Infectious Disease Life Cycle and Transmission Modes

• Aetiology


  Cause of the disease
• Pathogenesis


  Step by step development of the disease
• Microbemia


  Presence of bacteria in the blood of the host and microbe specified, e.g. viremia, fungemia, bacteremia
• Types of host defences

  • Non-specific
  • Specific
• Non-specific host defences


  • Work against all types of pathogens, regardless of their class
• Skin and mucous membranes


  • Skin is a barrier that prevents penetration of pathogens
  • Mucous membranes are a protective barrier
• White blood cells, inflammation, defensive proteins

  • Non-specific host defences
• Specific host defences


  • Designed to work against a specific type of disease
• White blood cells


  Beta and T lymphocytes
• Humoral and cell-mediated immunity


  Occurs due to previous exposure to the disease
• Antibodies


  Aid in the recognition of a previous infection, used in vaccine situations
• Vaccination


  Injecting a fragment of the pathogen or a weakened version into the body, so the immune system produces antibodies to fight it
• Bacterial pathogenesis


  • Multi-factorial process that depends on immune status of host, characteristics of bacterial species/strain, and number of organisms in the exposure
• Limited number of bacterial species responsible for majority of infectious diseases
• Some infections eradicated/reduced due to vaccines, antibiotics and effective public health measures. However, there are still many problems such resistance to antibiotics, bioterrorism and new infections.
• Infection - Life Cycle

  1. Transmission

  2. Adhesion

  3. Penetration

  4. Spread

  5. Survival in host
• Exotoxins


  The most toxic substances known, released from viable bacteria, produced by both Gram positive and negative bacteria
• Endotoxins


  Toxic lipopolysaccharide LPS of the outer membrane of Gram negative bacteria, secreted when the bacteria die
• Endotoxins


  • Omnipresent in environment, cause pyrogenicity, leukopenia, blood pressure changes, sepsis and lethal shock
• What bacteria are endotoxins found in?

  • Salmonella spp. and E. coli
• Exotoxins


  • Grouped into neurotoxins, cytotoxins, and enterotoxins
• Exotoxins are released from alive bacteria while endotoxins are released from bacteria when they die
• Exotoxins are proteins while endotoxins are lipopolysaccharides
• Methods of transmission


  • Contact
  • Airborne droplets
  • Common vehicle
  • Air
  • Vector
• Route of entry


  Entry site is important to each individual pathogen, as different pathogens will infect and multiply in different areas of the body
• Virulence factors


  • Adherence and colonisation factors
  • Invasion factors
  • Capsules and other surface components
• Skin infections


  • Usually enter through a break in the skin, can cause systemic infections if close to main circulation, cause erythema, oedema and other signs of inflammation, can be primary or secondary
• Types of respiratory tract infections

  • Upper (nasal cavity, pharynx, larynx)
  • Lower (trachea, lungs)
• Cause of upper respiratory tract infections

  • Organisms enter through inhalation of droplets, invade the mucosa, viral infections treated symptomatically, most caused by viruses like rhinovirus and coronavirus
• Lower respiratory tract infections


  • Organisms enter by inhalation, aspiration or haematogenous seeding, sputum and fluid should be examined for predominant organism
• Urogenital tract infections


  May cause painful, frequent urination, perineal pain, fever, chills, and back pain, most caused by bacteria from intestinal flora, E. coli causes 70% of infections, can ascend through urethra
• Chlamydia trachomatis


  Obligate intracellular bacteria that can cause ocular and genital infections, genital infections spread sexually, exist as infectious elementary bodies and intracytoplasmic reticulate bodies
• Conjunctiva


  • Thin, clear mucous membrane that protects the eye, usually free of microorganisms, damage can lead to opportunistic infections like conjunctivitis
• Conjunctivitis


  • Chlamydia trachomatis, Staphylococcus aureus