epithelial surfaces

• Respiratory tract

• Skin

• Alimentary tract

• Urogenital tracts

why epithelial surfaces are susceptible to environmental factors that can cause disease

• ​​Epithelial tissue at external surfaces susceptible to environmental insults trauma & infection

• Mucous membranes particularly susceptible due to thin walls (required for physiological function)

Mucous

• Water & electrolytes

• Glycoproteins & lipids

• Antimicrobial compounds

  • Transferrin

  • Lactoferrin

  • Lysozyme

• Antibodies (IgA)

• Antioxidants

  • Superoxide dismutase

  • Catalase

  • Glutathione peroxidase

Routes of infection

• Horizontal transmission

  • Direct contact

  • Aerosol (e.g. kennel cough)

  • Oro-faecal (e.g. handling faeces & infecting feed)

  • Vector-bourne (pathogen uses arthropod host to spread from animal to animal, e.g. ticks & lyme disease)

• Vertical transmission

  • Direct contact (pathogen crosses placental barrier to infect foetus, e.g. bovine virus diarrhoea - BVD)

adaptations of skin, respiratory tract & alimentary tract

Defensive barriers

Physical barriers - skin

• epithelial barriers

  • Physical - skin

  • Physiological - mucous membranes

  • Biochemical - gastric acid

  • Immunological - macrophages

• Keratinocytes replicate to generate stratified squamous keratinised epithelium, has receptors binding to microbes, causing signalling cascade, leading to production of various soluble factors (w/ antimicrobial properties)

• TRIGGERING INFLAMMATION - produce cytokines (immunological hormones) & chemokines (cell migration factors)

• Melanocytes protect from UV damage

• Langerhans cells - tissue-resident macrophages present in all layers of epidermis

• Sebaceous glands & sebum - water repellent, contains fatty acids & has anti-microbial components

Respiratory tract

• Mucociliary escalator

  • Continual movement of cilia stops microbes from forming colonies by moving them continually

  • Host become immunocompromised without this - can be caused by toxic compounds, potentially causing pneumonia

  • Viral infection, trauma (from dust) and toxins (ammonia) can occur if mucociliary escalator is compromised

• Alveolar macrophages

  • Main line of defence against inhaled microbial pathogens & inanimate particles

  • Derived from blood monocytes & interstitial macrophages

  • Short lifespan in alveoli

The respiratory system as a defensive barrier

• Conductive system:

  • Nasal cavity

  • Pharnyx

  • Larynx

  • Trachea

  • Bronchi

• Transitional system

  • Bronchioles

• Gas exchange system

  • Alveoli

Alimentary tract

• Oral cavity & oesophagus

  • Function - prehension, mastication & digestion (salivary enzymes)

  • Protection needed against trauma & infectious agents

  • Physical barrier - stratified squamous epithelium

  • Biochemical barrier - lysozyme in saliva

• Stomach

  • Sterilisation & digestion of food

    • HCl acid (pH 2)

    • Pepsin (protease)

    • Mucous (protect epithelium)

    • Regulatory peptides

• Small intestine

  • Cryptidins (similar to denfensins)

    • Toxic to bacteria

    • Synthesised by Paneth cells present in intestinal crypts

• Immune defence of GIT

  • Gut-associated lymphoid tissue - produces immune responses

  • Specific mucosal protective antibody (IgA)