Download Giardia lamblia

THE PROCESS OF INFECTION
• The growth/multiplication of a microbe in a
host
• Infection does not always result in injury of
the host (disease)
• Two main classes of infection by site:
– localized
– generalized (disseminated; systemic)
Localized Infections
• Organism enters the body and reaches target site of infection
• Organism adheres to or enters host cells and multiplies at site of
infection
• Infection spreads within the site (e.g., respiratory tract; intestines)
• Symptoms of illness appear
• Organism does not spread through the lymphatic system or reach
the bloodstream
• Infection subsides due to host defenses (e.g., immunity)
• Agent eliminated from the body; infected cells replaced; "cure"
Generalized Infections
• Organism enters the body and reaches target site of initial infection
• Organism adheres to or enters host cells and multiplies at initial site
of infection
• Infection spreads within site and to other sites via tissues, lymphatic
system, bloodstream (bacteremia, viremia, etc.) and possibly other
routes
• Symptoms of illness may appear
• Organisms infect other organs, tissues and cells; more spread via
bloodstream
• Symptoms of illness become severe
• Host defenses eliminate organisms leading to cure or disease
continues, possibly leading to irreversible damage or death
FACTORS INFLUENCING EXPOSURE
AND INFECTION: Agent Factors
Sources, Reservoirs, Transport and Persistence (in
the Environment)
Ability to Enter a Portal in the Human or Other Host
Ability to Reach and Proliferate at Site(s) of
Infection in the Host
Excretion of the Agent from the Host
Quantity and "Quality" (including virulence) of the
Infectious
Factors Influencing Exposure and Infection:
Environmental Factors
• Reservoirs: where organisms can live, accumulate or persist outside of
the host of interest; could be another organism or the inanimate
environment.
• Vehicles: inanimate objects/materials by which organisms get from one
host to another; includes water, food, objects (called fomites) and
biological products (e.g., blood).
• Amplifiers: Types of reservoirs where organisms proliferate; often
applied to organisms transmitted by the airborne route.
• Vectors: Living organisms bringing infectious organisms to a host.
– Mechanical vectors: Microbes do not multiply in the vector
• ex: biting insects infected with the infectious organism
– Biological vectors: Microbes must propagate in the vector before they
can be transmitted to a host.
Environmental Factors Influencing Survival or
Proliferation of Infectious Agents
• Physical: temperature, relative humidity, sunlight,
moisture content or water activity, climate and
weather, etc.
• Chemical and Nutritional: Antimicrobial
chemicals, nutrients for microbial proliferation.
• Biological: Antagonistic activity by other
organisms: antimicrobial agents, parasitism, etc.;
presence and state of a vector
Factors Influencing Exposure and Infection: Host
Factors and Host Susceptibility
• Opportunities for host exposure
– transmission routes
– host availability
• Susceptibility factors
– Dosage (quantity) and "quality" of infectious
organisms, including their "virulence";
– age
– immunity
– nutritional status
– immunocompetence and health status,
– genetics
– behavior (personal habits) of host.
Infectivity of Pathogenic Microorganisms and
Risk of Infection, Illness and Death
Infection

Illness
 Sequelae
Death
Secondary Spread
Infectious Dose
• Microbes differ in their infectious dose.
• Enteric and respiratory viruses are infectious at very low
doses
– as little as one cell culture infectious dose has a high
probability of infecting an exposed human.
– But, this may still require exposure to many virus particles.
• Most enteric bacteria are infectious at moderate (10s-100s
of cells) to high (1,000 cells) doses.
• Protozoan cysts may be infectious at low doses
– A few as 1-10 cysts of Giardia lamblia) or oocysts of
Cryptosporidium parvum
Outcomes of Infection
Microbes differ in their ability to produce
the different outcomes of infection:
• (i) infection without illness;
• (ii) infection with illness; (with or without
long-term sequelae) and
• (iii) infection, illness and then death
The Iceberg Concept: As Applied to Virus Infections
Type of
contact
Inhalation
Example
Type of
Agent
Virus
Source
Ingestion
Coccidiodo
mycosis
Typhoid
fever
Salmonella
food
poisoning
Fungus
Bacterium
Bacterium
Sexual
contact
Wound
Gonorrhea
Bacterium
Surgical
infections
Bacterium
Insect Bite
Malaria
Protozoan
Difficult to
avoid
contact
Lower
infecting
dose
Lower
infecting
dose
Person
Social
Avoid
Behavior
contact
Normal flora Aseptic
Avoid
surroundin techniques contact
gs
Mosquito
Insect
Eliminate
control
vector
Common
cold
Strategy for Preventive
Prevention Aim
None
Difficult to
avoid
contact
Aerosol
from
infected
persons
Soil
None
Water, food Sanitation
Food
Sanitation
Examples of Encounters and Disease Prevention
Type of
contact
Example
Type of
Agent
Source
Strategy for Preventive Aim
Prevention
Inhalation
Common cold
Virus
None
Difficult to
avoid contact
Ingestion
Coccidiodomycosis
Typhoid fever
Salmonella
food
poisoning
Gonorrhea
Fungus
Aerosol from
infected
persons
Soil
None
Hard to avoid
contact
Lower infecting
dose
Surgical
infections
Bacterium Normal flora
Aseptic
Avoid contact
surroundings techniques
Sexual
contact
Wound
Insect Bite Malaria
Bacterium Water, food
Bacterium Food
Sanitation
Sanitation
Bacterium Person
Social
Behavior
Protozoan Mosquito
Insect
control
Avoid contact
Eliminate
vector
Constitutive Defenses: Physical Barriers to Infection
System or Organ
Skin
Mucous
membranes
Cell Type
Squamous
Columnar nonciliated
(e.g., gastrointestinal tract)
Columnar ciliated
(e.g., trachea)
Cuboidal ciliated
(e.g., nasopharynx)
Secretory
Clearing Mechanism
Desquamation
Perstalsis
Mucociliary movement
Tears, saliva, mucus,
sweat
Flow of liquids
Constitutive Defenses: Chemical Barriers to Infection
System or Organ
Source
Skin
Mucous membranes
Sweat, sebaceous glands
Parietal cells of stomach
Secretions
Neutrophils
Lung
A cells
Salivary glands
Neutrophils
Small bowel and below
Liver via biliary tree
Gut flora
Substances
Organic acids
Hydrochloric acid, Low pH
Antimicrobial compounds
Lysozyme, peroxidase,
lactoferrin
Pulmonary surfactant
Thiocyanate
Myeloperoxidase
Cationic proteins
Lactoferrin
Lysozyme
Bile acids
Low molecular weight fatty
acids
Mortality Rates for Different Viruses in Healthy,
Immunocompetent Humans
(Rates Higher in the Immunocompromised)
VIRUSES:
• Adenovirus
• Enteroviruses
– Coxsackievirus B
– Echoviruses
• Hepatitis A virus
• Norwalk virus
• Rotavirus
% Mortality
0.01%
0.001 (average)
0.59-0.94
0.28
0.3
0.0001
0.01
Mortality Rates for Different Pathogens in
Healthy, Immunocompetent Humans
(Rates Higher in the Immunocompromised)
BACTERIA:
• Campylobacter jejuni
• E. coli
• Salmonella spp.
• Shigella spp.
% Mortality
0.1
0.2
0.1
0.2
• PARASITES:
• Giardia lamblia
• Entamoeba histolytica
0.0001
0.3
Transmission Categories of WaterAssociated Diseases
• Waterborne Microbial Diseases
• Water-washed or Water Hygiene Diseases
• Water Contact/Water Vector Habitat Diseases
Transmission Categories of Water-Associated
Diseases:
Waterborne Microbial Diseases
• Exposure mainly by ingestion of contaminated water
• Primarily enteric diseases transmitted by the fecaloral route
• Some are due to organisms that are not fecally
associated and also proliferate in water
– example: Legionella bacteria via aerosols and droplets
Transmission Categories of Water-Associated
Diseases:
Water-washed or Water Hygiene Diseases
• Diseases whose exposure is reduced by the use of
water for personal and domestic hygiene:
– washing: clothes, floors, other household chores
– bathing and other personal hygiene
– cleaning of cooking and eating utensils
• Includes many enteric organisms as well as diseases
of the skin and eyes (ex: trachoma) and insect
infestations (ex: scabies caused by mites;
pediculosis caused by lice).
Transmission Categories of Water-Associated Diseases:
Water Contact/Water Vector Habitat Diseases
• Exposure by skin contact with infested water
– example: schistosomiasis: the free-living larvae
released from aquatic snails (the intermediate
host) invade the skin.
• Exposure to water habitat "insect vector" diseases
– Insect vectors breed in or near water
• examples: malaria (parasite) and yellow fever
(virus) transmitted by mosquitoes