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Infection and Disease
UNIT 7
Cholera
• Cholera is an infection of the small
intestine caused by the bacterium
Vibrio cholerae
• The main symptoms are profuse watery
diarrhea and vomiting
• Worldwide it affects 3-5 million people
and causes 100,000-130,000 deaths a
year as of 2010
Cholera
• 1849 – cholera broke out in London
• Golden Square – worst affected district
and 500 people died in 10 days
• John Snow – physician that lived in the
area
• He did not believe that cholera was
spread by bad air and direct contact as
others did
Cholera
• John Snow noticed that people got
their water from a well that was
contaminated by a cesspool overflow
• A cholera patient lived in an apartment
that drained into the cesspool
• He concluded that the water was the
source of the disease
• The well was shut down and the
epidemic subsided
Cholera
• Cholera remains a global threat and is one of
the key indicators of social development.
• It no longer poses a threat to countries with
minimum standards of hygiene
• It remains a challenge to countries where
access to safe drinking water and adequate
sanitation cannot be guaranteed
• Almost every developing country faces
cholera outbreaks or the threat of a cholera
epidemic.
Host/Parasite Relationship
• Infection – refers to the relationship
between a host and a parasite – a
competition
– Host with good resistance – parasite is
removed
– Host without good resistance – disease
develops
• Disease - any change from the state of
good health
INFECTION
• Infectious Diseases = ½ all human
diseases
– Bacteria
– Viruses
– Protozoa
– Fungi
– Parasites
Normal Flora
• Population of Mo’s that infect the body
without causing a disease
• These relationships are a form of
symbiosis
• Can be permanent or for only limited
amounts of time
Normal Flora
• Mutualism – beneficial to both host and
MO
– Lactobacillus – lives in the human vagina,
obtaining nutrients and providing an acidic
environment to prevent other MO’s from
growing
Normal Flora
• Commensalism – beneficial to only the
MO
– E. coli lives in the human intestines to gain
nutrients, but doesn’t hurt us
Pathogenicity
• Ability of a parasite to gain entry into a
host and bring about disease
• Parasites vary greatly in pathogenicity:
– Some are known to always cause serious
human diseases (cholera, plague, typhoid
bacilli)
– Some are less pathogenic and cause
milder illnesses (common cold)
– Some are opportunistic – normally OK
until human defense mechanisms are
suppressed
Opportunistic Parasites
• Streptococcus pneumoniae
– commonly found on the surface of the
respiratory tract
– doesn’t cause any problems unless
tissues are damaged
– Then it can invade tissues and cause
pneumonia
Normal
Lung
Tissue
Infected
Lung
Tissue
Normal
Lung
Infected
Lung
Opportunistic Parasites
• Patients with acquired immune
deficiency syndrome (AIDS) are highly
susceptible to opportunistic infections
– Pneumoncystis carinii - commonly found
in the lungs of healthy people, it can cause
a lung infection in people with a weak
immune system.
– Toxoplasma gondii - usually minor but can
have serious or effects on an immunocompromised human
Opportunistic Parasites
• A shift in the body’s delicate balance of
controls may convert infection to
disease
• The distinction between pathogenic
and non-pathogenic organisms is
blurred when body defenses are
weakened:
– Cancer patients
– Transplant patients
– AIDS patients
Upsetting Nature’s Balance
• Borneo in the 1950’s had malaria
epidemic
• WHO sprayed insecticides that killed
the mosquitoes and stopped spread of
malaria
• But the insecticides also killed
houseflies
• Geckos ate the houseflies, ingested the
insecticides and died
Upsetting Nature’s Balance
• Cats ate the geckos, ingested the
insecticides and died
• Within weeks, the rat population soared
because the cats weren’t eating the rats
• Rats carry the fleas that transmit
bubonic plague and the plague spread
through Borneo
• WHO had to parachute boxes of
housecats onto the island – “Operation
Cat Drop”
Progress of Disease
• Period of Incubation – time between
when parasite enters host and first
symptoms appear
• Depends on # of MO’s, generation time,
host resistance
• Can be:
– Short: 1-3 days with cholera
– Moderate: 2 weeks with chicken pox
– Long: 3-6 years with leprosy
Progress of Disease
• Period of Prodromal Symptoms
• General symptoms appear:
– nausea
– fever
– headache
– tiredness
• The competition between host and
parasite has begun
Progress of Disease
• Period of Acme – acute stage of
disease
• Disease specific symptoms appear:
– Skin rash – scarlet fever
– Jaundice - hepatitis
• Patients suffer high fevers and chills
• Dry skin and pale expression result
from constriction of blood vessels to
conserve heat
Progress of Disease
• Period of Decline – symptoms subside
– sweating is common
– normal skin color returns as blood vessels
dilate
• Period of Convalescence – body
systems return to normal
Disease Transmission
• Direct Methods
• Close or personal contact with one who
has the disease
– Hand-shaking
– Kissing
– Sexual intercourse
• Can also mean exposure to droplets
– cough or sneeze
Indirect Methods
• Consumption of contaminated food or
water
– Foods are often contaminated during
processing
– Foods come from diseased animals
• Contact with Fomites – inanimate
objects that carry disease organisms
– Bed linens
– Dirty needles
Indirect Methods
• Arthropod Vectors
• Insects can carry disease agents from
one host to another
• Vectors:
– Mechanical vectors – carry MO’s on legs
and other body parts
– Biological vectors – insect itself is
diseased and inject MO’s when they bite
host (Lyme’s disease, Rocky Mountain
Spotted Fever)
=
=
Reservoirs/Carriers
• Reservoirs – continuing sources of
disease organisms necessary for a
disease to perpetuate
– Humans are the reservoir for smallpox
– By locating all persons with the disease,
the WHO was able to eradicate the disease
from the earth
• Carriers – organisms who have
recovered from a disease but continue
to shed disease agents (feces)
Epidemiology
• Endemic disease – occurs at a low level
in a certain geographic area
• Epidemic disease – breaks out in
explosive proportions within a
population
• Pandemic disease – occurs worldwide
Endemic to Epidemic
• For centuries Mongol tribesmen in
Siberia hunted marmots (rodents)
which can carry the plague
• They observed rules, knowing not to
hunt sluggish animals and to move
their camp if the animals appeared sick
• They kept the plague endemic – low
level in a certain area
Endemic to Epidemic
• In the early 1900’s, fur trappers moved
into the area from China
• They captured healthy as well as sick
marmots
• Plague broke out and they brought the
disease back to China
• In a 7 month period, 60,000 Chinese
people died - epidemic
Portal of Entry
• Site at which the parasite enters the host
• Key factor in disease establishment
• Certain parasites are only bad if they
enter the body at a specific site
– Tetanus occurs if C. tetani
spores are introduced to
anaerobic tissues of a
wound, but not if eaten
Portal of Entry
• Invasion- MO enters body by 1
invasion route:
– R.T. (Respiratory Tract)
– I.T. (Intestinal Tract)
– Skin and Mucous Membranes
Entry Portals
• Respiratory
– Nose, Throat, Trachea,
Bronchi, Lungs
– Inhale MOs
• Intestinal
– Mouth, Throat, Stomach, Intestines
– MOs ingested: Contaminated food, water,
objects
Entry Portals
• Skin
– MO enters via wound
• Mucous Membranes
– Eyes, Nose, Mouth,
Urogenital
– With or without
wound
Dose
• The number of parasites required in
order for disease to be established
• Typhoid bacilli – one million ingested
from contaminated water causes
disease
• Cholera bacilli – many times more
required to cause disease because it is
less resistant to stomach acid
Tissue Penetration
• Some bacteria have genes for tissue
penetration on their chromosome
• Invasiveness – the ability of a parasite
to penetrate tissue and cause structural
damage
• Certain bacteria are well known for
their invasiveness:
– Bacilli that cause typhoid fever
– Amebas that cause amebiasis
Enzymes
• Parasites produce enzymes directed at
host’s immune defenses
Enzyme
Effect
Coagulase
Resistance to
phagocytosis
Streptokinase
Prevents isolation of
infection
Hyaluronidase
Permits tissue
penetration
Leukocidin
Limits phagocytosis
Hemolysins
Induces anemia and
limits oxygen delivery
Toxins
• Microbial poisons that affect the
establishment and course of disease
• Exotoxins – protein molecules
produced and released by gram
positive bacteria
– Neurotoxins – affect the nervous system
(botulism)
– Enterotoxins – affect the GI tract
(staphylococci)
• Body can produce antitoxins against
them
Toxins
• Endotoxins – Parts of the bacterial cell
wall released when gram negative
parasite dies
• They do not stimulate an immune
response – no antitoxins
• Cause endotoxin shock
– Fever
– Body weakness and aches
– Coma
• Pathogenicity
– Ability of MO to produce disease enzymes
– Endo or Exotoxins
• Infection
– Is MO in the right place?
– Can MO invade tissues?
– Sufficient number
of MOs?
Defense of Entry Portals
Each entry route has developed its own
protective mechanisms
• Respiratory System
– Epithelial cells secrete thick, sticky
mucous, contains digestive enzymes
– Trap and dissolve invading MOs
– Also: Cilia – “wave” motion sweeps MOs
away from lungs
• Intestinal
– Saliva, Mucous
– HCl in stomach bile
– Digestive enzymes in intestines
• Skin
– Very thick, non-living outer layers
– Salt, oil, and acid secretions
• Mucous Membranes
– Mucous
– Tears with digestive enzymes
Circulatory System
• Principal method of
MO’s spreading
through body
• Also important
in defense
against MO’s
Blood – 3 Components
• Fluid – called serum
– an aqueous solution of minerals, salts,
proteins and other organic substances
• Clotting Agents – fibrinogen and
prothrombin
– if present in serum it is called plasma
• Cells
– RBCs (erythrocytes), WBCs (leukocytes),
platelets (thrombocytes)
RBCs - erythrocytes
• Are made in bone marrow and carry
oxygen to body tissues
• Oxygen is bound to the red pigment
hemoglobin
RBCs - Erythrocytes
• After ~ 120 days, RBCs disintegrate and
hemoglobin is converted to bilirubin
(yellow pigment) and taken to the liver
to be turned into bile
• Jaundice occurs if liver is damaged and
bilirubin enters the bloodstream
WBCs - Leukocytes
• Also made in bone marrow
• Granulocytes – have granules in
cytoplasm
– Neutrophils
– Basophils
– Eosinophils
• Agranulocytes – no granules
– Monocytes
– Leukocytes
Platelets - thrombocytes
• Small disc-shaped cells that originate
from cells in bone marrow
• Have no nucleus
• Function in blood clotting
Lymphatic System
• Lymph – the fluid that surrounds cells
and fills intercellular spaces
• It bathes the cells, supplying oxygen
and nutrients and collecting wastes
• It is pumped along tiny vessels through
muscle contraction
• Tiny vessels form larger vessels and
system empties into a large vein just
before the heart
Lymphatic System
• Lymph nodes – pockets of tissue
located along the lymph vessels
• Abundant in neck, armpits and groin
• They contain phagocytes (engulf
particles) and lymphocytes (respond to
substances in circulation)
• They become enlarged during times of
disease (swollen glands)
• Tonsils, adenoids, spleen, appendix
First Response to Invasion
• Inflammation
– Delivery of defensive WBCs and chemicals
in blood to attack site
– Surround site to retard invasiveness
– Remains until all damage is repaired
• Symptoms
– Redness: Normally 50% small arteries are
closed – they open up to increase blood
supply
– Heat: Increased blood raises temp
– Swelling: Pressurized capillaries expand
and leak WBCs and serum (with
chemicals) into tissue
– Pain: Increased fluid, heat, nerve damage
stimulates nerves