Prevention and control of infectious diseasesSession objectives;
• What is Infection& its process?
• What is the Disease process?
• The causative factors of disease (agents, host and environment).
• Gradient of infection(spectrum of disease)
• What is Communicable diseases?
• Dynamics of infectious disease
• Dynamic of disease transmission
• Reservoir definition & types
• Carriers definition & types (classification)
Prevention and control of infectious diseasesInfection:
Is the entry and development or multiplication of an infectious agent in the body of man or animals.
2. sub clinical infections
3. . latent
4. apparent illness
Disease process: which is the product of reaction (or struggle) between two forces:force of infection (path. M.O).
force of resistance (i.e., the immune response, host defense ).
The modern theory of diseaseepidemiological triad (agents, host and environment)
• Agent factorsThe disease agent is defined as a substance, living or nonliving, or a force, tangible or intangible, the excessive presence or relative lack of which may initiate or perpetuate a disease process.
It classified into:1. Biological agents
2. Nutrient agents
3. Physical agents
4. chemical agents;
(I). Endogenous (produced in the body)
(ii). Exogenous (arising out side the human host)
5. Mechanical agents; (friction, other forces)
6. Absence or insufficiency or excess of factor necessary to life
7. Social agent
• Host:A person or other animal, that affords subsistence or lodgement to an infectious agent under natural conditions.
• Host factors (intrinsic)It classified into:
1. Demographic characteristics (age, sex and ethnicity).
2. Biological characteristics (genetic factors, biochemical levels of the blood, blood group, enzymes and immunological factors).
3. Social and economical characteristics (occupation, education, marital status, economic status, etc.).
4. Lifestyle factors (living habits, nutrition, use of alcohol and smoking, behavioral pattern, etc.).
Host defenses (immune response) against infection are:natural body defense mechanisms (skin, blood, lymph nodes….)→non-specific
Passive immunity from mother to the fetus→ specific and last for few months.
Specific protective antibodies or cellular immunity as a result of previous infection or immunization (specific and active), while anti sera and IG (specific and passive).
• Environmental factors (extrinsic)It is defined as all that which is external to the individual human host, living and nonliving, and with which he is in constant interaction.
a. Physical environment(air, water, soil, housing, climate, heat light, noise, radiation, etc)
b. Biological environment(living things which (surrounds man, including man himself
c. Psychosocial environment (cultural values, customs, and habits, beliefs, attitudes, morals, religion, education, lifestyle, community life, and health services, social and political organization).
Gradient of infectionAlso referred as "spectrum of disease", variation in the manifestation of disease. That varies from sub clinical infections (at one end of the spectrum), to a fatal illnesses (at the other end), in the middle of the spectrum lie the illnesses ranging in severity from mild to sever.
Force of infection Force of resistanceVery high ←------------------------------------→ very weak
Very sever infection (fatal)
High ←--------------------------------------→ weakModerate infection
Bit higher ←----------------------------------→ moderateMild infection
Equal ←----------------------------→ EqualSub clinical infection
Very weak ←---------------------------------------→ very highNo disease
Which is more important?
• Sub-clinical infections are more important in epidemiological point of view because:• The no. of persons with sub-clinical infection may be more than no. of cases (polio).
• The sub-clinical are usually not aware of infection, not take a precaution measures, moving free in the community, while patients are known and avoided.
• They act as a source of infection and can introducing the disease to an area which is free of infection.
Communicable diseases (infectious diseases) are characterized by the existence of living infectious agent which are transmissible and in order to survive it must have a reservoir.
the infectious agents may be viruses, rickettsiae, bacteria, protozoa, fungi or helminthes
Dynamics of infectious disease:incubation period (IP).
prodromal period (PP).
illness (appearance of characteristic sign and symptoms).
**infectivity may occur at any of these periods, and according to type of diseases.Incubation period (I.P):
is the time interval between invasion by the infectious agent and the appearance of first sign or symptom of the disease in question.
I.P can be ranging from very short (few hrs), to very long (few months or years).
prodromal period (P.P):is the interval between the onset of symptoms and appearance of specific characteristic clinical manifestations.
I.P P.P exposure → onset of symptoms→ appearance of characteristic symptoms
communicable period:is the time during which the infectious agent may transferred directly or indirectly from an infected person to another person, or from an infected animal to man.
Infectivity is the ability to transfer the disease to the others.
sequence of events in infection: (dynamic of disease transmission)The sequence of events can be interrupted at any point in order to prevent or retard the development of disease.
A reservoiris defined as “any person, animal, arthropod, plant, soil or substance (or combination of these) in which an infectious agents lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host”.
• The reservoir may be of 3 types:• 1. human reservoir
• Cases (clinical, Sub-clinical infection & latent).
• Carrier (inadequate Rx, inadequate immune response)
• 2. animal reservoir
• Arthropods (tick, mite, flies).
• Lower vertebrates (dogs, bats, mouse, sheep).
• 3. non-living reservoir (soil in tetanus and anthrax)
Carrier:is define as an infected person or animal that harbors a specific infectious agent in the absence of clinical symptoms and serves as a potential source of infection for others, e.g typhoid.
• Carriers can classified according to following:• Course of the disease
• Incubatory carrier (measles, mumps, influenza)
• Convalescent carrier (typhoid,dysentry,diphtheria)
• Healthy carrier (sub clinical with no overt disease)
• Transient carrier (few wks as polio).
• Temporary carrier (6-12 months, dysentery).
• Chronic carrier (>yr, typhoid, hepatitis B).
• Permanent carrier (for a life, AIDS).
• The longer the carrier state, the greater the risk to the community.
• Portal of exit (urinary, intestinal, respiratory, others)