Chapter 10 Mechanisms of Infectious Disease.docx

Chapter 10 Mechanisms of Infectious Disease - Triad of disease model - Infectious diseases result from the interaction of an agent, a susceptible host, and environmental conditions that promote infection. - Host -- any organism capable of supporting the nutritional and...

Chapter 10 Mechanisms of Infectious Disease - Triad of disease model - Infectious diseases result from the interaction of an agent, a susceptible host, and environmental conditions that promote infection. - Host -- any organism capable of supporting the nutritional and physical growth requirements of another. - Inection -- the presence and multiplication within a host of another living organism with subsequent injury to the host. - Colonization -- the act of establishing a presence. - Normal microflora -- the multitude of bacteria harmlessly inhabited within the internal and exposed surfaces of the body. - Commensalism -- when the host is not adversely affected by the colonizing bacteria acquiring nutrition from it. - Commensal flora -- colonizing microorganisms - Mutualism -- an interaction in which both the microorganism and the host derive benefits from the interaction. - Example: Microorganisms in the intestinal tract extract nutrients from the host and secrete essential vitamin by-products of metabolism (e.g., vitamin K) that are absorbed and used by the host. - Parasitic relationship -- a relationship in which only the infecting organism benefits from the relationship and the host either gains nothing or sustains injury from the interaction. - Infectious disease -- injury or pathologic damage sustained as a result of a parasitic relationship between an infecting organism and its host. - Virulence -- disease-producing potential of a microorganism. - Pathogens -- microorganisms capable of causing disease. - The severity of infectious disease depends on many variables, including: - The health of the host at the time of infection - The virulence of the microorganism - Environmental conditions - Saprophytes -- harmless, free-living organisms obtaining their growth from dead or decaying organic material in the environment. - Opportunistic pathogens - microorganisms (including saprophytes and normal flora) capable of producing an infectious disease when the health and immunity of the host are weakened by illness, malnutrition, or medical therapy. - Common diseases caused by viruses: - Influenza - The common cold - Measles - HIV/AIDS - Common diseases caused by bacteria: - Strep throat - Some sinus and lung infections - Some food poisoning - Common diseases caused by fungi: - Athlete's foot - Yeast infections - Common diseases caused by protozoa: - Malaria - Sleeping sickness - Common diseases caused by worms: - Roundworms - Tapeworms (helminths) - Common diseases caused by prions: - Creutzfeldt-Jakob disease (associated with other neurodegenerative conditions) - Agents of infectious disease - Prions - Viruses - Bacteria - Fungi - Parasites - Prions -- protein particles that are able to transmit infection by self-propagation - Prion-associated diseases: - Creutzfeldt-Jakob disease - Multiple system atrophy - Kuru in humans - Prion-associated animal disease: - Bovine spongiform encephalopathy (mad cow disease) - Transmissible neurodegenerative diseases -- prion-associated diseases that are characterized by a slowly progressive, noninflammatory neuronal degeneration leading to loss of coordination (ataxia), dementia, and death over a period ranging from months to years. - The conversion of a cellular precursor protein (PrP^C^) into an abnormally folded protein (PrP^SC^) causes the protein to behave differently. - The PrP^SC^ is resistant to the action of proteases (enzymes that degrade excess or deformed proteins). - Accumulation of these misfolded proteins becomes toxic to cells; however, as they aggregate, they become less toxic to the cell and can then be captured in plaques, tangles, or inclusion bodies. - The stable pathogenic structure of PrP^SC^ is resistant to treatment. - Prions can reproduce despite the absence of genetic material. - It is believed that PrP^SC^ aggregates into amyloid-like plaques in the brain and spreads within the axons of the nerve cells, causing progressively greater damage of host neurons and the eventual incapacitation of the host. - Human transmission occurs primarily from eating infected meat or receiving an infected transplant organ or cornea.