Containment Strategies of Infectious Diseases in a Global Context
Institution
Vrije Universiteit Amsterdam (VU)
Book
Communicable Diseases
Complete summary of the selected chapters of the book, including the selected diseases, needed for the course Containment Strategies of Infectious Diseases in a global context. The summary is based on the 5th edition of Communicable diseases a global perspective by Roger Webber.
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. An
epidemic is the introduction of a new infection or the presence of an illness in excess of normal
expectancy. An endemic disease is constantly present in a geological area or population group. The
difference between communicable and non-communicable diseases used to be quite clear cut.
However, this strict boundary is becoming less well defined as new suspects of diseases are
discovered, or diseases, by their very nature, suggest a communicable origin. Such as, the link
between hepatitis B virus and hepatocellular cancer.
The agent can be an organism (virus, prion, bacteria, rickettsia, protozoa, helminth, fungus or
arthropod) or a physical or chemical agent (toxin or poison). Whether the organism reproduces
sexually or asexually is relevant fin treatment and control.
Reservoirs can be humans, animal, vectors or the inanimate environment. Another mechanism used
by parasites to survive is the development of special stages that resist destruction in an adverse
environment, this mechanism is called persistence. The production by the organism of a
developmental stage in the environment that is not infective to a new host is called latency.
Generally, virulence decreases, and passage through many experimental animals is a method used in
developing vaccines. For each infectious agent, a minimum number of organisms, the infective dose,
is required to overcome the defences of the host and cause the disease.
Communicable diseases fall into a number of transmission patterns. Direct transmission includes
person-to-person contact, as from dirty fingers, or via food and water. The adults of the
schistosomiasis parasite live in humans, but for transmission to another human the parasite must
undergo developmental stages in a snail intermediate host. Animals can either be intermediate
hosts, or they can be reservoirs. A vector carriers the infection from one host to another either as
part of the transmission process or mechanically. Zoonoses are infections that are naturally
transmitted between vertebrate animals and humans. These can be grouped according to the
intimacy of the animal to the human: domestic (pets and farm animals), synanthropic (close but
uninvited; rats), and exoanthropic (not close; monkeys). Also plants can serve as a method of
transmission.
A carrier is a person that can transmit the infective agent but is not manifesting the disease. They
could be in an early stage of the disease and not experiencing any symptoms, remain asymptomatic,
or can be convalescent carriers who continue to be infectious after the clinical disease has passed.
Whether an agent can successfully invade a host depends on a number of host factors: genetics, age,
and sex. Some disease will affect certain age groups, others only homozygotes. In addition, pregnant
woman are more susceptible to infections.
Active immunity follows an infection or vaccination, whereas passive immunity is the transfer of
antibodies (via placenta, passive vaccination). A persons resistance may be lowered by nutrition,
trauma, multiple infections or immunodeficiency.
Diseases are best prevented by educating people to overcome them, but resources are required to
achieve this. Education is a complex process, it is not just teaching people; they must be able to
understand to such an extent that they are able to modify their lives. The more people travel, the
more diseases are spread.
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, The physical environment exists of the topography and the climate, including temperature, rainfall
and wind. Temperature not only affects the presence or absence of disease, but often regulates the
amount; some parasites have shorter life cycles in higher temperatures. The pattern of life can be
determined by seasonality, whereas malnutrition occurs before the harvest, rainfall provides
mosquito breeding places, etc..
While most of the concern on increase in disease due to global warming has been expressed by the
Western world, it is more likely that most of the effects will be concentrated in the poorer regions of
the world, with an increase in vector-borne and diarrhoeal diseases, malnutrition and natural
disasters. A development of mapping is geographical information systems (GIS) using complex
computer software packages to analyse a range of coordinates to identify links between them. While
GIS collect information in detail for a small area, particular features identified as risk factors can be
looked for by remote sensing over larger areas (satellite images).
2 Communicable Disease Theory
In a communicable disease, the number of new cases occurring in a period of time is dependent on
the number of infectious persons within a susceptible population and the degree of contact between
them.
The force of infection = Number of infectious individuals X Transmission rate.
Number of newly infected individuals = Force of infection Х Number of susceptible individuals in the
population.
The proportion of susceptible individuals can either be reduced by mortality, immunity or
emigration, or increased by birth or immigration. Epidemic means an excess of cases in the
community from what normally expected, or the appearance of a new infection. The epidemic
threshold can either be the upper limit of cases expected at that particular time, an excess mortality,
or a combination of both the number of cases and mortality.
Latent period is the time interval from initial infection until the start of infectiousness. The
incubation period is the time interval from initial infection until the onset of clinical disease, this has
a particular range for each disease. And the period of communicability is the period during which an
individual is infectious.
Common-source epidemics can be divided into a point-source (food-poisoning), and an extended
epidemic (contaminated well). In a propagated-source epidemic, the agent is spread through serial
transfer from host to host.
In the investigation of any outbreak of a disease, the following information should be gathered:
person (age, sex, occupation, etc.), place (town, geographical features), and time (annual, seasonal).
To instil conformity on what is regarded as a case of the particular disease, a case definition is
developed.
Epidemics are suitably described by expressing them in attack rates. In a common-source epidemic,
the overall attack rate is used. Overall attack rate = Number of individuals affected during an
epidemic / Number (of susceptibles) exposed to the risk.
Contagiousness or the probability that an exposure will lead to a transmission is measured by:
Secondary attack rate = Number of cases within the period of one minimum and one maximum
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