Communicable Diseases

• Pathogen


  Organism that causes disease
• Disease


  Condition that impairs normal functioning of organism
• Host


  An organism that a pathogen can live in creating a habitat for it
• Types of pathogens


  • Bacteria
  • Fungi
  • Virus
  • Protocista
• Bacteria


  • Belong to kingdom of prokaryotae
  • Cells smaller than eukaryotic cells so can reproduce rapidly
  • Their presence can cause disease by damaging cells or releasing waste products that are toxic to host
  • In plants they live in vascular tissue and cause blackening and death of tissue
• Fungi in animals


  • Live in the skin of animals and its hyphae will form a mycelum to grow under skin surface
  • Can also send out hyphae that grow to surface of skin and release spores
• Fungi in animals


  • Athletes foot (humans)
  • Ringworm (cattle)
• Fungi in plants


  • Live in vascular tissue where can gain nutrients
  • Hyphae releases extracelluar enzyme e.g. cellulase to digest surrounding tissue causing decay
  • Leaves mottled in colour curl up and shrivel
  • Fruit and storage organs turn black and decay
• Fungi in plants


  • Black sigatoka (banana plants)
• Viruses


  • Invade cells and take over genetic machinery + other organelles in cells
  • Causes cell to manufacture more copies of virus
  • Host cell eventually bursts and releases new viruses infecting healthy cells
• Protocista


  • Cause harm by entering host cells and feeding on contents as they grow
• Protocista


  • Potato blight (potatoes)
  • Malaria (Animals)
• TB is transmitted by sufferer coughing catapulting droplets of saliva into air
• HIV attacks and destroys immune cells weakening the immune system, making it open to a range of opportunistic diseases and secondary infections
• HIV is transmitted through exchange of bodily fluids, sharing of hypodermic needles, across placenta during childbirth, from mother to baby during breast feeding, use of unsterilized surgical equipment
• Types of direct transmission in animals

  • Direct physical contact: touching infected person or contaminated surfaces
  • Faecal: oral transmission usually by eating/drinking contaminated
  • Spores: Resistant stage of pathogen. Can be carried in air or residue on surfaces or in soil
  • Social: overcrowding, poor ventilation, health, diet, homelessness
• Indirect transmission


  When disease transmitted from one organism to another via intermediate- air, water, food or vector
• Malaria transmission


  Person with malaria--> Gametes of plasmodium in blood--> Female anophele mosquito suck blood--> Plasmodium develops and migrates to mosquitos salivary glands--> uninfected person bitten--> Plasmodium migrates to liver--> Plasmodium migrates to blood
• Direct methods of transmission in plants

  • Pathogens in soil, infect plant by entering soil
  • Fungi make spores, means of sexual and asexual reproduction
  • Once pathogen inside plant may infect vascular tissue causing leaves to fall to ground and transmit pathogens through soil
  • Pathogens enter seeds and fruit and spread to other fruits
• Indirect methods of transmission in plants

  • Result of insect attack
  • Spores of bacteria attach to burrowing insect which attacks infected plant
  • Insect= vector
• Factors affecting disease transmission

  • Overcrowding
  • Climate
  • Social
• Types of passive defence in plants


  • Physical
  • Chemical
• Physical defences in plants

  • Cellulose cell wall
  • Lignin
  • Waxy cuticle
  • Bark
  • Stomatal closure
  • Callose
  • Tylose formation
• Chemical defences in plants

  • Terpanoids
  • Phenols
  • Alkaloids
  • Hydrolytic enzymes
• Active defences in plants

  • Cell walls thickened and strengthened
  • Deposition of callose between plant cell wall and cell membranes near invading pathogen
  • Oxidative bursts
  • Necrosis
  • Canker
• Chemicals in active plant defence

  • Terpenoids
  • Phenols
  • Alkaloids
  • Defensins
  • Hydrolytic enzymes
• Skin as primary defence

  • Acts as physical barrier, epidermis has cells called keratinocytes which migrate out to surface of skin, cytoplasm replaced by keratin known as keratinisation
• Mucous membranes as primary defence

  • Protect body openings, some secrete mucus which trap pathogens and have lysosomes enzymes, goblet cells in airways line passage and trap pathogens, ciliated cells waft mucus along to top of trachea to be swallowed or coughed out
• Blood clotting as primary defence


  • Body prevents excess blood loss, clot acts as temporary seal, prevents infection and first step in skin reparation, requires calcium ions and 12 cofactors to initiate and see through the clotting cascade
• Inflammation as primary defence

  • Presence microorganisms detected by specialised cells- mast cells, release histamine (cell signalling), causes vasodilation make walls more permeable to WBC, plasma and WBC leave blood and enter tissue fluid causing increased swelling, excess fluid drained to lymphatic system meaning pathogens contact with lymphocytes initiating specific immune response
• Expulsive reflexes as primary defence

  • Sneeze- mucus membrane in nostrils irritated, Cough- irritation in respiratory tract, both attempt to expel foreign objects
• Other primary defences


  • Eyes- antibodies and enzymes in tear fluid
  • Ear canal- lined by wax
  • Female reproductive system- mucus plug in cervix, maintain acidic conditions
• Antigens


  How pathogens are recognised in the body
• Opsonins


  Protein molecules that attach to antigens on surface of pathogen, enhancing ability of phagocytic cells to engulf pathogen
• Neutrophils


  • Most common phagocyte, made in bone marrow, have multilobed nucleus, travel in blood and squeeze out into tissue fluid, short lived but released large numbers, have lots lysosomes, engulf and digest pathogens, dead neutrophils collect and form pus
• Macrophages


  • Larger, made in bone marrow, found in lymph nodes, dendritic cells found in peripheral tissue, when macrophage encounters pathogen doesn't digest all of it, antigen saved and moved to special protein complex on surface of cell, cell becomes antigen presenting cell so informs other cells in immune system
• Phagosome


  A vesicle that forms around pathogen that separates from the cell membrane, and then fuses with lysosomes, digesting of the pathogen
• Phagocyte mode of action


  Phagocyte envelopes and engulfs the pathogen, membrane folds inwards: phagocytosis, pathogen is trapped inside in vacuole called the phagosome, lysosome fuse with the phagosome forming phagolysosome, release enzymes into it called lysins, lysins digest the bacterium, products of the digestion are entirely harmless, nutrients can then be absorbed into the cytoplasm or exocytosed into extracellular fluid
• B and T cells


  • Cells produced by the immune system to fight pathogens
• Pus is formed from dead neutrophils that collect