Infectious disease life cycle and transmission modes (L1)

• 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
• Host defences

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


  • Skin and mucous membranes
  • White blood cells, inflammation, defensive proteins
• Specific host defences


  • White blood cells – B and T lymphocytes
  • Humoral and cell-mediated immunity
  • Antibodies aid in the recognition of a previous infection used in vaccine situations
• Bacterial pathogenesis


  • Multi-factorial process that depends on immune status of host, characteristics of bacterial species or strain, 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, effective public health measures
  • Problems include resistance to antibiotics, bioterrorism, new infections
• Infection – Life cycle

  1. Transmission

  2. Adhesion

  3. Penetration

  4. Spread

  5. Survival in host
• Epidemiologic Triangle


  • Disease
  • Host
  • Agent
  • Environment
• Host


  • Age
  • Sex
  • Genotype
  • Behaviour
  • Nutritional status
  • Heath status
• Agent


  • Infectivity
  • Pathogenicity
  • Virulence
  • Immunogenicity
  • Antigenic stability
  • Survival
• Environment


  • Weather
  • Housing
  • Geography
  • Occupational setting
  • Air quality
  • Food
• Injury mechanisms


  • Exotoxins
  • Endotoxins
  • Siderophores

  
• Exotoxins vs Endotoxins


  Exotoxins are proteins released from viable bacteria, while endotoxins are toxic lipopolysaccharides from the outer membrane of Gram-negative bacteria released after bacterial lysis

  
• Methods of transmission


  • Contact
  • Common Vehicle
  • Air
  • Vector
• Contact transmission


  Spread by airborne droplets (<3 feet), transmission directly from source to susceptible host
• Common vehicle transmission


  Transmission by a common inanimate vehicle, with multiple cases resulting from such exposure, e.g. Food, water
• Air transmission


  By droplet nuclei/dust (>3 feet)
• Vector transmission


  Arthropods are vectors, either internalised or not
• Entry site is important to each individual pathogen, once infection is established, different pathogens will go on to infect and multiply in different areas in the body or remain at site of entry
• Virulence factors


  • Adherence and colonization factors like attachment mechanisms such as pili
  • Invasion factors that enable bacterium to invade eukarytoic cells
  • Capsules and other surface components that are protective and resistant to phagocytosis and intracellular killing by host cells
• Skin infections


  Organisms usually enter through a break in the skin, e.g. an insect bite, may cause erythema, oedema, and other signs of inflammation, can be either primary or secondary
• Respiratory tract infections


  • Upper respiratory tract infections
  • Lower respiratory tract infections
• Upper respiratory tract infections

  Organisms enter through respiratory tract by inhalation of droplets and invade the mucosa, viral infections are treated symptomatically
• Lower respiratory tract infections


  Organisms enter the distal airway by inhalation, aspiration or by haematogenous seeding
• Pneumonia


  • Infectious agents gain access to the lower respiratory tract by the inhalation of aerosolized material, sputum should be examined for a predominant organism
• Urogenital tract infections


  May cause painful, frequent urination with a feeling of incomplete emptying of the bladder, perineal pain, fever, chills, and back pain, most are caused by bacteria from the intestinal flora like E. coli, can ascend through the urethra to infect the bladder and renal pelvis, antimicrobial agents cure most but recurrence is common
• Chlamydia trachomatis infections


  Chlamydiae are obligate intracellular bacteria that exist as infectious elementary bodies and intracytoplasmic, reproductive reticulate bodies, genital tract infection serves as a source of infectious elementary bodies for the eyes
• Conjunctivitis


  The conjunctiva is usually free of microorganisms but damage can increase microbial adhesion or reduction in tear flow, leading to opportunistic bacteria causing infections like Chlamydia trachomatis and Staphylococcus aureus

  
• Antigenic stability

  The ability of a virus to maintain its antigenic structure, or the part of the virus that triggers an immune response in the host. A virus with high antigenic stability is less likely to mutate, which means that the immune system can recognize and fight it more effectively. This is important in the development of vaccines, as a stable virus is less likely to evade the immune response induced by the vaccine.
• Transmissibility

  The ease with which a virus can spread from one host to another. A virus with high transmissibility is more easily passed from person to person, often through respiratory droplets or other bodily fluids. Transmissibility is influenced by a variety of factors, including the mode of transmission, the duration of infectivity, and the severity of symptoms.
• Endotoxins

  • Found everywhere in environment
  • Makes up the outer layer of outer membrane of Gram negative bacteria
  • Released after lysis of bacteria
  • Biological effects include pyrogenicity, leukopenia, blood pressure (could culminate in sepsis and lethal shock)
• What is the basic structure of an endotoxin?

  • oligosaccharide side chains
  • core polysaccharide
  • lipid A
• Exotoxins

  • proteins produced inside gram positive and gram negative bacteria
  • are secreted/released into the surroundings after cell lysis
• What are the three categories of exotoxins?

  • Neurotoxins
  • Cytotoxins
  • Enterotoxins
• Common cold

  • most are caused by viruses
  • virus directly invades epithelial cells of the respiratory mucosa
  • after a cold there is an increase in leukocyte infiltration and nasal secretion
  • after 48-72h incubation, classic symptoms arise such as headache, runny nose, cough