Ecology of the disease

or
dynamic of Disease Transmission
chapter two
 Ecology
It is the study of interrelationship ( within ecosystem) of living
things to the natural or man – made environment.
Ecology is a biological discipline , which deals with the mutual
relationship of various organisms in an environment and their
reaction to animate and inanimate subject.
interactions between man and environment he lives in. Many
results may come out due to the interactions.
Human disease does not arise in a vacuum. It results from an
interaction of the host (a person), the agent (e.g., a bacterium),
and the environment (e.g., a contaminated water supply).
 Although some diseases are largely genetic in origin
 Conti………..
Three causative factors of disease may be
classified as Agent, Host and Environment.
These three factors are referred to as ‘’
Epidemiological triad “
The health or disease conditions are expression
of dynamic relationship among the ecological
factors namely agent, host and environment.
Disease occur when host , agent and
environment are not in balance---------------
 Conti…….
Due to new agent.
Due to change in existing agent
Due to change in number of susceptibilities in the
population.
Due to environmental changes that effect the
agent or growth of agent.
 There must be a unique combination of events,
i.e. a harmful agent that comes into with a
susceptible host in a proper environment
Occurrence of diseases
Conti…………
 Conti…….
• Condition may vary in the agent: Agent may be
virulent or weak.
• Condition may be in the host : Host may be
resistant or susceptible
• Condition may vary in environment :
Environment may be favorable or unfavorable.
Environment act as a central factor in
epidemiological triad , because it has got
influence over both agent and host.
 Uses of epidemiological triad
• To identify the weakest link
• To identify the most appropriate measure for disease
prevention and control
• To study natural history of disease
Challenges of epidemiology
• Instant global transmission of pathogens
– Population overcrowding
– Ease of travel
– Importation of foods
 Therefore epidemiology has provided new opportunities for
prevention, treatment , planning and improving effectiveness
and efficiency of health services.
 Agent
• An agent is a substance , living or non-living ,
or a force, tangible or intangible, the relative
excess or lack of which or the mere presence
of which may initiate or perpetuate disease or
health related state or event which may or
may not be immediately manifested. Tangible
force can least be felt but the intangible force
can no way be felt.
 Classification of agent
Nutritional : Excess – Obesity, Deficiency-PEM,
Iodine deficiency
Chemical : Endogenous- e.g. Urea, uric acid,
Bilirubin , Exogenous- e.g. Allergens , Gases,
Fumes, Drugs
Physical : Radiation – Burn, Bone marrow
deficiency, Mechanical – Sound, cut injury,
Abrasion
 Conti……
Infectious : Bacteria, Fungus, protozoa,
Rickettsia
a) Of these agents, infectious agents are
important because : a) The disease most
commonly occurring are by infectious agents.
b) They can be easily detected , as their
pathological characteristics are well
understood
 Host
• A host is any organism that gives shelter to another
organism .But in epidemiology we should only mean the
human to be the host.
• A person or other living organism on which another lives as
parasite is the host..
o Obligate host – an obligate host means the only host
e. g. man in measles and typhoid fever.
o Primary or definitive host – Host in which parasite passes its
sexual host.
o Transport host – is a carrier in which the organism remains
alive but does not undergo development
Characteristics of agent, host and
environment
 Host response
 When a agent invades a susceptible host ,
some reactions are evoked inside the host.
The response or reaction of host to entry of
agent is called host response.
 Host response can be divide into two parts ;
specific or immunity and non- specific
 Environment
• Environment is not merely surroundings but
includes all external conditions, which have an
effect on the life and development of human
host.
• Environmental factors – which can influence
health status of populations.
• In epidemiology environment means external
environment
 Conti………
• Human disease does not arise in a vacuum.
• It results from an interaction of the host (a
person), the agent (e.g., a bacterium), and the
environment (e.g., a contaminated water supply).
• Although some diseases are largely genetic in
origin, virtually all disease results from an
interaction of genetic and environmental factors,
with the exact balance differing for different
diseases.
 Conti………….
• Disease has been classically described as the
result of an epidemiologic triad.
• According to this diagram, it is the product of
an interaction of the human host, an
infectious or other type of agent, and the
environment that promotes the exposure.
• A vector, such as the mosquito or the deer
tick, is often involved.
 Cont……
• For such an interaction to take place, the host
must be susceptible. Human susceptibility is
determined by a variety of factors including
genetic background and nutritional and
immunologic characteristics.
• The factors that can cause human disease
include biologic, physical, and chemical factors
as well as other types, such as stress, that may
be harder to classify.
 MODES OF TRANSMISSION
• Diseases can be transmitted directly or
indirectly. For example, a disease can be
transmitted person to person (direct
transmission) by means of direct contact.
• Indirect transmission can occur through a
common vehicle such as a contaminated air or
water supply, or by a vector such as the
mosquito.
 Modes of Disease Transmission
1. Direct
a) Person-to-person contact
2. Indirect
a) Common vehicle
(1) Single exposure
(2) Multiple exposures
(3) Continuous exposure
b. Vector
 Cont…….
• Thus, different organisms spread in different
ways, and the potential of a given organism
for spreading and producing outbreaks
depends on the characteristics of the
organism, such as its rate of growth and the
route by which it is transmitted from one
person to another.
 CLINICAL AND SUBCLINICAL DISEASE
• It is important to recognize the broad spectrum of
disease severity. Here we will discus the iceberg
concept of disease. Just as most of an iceberg is
underwater and hidden from view with only its tip
visible, so it is with disease:
• only clinical illness is readily apparent
• But infections without clinical illness are important,
particularly in the web of disease transmission,
although they are not visible clinically.
• The iceberg concept is important because it is
not sufficient to count only the clinically
apparent cases we see; for example, most cases
of polio in pre-vaccine days were subclinical—
that is, many people who contracted polio
infection were not clinically ill.
• Nevertheless, they were still capable of
spreading the virus to others. As a result, we
cannot understand and explain the spread of
polio unless the pool of inapparent cases is
recognized.
 Stages of the disease
• As clinical and biologic knowledge has increased
over the years, so has our ability to distinguish
different stages of disease.
• These include:
 clinical disease
nonclinical disease
Clinical Disease
 Clinical disease is characterized by signs and
symptoms
 Nonclinical (Inapparent) Disease
• Nonclinical disease may include the following:
1. Preclinical Disease. Disease that is not yet clinically
apparent but is destined to progress to clinical disease.
2. Subclinical Disease. Disease that is not clinically apparent
and is not destined to become clinically apparent. This
type of disease is often diagnosed by serologic (antibody)
response or culture of the organism.
3. Persistent (Chronic) Disease. A person fails to “shake off”
the infection, and it persists for years, at times for life.
 Conti…….
4) Latent Disease. An infection with no active multiplication of the
agent.
CARRIER STATUS
 A carrier is an individual who harbors the organism but is not
infected as measured by serologic studies (no evidence of an
antibody response) or by evidence of clinical illness.
One of the best-known examples of a long-term carrier was
Typhoid Mary, who carried Salmonella typhi and died in 1938.
Over a period of many years, she worked as a cook in the New
York City area, moving from household to household under
different names. She was considered to have caused at least 10
typhoid fever outbreaks that included 51 cases and 3 deaths.
 ENDEMIC, EPIDEMIC, AND PANDEMIC

I. Endemic is defined as the habitual presence

of a disease within a given geographical area.
II. Epidemic is defined as the occurrence in a
community or region of a group of illnesses
of similar nature, clearly in excess of normal
expectancy, and derived from a common or
from a propagated source .
III. Pandemic refers to a worldwide epidemic.
 IMMUNITY AND SUSCEPTIBILITY
• The amount of disease in a population
depends on a balance between the number of
people in that population who are susceptible,
and therefore at risk for the disease, and the
number of people who are not susceptible, or
immune, and therefore not at risk.
• They may be immune because they have had
the disease previously or because they have
been immunized.
 Cont……..
• They also may be not susceptible on a genetic
basis. Clearly, if the entire population is
immune, no epidemic will develop.
• But the balance is usually struck somewhere in
between immunity and susceptibility, and
when it moves toward susceptibility, the
likelihood of an outbreak increases.
 HERD IMMUNITY
• Herd immunity may be defined as the
resistance of a group of people to an attack by
a disease to which a large proportion of the
members of the group are immune.
• If a large percentage of the population is
immune, the entire population is likely to be
protected, not just those who are immune
 Cont……….
• Why does herd immunity occur?
• It happens because disease spreads from one
person to another in any community. Once a
certain proportion of people in the community
are immune, the likelihood is small that an
infected person will encounter a susceptible
person to whom he can transmit the infection
 Cont……..
• Why is the concept of herd immunity so
important?
 When we carry out immunization programs, it
may not be necessary to achieve 100%
immunization rates to immunize the
population successfully. We can achieve highly
effective protection by immunizing a large part
of the population; the remaining part will be
protected because of herd immunity.
 Cont……
• For herd immunity to exist, certain conditions
must be met. The disease agent must be
restricted to a single host species within which
transmission occurs, and that transmission
must be relatively direct from one member of
the host species to another.
• Let us consider poliomyelitis immunization
and herd immunity. From 1951 to 1954, an
average of 24,220 cases of paralytic
poliomyelitis occurred in the United States
each year. Two types of vaccine are available.
• The oral polio vaccine (OPV) not only protects
those who are vaccinated, but also protects
others in the community through secondary
immunity, produced when the vaccinated
individual spreads the active vaccine virus to
contacts.
 Cont…….
• However, even inactivated poliovirus vaccine
(IPV), which does not produce secondary
immunity (does not spread the virus), can
produce herd immunity if enough of the
population is immunized; even those who are
not immunized will be protected because the
chain of transmission in the community has
been interrupted.
 INCUBATION PERIOD
• The incubation period is defined as the interval
from receipt of infection to the time of onset
of clinical illness.
• If you become infected today, the disease with
which you are infected may not develop for a
number of days or weeks.
• During this time, the incubation period, you feel
completely well and show no signs of the
disease.
 Cont…….
• Why doesn’t disease develop immediately at the time of
infection?
• It may reflect the time needed for the organism to replicate
sufficiently until it reaches the critical mass needed for
clinical disease to result.
• It probably also relates to the site in the body at which the
organism replicates— whether it replicates superficially,
near the skin surface, or deeper in the body.
• The dose of the infectious agent received at the time of
infection may also influence the length of the incubation
period.