Host Pathogens Disease

1. HOST-PATHOGEN
RELATIONSHIPS

2. Terms "infection" and "disease" are not
synonymous
Infection 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.

3. Chain of infection

4. 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

5. 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.

6. 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.

7. 7
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
Black (2008)

8. 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,
water
Source and Reservoir: example:
Yellow fever in jungle areas:
• reservoir = monkeys
• source = mosquitoes (vector)
carrying virus from infected
individuals

9. 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.

10. 10
Most worrisome infectious agents
are those that are both very
contagious and very virulent.
Black (2008)

11. 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

12. 12
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
Black
• microorganisms entering common bile duct from GI tract (2008)
• S/S abdominal pain, jaundice

13. 13
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
Black (2008)

14. 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

15. 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.

16. 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

17. 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

18. 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

19. 19
Spread of Infectious Agents In
Body via the bloodstream
• Travel
– 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)

20. The Natural Course of Disease
sequence of events in infectious
disease
ncubation
interval
between
exposure &
1st
appearance
of disease
s/s
Prodromal
1st
disease s/s
Fastigium Defervescence Convalescence
disease
at its peak
.
disease s/s
decline
recovery
period
Defection
pathogen
killed off or
brought into
remission

21. Number of microorganisms or
intensity of signs or symptoms
FIGURE 14.10 THE STAGES OF INFECTIOUS DISEASES
Incubation Prodromal
period
period
(no signs or (vague,
symptoms) general
symptoms)
Illness
(most severe signs
and symptoms)
Time
Decline
Convalescence
(declining signs (no signs or
and symptoms) symptoms)

22. 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)
22

23. 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