Infectious Diseases
prof.Asist: Heavin Hannan
Disease
• Does not allow the body to
function normally
• Can affect individual organs or
an entire body system
• Divided into two groups –
infectious and non-infectious
Infectious Disease
• A disease that can be passed
from organism to organism
• Caused by tiny organisms
called PATHOGENS
Infectious Diseases
What is an infectious diseases??
Infectious disease is:
①
A group of common diseases
②
Caused by different pathogens
③
Possessing infectivity.
④
To form epidemic.
⑤
Infectious disease is a threat to the health of people.
Infectious Diseases
What is an infectious disease??
A case is a risk factor …
Infection in one person can be transmitted to others
Infectious Diseases
What is an infection??
Definition of infection
①
Complex process of interaction between pathogen
and human body
②
Infection is composed of three factors: pathogen,
host and environment
③
There are commensalisms and opportunistic
infection
4 KINDS OF
PATHOGENS
• Bacteria
• Viruses
• Fungi
• Protist
Means of Transmission of Infectious
Diseases
Contact
Requires direct or indirect contact (fomite,
blood, or body fluid)
Food or
Water
Ingestion of contaminated food or water
Airborne
Inhalation of contaminated air
Vector-borne Dependent on biology of vector as well as
infectivity of organism
Perinatal
Sexual
Similar to contact infection, however, the
contact may occur in utero or during delivery.
transmission by sexual intercourse.
Factors Influencing Disease Transmission
Agent
Environment
• Infectivity
• Weather
• Pathogenicity
• Housing
• Virulence
• Geography
• Immunogenicity
• Occupational setting
• Antigenic stability
• Air quality
• Survival
• Food
• Age
Host • Sex
• Genotype
• Behaviour
• Nutritional status
•Health status
Infectious Diseases
Factors Influencing Disease Transmission( Agent)
Pathogenicity
What does pathogenicity mean???
It means the ability of a microbiological agent to
induce disease
The pathogenicity of pathogen is related to :
1.
Invasiveness
2.
virulent
3.
Number of pathogen
4.
Mutation (variability)
Infectious Diseases
Factors Influencing Disease Transmission( Agent)
Infectivity:
 Ability
of agent to cause infection
 Number
 In
of infectious particles required
person-to-person transmission, secondary attack rate is a
measure of infectivity
Virulence :
Severity of the disease after infection occurs.
Measured by case fatality rate or proportion of clinical cases that
develop severe disease.
Infectious Diseases
Factors Influencing Disease Transmission( Agent)
Immunogenicity
 Ability of an organism to produce an immune response that provides
protection against reinfection with the same or similar agent
 Can be life long or for limited periods
 Important information for development of vaccines
Infectious Diseases
Barriers for Defense Against Infection:
1. Skin:

Prevents entry of infectious organisms, unless injured.

Severe burn patients who die are usually killed by infections. So much skin is
damaged they are very vulnerable to infections.
2. Mucus membrane:

Mucous is usually rich in enzymes that will kill many pathogens
3. Cilia:

These are hair-like structures lining the respiratory tract. They work to sweep
foreign particles out of the respiratory tract.

Damaged by smoking, leaving smokers more vulnerable to infections.
4. Coughing:

Helps remove foreign material from respiratory tract.
5. Personal Hygiene

Helps reduce the number of pathogenic organisms on the skin and other
surfaces of the body.
Incubation period
 Time between exposure and onset of symptoms or signs of infection.
 Each disease has typical incubation period but varies widely.
 Requires replication of the organism to some threshold level for
producing symptoms
Bacterial Diseases









Tuberculosis
tetanus
Gonorrhea
Diptheria
Streptococcal Infections
Pneumonia (can also be viral or fungal)
Pertussis
Bubonic Plague
There are many others.
Normal Micro flora & its importance
1. Prevent the growth of pathogens
2. Stimulate the immune system to produce
antibodies that cross-react with invading
pathogens
3. Aid in digestion of cellulose in ruminants.
4. Produce essential nutrients
Koch’s Postulates
1. The same pathogen must be present in
every case of the disease.
2. The pathogen must be isolated from the
diseased host and grown in pure culture;
3. The pathogen from the pure culture must
cause the disease when it is introduced into
a healthy but susceptible organism.
4. The pathogen must be isolated from the
inoculated animal and be shown to be the
original organism.
Modifications to Koch’s Postulates
1. Some infectious agents cannot be
cultured
2. Some pathogens have non-virulent
strains whose presence does not link
them to a disease. E.g. non
encapsulated Diplococcus pneumoniae
Types of Pathogens
1. Bacteria
1. Gram positive
2. Gram negative
3. Acid-Fast e.g. Mycobacteria
1. Spherical described as cocci
2. Rod shaped described as bacilli
Gram Positives
•
Unique Features
–
–
–
–
Thick peptidoglycan wall
No periplasmic space
No outer membrane (capsule)
E.g. Streptococcus pyogenes,
Staphylococcus aureus, Bacillus anthracis,
Clostridium tetani
Gram Positive wall
Gram Negatives
• Unique features
– Thin peptidoglycan wall
– Has periplasmic space containing different
degradative enzymes such as
deoxyribonucleases, -lactamases and
proteases
– Outer membrane containing lipid A, an
endotoxin
– E.g. Neisseria, Salmonella typhi, E. coli,
Yersinia pestis, Vibrio cholerae
Gram Negative wall
Types of pathogens
2. Parasites (Eukaryotic Pathogen)
1. Fungi e.g. Candida, Aspergillus
2. Protozoa e.g. Plasmodium, Schistosoma
3. Worms e.g. Ascaris, Taenia
Types of pathogens
3. Viruses
1. Are pieces of DNA or RNA surrounded by
protein coat. The may be:
- Encapsulated e.g. HIV
- Non-encapsulated e.g.adenoviruses
Modes of transmission
1. Direct contact e.g. touching,
handshaking, or sexual intercourse
2. Indirect contact e.g. food, water or
droplets in air;
3. Animal vectors e.g. insect bites in
malaria, plague, dog bite in rabies
Pathogens can come from
• Another person
• Contaminated objects
• Animal bite
• Environment
Pathogenesis
• Sequence of activities
1. Transmission of causative agent to
susceptible host.
2. Adherence of the agent to a target tissue;
3. Colonization and invasion.
4. Damage to host by toxins or other
mechanisms.
5. Exit from host.
6. Survival outside host long enough for step
1 to occur.
Virulent Factors
•
•
For all pathogens there is an infective
dose (about 10 to 100 in Shigella and 1
million in Salmonella )and a lethal dose.
Virulent factors that confer pathogenicity
include;
1.
2.
3.
4.
5.
6.
Pili that facilitate attachment;
Capsules that interfere with phagocytosis
Exotoxins
Endotoxins
Proteases that break down antibodies
Ability to vary antigens to evade antibodies
Bacterial Pathogenesis
1. Toxin production. Toxins fall into two
categories; exotoxins and endotoxins.
2. Invasiveness, where bacteria grow to
large numbers locally and produce
enzymes that damage host tissues.
exotoxins
1. Heat labile (60-100 degrees for 30 mins)
proteins produced and released by both
gram positive and gram negative
bacteria.
2. Produced by bacteria such as
Clostridium (neurotoxins) and Bacillus
(enterotoxin) (+) and E. coli and Vibrio
(enterotoxin) (-)
endotoxins
1. Are heat stable (100 degrees for 1 hr)
lipopolysaccharide produced only by
gram –ve bacteria. They remain attached
to cell wall.
2. Cause fever and shock and is of lower
toxicity compared to exotoxins.
3. Produced by bacteria such as
Salmonella
cholera
1. Causative Agent: Vibrio cholerae
2. Symptoms: severe diarrhoea up to 20
liters a day of “rice water stool”, vomiting,
muscle cramps caused by loss fluid and
electrolytes.
3. Pathogenesis: Vibrio adheres to the
small intestinal lining, multiply and
produce the enterotoxin choleragen
which causes increase secretion of water
and electrolyte from the cells results
Cholera
4. Epidemiology: Feacally contaminated
water, crabs and vegetables fertilized
with human faeces. Has been eradicated
most developed countries but a new
strain discovered in 1992 is threatening
another pandemic.
Cholera
5. Incubation period: 12-48 hours
6. Lab diagnosis: Microscopy, culture of
sample from faeces or vomit.
7. Prevention: Purification of water, washing
of hands.
8. Treatment: administration of solution of
glucose and electrolyte orally or
intravenously; tetracycline antibiotic orally
malaria
1. Causative Agent: Plasmodium (4 species)
2. Symptoms (Clinical features): fever, chills,
anaemia, headache, nausea, shivering,
convulsions (esp. in under 5 yr olds)
enlarged spleen.
3. Pathogenesis: site of action of pathogen
include: liver, RBC, brain. The vector,
female Anopheles mosquito, transfer
pathogen during feeding.
malaria
4. Epidemiology: Endemic in 91 tropical and
subtropical countries. Invade the liver 1st
and move to reproduce in RBCs resulting
in their rupture and the associated chills.
5. Incubation Period: 1 – 2 weeks.
6. Lab diagnosis: Microscopy.
An Infected RBC
Malaria - Prevention
1. Reduce the number of mosquitoes;
destruction of larvae and adult
mosquitoes by biological and chemical
control methods
2. Avoid being bitten; protective clothing
and creams, treated bed nets
3. Use of drugs to prevent infection
Tuberculosis
1. Pathogen: M. tuberculosis (pulmonary TB);
M. bovis(GI TB)
2. Transmission: airborne droplets (NB MTB is
dessication resistant and survives in dried
sputum); unpasteurized milk.
3. Clinical features: prolonged coughing
sometimes with bloody sputum, shortness of
breath, fever, sweating , weight loss
Tuberculosis
4. No toxin production. Pathogenicty is by
invasiveness that produce characteristic
lesions in the lungs.
5. Epidemiology: pathogen triggers acute
inflammatory response + forms tubercle –
giant cells containing MTB and surrounded
by epithelial cells. Tubercles heals by
fibrosis and calcification. Can desseminate
via bloodstream to other internal organs