All reading material for the course Containment Strategies of Infectious Diseases in Global Context. An elective for MPA, Health Sciences and Biomedical Sciences Master
Table of contents
Chapter 1 – Elements of Communicable Diseases........................................................................................... 3
, All new and emerging diseases..........................................................................................................................37
,Communicable Diseases a Global
Perspective
Webber
Chapter 1 – Elements of Communicable Diseases
A communicable disease is an illness that is transmitted from a person, animal or inanimate source
to another person either directly, with the assistance of an intermediate host or by a vector.
- Epidemic: the introduction of a new infection or the presence of an illness in excess of
normal expectancy.
o Seasonal for instance with influenza
o Every new infection is epidemic when first introduced
- Endemic: constantly present in a geographical area or population group
o Children are particularly vulnerable
The border between communicable and non-communicable diseases used to be very distinct but is
becoming less defined as new suspect organisms are discovered. various cancers caused by
viruses or parasites (hep-B, Epstein-Bar, malaria)
There needs to be a causative agent, which requires a means of transmission from one host to
another, but the outcome of infection will be influenced by the environment in which the disease is
transmitted and the response of the host.
The agent
- Can be an organism or a physical or chemical agent
- If it is an organism, the agent needs to multiply, find a means of transmission, and survive.
o Multiplication: relevant for treatment and control
Asexual: a succession of exact or almost exact replicas are produced. If a
treatment is found, it is effective to all strains, until a mutation is present.
The strain of the organism is either successful or unsuccessful in invading the
host. Only one needs to be transmitted.
Sexual: natural selection acts on individuals, some will be able to invade the
host and others will not. Some are adapted well enough to succumb
treatment and control measures whereas others are not. Treatment can be
focused on one sex, or of the two sexes not meeting. Both male and female
must meet before reproduction can take place.
o Survival: agents survive by finding a suitable host within a certain period of time
Reservoir: when adaptation occurs so that the reaction of the host
diminishes, and the adaptability of the parasite increases. The parasite is
able to live in the host and the host becomes a reservoir from which
parasites attack new hosts of the same species or attempt to colonize
different species.
Persistence: the development of special stages that resist destruction in an
adverse environment
Latency: the production by the organism of a developmental stage in the
environment that is not infective to a new host.
Transmission:
, - Direct: Person-to-person contact: dirty fingers, via food and water, droplet infection in
respiratory diseases, auto-infection when humans contaminate themselves
- Human reservoir with intermediate host: for transmission to another human, an
intermediate host is needed.
- Animals as intermediate host or reservoir
- Vectors: a vector carries the infection from one host to another, either as part of the
transmission process or mechanically.
o Direct: insect to human malaria
o Insect to animal with humans entering the cycle as an abnormal host bubonic
plague
o Insect to animal, including humans from which it is transmitted to other humans by
the same or another vector yellow fever
Snails are intermediate hosts not vectors!
Zoonoses: infections that are naturally transmitted between vertebrate animals and humans
- Domestic: pets and farm animals
- Synanthropic: animals that live in close association with humans but are not invited rats
- Exoanthropic: animals that are not in close association with humans
A carrier is a person that can transmit the infective agent but is not manifesting the disease
- Incubating or prodromal carriers: infectious but unaware that they are in the early stage of
the disease
- Asymptomatic carriers
- Convalescent carriers: infectious after the clinical phase has passed
The number of carriers may be far greater than the number of sick people. Carriers are not restricted
so they transmit the disease widely.
The agent is undetectable because the numbers are very low or the agent is excreted at infrequent
intervals.
Cyclical subclinical transmission: when infection is transmitted within a family or throughout a
community that is tolerant or has a high level of immunity. The members are asymptomatic.
Host factors:
- Genetic susceptibility: widely diverse heterozygotes are at least 20% better at resisting
infection than related zygotes.
- Age: childhood diseases chickenpox, measles, diphteria
- Sex: this has also to do with occupation and behaviour
- Pregnancy
- Inherent defence mechanisms: skin, mucous-secreting membranes, acidity of the stomach or
the quality of the immune system
- Immunity: experience of precious infection leading to a cellular and humoral response. B-cell
activation stimulate memory-cells to produce long-term protection.
o Active: after an infection with or without symptoms or vaccination with attenuated
organisms. immunity is most completely developed against the viral infections and
may be permanent.
o Passive: the transfer of antibodies from mother to child or from animals or
convalescent human serum. Passive immunity is short.
- Resistance to infection may be lowered by the following:
o Nutrition
o Trauma and debilitating conditions
o Multiple infections
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