacteria (see Ch. 2). Endotoxin exerts profound biologic effects on the host and may be lethal. Because it is omnipresent in the environment, endotoxin must be removed from all medical supplies destined for injection or use during surgical procedures.

Exotoxins• The term endotoxin was coined in 1893 by Pfeiffer

to distinguish the class of toxic substances released after lysis of bacteria from the toxic substances (exotoxins) secreted by bacteria. Few, if any, other microbial products have been as extensively studied as bacterial endotoxins. Perhaps it is appropriate that a molecule with such important biologic effects on the host, and one produced by so many bacterial pathogens, should be the subject of intense investigation.

Other Factors • Plasmids• Bacteriophages• Compatibility

Other Bacterial factors • Coagulase• Fibrinolysin• Hyaluronidase

Bacterial Appendages

• Biofilms• Free floating bacteria come in

contact with medical devices and attach to them with pili

Infecting Dose•Minimum infective dose

•Minimal lethal dose

Types of Infectious diseases

•Bacteraemia •Septicaemia•Pyemia

Other types•Endemic disease•Epidemic disease•Pandemic disease

Hospital acquired infections

• Infection which was neither present nor incubating at the time of admission

• Includes infection which only becomes apparent after discharge from hospital but which was acquired during hospitalisation (Rcn, 1995)

• Also called nosocomial infection

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Basic steps in Prevention of Infection

• There are possible treatment and prevention to stop the infection cycle. This is through adequate hygiene, sanitary environment maintenance and health education.

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Antimicrobial agents In Infection

• Anti-infective drugs such as antibiotics, antiviral, antifungal and ant tubercular drugs suppress infection. It can be administered by mouth, topically or intravenously depending on the infection extent and severity. Sometimes, if drug resistance is known, multiple drugs are used to stop drug resistance and increase drug effectiveness. Antibiotics only work for bacterial infection and have no effect on viral ones.

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INSPITE OF MANY ADVANCES THERE IS NO STEP GREATER THAN HAND WASHING IN CONTROL OF INFECTIOUS

DISEASES

• Program Created by Dr.T.V.Rao MD for Medical and Paramedical Professional for Global education on Infectious Diseases

• Email • doctortvrao@gmail.com