Patterns of Microbe-Human Interactions in Causing Infection and Disease

 Contact, Infection, Disease- A Continuum ◦ Body surfaces are constantly exposed to microbes ◦ Inevitably leads to infection: pathogenic microorganisms penetrate the host defenses, enter the tissues, and multiply ◦ Pathologic state that results when the infection damages or disrupts tissues and organs- disease ◦ Infectious disease: the disruption of a tissue or organ caused by microbes or their products

 Infection is the invasion of the host by a pathogen  Disease results only if the invading pathogen alters the normal functions of the body  Disease is also referred to as morbidity

 Diseases can be classified in number of ways ◦ The taxonomic groups of the causative agent ◦ The body system they affect ◦ Their longevity and severity ◦ How they are spread to their host ◦ The effects they have on populations (rather than on individuals)

The study of epidemics. The study of distribution and prevalence of infectious disease in a given population. The study of comparisons of baseline infections to cases. Interested in protecting the public from outbreaks of infection.

Epidemiologists concerned with virulence, portals of entry and exit, and the course of the disease Also interested in surveillance: collecting, analyzing, and reporting data on the rates of occurrence, mortality, morbidity, and transmission of infections Reportable diseases: by law, must be reported to authorities Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia – Weekly notice: the Morbidity and Mortality Report – Shares statistics with the World Health Organization (WHO)

All outbreaks follow a pattern. If we can understand the pattern, then we can intervene to prevent further cases.

Short term increase in a given infectious disease in a give population.

Presence of infectious disease in a given population or in a given geographical location all the time. Ebola in Africa Lyme disease in Texas Rabies in Austin (bats)

Worldwide or global increase in an infectious disease over a long period of time. HIV/AIDS Tuberculosis

Figure 13.16

Morbidity – number of cases of specific infectious disease in a give population at any given time. Mortality – number of deaths from a specific infectious disease in a given population at any given time.

Prevalence – number of cases at any give time (people are either getting better or becoming infected.) Incidence – number of new cases of a specific infectious disease. This is important because it can tell us how fast the disease is spreading.

◦ Track occurrence of diseases using two measures  Incidence – number of new cases of a disease in a given area during a given period of time  Prevalence – number of total cases of a disease in a given area during a given period of time ◦ Occurrence also evaluated in terms of frequency and geographic distribution

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◦ Exceptions to Koch’s postulates  Using Koch’s postulates is not feasible in all cases  Some pathogens can’t be cultured in the laboratory  Some diseases are caused by a combination of pathogens and other cofactors  Ethical considerations prevent applying Koch’s postulates to pathogens that require a human host  Difficulties in satisfying Koch’s postulates  Diseases that can be caused by more than one pathogen  Pathogens that are ignored as potential causes of disease

Epidemiologists work to figure out what is going on so they can intervene. Every infectious disease has a pattern (not random.) Epidemiologists want to know: Place People Time

◦ Following infection, a sequence of events called the disease process occurs ◦ Many infectious diseases have five stages following infection  Incubation period  Prodromal period  Illness  Decline  Convalescence

Exposure – Preclinical symptoms – before you go to the doctor. You may not realize that you have an infectious agent, but your body has begun to have immune response. A preclinical symptom can be any cytopathic effect. Clinical symptoms – symptoms that can be appreciated. In TB clinical symptoms could include: cough, blood in phlegm, fever, night sweats. Incubation period – spans from exposure to the time when clinical symptoms are detected. Intervention – this is period when people seek medical attention. Options during intervention include: quarantine and medicate individuals that were exposed or vaccination. Outcome – possible outcomes: Get better (convalesce) Death Impairments (mental and physical)

◦ Descriptive epidemiology  Careful tabulation of data concerning a disease  Record information about the location and time of the cases of disease  Collect patient information  Try to identify the index case (or first case) of the disease

◦ Analytical epidemiology  Seeks to determine the probable cause, mode of transmission, and methods of prevention  Useful in situations in which Koch’s postulates can’t be applied  Often retrospective – investigation occurs after an outbreak has occurred

◦ Experimental epidemiology  Involves testing a hypothesis concerning the cause of a disease  Application of Koch’s postulates is experimental epidemiology

 Transmission is either from a reservoir or a portal of exit to another host’s portal of entry  Three groups of transmission ◦ Contact transmission  Direct, indirect, or droplet ◦ Vehicle transmission  Airborne, waterborne, or foodborne ◦ Vector transmission  Biological or mechanical

 Pathogens leave host through portals of exit  Many portals of exit are the same as portals of entry  Pathogens often leave hosts in materials the body secretes or excretes

Communicable disease: when an infected host can transmit the infectious agent to another host and establish infection in that host – Transmission can be direct or indirect – Contagious agent: highly communicable Noncommunicable disease: does not arise through transmission of the infectious agent from host to host – Acquired through some other, special circumstance – Compromised person invaded by his or her own microbiota – Individual has accidental contact with a microbe in a nonliving reservoir

Contact transmission Indirect transmission – Vehicle: any inanimate material commonly used by humans that can transmit infectious agents (food, water, biological products, fomites) – Contaminated objects (doorknobs, telephones, etc.) Food poisoning Oral-fecal route – Air as a vehicle Indoor air Droplet nuclei Aerosols

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Reservoir: the primary habitat in the natural world from which a pathogen originates Source: the individual or object from which an infection is actually acquired Living Reservoirs – Carrier: an individual who inconspicuously shelters a pathogen and spreads ith to others without any notice Asymptomatic carriers Incubation carriers Convalescent carriers Chronic carrier Passive carrier

 Vector: a live animal that transmits an infectious agent from one host to another ◦ Majority are arthropods ◦ Larger animals can also be vectors  Biological vector: actively participates in a pathogen’s life cycle  Mechanical vectors: transport the infectious agent without being infected

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 Zoonosis: an infection indigenous to animals but naturally transmissible to humans ◦ Human does not contribute to the persistence of the microbe ◦ Can have multihost involvement ◦ At least 150 worldwide

 Human hosts in regular contact with environmental sources  Soil  Water