Susceptible host and host defence

Dr. Gneyaa Bhatt
Assistant Professor,
Deptt. Of Community Medicine
1GSB/ Susceptible host-defences

Objectives
Host factors
Successful parasitism
Incubation period & latent period
Serial interval, generation time,
communicable period
Secondary Attack Rate
Various Host Defences
GSB/ Susceptible host-defences 2

Susceptible Host
Source/Reservoir Mode of Transmission
3 strategies:
1) control of reservoir/source of infection
2) Breaking channel of transmission
3) Protect the susceptible host
Dynamics of disease transmission/
Chain of Transmission

A person lacking
effective resistance
to a particular
pathogenic
organism
Susceptible host

Host factors
(a) Host Attributes which Affect the Probability of Being
Exposed to the Infectious Agent :
 age (e.g. young children);
 Sex (e.g. females)
 Economic status (poverty),
 Occupation (e.g. agricultural workers and veterinarians likely
to be exposed to certain zoonoses),
 Education,
 Living conditions (Poor housing, overcrowding, lack of
sanitary eating and drinking facilities),
 Life style and behavioural factors (e.g. permissive attitude
toward sex will increase the probability of exposure to reservoir
of STDs),
 use of Personal protective measures (e.g. use of mosquito
nets etc)

 (b) Host Factors that Influence Occurrence of
Infection and Disease :
 status of host immunity;
 Age;
 Genetic make up
 Availability & utilization of health services

Susceptible host
 An infectious agent seeks a susceptible host
aiming “successful parasitism”.
 Four stages are required for successful
parasitism:
1. Portal of entry
2. Site of election inside the body
3. Portal of exit
4. Survival in external environment

Incubation and Latent periods
 Incubation period: time from exposure to
development of disease.
 Incubation period is the time period between
 the entry of infectious agent (or its toxin) into the
human body and the point when the earliest
clinical manifestations (first sign or symptom ) of
the disease are apparent.

 During this period, the host does not exhibit any
outwardly clinical manifestations, though
immunological and histopathological changes within
the body would definitely occur.
 If, during this period, the organism is also shed from
the body of the host, the host qualifies to be an
“incubatory carrier”.
 Incubation period is usually measured in terms of
“median incubation period”, i.e. the time in which half
of the infected subjects will develop clinical
manifestations, following entry of the organism into the
body.

 Alongwith the median incubation period, a “range” is
also given which indicates the minimum and
maximum incubation periods.
A) Short
B) Medium
C) Long
 Latent period: the period used in NCD & it is
between disease initiation to disease detection.

 Incubation period of a disease is found out by studying
the time taken for onset of secondary cases following
exposure to the index case, in family groups or in
closed communities, or else during investigation of
“common - vehicle, point source epidemics”.
 Different diseases have different values of median
incubation period and range, and a specialist in Public
health should remember them well.

Importance of Incubation Period
A) Tracing the source of infection and contacts
B) Period of surveillance/ quarantine- equal to
maximum incubation period
C) immunization
D) Identification of point source or propagated
epidemics
E) Prognosis e.g. rabies, tetanus

Serial interval
 Serial interval: the gap in time between the onset
of the primary and the secondary cases.

 Generation Time :
 The generation time is the duration between the
entry of infectious agent into the body to the peak
infectivity of the host.

Infectious (communicable)
period
 Infectious (communicable) period:
length of time a person can transmit disease (sheds the
infectious agent).
 Important measure of communicability is Secondary
Attack Rate (SAR).

 The relationship between the various landmarks of a typical
infectious disease is depicted as follows :
 I -----------I ---------- I -----------I ----------- I ----------- I ---------- I
A B C D E F G
 A = Entry of agent into host; B = Shedding of agent starts;
C = Clinical manifestations start;
 D = Maximum infectivity of host;
 E = Clinical disease ends;
 F =Shedding of agent ends; G = Convalescence ends;

 The relationship between the various landmarks of a typical
infectious disease is depicted as follows :
I -----------I ---------- I -----------I ----------- I ----------- I ---------- I
A B C D E F G
 A = Entry of agent into host; B = Shedding of agent
starts; C = Clinical manifestations start; D = Maximum
infectivity of host; E = Clinical disease ends; F
=Shedding of agent ends; G = Convalescence ends;
 A to C = Incubation period;
A to D = Generation time;
B to F= Period of Communicability;
B to C = Incubatory carrier phase;

Secondary Attack rate (%)
No. of exposed persons
developing the disease within
the range of incubation period
Total no. of exposed/ susceptible
SAR = X 100

Secondary Attack rate (%)
 The denominator consists of all persons who are
exposed to the case.
 It is restricted only to the susceptible contacts, if
means are available to distinguish the susceptible
persons immune.
 The primary case is excluded from both the numerator
& denominator.

Find Out Secondary Attack Rate
A 3 years old Ramesh went to his uncle’s home on 1st Sept. His
uncle’s son developed rashes of measles on 5th sept. On 6th
Sept. Ramesh came back home & started going to school
next day. Ramesh developed rash on 12th Sept.
Total no. of enrolled students in his class- 50
Out of which
15 were vaccinated previously against measles,
9 already had measles in past.
 20 students developed rashes of measles between 20th
Sept.- 23rd Sept. while 3 students developed rashes of
measles between 2nd Oct.-5th Oct.

A 3 years old Ramesh went to his uncle’s home on 1st Sept. His
uncle’s son developed rashes of measles on 5th sept. On 6th
Sept. Ramesh came back home & started going to school
next day. Ramesh developed rash on 12th Sept.
Total no. of enrolled students in his class- 50
Out of which
15 were vaccinated previously against measles,
9 already had measles in past.
 20 students developed rashes of measles between 20nd
Sept.- 23rd Sept. while 3 student developed rashes of
measles between 2nd Oct.-5th Oct.
Numerator- 20 Denominator- 50-1-15-9 =25
SAR= 20 X 100 = 80%
25 25GSB/ Susceptible host-defences

Secondary Attack rate- Uses
1. Spread of infection within a family, household or
any closed aggregate who have had contact with
a case of disease. The communicable disease
with high SAR in contacts is considered to be
highly infectious.
2. The SAR calculated in different groups (age,
occupation) helps in identifying the susceptible.
3. Fall in SAR after intervention indicates
usefulness of preventive measures e.g. SAR in
vaccines v/s non- vaccines.

Secondary Attack rate- Limitation
1. Disease with long infective period.
2. Infectious disease with long incubation period
where primary case is infective for long period of
time.
3. Disease which cannot recognized on clinical
examination alone or disease with many sub-clinical
cases.

Host Defense Mechanisms
 Non-specific host defense mechanisms serve to protect
the body from a variety of foreign substances or pathogens
(first and second line of defense)
 Specific host defense mechanisms are directed against a
particular foreign substance or pathogen that has entered the
body (third line of defense)

Specific Host Defense Mechanisms
 1. Active immunity.
2. Passive immunity.

Specific Host Defense Mechanisms (Cont’d)
 Active Immunity:
a. Natural active immunity: that is acquired
in response to the entry of a live pathogen into the
body (i.e., in response to an actual infection)- it has
long duration
b. Artificial active immunity: immunity that
is acquired in response to vaccines. Its duration for
many years; but must be reinforced by boosters
inculcations.

 Active Immunity
1. Humoral immunity
2. Cellular immunity
3. Combination of the above
 Acquired in 3 ways:
1. Following clinical infection e.g. chickenpox, rubella
2. Following subclinical or inapperant infection e.g.
polio, diphtheria
3. Following immunization

Immune response
A) Primary response:
- Production of memory cells
B) Secondary (booster)response:

Humoral immunity
 Comes from the B cells which produces antibodies.
 Specific for specific antigen.
 5 different classes of Ig
 Ig G, Ig M, Ig A, Ig D, Ig E

Cellular immunity
 Mediated by T cells
 Imp. In many diseases like TB, Leprosy
Combination of Humoral & Cellular
immunity

Specific Host Defense Mechanisms (Cont’d)
 Passive Immunity:
a. Natural passive immunity: immunity
that is acquired by a fetus when it receives
maternal antibodies in utero or by an infant when
it receives maternal antibodies contained in
colostrum. Its duration from 6 months- 1 year.
b. Artificial passive immunity: immunity
that is acquired when a person receives antibodies
contained in anti-sera or gamma globulin. Its duration
from 2 – 3 weeks.

 Passive Immunity
1. Normal human Ig
2. Specific human Ig
3. Animal antitoxins or antisera
 Induced by:
1. By administering an Ig or Antisera
2. By transfer of maternal Abs across placenta or
human milk
3. By transfer of lymphocytes- experimental

Comparison between Active &
Passive Immunity

Herd Immunity
 It is the level of immunity that is present in a
population against an infectious agent.
 It may be defined as “the resistance of a group to attack
by a disease to which a large proportion of the
members are immune, thus decreasing the probability
that a person having the infectious agent will transmit
it to another susceptible person in the same
population”.

 Elements
1. Occurrence of clinical & sub clinical infection in
herd
2. Immunization of herd
3. Herd structure
 Herd immunity determined by serological surveys.
 Tetanus – no herd immunity
 Polio, diphtheria- herd immunity present

 In general, while dealing with childhood infectious
diseases amenable to prevention by immunization,
 vaccination coverage of about 85% is likely to provide
adequate herd immunity, which will effectively block
the disease transmission, even if remaining 15%
children are not immunized (though there may be
many exceptions to this generally held belief).

Thank you

Short notes
 Q- Herd Immunity
 Q- Incubation Period
 Q- Secondary Attack Rate

Susceptible host and host defence

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Susceptible host and host defence

Similar to Susceptible host and host defence (20)

Recently uploaded

Recently uploaded (20)

Susceptible host and host defence