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Chapter 12: Communicable diseasesChapter 12.1: Animal and plant pathogens
Communicable diseases are caused by infective organisms known as pathogens.
Pathogens include bacteria, viruses, fungi and Protoctista.
Each has characteristics that affect the way they are spread and the ways we can attempt to prevent or cure
the diseases they cause.
A communicable disease can be passed from one organism to another.
In animals they are most spread from one individual of a species to another, but they can also be spread
between species.
Communicable diseases in plants are spread directly from plant to plant.
Vectors, which carry pathogens from one organism to another, are involved in the spread of several
important plant and animal diseases.
Common vectors include water and insects.
Communicable diseases are also a major problem in domestic and wild animals, and in the plants on which
life on Earth depends.
Types of pathogens
Bacteria
There are probably more bacteria than any other type of organism.
A small proportion of these bacteria are pathogens, causing communicable diseases,
Bacteria are prokaryotes, so they have a cell structure that is very different from the eukaryotic organisms
they infect.
They do not have a membrane-bound nucleus or organelles.
Bacteria can be classified in two main ways:
o By their basic shapes - they may be rod shaped, spherical, comma shaped, spiralled, and corkscrew
o By their cell walls - the two main types of bacterial cell walls have different structures and react
differently with a process called Gram staining.
Following staining Gram-positive bacteria look purple blue under the light microscope, for
example methicillin- resistant Starrylococcus aureus (MRSA).
Gram negative bacteria appear red, for example the gut bacteria Escherichia coli (Buch).
This is useful because the type of cell wall affects how bacteria react to different antibiotics a
compound that kills or inhibits the growth of bacteria.
Viruses
Viruses are non-living infectious agents.
At 0.02-0.3 um in diameter, they are around 50 times smaller in length than the average bacterium.
The basic structure of a virus is some genetic material surrounded by protein.
Viruses invade living cells, were the genetic material of the virus takes over the biochemistry of the host cell
to make more viruses.
Viruses reproduce rapidly and evolve by developing adaptations to their host, which makes them very
successful pathogens.
All naturally occurring viruses are pathogenic and cause disease in every other type of organism.
There are even viruses that Attack bacteria, known as bacteriophages.
They take over the bacterial cells and use them to replicate, destroying the bacteria at the same time.
Bacteriophages are used to identify and treat some diseases
Protoctista
The Protoctista are a group of eukaryotic organisms with a wide variety of feeding methods.
They include single-celled organisms and cells grouped into colonies.
A small percentage of Protoctista act as pathogens, causing devastating communicable diseases in both
animals and plants.
The protists which cause disease are parasitic - they use people or animals as their host organism.
Pathogenic protists may need a vector to transfer them to their hosts – malaria and sleeping sickness are
examples - or they may enter the body directly through polluted water - amoebic dysentery and Giardia are
examples of these.
Fungi
Fungal diseases are not a major problem in animals, but they can cause devastation in plants.
, Fungi are eukaryotic organisms that are often multicellular, although the yeasts which cause human diseases
such as thrush are single-celled.
Fungi cannot photosynthesise and they digest their food extracellularly before absorbing the nutrients.
Many fungi are saprophytes which means they feed on dead and decaying matter.
However, some fungi are parasitic, feeding on living plants and animals.
These are the pathogenic fungi which cause communicable diseases.
Because fungal infections often affect the leaves of plants, they stop them photosynthesising and so can
quickly kill the plant.
When fungi reproduce, they produce millions of tiny spores which can spread huge distances, this
adaptation means they can spread rapidly and widely through crop plants.
Fungal diseases of plants cause hardship and even starvation in many countries around the world.
Pathogens - modes of action
Damaging the host tissues directly
Many types of pathogen damage the tissues of their host organism.
It is this damage, combined with the way in which the body of the host responds to the damage, that causes
the symptoms of disease.
Different types of pathogens attack and damage the host tissues in different ways:
Viruses take over the cell metabolism. The viral genetic material gets into the host cell and is inserted into
the host DNA. The virus then uses the host cell to make new viruses which then burst out of the cell,
destroying it, and then spread to infect other cells
Some Protoctista also take over cells and break them open as the new generation emerge, but they do not
take over the genetic material of the cell. They simply digest and use the cell contents as they reproduce.
Proctitis which cause malaria are an example of this.
Fungi digest living cells and destroy them. This combined with the response of the body to the damage
caused by the fungus gives the symptoms of disease.
Producing toxins which damage host tissues:
Most bacteria produce toxins that poison or damage the host cells in some way, causing disease.
Some bacterial toxins damage the host cells by breaking down the cell membranes, some damage or
inactivate enzymes and some interfere with the host cell genetic material so the cells cannot divide.
These toxins are a by-product of the normal functioning of the bacteria
Some fungi produce toxins which affect the host cells and cause disease.
12.2: Animal and plant diseases
Plant diseases
Plant diseases threaten people, because when crop plants fail. people suffer.
They may starve, economies may struggle, and jobs are lost.
Plant discases threaten ecosystems too - entire species can be threatened.
Animal diseases
The diseases that affect animals - and in particular human beings have a profound effect on human health
and wellbeing - and on national economies. Communicable diseases range from mild to fatal.
Examples include:
Tuberculosis
A bacterial disease of humans, cows, pigs, badgers, and deer commonly caused by
Mycobacterium tuberculosis and M. hovis. TB damages and destroys lung tissue and suppresses the immune
system, so the body is less able to fight off other diseases.
people also suffering from diseases such as TB, because people allected by HIV/AIDS are much more likely to
develop TB infections, also in people TB is curable and preventable
Bacterial meningitis
A bacterial infection of the meninges of the brain which can spread into the rest of the body causing
septicaemia and rapid death. It mainly affects very young children and teenagers aged 15-19.
They have different symptoms but in both, a blotchy red/purple rash that does not disappear when a glass is
pressed against it is a symptom of septicaemia and immediate medical treatment is needed.
Up to 25% of those who recover have some permanent damage. Antibiotics will cure the disease if delivered
early.
Vaccines can protect against some forms of bacterial meningitis.
HIV/AIDS
caused by HIV which targets T helper cells in the immune system of the body
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