L3: General Epidemiology of Communicable Diseases (Reservoir - Mode of Transmission - Host)

Question 1

Def of Reservoir

Answer 1

Is the habitat in which the agent normally lives, grows, and multiplies.

Question 2

Types of Reservoir

Answer 2

(human/ animal/ environment)

Question 3

Types of Human reservoirs of infection

Answer 3

Question 4

Classifications of carriers

Answer 4

• According to relation to clinical picture of the diseases
• According to duration of communicability

Question 5

Classifications of carriers According to relation to clinical picture of the diseases

Answer 5

Question 6

Classifications of carriers According to duration of communicability

Answer 6

Question 7

what is Zoononsis?

Answer 7

The term zoonosis refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans.

Question 8

Examples of Zoonotic Diseases

Answer 8

Question 9

Examples of Environmental reservoirs

Answer 9

• Environmental media as plants, soil, and water can contain organisms
• Examples: Outbreaks of Legionnaires disease are often traced to water supplies.

Question 10

Characters of Portal of exit

Answer 10

• It is the path by which a pathogen leaves its host.
• The portal of exit usually corresponds to the site where the pathogen is localized.
• Examples: Influenza viruses are localized and exit from the respiratory tract.

Question 11

Characters of Period of infectivity

(Communicability)

Answer 11

• The period where the reservoir is infectious.
• It is variable.
• It may start from last days of incubation period (incubatory carrier), or with the start of symptoms.
• It may end with clinical cure, or end with convalesce, or extend after convalescence (chronic carrier)

Question 12

Def of Modes of transmission

Answer 12

It is how infection is transmitted

Question 13

Types of Modes of transmission

Answer 13

1. Droplet (respiratory) infection
2. Food and water borne infection
3. Contact infection (through skin &/or mucus membranes)
4. Vertical infections

Question 14

Types of Droplet (respiratory) infection

Answer 14

Question 15

MOT of Food and water borne infection

Answer 15

Causative agents in excreta are transmitted either:

• Directly by hands (hand to mouth)
• Indirectly by ingestion of contaminated food and water (vehicle borne).

Question 16

Examples of Food and water borne infection

Answer 16

• Viral: Poliomyelitis
• Bacterial: salmonellosis
• Parasitic: Entamoeba histolytica.

Question 17

MOT of Contact infection (through skin &/or mucus membranes)

Answer 17

• Transmission through skin or mucus membranes by the organism or a vector.

Question 18

Examples of Contact infection (through skin &/or mucus membranes)

Answer 18

A. Penetration of undamaged skin or mucus membrane by:
- The organism as bilharziasis.
- The vector as malaria and plague.

B. Penetration of injured skin or mucus membrane by the organism as tetanus.

Question 19

what are vectors?

Answer 19

Vectors are animate object that transmit an infectious agent.

Question 20

Vectors may carry an infectious agent through

Answer 20

• Purely mechanical means, as flies carrying Shigella
• Biological transmission where the vector support growth or changes in the agent, as malaria agent which undergoes maturation in the mosquito before
  it can be transmitted to humans.

Question 21

Method of transmission of Vertical Infections

Answer 21

Infection from mother to child during:
- Pregnancy (as syphilis)
- Birth (as HBV)
- Breast feeding (as HIV).

Question 22

Modes of transmission can be also classified into …….

Answer 22

Question 23

Def of Host

Answer 23

Is the final station in the chain of infection

Question 24

Characters of Portal of entry

Answer 24

• the way by which a pathogen enters a susceptible host.
• It must provide access to tissues where pathogens can multiply or toxins can act.
• Often, infectious agents use the same portal to enter a new host that used to exit the source host.

Question 25

Examples of Portal of entry

Answer 25

Influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host.

Question 26

Characters of IP

Answer 26

• Period (time) between the entry of causative agent (or its toxin) into human body and the appearance of earliest clinical manifestations of the disease.

Question 27

Epidemeological Importance of Incubation period

Answer 27

• To trace source of infection.
• To control contacts of cases (by surveillance for maximum IP from last case).
• To set up international measure.

Question 28

Variation of Incubation period

Answer 28

• It varies from disease to another and even from patient to another with the same infection.
• Shortest IP: is (0.5-8 hours) in staph food poisoning.
• longest IP: can reach up to 10 years as in leprosy.

Question 29

What does Incubation period depend on?

Answer 29

It depends on many factors:
 Portal of entry.
 Rate of agent growth in host
 Dosage of agent.
 Host resistance.

Question 30

what does Host Susceptibility and resistance depend on?

Answer 30

Question 31

Important periods in epidemiology

Answer 31

Question 32

Importance of understanding infection cycle for public health

Answer 32