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 thepattern, 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 AfricaLyme disease in TexasRabies in Austin (bats)

Worldwide or global increase in an infectious disease over a long period of time.

HIV/AIDSTuberculosis

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

[INSERT FIGURE 14.17]

◦ 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:

PlacePeopleTime

◦ 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)

-----------Incubation period ---------- exposure preclinical clinical Tx outcomes symptoms symptoms (treatment) 1. convalese 2. death 3. impairment

◦ 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

[INSERT FIGURE 14.13]

• 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

[INSERT TABLE 14.10]

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