Chapter 16: Mechanisms of Infectious Disease

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Infectious Disease

the disease state brought about by the interaction with another organism

Host

any organism capable of supporting the nutritional and physical growth requirements of another

Colonization

the presence and multiplication of a living organism on or within the host. An example, at the onset of a cold, a respiratory virus causes illness, it has colonized and is multiplying

Microflora

the bacteria inhabiting exposed surfaces of the body. An example would be micro-organisms found on the surface of your skin

Pathogen

micro-organisms so virulent that they are rarely found in the absence of disease

Saprophyte

free-living organisms obtaining their growth from dead or decaying organic material from the environment. There are typically harmless but like all micro-organisms including normal flora, they too can cause opportunistic disease in a weaken host

Virulence

the organism has disease-inducing potential

Mutualism

the microorganism and the host both derive from the interaction, for example the flora of the gut

Commensalism

a situation where organisms acquire nutrition needs and shelter but the host body is not affected for example normal skin flora

Parasitism

a situation whereby only the invasive organism benefits from the relationship for example, a tapeworm

Prions

protein particles that have the capability to cause infectious disease but are unique that they do not contain RNA and DNA or a genetic master plan when reproducing or coding for essential proteins and enzymes necessary in most cases for survival, Examples: Creutzfeldt-Jakob disease and Mad Cow disease. They typically cause neuronal degeneration, ataxia, dementia and death

Viruses

are the smallest pathogens, they are unique as they have no organized cellular structure, they have a protein coat surrounding a nucleic acid core of DNA or RNA, but never both, incapable of replication outside a living cell, they depend on living cells for reproduction. Examples: Rotovirus', herpes virus', CMV, EB virus

Parasites

infect and cause diseases in animals and humans. Animals can also transmit disease to humans by parasitic infection. Protozoans (transmitted by sexual contact, food water or arthropod vector such as through a mosquito) Examples: malaria, amebic dysentery, Helminths (roundworms, tapeworms, flukes etc.) Arthropods (ticks, mosquito, flea vectors)

Bacteria

classified according to microscopic appearance, as either aerobic or anaerobic, according to staining of the cell

Rickettsiaceae, Anaplasmataceae, Chlamydiaceae, and Coxiellai

oraganisms that combine charateristics of viral and bacterial agents to produce disease in humans. Obligate intracellular pathogens like the viruses, produce a rigid peptidoglycan cell wall, reproduce asexually by cellular division, contain RNA and DNA similar to the bacteria

Fungi

saprophytes found on all creatures and part of the normal flora of humans, they are usually harmless however can cause disease in a weakened host, for example: dermatophytes, ringworms, candida albicans, they are divided into two groups: yeast and molds that reproduce by budding, because their cell wall is unlike bacteria, antibiotics do not work

Epidemiology

the study of factors, agents and circumstances that influence the transmission of infectious disease among humans

Incidence

the number of new case of an infectious disease that occur within a defined population

Prevalence

the number of active cases at any given time

Endemic

found in a particular geographic region, the incidence and prevalence are expected relatively stable

Epidemic

abrupt and unexpected increase in the incidence of disease over endemic rates

Pandemic

spread of disease beyond continental boundaries

Organisms Portal of Entry

penetration-disruption of bodily integrity causing potential for invasion i.e. surgical incision, cut. Direct contact-exposure of pathogens with intact membranes i.e. STDs. Ingestion-invasion via oral or GI pathway. Inhalation-invasion by inhalation i.e. bacterial PNA, meningitis (2), TB, flu, measles, mumps, chicken pox, the common cold

Infectious Disease defined by Location

Nosocomial infection develop in hospitalized patients. Community-acquired infections are acquired outside of healthcare facilities. A host may be the victim or an object, or substance from which the infectious agent was acquired

Specific

reflects the site of infection (e.g., diarrhea, rash, convulsions, hemorrhage, pneumonia)

Nonspecific

can be shared by a number of diverse infectious diseases (e.g., symptoms such as fever, myalgia, headache)

Obvious

predictable patterns (e.g., chicken pox and measles)

Covert

may require lab testing to detect (e.g., hepatitis or increased white blood cell count)

Incubation Period

phase during which the pathogen begins active replication without producing recognizable symptoms in the host, the duration can be long or short in some cases

Prodromal Stage

initial appearance of symptoms in the host, although the clinical presentation during this time may be only a vague sense of malaise. The host may experience mild fever, myalgia, headache, and fatigue. The duration can vary considerably from host to host.

Acute Stage

period which the host experiences the maximum impact of the infectious process corresponding to rapid proliferation and dissemination of the pathogen. During this phase, toxic byproducts of microbial metabolism, cell lysis, and the immune response mounted by the host combine to produce tissue damage and inflammation. The symptoms of the host are pronounced and more specific than in the prodromal stage, usually typifying the pathogen and sites of involvement

Convalescent Stage

characterized by by the containment of infection, progressive elimination of the pathogen, repair of damaged tissue, and resolution of associated symptoms. The duration can either long or short depending on the type of pathogen and the voracity of the host's immune system.

Resolution Stage

the total elimination of a pathogen form the body without residual signs or symptoms of disease

Virulence Factors

substances or products generated by infectious agents that enhance the ability to cause disease

Exotoxins

proteins released during bacteria growth that damage/kill host cells (botulism, tetanus, diptheria)

Endotoxins

potent activators from gram negative bacteria that cause problems such as endotoxemia

Adhesion Factors

no interaction between microorganisms and humans can progress to infection or disease if the pathogen is unable to attach to and colonized the host

Evasive Factors

work to counteract the immune system or take advantage of immune incompetence (H. pylori, HIV)

Invasive Factors

penetrate anatomical barriers allowing entry

Antibacterial Agents

usually called antibiotics. Most antibiotics are usually produced by other microorganisms, primarily bacteria and fungi, as byproducts of metabolism and usually are effective only against other prokaryotic organisms

Antiviral Agents

viral replication requires the use of eukaryotic host cells enzymes, and the drugs that effectively interrupt viral replication are likely to interfere with host cell reproduction as well

Anti-fungal Agents

the target site of the two most important families are the cytoplasmic membranes of yeast or molds, they differ from human cells membranes in that they contain the sterol ergosterol instead of cholesterol

Anti-parasitic Agents

because of the extreme diversity of human parasites and their growth cycles, a review of therapies and agents would be highly impractical and lengthy. Similar to other infectious diseases caused by eukaryotic microorganisms, treatment of parasitic illnesses is based on exploiting essential components of the parasite's metabolism or cellular anatomy that are not shared by the host

Bacterial Resistance Mechanisms

inactivate antibiotics, genetically alter antibiotic binding sites, bypass antibiotic activity, changes in bacterial cell wall of bacteria cause resistance

Intravenous Immunoglobin & Cytokine Therapy

methods supplementing or stimulation the host's immune response so that the spread of a pathogen is limited or reversed from example, the use of IgG IV. Pathogen-specific antibodies infusions facilitate neutralizations, phagocytosis, and clearance of infectious agents above and beyond the capabilities of the diseased host

Nonpharmacologic Intervention

providing access to an infected site by antimicrobial agents (drainage of an abscess), cleaning of the site (debridement), removing infected organ or tissue (e.g., appendectomy)

Antibiotic Mechanisms

inference with a specific step in bacterial cell wall synthesis, inhibition of bacterial protein synthesis, interruption of bacterial nucleic acid synthesis, interference with normal bacterial metabolism

Bactericidal

causes irreversible and lethal damage to the bacterial pathogen

Bacteriostatic

inhibitory effects on bacterial growth are reversed when the agent is eliminated

Category A Agents

plague, tularemia, smallpox, hemorrhagic fever viruses

Category B Agents

agents of food-borne and water-borne disease, zoonotic infections, viral encephalitides, toxins from castor bean

Category C Agents

mycobacterium tuberculosis, nipah virus and hantavirus, tick-borne and yellow fever viruses, cryptosporidium parvum

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