General Epidemiology (Part II)

Question 1

What is included in the infection cycle?

Answer 1

1. Infectious Agent
2. Reservoir
3. Portal of exit
4. Mode of transmission
5. Portal of entry
6. Susceptible host

Question 2

Mention the etiological / causative agents with an explanation for each.

Answer 2

1. Infectivity:
   organism that invade the host, causing infection
2. Virulence:
   - The potential power of the organism to cause a disease through its invasive and/or toxic properties
3. Invasiveness:
   - Is the ability of the organism to go into the body fluids or tissue cells.
4. Resistance:
   - viability of the pathogenic organism outside the body of the host.

e.g. Spore-bearers remain viable in the form of highly resistant spores for years.

5. Antigenicity (immunogenicity):
   - Ability of the agent to induce antibody production in the host.

Question 3

Define reservoir .

Answer 3

site where the infectious organism resides, metabolizes, and multiplies

Question 4

Define Source .

Answer 4

the site from which it is transmitted to a susceptible host.

Question 5

How the reservoir and source can be different or same ?

Answer 5

1- Different reservoir and source ——> eg. —-> reservoir: S. typhi could be the gallbladder of an infected individual

Source : food contaminated by the carrier

2- Same reservoir and source : ——> eg.

permanent nasal carrier of S. aureus the organisms disseminate from this site of carriage

Question 6

List the types of reservoirs.

Answer 6

1. Human Reservoir: (Case or Carrier)

2. Animal Reservoir

Question 7

Describe the cases of human reservoir

Answer 7

a. Severe form:
- the patient gets a severe attack of the disease and looks seriously ill.
- may end fatally before showing the characteristic picture of the disease.

b. Moderate form:
- the patient shows the classical picture of the disease
- can be easily diagnosed.

c. Mild form:
- the manifestations are vague or mild
- The characteristic picture is lacking and the patient himself may be unaware of his illness and finds it unnecessary to seek medical care.

d. Atypical or modified form:
- the patient may show atypical manifestations
- do not conform to the typical or classical picture of the disease
- the difficulty or uncertainty of clinical diagnosis.

e. Chronic form:
- some diseases may run a chronic course
- when treatment is inadequate or neglected

Question 8

Who is the “carrier” ?

Answer 8

an infected person who carries the infectious agent in his body without showing any clinical manifestations of disease.

Question 9

Describe the types of carriers in human reservoir

Answer 9

a. Incubatory Carriers:
The causative organism excreted during the incubation period

b. Convalescent Carrier:
In some infectious diseases, the recovered cases will continue to harbor and shed the causative agent for a variable period of time (weeks, months, years or for life)

c. Contact Carrier:
individual who contracts infection through contact with a patient without falling ill and he may disseminate infection to others

Question 10

Why a carrier is considered to be an important source of infection ?

Answer 10

1. • Looks healthy
     - are not recognized
     - they are free to move in the community exposing others to infection
2. The number of carriers is > than clinical cases of the disease
   - Many individuals may get infected
   - A minority falls ill
3. Carrier state may be of long duration in some convalescent carriers ( lasting for years or life )

Question 11

Elucidate the animal reservoir and give examples.

Answer 11

Animal Reservoir ( 2nd type of reservoir ) :

• Zoonoses —> are infectious diseases naturally transmitted between vertebrate animals and man.
• Cattle, domestic animals and rodents form the main animal reservoirs

Eg: - bovine tuberculosis

• brucellosis
• salmonellosis
• anthrax
• plague
• Weil’s disease
• rabies

Question 12

Modes of disease transmission

Answer 12

• Droplet
• Food borne
• Arthropod borne
• Contact

Question 13

What are the “Patterns of spread of communicable diseases”

A- Among Humans ?

Answer 13

• Among Humans:
  1. Sporadic: disease occurs in of scattered individual cases “not related”.
  2. Endemic: constant presence of a particular disease in a locality.

3. Epidemic:
   - occurrence of an unusually increased number of cases in a specific place & community, within a limited time.
   - The cases can be traced to a common source

Question 14

Epidemics may arise from ?

Answer 14

1- An endemic disease shows epidemic waves at any time when favoring conditions are available.

2-Disease long absent from a community.

3- Disease not previously recognized but 1 st introduced in the locality from abroad

Question 15

Types of epidemics

Answer 15

1- Outbreak: ——> an epidemic within a closed or confined community ( hospital, school etc..)

2- Pandemic: ——> an epidemic that spreads over some countries of the world.

Question 16

What are the “Patterns of spread of communicable diseases”

B- Among Animals ?

Answer 16

• Enzootic: endemic ( local) spread between animal populations
• Epizootic: epidemic spread between animal population

Question 17

What are the 2 opposing forces working in the susceptible host during its invasion ?

Answer 17

1- Number and virulence of the invading organisms

2- Host resistance

Question 18

Classify immunity & its subtypes

Answer 18

Immunity is divided into 2 main types :

1. Innate Immunity ( non - specific)
2. Adaptive Immunity ( acquired ) , which is also subdivided into:

2a. Natural : ——> Passive ( maternal)
———> Active ( Infection)

2b. Artificial : ——> Passive ( antibody transfer)
———> Active ( immunization )

Question 19

What is “ Acquired Immunity” ?

Answer 19

It is a specific immunity caused by the production of specific antibodies for particular organisms and may be for different strains of the same organism.

It may be :
A. naturally acquired
B. artificially induced

Question 20

State the :

A. Naturally Acquired Immunity ——> “Passively acquired from the mother “

Answer 20

1. Passively acquired from the mother:

a. Transplacental:
- The infant is born with temporary immunity to some infectious diseases such as measles, diphtheria, poliomyelitis and mumps
- antibodies are available in maternal blood
- It is valuable for protecting the infant in his early months of life and fades in about six months.

b. Colostrum:
- Rich in antibodies
- Antibodies acquired from mother’s milk
in small amount

Question 21

State the

A. Naturally Acquired Immunity ——> “Actively acquired natural immunity through infection”

Answer 21

2. Actively acquired natural immunity through infection:

a. Subclinical infection:
- Repeated exposure to subclinical infections in endemic areas —> causes building up of immunity against the endemic diseases in this community

b. Manifested disease:
Attacks of infectious diseases are followed by variable degrees of immunity according to the nature and antigenic stability of the organism:

1. Absolute immunity : is unknown except for yellow fever where second attacks are not recorded.
2. Solid immunity:
   - high degree of long lasting immunity
   - second attacks are quite rare
   - occurs with e.g. measles
3. Moderate immunity : in case of enteric fever.
4. Weak and short-lived immunity : repeated attacks are common in common cold, influenza and gonorrhea.
   c. Infection foci (premunition immunity):

the existence of a focus of infection within the body that gives a continuous stimulus for antibody formation while it fades once the focus disappears. (BCG vaccine)

Question 22

State the

B. Artificially acquired immunity (immunization)?

Answer 22

1. Active immunization (vaccination):

depends on stimulation of the immune system of the body to produce specific antibodies through the administration of a suitable antigen of the infective organism or its products

2- Passive immunization (Sero-prophylaxis):

Through inoculation of immune serum containing ready
formed antibodies preparations which give temporary protection against infection

Question 23

What are the forms of the immunizing agents ?

Answer 23

1. Vaccines :- killed or attenuated bacterial or viral preparation
2. Toxoids :- detoxicated exotoxin preparations

Question 24

Antibodies produced are maintained at a high level for a variable period of time which may be few months or several years according to ________ and ________

Answer 24

According to:

1- The antigen used

2- The body response

Question 25

Factors Influencing Antibody Formation after active immunization ?

Answer 25

1. Antigen given:
   a. Dose
   b. Number of doses
   c. Spacing of doses
   d. Booster dose
2. Mixtures of Antigens
3. Agammaglobulinaemia or hypogammaglobulinaemia
4. Presence of corresponding antibodies
5. Anamnestic reaction

Question 26

The body responds when :

a. Only one antigen is given separately
b. More than one antigen is given separately
c. More than one antigen is given simultaneously

Answer 26

c. More than one antigen is given simultaneously

eg. preparation of :
diphtheria toxoid + tetanus toxoid+ pertussis vaccine (DPT) = gives excellent results and easier immunization for children

Question 27

What is Agammaglobulinaemia ?

Answer 27

• A rare congenital condition affecting males
• Total absence or presence of gamma globulins in serum
• the tissues contain few or no plasma cells

Question 28

What is the “Anamnestic reaction “?

Answer 28

It is non-specific stimulation of antibody formation through an obscure mechanism

• Previously immunized with a particular antigen ——> show a rise in the specific antibody after giving a different antigen ————> intercurrent infection or fever

Question 29

Define Herd immunity & the factors that it arises from.

Answer 29

Definition:
Immunity of the population as a whole against the particular disease

May arise from :

1- Endemicity of the disease and building up of subclinical immunity

2- Wide application of active immunization e.g. diphtheria and measles

Question 30

Antibody response to an introduced antigen is less in the presence of _______

Answer 30

Antibody response to an introduced antigen is less in the presence of :
“ preformed antibody “

Question 31

Don’t immunize infants against measles before _______

Answer 31

six months

Question 32

What are the types of passive immunization in human ?

Answer 32

Human = homologous

a. Human normal immunoglobulin:
“g globulin”

b. Human specific immunoglobulin: “hyper-immune g globulin”

Question 33

What are the types of passive immunization in animals anti-sera ?

Answer 33

a. Antitoxic sera: For prevention & treatment of :
- tetanus
- diphtheria
- gas gangrene
- botulism

b. Antiviral sera as anti-rabies

Question 34

Differences between human & animal anti-sera

Answer 34

Human anti-sera :

——> Dose : Less
——> Protective period : Long
——> Test of sensitivity: No need

While in the Animal anti-sera :

——> Dose : More
——> Protective period : Less
——> Test of sensitivity: Must