L3: General Epidemiology of Communicable Diseases (Reservoir - Mode of Transmission - Host) Flashcards
Def of Reservoir
Is the habitat in which the agent normally lives, grows, and multiplies.
Types of Reservoir
(human/ animal/ environment)
Types of Human reservoirs of infection
Classifications of carriers
- According to relation to clinical picture of the diseases
- According to duration of communicability
Classifications of carriers According to relation to clinical picture of the diseases
Classifications of carriers According to duration of communicability
what is Zoononsis?
The term zoonosis refers to an infectious disease that is transmissible under natural conditions from vertebrate animals to humans.
Examples of Zoonotic Diseases
Examples of Environmental reservoirs
- Environmental media as plants, soil, and water can contain organisms
- Examples: Outbreaks of Legionnaires disease are often traced to water supplies.
Characters of Portal of exit
- 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.
Characters of Period of infectivity
(Communicability)
- 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)
Def of Modes of transmission
It is how infection is transmitted
Types of Modes of transmission
- Droplet (respiratory) infection
- Food and water borne infection
- Contact infection (through skin &/or mucus membranes)
- Vertical infections
Types of Droplet (respiratory) infection
MOT of Food and water borne infection
Causative agents in excreta are transmitted either:
- Directly by hands (hand to mouth)
- Indirectly by ingestion of contaminated food and water (vehicle borne).
Examples of Food and water borne infection
- Viral: Poliomyelitis
- Bacterial: salmonellosis
- Parasitic: Entamoeba histolytica.
MOT of Contact infection (through skin &/or mucus membranes)
- Transmission through skin or mucus membranes by the organism or a vector.
Examples of Contact infection (through skin &/or mucus membranes)
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.
what are vectors?
Vectors are animate object that transmit an infectious agent.
Vectors may carry an infectious agent through
- 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.
Method of transmission of Vertical Infections
Infection from mother to child during:
- Pregnancy (as syphilis)
- Birth (as HBV)
- Breast feeding (as HIV).
Modes of transmission can be also classified into …….
Def of Host
Is the final station in the chain of infection
Characters of Portal of entry
- 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.
Examples of Portal of entry
Influenza virus exits the respiratory tract of the source host and enters the respiratory tract of the new host.
Characters of IP
- Period (time) between the entry of causative agent (or its toxin) into human body and the appearance of earliest clinical manifestations of the disease.
Epidemeological Importance of Incubation period
- To trace source of infection.
- To control contacts of cases (by surveillance for maximum IP from last case).
- To set up international measure.
Variation of Incubation period
- 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.
What does Incubation period depend on?
It depends on many factors:
Portal of entry.
Rate of agent growth in host
Dosage of agent.
Host resistance.
what does Host Susceptibility and resistance depend on?
Important periods in epidemiology
Importance of understanding infection cycle for public health
