HOST-PATHOGEN RELATIONSHIPS

Terms "infection" and "disease" are not synonymousInfection results when a pathogen invades and begins growing within a host.

Disease results when, as a consequence of invasion and growth of a pathogen, tissue function is impaired. •Our bodies have defense mechanisms to prevent infection and, should those mechanisms fail, to prevent disease after infection occurs.

Chain of infection

Host Infection and Pathogens

Occurrence and Spread of Infection:• Infection occurs when micro-organismsm (pathogen) invade sterile

body tissues• infectious disease occurs when infection is associated with

clinically manifested tissue damage• Colonization- presence of organisms on a body surface or

in a lumen, but not producing disease• All persons have bacteria (and some fungi) on skin surfaces or

in the oral cavity

• Invasion of tissues (avoids immune system) organisms have moved into tissues to cause disease

• Toxin production (endotoxin or exotoxin) toxins can be produced in body or ingested

Host• A person (or animal) who permits lodgment of an

infectious disease agent under natural conditions. • Once an agent infects the host, the degree & severity of the

infection will depend on the host’s ability to fight off the infectious agent.

Defense mechanisms present in host:

• Resistance: ability of the host to prevent infection from occurring & infectious disease from developing

• Resistance is normally aided by:• Barriers to infection: intact, functional epithelial surfaces (respiratory

tract, gastric acid, antibacterial action of bladder secretions and saliva of oral cavity)

• Immune system

• Nonspecific • Examples: skin, mucosal surfaces, tears, saliva, gastric juices, & the

immune system.

• Nonspecific defense mechanisms such as immunity may decrease as we age.

• Disease-specific• Immunity (resistance) against a particular agent.

Resistance• Resistance diminished by:

• Debilitation from malnutrition (poor diet, alcoholism)• Cancer• Poorly functioning immune system (congenital or acquired)• Drug therapy – corticosteroids, antibiotics• Previously damaged or abnormal anatomical structure

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Environment: Source & Reservoir• Source: immediate site from

which pathogen transferred to host. Can be human, animal, inanimate (water, soil, food)

• Reservoir: where infectious organisms live & multiply; • humans often reservoir and host.• Common ones: feces, food,

waterSource and Reservoir: example:

Yellow fever in jungle areas: • reservoir = monkeys• source = mosquitoes (vector)

carrying virus from infected individuals

Why do pathogens cause disease and some don't?

• Virulence! the ability of an organism to cause infectious disease

• Some infectious agents are easily transmitted (very contagious), but they are not very likely to cause disease (not very virulent). • Ex: polio virus : probably infects most people who contact it, but only

about 5 - 10 % of those infected actually develop clinical disease.

• Other infectious agents are very virulent, but not terribly contagious.

• Ex; Ebola hemorrhagic fever virus virulence very high (50 - 90 % fatality rate among those infected); however, virus not transmitted easily by casual contact.

Most worrisome infectious agents are those that are both very contagious and very virulent.

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The Movement of Pathogens Out of Hosts: Portals of Exit to Portals of Entry

• 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

Sites of Entry / Cause / S/S• Ingestion into gastrointestinal tract;

• microorganisms contaminating food or water • Salmonella, Vibrio cholera

• S/S abdominal pain, nausea, vomiting, diarrhea

• Inhalation into respiratory tract; • microorganisms in air• S/S cough, chest pain, shortness of breath, coughing blood

• Ascension into urinary tract; • microorganisms that enter bladder through urethra or catheter• S/S painful urination, blood in urine, pelvic pain, flank pain

• Ascension into biliary tree• microorganisms entering common bile duct from GI tract• S/S abdominal pain, jaundice

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Sites of Entry / Cause / S/S• Crossing of mucosal surfaces

• microorganisms that penetrate oral, anal, genital, or conjunctival linings• S/S Human papillomavirus, HIV, herpes simplex virus, Neisseria

gonorrhea

• Experience local irritation, ulceration, pain, redness

• Entrance through wound sites• Direct inoculation of micro-organisms leads to direct spread

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Three groups of transmission• Contact transmission• Vehicle/ fomite transmission• Vector transmission

• Biological or mechanical

Fomites: inanimate objects that transmit disease

Vector: insect or small animal carrier of disease

Contact transmission• Direct: (person-to-person): through direct physical contact

(skin-to-skin or body fluids) eg.., STDs, cold sores• Indirect: pathogens or agents transferred via intermediate

item, organism etc. to susceptible hosts.

Vehicle/ fomite transmission

Airborne: pathogen suspended in droplets or dust; can remain in air for hours to days

e.g., cold virus travels in droplets when person sneezes, talks

Vehicle-borne: pathogen transmitted from source to susceptible individual via intermediate object (fomite)

e.g., contaminated medical equipment

Vehicle/ fomite transmission

Food or Waterborne: Pathogen transmitted to susceptible individual via or food water. • intoxication: Chemical or toxin causes body malfunction.

• e.g., lead, botulism

• infection: Ingestion of pathogenic organism that grows in GI tract. • e.g., V. cholera transmitted in water sources; E. coli in hamburger

Vector borne transmission

Pathogen transmitted to susceptible individual via animal or insect. • mechanical transmission (external): use

a host for transport;• e.g., fly landing on food at picnic after it

picked up some e.coli

• biologic transmission (internal) carried inside vector; use vector for part of life cycle • e.g. malaria parasite Plasmodium in

blood ingested by Anopheles mosquito after bite

Spread of Infectious Agents In Body• Travel via the bloodstream

– Septicemia

• Travel via the lymphatic system– Enlarged tender lymph nodes suggest possible infection at site

• Travel via the body cavity– Can spread in cerebrospinal fluid, peritoneal fluid, joint space

• Crossing of the placenta to fetus– Basis for congenital infection

Black (2008)

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The Natural Course of Disease sequence of events in infectious disease

Incubation Prodromal Fastigium Defervescence Convalescence Defection

interval between exposure & 1st appearance of disease s/s

1st disease s/s

disease at its

peak.

disease s/sdecline

recovery period

pathogen killed off or brought into remission

FIGURE 14.10 THE STAGES OF INFECTIOUS DISEASES

Incubationperiod

(no signs orsymptoms)

Prodromalperiod(vague,general

symptoms)

Illness(most severe signs

and symptoms)

Decline(declining signsand symptoms)

Convalescence(no signs orsymptoms)

Time

Nu

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of

mic

roorg

an

ism

s o

rin

ten

sit

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f sig

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ms

Barriers to Decreasing Disease Transmission

• Increases in host susceptibility due to changes in demographics and behavior

• Microbial adaptation and change (eg. drug resistance)• Emergence of new diseases• Breakdown of public health measures• International travel and commerce• Changes in the environment, technology, and industry

Cohen, M. L. (2000). Changing patterns of infectious disease. Nature, 406(6797), 762-767. doi:10.1038/35021206)

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Human Microbiome Project (HMP)aims to characterize the microbial communities found at several different sites on the human body, including nasal passages, oral cavities, skin, gastrointestinal tract, and urogenital tract, and to analyze the role of these microbes in human health and disease.

Human Microbiome Project: Analyzing microbes that play a role in health and disease

http://www.youtube.com/watch?v=axB_8O4WHYg

3 min