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Unformatted text preview: Chapter 33 & 36 (parts): Chapter 33 & 36 (parts): Epidemiology and Pathogenesis Pathogens Pathogens Pathogens are capable of causing disease in an otherwise healthy host with a normal immune system Opportunistic pathogens cause disease when the host is compromised or the bacteria find themselves in an unusual location Pseudomonas aeruginosa in burn patients: Physical/chemical barriers are down Organism invades epithelial tissues and can also easily penetrate to the bloodstream Defining a Pathogen Defining a Pathogen Recall Koch’s postulates of disease: The specific causative agent must be present in all cases of the disease. The agent must be isolated from host & grown in pure culture. When inoculated into susceptible host, this agent must cause the same disease. The agent must be reisolated from the inoculated, diseased host. CDC Classifications of CDC Classifications of Pathogens Biosafety level 1: Not known to cause disease
Biosafety level 2: Moderate risk Organisms which could cause disease by ingestion, mucus membrane exposure, or percutaneous injury Examples: Salmonella, Listeria, Yersinia Organisms with possible aerosol transmission and/or which may have serious or lethal consequences Examples: West Nile Virus, Hanta Virus, SARS Virus Organisms which pose lifethreatening diseases, aerosol transmitted organisms, and organisms with unknown modes of transmission Examples: Ebola Virus, Smallpox Virus Biosafety level 3: High risk Biosafety level 4: Highest risk Disease Terms Disease Terms Contamination: microorganisms are present Infection: multiplication of pathogen in or on host Infestation: presence of larger parasites such as worms or arthropods, in or on host Disease: a disturbance in the state of health of a body Pathogenicity: capacity to produce disease Virulence: the measure of pathogenicity Avirulent: unable to cause disease Attenuation: the weakening of the diseaseproducing ability Sepsis: presence of bacteria or their toxins in the blood or tissues; does not have to be pathogen presence Septicemia: Blood poisoning associated with persistence of pathogens and/or their toxins in the bloodstream Septic shock: sepsis with hypotension Systemic: spread throughout the body Epidemic: disease affects many people in a very short time; example: cholera Pandemic: a worldwide epidemic; example: AIDS Endemic: disease is always present and infects about the same percentage of the population year after year; example: chicken pox Epidemiology Terms Epidemiology Terms Sporadic: an occasional case of the disease is observed and is isolated to specific individuals Outbreak: a cluster of cases within a brief time period and within a specific geographical region; can develop into epidemics if uncontrolled spread occurs Morbidity: illness and disability due to disease Mortality: death due to a disease Specific types of outbreaks: Common source: All infections stem from contact with one source of the infection, often at the same time Example: a case of food poisoning Church picnic and the contaminated potato salad Propagated: Infections stem from a single infected person or a carrier who contacts new individuals, often at various points in time, and pass on the disease. Example: SARS transmission Aerosol contact in planes, trains, office buildings, etc. Determinants of Infectious Disease and Disease Progression Determinants of Infectious Disease and Disease Progression
1. Portal of Entry The pathogen must find an entry into its host Portals:
Urogenital tract Respiratory tract Gastrointestinal tract Skin Wounds 2. Modes of transmission: The organism must have an established means of continuing its life cycle or future generations Nosocomial infections: acquired while in hospital Communicable diseases: infected host transmits disease to new hosts Carriers: individuals that harbor pathogen subclinically Reservoir: primary natural habitat from which pathogen originates Animals or insects which serve as reservoirs are called vectors – biological vs. mechanical Transmission Routes Transmission Routes Airborn: Respiratory pathogens Droplet infections, aerosols Physical barrier broken by cut, wound, animal bite Usually associated with skin and underlying tissues, but can be disseminated through the body Entry through insect bite/wound via saliva injection, defecation or regurgitation into wound Poor hygiene or sanitation Incomplete cooking Direct contact: Arthropodborne: Oralfecal contamination (Waterborne/Foodborne): Common Vectors: Disease & Common Vectors: Disease & Transmission
Organism Insects Disease(s) Malaria West Nile Virus Transmission via Bites (saliva injection) Bites (saliva injection) wound Mammals Birds Rabies Psittacosis Influenza Salmonella Salmonella African Sleeping Sickness Regurgitation of blood in Bites (saliva injection) Direct contact / airborn Airborn Oralfecal contact Oralfecal contact Reptiles & Amphibians Determinants of Infectious Disease and Disease Progression Determinants of Infectious Disease and Disease Progression
3. Size of inoculum (level of contamination) Varies for different pathogens < 10 cells for bacillary dysentery (Shigella) 1000 cells for gonorrhea 1,000,000 cells for cholera 4. Attachment and Colonization Once transmitted into the host, the pathogen has to manage to stay Attachment is aided by the presence of virulence factors like: Fimbriae Glycocalyx Surface proteins Adhesinreceptor binding Colonization refers to ability of microbe to reproduce itself inside the host, but not cause damage in doing so 5. DAMAGE! Determinants of Infectious Disease and Disease Progression Determinants of Infectious Disease and Disease Progression
Invasion into cells or tissues: often involves specific enzymes to counter host defenses Coagulase causes fibrinogen in plasma to coagulate, protecting Staphylococcus aureus from phagocytes Collagenase hydrolyzes collagen in connective tissue Fibrinolysin breaks down clots, allowing some bacteria to leave clotted area Hemolysins and leukocidins lyse RBC’s as source of iron and WBC’s to escape phagocytosis Multiplication within body cells which leads to tissue destruction and organ failure Rickettsia prowazekii is an intracellular parasite of endothelial cells Production of endotoxin causing vasodilation (drop in blood pressure), endotoxic shock, and vascular collapse Associated with Gram negatives and lipopolysaccharides Released only when cell lyses Endogenous pyrogens Determinants of Infectious Disease and Disease Progression
5. DAMAGE! (cont.) Production of exotoxins which are excreted and directly damage tissues Diphtheria toxin:
Single molecule sufficient to kill cell: AB exotoxin B portion binds receptor A portion has catalytic activity Catalyzes ADPribosylation of EF2 Stops protein synthesis Main targets are heart and respiratory tract Other exotoxins: Cholera toxin: Also an AB exotoxin Activates adenylate cyclase Causes hypersecretion of Cl ions and water loss Neurotoxins Enterotoxins Sitespecific exotoxins: Membrane–disrupting toxins: Hemolysins and leukocidans Determinants of Infectious Disease and Disease Progression Determinants of Infectious Disease and Disease Progression 6. Portals of exit Success of pathogen depends on its ability to move to new host Sometimes the exit portal is the same as the portal of entry (respiratory pathogens) Sometimes the exit portal is different than portal of entry Enteric pathogens have an oralfecal route: enter through ingestion orally and exit with fecal material Viruses: Evading Host Defenses Evading Host Defenses Envelope themselves in host’s own cytoplasmic membrane
Antigens presented on surface look like own and virus escapes antigenic detection Antigenic shift of segmented viruses Influenza AIDS virus infects T cells Suppress host’s defense cells through infection Bacteria: Capsules help evade complement system activation which aids in phagocytosis Some bacteria are capable of surviving in macrophages, monocytes, or neutrophils after phagocytosis Phase variation in surface antigens (flagella or capsules) switch one gene off and turn on a duplicate gene with slight protein differences Past, Present & Future of Disease Past, Present & Future of Disease Measles kills 1.5 million children each year Single dose of vaccine costs 12¢ As of 1995, more than 14 million children die each year from: measles, whooping cough, tetanus, diarrhea, and pneumonia Most of these diseases are vaccinepreventable This is enough people to fill Sanford stadium for 6 home football games a year for 25 years At least 4 factors complicate eradication: Available medical expertise not always applied Infectious agents highly adaptable Changes in human activities or social conditions promote emerging diseases Example: parents not having children immunized because disease is ‘rare’ these days results in lessened herd immunity and outbreaks of disease Immigration and foreign travel introduce new or recurring strains of pathogens: the world is a global village ...
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This note was uploaded on 03/24/2009 for the course MIBO 3500 taught by Professor Dustman during the Fall '09 term at University of Georgia Athens.
- Fall '09