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Webber summary chapter 1-4
A summary of Webber Chapter 1-4.
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SUMMARY – COMMUNICABLE DISEASES (WEBBER)Chapter 1 – Elements of Communicable Diseases
Communicable disease: 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: introduction of a new infection or the presence of illness in excess of normal expectancy.
Can be:
- Seasonal (influenza)
- The number of susceptible persons is sufficient for a new epidemic to take place (measles)
ANY unknown infection will be epidemic when first introduced
Endemic: constantly present in a geographical area or population group (e.g. malaria in tropical
countries)
Communicable diseases are dependent on:
- A person’s susceptibility (e.g. children meeting an infection for the first time; isolated
communities)
- Conditions that encourage transmission (poor hygiene and overcrowding)
- Invariably associated with poverty
With epidemic diseases, whole populations can be devastated – often young and growing population
recovers. With endemic diseases, children are particularly vulnerable; a high birth rate often tries to
compensate for this.
A tropical environment is more favourable to many diseases than the cooler regions. There is no
difference between the divided parts of the world except for the resources that each is able to devote
to the improvement of their health. Communicable diseases could be reduced to manageable problems
if enough resources (financial and educational) could be spent on them.
In time nearly all diseases will be shown to have a transmissible factor in their causation. Avoiding
infection is the best strategy, however, our cleanliness might be responsible for the increase in
allergies, type I diabetes, inflammatory bowel disorders, and MS in developed countries. Minor
infections can stimulate the immune system.
Key to communicable diseases: think of it in terms of:
1. Agent
2. Transmission
3. Host
4. Environment
FIGURE 1.1
There needs to be a causative agent which requires a means of transmission from one host to
another; the outcome of the infection will be influenced by: the environment in which the disease is
transmitted and the response of the host.
THE AGENT
- Organism (virus, prion, bacteria, fungus, rickettsia (gram-negative bacteria in ticks, lice,
fleas, mites, etc.), protozoa, helminth, arthropod) needs to multiply, find a means of
transmission, and survive.
- Physical or chemical agent (toxin of poison)
1
,Multiplication
Sexual or asexual. Important in the treatment and control. Asexual; successful against all
individuals in a strain (unless a mutation occurs). Sexual reproduction can lead to individuals of
different vigor – some will succumb to treatment while others will not. Having 2 sexes can be a
disadvantage: attacking only one sex means a reduced chance of meeting with individuals from the
other sex.
- Asexual: a succession of exact or almost exact replicas. Natural selection will act on batches
or strains (and not individuals).
- Sexual: offers great variety within cells of single organisms and from one organism to another
acts on individuals
Wuchereria bancrofti: one of the most successful parasites, contrary to its disadvantage in which it
requires finding an individual from the other sex by migrating through the body
Survival
Parasitic agents survive by finding a suitable host within a certain period. It is within the parasite’s
interest to improve its reproductive capability, so if a new mutation arises which is beneficial to this
end, this will be selected (generally to the hosts’ disadvantage).
Reservoir: storage place for water and serves as an appropriate term to describe a suitable place for
storing agents of infection. Can also be humans, animals, vectors, or the inanimate environment (e.g.
soil water).
After the invasion of a host:
1. Latent/waiting period – production of sufficient organisms
2. Main attack
3. Host survives and no elimination? = continuation of living and reproduction in the host.
Few ill effects – can form an established population and continue with minimal reaction from
the host = host acts as a reservoir
Biological arms race: a continuing challenge between parasite and host.
Some parasites might be to the advantage of a host, but our hygienic surrounding decreases these
parasites = increase in allergic diseases. Sometimes is preferred to be introduced to a parasite.
Some parasites can develop stages that resist destruction in an adverse environment by the
development of spores (as with anthrax and tetanus bacilli). Destruction of persistent organisms
requires antiseptics or sufficient heat for a longer period of time.
Latency: a developmental stage in the environment that is not infective to a new host. It allows the
parasite to wait until suitable conditions occur, before changing in the infective form (e.g. hookworms,
such as Ascaris).
Effect
If enough agent survives, it will produce illness. The severity is determined by:
1. Toxicity
2. Virulence
The toxic reaction is produced by foreign proteins which consist of or produce in their respiratory or
reproductive processes. Some agents have a very marked effect on their host, while others have a mild
one.
Spanish flu: first influenza epidemic killed 20-40 million people worldwide, while further infections
are often mild with mortality almost only in the young and aged. Virulence can increase or decrease
owing to its passage through several individuals.
2
, Some organisms produce toxins when they grow in food; causing illness at distance, e.g. Clostridium
botulinum.
Excreted load and infective dose
The number of organisms excreted can vary considerably according to the type of infection or the
stage of the disease.
Infective dose: for each infectious agent, a minimum number of organisms (infective dose) is
required to overcome the defenses of the host and cause the disease. Sometimes a large dose is
required (vibrio cholerae) or very few (E. Histolytica). When this dose is surpassed, the severity of a
disease is often the same, while in others there is a correlation between:
DOSE VS. SEVERITY OF ILLNESS
Sometimes, the quantity of infected food items that are consumed affects the severity of the disease.
A low dose may produce no symptoms be can be sufficient to induce immunity. Infections with a low
infective dose can spread by person-to-person contact. Typhoid and cholera need a high infective dose
to produce disease.
- Improving water quality
- Reduction of pathogens in the sewage
Beneficial to communities in protecting them from these infections
TRANSMISSION
There are several types of transmission:
1. Direct
Person-to-person contact (e.g. dirty fingers, food, and water as in diarrhoeal diseases).
Can also occur through droplet infection (respiratory tracts) and autoinfection – which
is that humans contaminate themselves directly from their external orifices
2. Intermediate host
When parasites that live in humans must undergo developmental stages in an
intermediate host (such as snails in schistosomiasis – these are intermediate hosts and
not vectors, while they do not carry the infection from one host to the other) for
transmission to another human
3. Vector
Carries the infection from one host to another; either as part of the transmission
process (mosquito) or mechanically (house fly – by its feet and mouth). All vectors
are insects or arachnids (tickets/mites). Cycles of transmission:
1. Direct insect to human (malaria)
2. Insect to an animal, with humans entering the cycles as abnormal hosts
(e.g. bubonic plague)
3. Insect to an animal, including humans, transmitted to other humans by
the same or another vector (e.g. yellow fever/East African sleeping
sickness)
4. Animal
Animals can be intermediate hosts and reservoirs. Intermediate hosts: such as beef and
pork tapeworms (Taenia saginata, Taenia solium) where cysticerci must develop in
the animal muscle before they infect humans. Reservoirs: infection is maintained in
wild rodent reservoirs, from which for instance dogs become infected – bringing the
infection into the home of a vulnerable human
5. Plants
Vegetable material that is eaten by the host; serves as a method of transmission. Can
be specific or non-specific plants
6. Carriers
3
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