Disease Principles Hugh B Fackrell Filename Disease Principles
Disease Principles • Hugh B. Fackrell • Filename: Disease. Principles. ppt 1 1/29/202
Outline • • • 2 Infection vs Disease Pathogenicity Virulence Pathogenic factors Latency Dormancy Communicability 1/29/202
Saprophyte • nutrient source is non-living • can become parasite 3 1/29/202
Parasitism • host/microorganism interaction • Mutualism - both host and parasite benefit. • Commensalism - the parasite does no damage to the host. • Pathogen - the parasite damages the host. • Opportunism - the parasite takes advantage of the weakened condition of the host. 4 1/29/202
Microbial Infection vs Disease • Infection: colonization of the body with microbe – usually non pathogenic – indigenous or commensal – beneficial • Disease: breach of host defenses – microbes infect tissues not normally exposed 5 1/29/202
Infection: Benefits • Metabolites – E. coli makes vitamin K • Bacterial antagonism – E. coli blocks colonization of gut by S. aureus – evidence: antibiotic sterilization before abdominal surgery – subsequent Staphylococcal infections – enteritis • Encourage immune system – axenic animals have poor immunity 6 1/29/202
Infection: Adverse Effects • Staphylococcal infection – Produces penicillinase – Concurrent infection of Neisseria gonorrhaeae – Becomes disease gonorrhea 7 1/29/202
Microbial Disease • Microbial disease is the exception – imbalance favours the microbe • 3% of all microbes pathogenic – majority of known microbes • >95% do not cause disease – virtually unknown 8 1/29/202
Location of Microbe in Host • Location often decides outcome of Infection vs disease • Streptococcus – Infection: nasopharynx – Disease: heart – bacterimiae after tooth extraction • E. coli – Infection gut – Disease: cystitis in urinary tract 9 1/29/202
Contagious Transmission • can be transmitted from one host to another (communicable) • some infections acquired from indigenous flora are categorized as communicable. 10 1/29/202
Communicability • Communicable: spread directly or indirectly from one host to another – chickenpox, measles, tuberculosis, typhoid fever • Contagious: easily communicable – eg chickenpox, measles, sore throat • Non communicable: Not spread from host to host – tetanus 11 1/29/202
Dormancy • Latency = Dormancy: causative microbe remains inactive in the host for some time but later becomes active to produce the signs and symptoms of the disease 12 1/29/202
Carrier State • Individual infected – results from a previous disease state (may be temporary) – the host is a true carrier • • • microbe in balance with that individual No overt signs or symptoms reservoir for infection of others – Typhoid Mary 13 1/29/202
Koch’s Postulates • The organism should be found in all cases of the disease and its distribution in the body should be in accordance with the lesions observed. • The organism should be cultivated outside the body of the host, in pure culture, for several generations of the pathogen. • The disease should be reproduced in susceptible animals. 14 1/29/202
Etiology Science of the causes of disease • • the nature of the host - species the condition of the host the nature of the disease agent the transmission of the agent 15 1/29/202
Etiology of Disease • Specimen from patient containing infectious agent cultured. • Pure culture obtained and described; identified if possible. • Inoculation and observation of test animal. • Many organs removed and cultured. 16 1/29/202
Etiology of Disease (2/2) • Isolation and identification of the test organism. • Inoculation of a second test animal. • Culture of second test animal. • Antibodies in blood of human or animal. • Immunity developed to the infecting agent in recently recovered animal. • Animal protected by vaccine or toxoid. 17 1/29/202
Pathogenicity • Ability or potential to cause disease attributed to a genus or species • Dependent on ability to – enter the host – adapt and multiply in the host – exit from the host – transmit to new host 18 Keppie Smith 1/29/202
Portal of Entry • • May multiply at entry site> lesion Often enters multiplies elsewhere 2/3 Respiratory 1/3 intestine, urethra, conjunctiva and skin 19 1/29/202
Multiplication in Host • Rate of multiplication different in vivo and in vitro • time to overt symptoms • Carrier state • reservoir gall bladder • Temperature – viruses – aspirin 20 1/29/202
Exit from Host • Usually by same route a entry 21 1/29/202
Bacterial transmission • Droplets • Fomites • Direct contact – sexual – non sexual • Bites – insects – animals 22 1/29/202
Types of Pathogens • pathogens characterized as “frank” Salmonella • opportunistic pathogens - E. coli in urinary tract • non-pathogens - such as Serratia marcescens may become infectious agents 23 1/29/202
Virulence • DEGREE of pathogenicity shown by a specific strain of an organism – C. diptheriae > diptheria • variables include: – number of infecting bacteria – route of entry into body – specific host defense mechanisms – non-specific host defense mechanisms – virulence factors of the bacterium 24 1/29/202
Virulence Measurement • Measures the pathogenicity of a isolate – variable among strains • Measure of Virulence – Median Dose – Minimum Effective Dose 25 1/29/202
Percent Susceptible Median Dose Amount that affects half the population [Pathogen] 26 1/29/202
Cause of Virulence Variation • Dose of pathogen • Virulence/Pathogenic factors 27 1/29/202
Dose of Pathogen • Typhoid Fever – S. typhosa contaminated water – 1 -100 bacteria no effect • Boils – Staphylococcus aureus – just on surface 7 x 10 6 cells – on suture or scratch ~1000 cells – “stictch abcesses” 28 1/29/202
“Pathogenic Factors” • Gene mutation – S. pneumoniae- capsule – Shigella > dysentery • Lysogeny – scarlet fever – diptheria • Capsules – Klebsiella > polysaccharide – Anthrax > protein capsule 29 1/29/202
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