communicable diseases

• Chemical defences in plants:

  Many plants produce powerful chemicals that either repel the insect vectors of disease or kill invading pathogens.
• Examples of plant defensive chemicals:

  • insect repellants
  • insecticides
  • antibacterial compounds (e.g antibiotics)
  • antifungal compounds
  • general toxins
• Non-specific animal defences against pathogens:

  • the skin (prevents entry of pathogens and produces sebum, an oily substance that inhibits the growth of pathogens)
  • lysozymes in tears and urine
  • acid in our stomach
  • many of the body tracts are lined by mucous membranes that secret mucus to trap pathogens
• Non-specific animal defences against pathogens: Blood clotting and wound repair.

  • thromboplastin (enzyme that triggers a cascade of reactions resulting in the formation of a blood clot)
  • serotonin (makes the smooth muscle in the walls of the blood vessels contract, so they narrow and reduce the supply of blood to the area)
• In blood clotting, the clot dries out forming a hard scab that keeps pathogens out. This is the first stage of wound repair. Epidermal cells below the scab start to grow, sealing the wound permanently, while damaged blood vessels regrow.
• Inflammatory response:

  Most cells are activated in damaged tissues and release chemicals called histamines and cytokines.
• Inflammatory response:

  • histamines make the blood vessels dilate, causing localised heat which helps prevent pathogens reproducing
  • histamines make blood vessels walls more leaky so blood plasma is forced out (once forced out of the blood it is known as tissue fluid. tissue fluid causes swelling and pain)
  • cytokines attract white blood cells to the site. They dispose of pathogens by phagocytosis.
• Phagocytosis:

  1. pathogens produce chemicals that attract phagocytes
  2. phagocytes recognise non-human proteins on the pathogen
  3. the phagocyte engulfs the pathogen and encloses it in a vacuole called a phagosome
  4. the phagosome combines with a lysosome to form a phagolysosome
  5. enzymes from the lysosome digest and destroy the pathogen
• Natural active immunity = body has acted itself to produce antibodies
• Natural passive immunity = some antibodies cross the placenta from the mother to her fetus. The first milk a mother makes is called colostrum which is very high in antibodies.
• Artificial passive immunity = antibodies are formed in one individual and extracted and then injected into the bloodstream of another individual
• Artificial active immunity = vaccinations
• Types of pathogens:

  • bacteria
  • viruses
  • protista
  • fungi
• 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 protoctists 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.
• 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.
• 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.
• Some fungi produce toxins which affect the host cells and cause disease.
• Plant diseases:

  • ring rot (bacterial disease)
  • tobacco mosaic virus (virus)
  • potato blight (fungus)
  • black Sigatoka (fungus)
• Animal diseases:

  • tuberculosis (bacteria)
  • bacterial meningitis (bacteria)
  • HIV/AIDS (virus)
  • influenza (virus)
  • malaria (protoctista)
  • ring worm (fungus)
  • athletes foot (fungus)
• Direct transmission in animals:

  • direct contact
  • inoculation
  • ingestion
• Indirect transmission in animals:

  • fomites (inanimate objects)
  • droplet infection (inhalation)
  • vectors
  • water
• Factors affecting transmission of disease in animals:

  • overcrowded conditions
  • poor nutrition
  • compromised immune system
  • poor disposal of waste
• Direct transmission in plants:

  This involves direct contact of a healthy plant with any part of a diseased plant.
• Indirect transmission in plants:

  • soil contamination (infected plants often leave pathogens in the soil)
  • vectors (wind, water, animals, humans)
• Factors affecting transmission of disease in plants:

  • overcrowding
  • poor mineral nutrition
  • damp, warm conditions
  • planting varieties of crops susceptible to disease