Pathogens that cause disease

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Pathogens that cause disease

• A summary of Chapter 7

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Definitions

• Disease Any change that impairs the normal
  function of an individual in some way.
• Parasite An organism that lives on or in
  another organism and feeds from it, usually
  without killing it.
• Host The organism or cell that a particular
  parasite lives in or on.

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Definitions continued

• Pathogen An organism that is able to cause
  disease in a host
• Vectors An insect or other animal that carries
  a pathogenic organism from one host to another
• Infectious Diseases caused by an agent that can
  be passed from one organism to another.
• Contagious Diseases easily passed through the
  population.

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Pathogens

• Types of pathogen can be split into two groups
• Cellular pathogens are organisms that cause
  disease and include bacteria, protozoans,
  oomycetes, fungi, worms and arthropods.
• Non-cellular pathogens are not actually organisms
  but can cause disease and include viruses,
  viroids and prions.

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Bacteria

• Bacteria are prokaryotic cells that are found
  almost everywhere.
• Some are autotrophs but most are heterotrophs.
• They can replicate very quickly by binary
  fission.
• Can be pathogens of both plants and animals.
• Some people can be infected by bacteria and not
  show symptoms, these people are known as carriers.

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Bacteria classification
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Bacteria can be transmitted by

• Direct contact with an infected host
• Contact with objects/foods that have been
  contaminated
• Inhalation of dust or water droplets that have
  come into contact with, or produced by an
  infected host
• Contact with vectors carrying the bacterium.

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How do bacteria cause disease?

• Some cause disease purely because their large
  numbers change normal functioning of their hosts
  tissue.
• Others directly attack and destroy host tissues.
• In other cases, the bacteria produce poisons
  (toxins) which may kill or seriously affect
  normal functioning of the hosts cells.

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Common diseases caused by bacteria include

• Anthrax
• Bubonic plague
• Salmonella poisoning
• Gonorrhoea
• Cholera
• Leprosy
• Syphilis

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Controlling bacteria

• Bacteria can be controlled with a combination
  of
• Antiseptics
• Disinfectants
• Antibiotics
• Antimicrobial agents.

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Protozoa

• Protozoa are animal-like protists
• This means they are single-celled eukaryotic
  organisms
• The three classes of protozoa are classified
  according to their structure

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The most common protozoa that cause disease

• Plasmodium this is the organism responsible for
  malaria. It is classified as a sporozoan. It
  relies on a certain species of mosquito for its
  spread. It therefore has two hosts in its life
  cycle. The primary (adult) host is the human and
  the secondary (larval) host is the mosquito.

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Plasmodium Life Cycle
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Animation

• http//library.thinkquest.org/05aug/01434/malaria.
  html

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The most common protozoa that cause disease

• Trypnasoma this protozoan is a flagellate and
  is the cause of African sleeping sickness. It
  relies on the tsetse fly for its transmission
  between hosts.
• Entamaoeba histolytica this protozoan is a
  sarcodinian and is capable of causing a severe
  form of dysentery. It is spread through
  contamination of food or water.

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Oomycetes

• These are fungus-like organisms that cause blight
  and downy mildew on plants. They were once
  thought to be fungi but they are motile and have
  many cellular processes not found in fungi so
  they are now classified in the kingdom Protista.
• When spores are released on a leaf they may be
  carried in water droplets to other leaves or they
  may be blown to reinfect the same host and to
  infect other plants.

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Two main groups

• Yeasts these are uni-cellular and can cause
  diseases such as thrush which is normally passed
  on by direct contact.
• Moulds this category contains those fungi which
  consist of branching filaments known as hyphae.
  Typical members of this group which cause disease
  are rusts and smuts (particularly harmful to
  cereal crops) and human ringworm.

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Fungi

• Fungi are heterotrophs and obtain their nutrients
  from decomposition of dead matter. Like
  bacteria, many species of fungi are important
  decomposers in biological systems. When fungi
  attack living plant or animal tissue, they can
  then be regarded as disease causing organisms.

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Treatment

• Fungal infections can be treated with fungicides
  and antifungal treatments
• Many of these treatments have been found by
  accident.

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Worms

• Parasitic worms can be classified as either
• Flatworms (Platyhelmithes) such as tapeworms,
  hookworms and blood flukes.
• Roundworms (Nematodes) such as dog heartworm,
  Ascaris and filarial nematodes (which cause
  elephtiasis). In plants round worms infect roots
  and are major pests of orchards and crops.

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Arthropods

• Insects are major vectors of disease in plants
  and animals.
• Ticks and mites are examples of arthropods that
  infect humans.
• Lerp insects are examples or arthropods that
  infect plants to produce galls.

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Non-Cellular Pathogens

• These are infectious agents that cannot be
  regarded as organisms. These were poorly
  understood until the invention of the electron
  microscope in the late 1930s.

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Viruses

• Viruses infect all types of organisms.
• They are obligate intracellular parasites,
  meaning they cannot replicate outside of cells.
• A single virus particle (virion) is composed of
  genetic material either DNA or RNA, enclosed in a
  protein coat known as a capsid
• Viruses do not have cytoplasm, membranes or any
  organelles
• Viruses produce new virions by taking over the
  metabolic pathways of the host cell.

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Classifying viruses

• Different features are used to classify viruses.
  These include
• The kind of cell they use for reproduction
• The kind of nucleic acid they contain
• The different structures that make up the protein
  coat

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Life cycle of a bacteriophage
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Bacteriophage

• The type of virus that infects a bacterial cell.

• Its protein coat becomes attached to the
  bacterial wall and its DNA is injected into the
  bacterial cell
• The phage DNA takes over the bacterial cell and
  uses the energy and organelles of the cell to
  make new bacteriophages
• The host bacterial cell ruptures and the
  bacteriophages are available to infect other
  bacterial cells

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Human viruses

• Details about the way in which mammalian cells
  are infected with viruses are not fully known.
  Some inject their nucleic acid in much the same
  way as bacteriophages. In other cases, the
  complete virus may be surrounded by a protein
  coat and enclosed by part of the plasma membrane
  of a cell.

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Life cycle of a virus

• In some viral infections, the complete virus
  enters the infected cell. New viruses are
  assembled and then they leave the cell by budding
  from the cell within a portion of the hot cells
  membrane. This process does not necessarily kill
  the host cell and results in a persistent
  infection of cells as in the case of the herpes
  virus.
• http//www.whfreeman.com/kuby/content/anm/kb03an01
  .htm

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Common Human Viruses

• Herpes complex
• HIV
• Chicken pox
• Influenza
• Measles
• Rabies
• Warts

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Treatment

• Treating viral infections is more difficult that
  treating bacterial infections because it is
  difficult to attack a virus without attacking
  host cells as well.
• There has been some success with developing
  antiviral therapies that interfere with the
  un-coating of coated viruses, preventing nucleic
  acid synthesis and preventing the assembly and
  release of viral particles.

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Viroids

• Viroids are tiny circular single-stranded RNA
  molecules, about one-tenth the size of the
  smallest virus.
• They have no protein coat or membrane capsule.
• They operate in a similar way to viruses.
• Viroids have so far only been identified in
  plants.

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Prions

• Prions are infectious proteins that have only
  recently been recognised as being responsible for
  several degenerative brain diseases like mad-cow
  disease which causes holes to form in the
  brain tissue and leads to aggressive behaviour.
• Like viruses, prions are able to replicate in
  cells, but they are composed of protein only and
  contain no genetic material.

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How do prions infect?

• It appears that we all contain the genetic
  instructions to make normal prion protein. The
  protein occurs mainly in nerve cells and its
  function is unknown.
• If we become infected with a defective prion it
  converts normal protein into prion protein. This
  is the equivalent of a prion replicating itself.
• Cells do not kill prions. Prions eventually
  cause a cell to burst and are free to infect
  other cells. The bursting of nerve cells results
  in the holes seen in infected brains.

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Transmission

• Prions can be transferred by consumption of
  infected tissues, as occurred with the mad-cow
  epidemic in England were cow where fed a food
  supplement which contained sheep carcasses that
  were infected with scrapie.
• Human to human infection can also occur through
  body fluids
• There is a genetic link