General Epidemiology (Part II) Flashcards
What is included in the infection cycle?
- Infectious Agent
- Reservoir
- Portal of exit
- Mode of transmission
- Portal of entry
- Susceptible host
Mention the etiological / causative agents with an explanation for each.
- Infectivity:
organism that invade the host, causing infection - Virulence:
- The potential power of the organism to cause a disease through its invasive and/or toxic properties - Invasiveness:
- Is the ability of the organism to go into the body fluids or tissue cells. - 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.
- Antigenicity (immunogenicity):
- Ability of the agent to induce antibody production in the host.
Define reservoir .
site where the infectious organism resides, metabolizes, and multiplies
Define Source .
the site from which it is transmitted to a susceptible host.
How the reservoir and source can be different or same ?
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
List the types of reservoirs.
- Human Reservoir: (Case or Carrier)
2. Animal Reservoir
Describe the cases of human reservoir
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
Who is the “carrier” ?
an infected person who carries the infectious agent in his body without showing any clinical manifestations of disease.
Describe the types of carriers in human reservoir
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
Why a carrier is considered to be an important source of infection ?
- Looks healthy
- are not recognized
- they are free to move in the community exposing others to infection
- Looks healthy
- The number of carriers is > than clinical cases of the disease
- Many individuals may get infected
- A minority falls ill - Carrier state may be of long duration in some convalescent carriers ( lasting for years or life )
Elucidate the animal reservoir and give examples.
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
Modes of disease transmission
- Droplet
- Food borne
- Arthropod borne
- Contact
What are the “Patterns of spread of communicable diseases”
A- Among Humans ?
- Among Humans:
1. Sporadic: disease occurs in of scattered individual cases “not related”.
2. Endemic: constant presence of a particular disease in a locality.
- 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
Epidemics may arise from ?
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
Types of epidemics
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.
What are the “Patterns of spread of communicable diseases”
B- Among Animals ?
- Enzootic: endemic ( local) spread between animal populations
- Epizootic: epidemic spread between animal population
What are the 2 opposing forces working in the susceptible host during its invasion ?
1- Number and virulence of the invading organisms
2- Host resistance
Classify immunity & its subtypes
Immunity is divided into 2 main types :
- Innate Immunity ( non - specific)
- Adaptive Immunity ( acquired ) , which is also subdivided into:
2a. Natural : ——> Passive ( maternal)
———> Active ( Infection)
2b. Artificial : ——> Passive ( antibody transfer)
———> Active ( immunization )
What is “ Acquired Immunity” ?
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
State the :
A. Naturally Acquired Immunity ——> “Passively acquired from the mother “
- 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
State the
A. Naturally Acquired Immunity ——> “Actively acquired natural immunity through infection”
- 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:
- Absolute immunity : is unknown except for yellow fever where second attacks are not recorded.
- Solid immunity:
- high degree of long lasting immunity
- second attacks are quite rare
- occurs with e.g. measles - Moderate immunity : in case of enteric fever.
- 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)
State the
B. Artificially acquired immunity (immunization)?
- 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
What are the forms of the immunizing agents ?
- Vaccines :- killed or attenuated bacterial or viral preparation
- Toxoids :- detoxicated exotoxin preparations
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 ________
According to:
1- The antigen used
2- The body response
