Epidemiological Characteristics of Infectious Diseases

Epidemiological Characteristics of Infectious Diseases
Prepared by: Dr. Hala A. Abed
 To describe any infectious disease, you should comment on:
 Definition of the disease( bacterial ,viral, zoonotic
 Magnitude of the problem.
 Epidemiology of the disease:
 Agent.
 Reservoir: man only, animal only or man& animal.
 Mode of transmission.
 Incubation period.
 Susceptibility:
*Distribution of the disease according to time , place person.
* Immunity: natural( active& passive) and acquired (active&
* Herd Immunity.
 Diagnosis : - Clinical picture, complication and
 Prevention
 Control
Incubation Period(IP):
Definition: It is the time between exposure to an infectious
agent and the onset of symptoms or signs of infection.
Each infectious disease has a typical IP that requires
multiplication of the infectious agent to a threshold necessary to
produce symptoms or laboratory evidence of infection.
 Causes of variability of IP:
 The dose or inoculum of the infectious agent.
 The route of inoculation.
 The rate of replication of the organism.
 Host factors(e.g. resistance, immunity, co-morbidity).
 Duration of IP:
 Shortest IP: few hours as in staphylococcal food poisoning.
 Diseases with short IP period 1-5 days (GISS + DC)are:
(Gonorrhea,Influenza,Shigella, Scarlet fever,Diphteria & Cerebrospinal
 Diseases with medium IP 6-12 days (3PRAM) are:
(Pertussis, Plague, Poliomyelitis, Relapsing fever, Anthrax & measles).
 All other diseases have IP period 2-3 weeks except:
-Cholera 5days.
-Plague 6days.
-yellow fever 6days.
 IP of parasitic diseases is 4 weeks.
 Diseases with long IP are:
-Hepatitis A Virus 1.5 months.
- Hepatitis B Virus 2-6 months.
 Very long IP:
-Leprosy 2-5years.
-AIDS 6m- 10years.
Epidemiological importance of IP:
Control of infectious diseases to trace source of infection.
Control of contact of cases either by surveillance for
maximum IP.
International measures for qurantinable diseases.
Specific protection of exposed individuals as in:
Cases of exposure to small pox early in IP person could be
vaccinated as the immunity is produced after 8 days while IP is 14
N.B Extrinsic incubation period: This the period taken by the
infectious agent outside the human body until it becomes infective
again to a new individual.
 Latent period: The time that elapses between the entry of the
agent in the human body to the point when the shedding of the
organism starts.
 Period of communicability: This the duration for which the host
sheds the organism.
 Generation time: The duration between the entry of the
infectious agent into the body to the peak of infectivity of the
host, it is equal to( latent period + period of maximum
Biological Characteristics
of the Organism
 Definition: It is the ability of the agent to cause infection in
a susceptible host.
 Measurement: The proportion of susceptible individuals
who develop infection after exposure ( secondary attack
rate) is a measure of the infectivity of an organism.
 Pathogenicity:
 Definition: It is the ability of the organism to induce disease.
 Measurement: The proportion of individuals with asymptomatic
or clinically inapparent infections is the measure of the
pathogenicity of the organism.
 Virulence:
 Definition: it is the severity of the disease after infection
 Measurement: It can be measured by case fatality rate or the
proportion of clinical cases that develop sever disease.
 Immunogenecity:
 Definition: It is the ability of the organism to produce an
immune response after infection.
 Advantage of Immunogenecity:
- Some organisms such are measles, yellow fever lead to solid
- Studies of the antigens that produce protective immunity after
natural infections have led to development of effective
- Antibodies may occur that are markers of a previous or current
 Disadvantage of Immunogenecity:
-Immune response that is provoked by some agents may not be
protective from future infections.
- Sometimes the immune response may be harmful to the host
e.g. Rheumatic fever.
Infectious process
 Infection: the entry and development or multiplication of an
infectious agent in the body of man or animals.
 Infestation: the presence of living infectious agents on the
exterior surface of the body(e.g. scabies).
 Contamination: The presence of living infectious agents
upon articles( soiled articles).
 Endemic:……………………………………
 Epidemic:…………………………………..
 Outbreak:………………………………….
 Pandemic:………………………………….
 Sporadic:……………………………………
 Non-infectious diseases:………………..
Classification of infectious
diseases according to their
a) Man only:
 Cases: Varicella- H.Z-Pertussis-measles-Epidemic typhusInfective conjunctivitis-scabies- Syphilis-GonorrheaLeprosy.
 Cases& Carriers: Diphteria-Mumps-RubellaStrept.Pharingitis-Meningocoocal meningitis-EntericaPoliomylitis-Shigellosis-Cholera-Viral Hepatitis.
b) Animal only: Brucellosis- Botulism-Plague-Q feverAnthrax-Rabies- Rift valley fever- Foot and mouth
disease- Encephalitis.
c) Man and Animal: T.B- Influenza- Infectious food
poisoning – Yellow fever- Tetanus- Gas gangrene.
** Infectious diseases with four types of carriers: EntericaPoliomyelitis- Cholera.
** Infectious diseases with highest incidence at 5-15years
old: Mumps- Rheumatic- Pertussis- M.Meningitis- Influenza- HAV.
** Infectious diseases produce neurotoxin: Tetanus- BotulismShigella shiga.
Specific prevention of
infectious diseases:
a) Seroprophylaxis only: Botulism only.
b) Active immunization only: Enterica-Yellow feverPneumococcal.pn-H. Influenza b-Epidemic typhus.
c) Immunization ,both forms: Diphtheria, Measles-MumpsRubella, Varicella, Poliomyelitis- Hepatitis A , B-Rabies.
d) Chemoprophylaxis: Strept. Pharyng- Puerperal sepsisSyphilis- Gonorrhea.
e) Seroprophylaxis & Chemoprophylaxis: Gas gangrene
f) Active immunization & Chemoprophylaxis: InfluenzaM.Meningitis-T.B-Cholera-Plague- Leprosy.
g) Immunization ,both forms& Chemoprophylaxis:
** Protective value of vaccines:
Absolute: Yellow fever- Varicella- Rabies vaccines.
Solid : M.M.R- DT toxoid- HBV (99%).
High protection: BCG- Sabin vaccine- Pertussis vaccine (8090%).
Moderate: TAB vaccine- Cholera vaccine (40-60%).
** Protective period of vaccines:
 Short period for some months: Cholera – Plague vaccines.
 About 3 years: TAB vaccine.
 3-5 years: DPT- Tetanus toxoid.
 5 or more years: BCG, Epidemic typhus vaccine.
 Life time protection: Yellow fever & MMR vaccines.
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