A process whereby the parasite enters
into a relationship with a host is known as infection or disease. In infection
or disease, there is an invasion of the body by pathogenic microorganisms. If
the disease is transmitted directly or indirectly from one person to another,
it is known as communicable disease. A contagious disease is one that is
directly transmitted from one person to another.
Types of infection based on how
often they occur
1. Endemic
– constantly present in locality (e.g. common cold)
2. Epidemic
– disease occurring in excess of normal (e.g. measles and influenza)
3. Pandemic
– an epidemic that has become worldwide (e.g. pandemic influenza)
4. Sporadic
– if a particular disease occurs only occasionally (e.g. anthrax)
Types of infection according to
distribution in the host
1. Local
– the signs and symptoms are confined to one area
e.g. infected
wound on the dorsum of the feet
2. Focal
– when the organism involved are originally confined to one area which serves
as a continuous source of further dissemination of the organism itself or of
toxic materials to other parts of the body (e.g. wound infection caused by C.
tetani, this serves as a focus for spread of tetanus toxin throughout the CNS)
3. Systemic
or Generalized – general invasion of the entire body
a. Bacteremia
– when the organism invade the bloodstream with no active multiplication
b. Septicemia
– when the organism invade the bloodstream with active multiplication
c. Pyemia
– when pus–forming organisms repeatedly invade the bloodstream and localize at
different parts of the body.
d. Sapremia
– toxic results of the presence in the blood of the product formed by the
growth of saprophytes on diseased or injured tissue.
e. Toxemia
– when bacteria are localized in one area, but produce a toxin which spreads
and is absorbed by the body cells.
Other types
1. Primary
– initial infection causing the illness (e.g. measles)
2. Secondary
– infections produced by those organisms causing infections only after the
primary infection has weakened the body’s defense (e.g. post measles pneumonia)
3. Mixed
– caused by two or more organisms (e.g. wound infection)
4. Acute
– those infection which develop rapidly and usually result in high fever and
severe sickness (e.g. whooping cough)
5. Chronic
– those infections which develop with milder but longer lasting symptoms (e.g.
tuberculosis)
6. Inapparent
– an illness in which the symptoms are so mild that it goes undetected and thus
diagnosed (e.g. poliomyelitis and Hepatitis A)
Essential steps for infection
to take place
1. Infectivity
or entrance of the parasite into the host
Fomites – are non–living
or inanimate vectors that transmit pathogens
Filth – are polluted
materials in the environment which carry microorganisms but are neither true
fomites nor insects (e.g. dust, dirt from animal droppings)
Many disease–causing
microorganisms have an obligate portal entry and can cause infection only if
they enter via such portal (e.g. Shigella bacillus – causes dysentery if it
enters by mouth to GIT, has no effect when rubbed into an open wound.
Other
microorganisms can enter the body tissues via almost any portal of entry and
produce disease (e.g. Pseudomonas aeruginosa)
2. Invasiveness
or establishment and multiplication of the parasite in the host.
From the
various portal of entry, they spread:
a. Directly
through the tissue by contiguity.
b. Via
the bloodstream and lymphatics
3. Perpetuation
of species is achieved by
a. Satisfactory
portal of exit from the host
b. Effective
mechanism for transmission of disease (by direct contact, airborne, dust–borne,
indirect contact through vectors or inanimate objects)
Phases in the course of
infectious diseases
1. Incubation
period – interval between the time the infection received and the appearance of
the disease.
2. Prodromal
period – short interval (prodrome) that sometimes follow the period of incubation
described by such ill–defined symptoms
as headache or malaise.
3. Invasion
period – disease reaching its full development and maximum intensity.
4. Fastigum
or acne – disease at its height
5. Defervescence
or decline – the stage at which manifestations subside
6. Convalescent
period – the period in which the symptoms may disappear and recovery begins
despite the fact that virulent microorganisms are still being discharged from
the body.
Attributes of microorganisms
which enable them to cause disease
Pathogenicity
– ability of the microorganism to produce disease
Two general
classes of pathogenic microorganisms
a. True
pathogens – are able to invade the tissues of healthy individuals through some
inherent power of their own.
b. Opportunistic
pathogens – are capable of producing disease only when given a special
opportunity to enter the body tissues through an accidental injury to the skin
or when the natural resistance to infection is abnormally low.
Virulence –
the degree of pathogenicity. This is measure in terms of:
a. Number
of microorganisms present
b. Micrograms
of toxins necessary to kill or injure tissue cells.
Factors which influence
virulence
1.
Toxic factors
Toxigenicity –
ability to produce toxic substances
Toxins – are
poisonous substances at high molecular weight produced by pathogenic
microorganisms.
Feature Exotoxins Endotoxins
Bacterial
source excreted by gram (+) released from the cell wall of
lysed
gram (–) bacteria
Chemical
nature protein lipopolysaccharide
Heat
intolerance inactivated easily at withstand autoclaving
61 – 80oC
Immunology can be converted to can not form toxoids, not
Toxoids and
readily neutralized by
antitoxins
Neutralized
by antitoxins
Pharmacology specific for a particular various effects, but mostly
type of cell
function symptoms of
generalized
shock/hypersensitivity
Lethal dose small amount larger amount
Microbial
disorders caused by exotoxins
a. Diphtheria
(Corynebacterium diptheriae) inhibits protein synthesis which brings about
disruption of normal physiologic function
b. Tetanus
(Clostridium tetani) affects synaptic transmission and increases reflex
excitability in neurons.
c. Gas
gangrene (Clostridium perfringens) necrotizing and hemolytic, affects also the
CNS.
d. Staphylococcal
food poisoning (Staphylococcus aureus) stimulate neutral receptors and
increased vomiting within hours.
e. Cholera
(vibrio cholera) produces massive absorption of water and electrolytes.
f.
Streptococcal erythrogenic
toxins (Streptococcus pyogenes) produces the typical punctuate maculopapular
rash of scarlet fever
2.
Mechanical
factors
The
multiplication of microorganisms may result in blocking of the capillaries of
the heart, lungs and brain.
3.
Miscellaneous
factors
a. Neurotoxin
or necrotizing factors – kills tissue cells
b. Hypothermic
factors – lowers body temperature to produce shock
c. Edema–producing
factors
4.
Enzymatic and
other related factors
Factors Effects
Hyaluronidase /
Duran facilitate spread of pathogens
Reynals Factor) and toxic material through
tissues
Coagulase
(clotting causes resistance
to phagocytosis
Factor) accelerates
formation of blood clot
Hemolysins causes lysis of RBC
and other tissue cells
Lecithinase or
alpha similar to
hemolysins
Toxins
Leucodin kills leukocyte
Collagenase dissolves collagen
Steptokinase / dissolves
human fibrin; easily dissolve blood clots
Streptodornase
Capsule resists
phagocytosis
Relationship between factors of
infection
D
is a function of NV/R
Where: D – diseases
N
– number of disease producing microbes
V
– virulence of the organism
R
– resistance of the host
The formula, signifies that, in any
case of infection, it is the total disease–producing power of the organisms
(which depends on their number and virulence), balanced against resistance of
the infected individual, that determines the nature and severity of the
disease.
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