Download Communicable diseases: epidemiology surveillance and response

Communicable diseases:
epidemiology
surveillance and response
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Definitions
1. A communicable (or infectious) disease is
one caused by transmission of a specific
pathogenic agent to a susceptible host.
• Infectious agents may be transmitted to humans
either:
• Directly, from other infected humans or
animals,
• Indirectly, through vectors, airborne particles or
vehicles.
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2. Vectors are insects or animals that carry the
infectious agent from person to person.
3. Vehicles are contaminated objects or
elements of the environment (such as
clothes, water, milk, food, blood, plasma,
parenteral solutions or surgical instruments).
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• 2- Contagious diseases are those that can be
spread between humans without an
intervening vector or vehicle.
• Malaria is therefore a communicable but not a
contagious disease, while measles are both
communicable and contagious.
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• Some pathogens cause disease not only
through infection but through the toxic effect
of chemical compounds that these produce.
• For example, Staphylococcus aureus is a
bacteria that can infect humans directly, but
staphylococcal food poisoning is caused by
ingestion of food contaminated with a toxin
that the bacteria produces.
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Role of epidemiology
• Epidemiology
developed
the
study
of
outbreaks of communicable disease and the
interaction between agents, hosts, vectors and
reservoirs.
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• Describe the circumstances that tend to make
epidemics in human populations (war, migration,
famine and natural disasters)
human
ability
to
control
has increased
the
spread
of
communicable disease through surveillance,
prevention, quarantine and treatment.
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Threats to human security and
health systems
• Communicable diseases pose an acute threat
to individual health and have the potential to
threaten collective human security.
• low-income countries continue to deal with
the problems of communicable diseases,
deaths due to chronic diseases are rapidly
increasing.
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• Although high-income countries have
proportionally less communicable disease
mortality, these countries still bear the costs
of high morbidity from certain communicable
diseases. For example, in high-income
countries, upper respiratory tract infections
cause significant mortality only at the
extremes of age (in children and elderly
people).
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Epidemic and Endemic disease
• Epidemics
• Epidemics are defined as the occurrence of
cases in excess of what is normally expected in
a community or region.
• When describing an epidemic, the time
period, geographical region and particulars of
the population in which the cases occur must
be specified.
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• The number of cases needed to define an
epidemic varies according to:
1.
2.
3.
4.
5.
the agent,
the size of population exposed
type of population exposed
Susceptibility of population exposed
time and place of occurrence.
6.
usual frequency of the disease in the area among the specified
population during the same season of the year.
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Endemic diseases
• Communicable diseases are termed endemic
when they have a relatively stable pattern of
occurrence in a given geographical area or
population group at relatively high prevalence
and incidence.
• If conditions change in the host, the agent or
the environment, an endemic disease may
become epidemic.
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• Endemic diseases such as malaria are among
the major health problems in low-income
tropical countries.
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Chain of infection
• Communicable diseases occur as a result of
the interaction between:
• The Infectious Agent
• The Transmission Process
• The Host
• The Environment.
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• The control of such diseases may involve
changing one or more of these components,
• Knowledge of each factor in a chain of
infection may be required before effective
intervention can take place.
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The infectious agent
• A large number of microorganisms cause
disease in humans.
• Infection is the entry and development or
multiplication of an infectious agent in the
host.
• Infection is not equivalent to disease, as some
infections do not produce clinical disease.
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• The specific characteristics of each agent are
important in determining the nature of the
infection, which is determined by such factors
as:
1. The pathogenicity of the agent: its ability to
produce disease, measured by the ratio of the
number of persons developing clinical illness to
the number exposed.
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2. Virulence: a measure of the severity of disease,
which can vary from very low to very high. Once
a virus has been attenuated in a laboratory and
is of low virulence, it can be used for
immunization, as with the poliomyelitis virus.
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3. Infective dose: the amount required to cause infection in
susceptible subjects.
4. The reservoir of an agent: its natural habitat, which may
include humans, animals and environmental sources.
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5. The source of infection: the person or object from
which the host acquires the agent.
– Knowledge of both the reservoir and the source is
necessary if effective control measures are to be
developed.
– An important source of infection may be a carrier
(an
infected person who shows no evidence of clinical
disease).
– The duration of the carrier state varies between agents.
– Carriers can be asymptomatic throughout the course of
infection or the carrier state may be limited to a particular
phase of the disease.
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Transmission
• The second link in the chain of infection is the
transmission or spread of an infectious agent
through the environment or to another
person.
• Transmission may be Direct or Indirect.
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Direct transmission
• Direct transmission is the immediate transfer
of the infectious agent from an infected host
or reservoir to an appropriate entry point
through which human infection can take
place.
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• This may be by direct contact such as touching,
kissing or sexual intercourse, or by the direct
spread of droplets by sneezing or coughing.
• Blood transfusions and transplacental infection
from mother to fetus are other important means
of direct transmission.
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Indirect transmission
• Indirect transmission may be vehicle-borne,
vector-borne or airborne.
1. Vehicle-borne
transmission
occurs
through
contaminated materials such as food, clothes,
and cooking utensils.
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2. Vector-borne transmission occurs when the
agent is
carried by an insect or animal (the
vector) to a susceptible host; the agent may or
may not multiply in the vector.
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3.
Airborne transmission occurs when there is dissemination
of very small droplets to a suitable point of entry, usually
the respiratory tract. Dust particles also facilitate airborne
transmission, for example, of fungal spores.
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• Direct transmission can be interrupted by
preventing contact with the source;
• indirect transmission requires different
approaches, such as the provision of mosquito
nets, adequate ventilation, cold storage for
foods or sterile syringes and needles.
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Host
• The host is the third link in the chain of
infection and is defined as the person or
animal that provides a suitable place for an
infectious agent to grow and multiply under
natural conditions.
• The points of entry to the host vary with the
agent and include the skin, mucous
membranes, and the respiratory and
gastrointestinal tracts.
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• The reaction of the host to infection is
extremely variable, being determined by the
interaction between host, agent and mode of
transmission.
• The spectrum of this reaction ranges from no
apparent signs or symptoms to severe clinical
illness.
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• The incubation period : the time between
entry of the infectious agent and the
appearance of the first sign or symptom of the
disease.
• varies from a few hours (staphylococcal food
poisoning) to years(AIDS).
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• The consequences of infection are largely
determined by the host's resistance.
• Such resistance is usually acquired through
previous exposure to or immunization against
the agent.
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• Immunization (or vaccination) is the
protection of susceptible individuals from
communicable disease by the administration
of a vaccine, which can be:
• a living modified infectious agent (as for measles)
• inactivated organisms (as for pertussis)
• an inactive toxin (as for tetanus)
• bacterial polysaccharides.
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• Antibodies – which are formed as part of the
natural immune response to pathogens – can
be pooled from blood donations and given as
post-exposure prophylaxis for a few diseases
(such as rabies, diphtheria, varicella-zoster
and hepatitis B) to people that have not been
adequately immunized.
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• This is called passive immunization, and is
done on a much smaller scale than active
immunization due to its risks, indications and
cost.
• Passive transmission of maternal antibodies
through the placenta can also confer
resistance to infection in the fetus.
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Environment
• The environment plays a critical role in the
development of communicable diseases.
• General sanitation, air pollution and water
quality are among the factors that influence
all stages in the chain of infection.
• Socioeconomic factors – such as population
density, overcrowding and poverty – are of
great importance.
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