Central Nervous System Infectious Inflammation

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Routes of Entry

  • CNS inflammation is usually the result of infection.
    • This may be caused by:
      • Bacteria
      • Fungi
      • Protozoa
      • Viruses
  • Organisms must first enter the CNS in order to establish infection.
    • There are several routes of entry that allow this:
      1. Haematogenous entry
        • This is the most common route.
      2. Entry via the peripheral nerves
        • Organisms track within the axoplasm of axons.
        • For example, Listeria monocytogenes.
      3. Penetrating trauma
        • For example, dehorning wounds, skull fracture or tail docking.
      4. Direct spread of infection
        • From the nasal cavity, middle ear or paranasal sinuses.

Localisation of Infectious Organisms

  • After entry, organisms may establish in one or more of four main areas:
    1. Epidural space
      • Infection tends to manifest as abscess formation.
    2. Subdural space
      • Manifests as abscess formation.
      • Fairly uncommon.
    3. Leptomeninges
      • Causes leptomeningitis, which may be:
        1. Suppurative
          • The most common form.
          • Neutrophils are the predominant cell type.
          • Caused by bacteria
            • E.g. E. coli and Streptococcus
          • There are often no gross lesions, but the brain may appear swollen and the meninges opaque.
          • Usually results in death.
        2. Eosinophilic meningoencephalitis
          • The classic example of this is porcine salt poisoning, when water has been restricted and the suddenly replenished.
          • Perivascular eosinophilic cuffing is seen in the cerebrum and meninges.
        3. Lymphocytic
          • Usually of viral origin.
        4. Granulomatous
          • Caused by fungal diseases and Mycobacteriosis.
    4. CNS parenchyma

Bacterial Infections

Pneumococcal meningitis. Image courtesy of BioMed Archive
  • Bacterial infections typically result in abscesses.
    • These may be single or multiple depending on the route of entry, and vary in size.
    • They contain a central, liquefied cavity.
  • There are differences between cerebral abscesses and those occuring elsewhere.
    • Encapsulation is slow.
      • This is due to a lack of fibroblasts.
      • There is therefore less collagen in the capsule.
    • Astrocytic glial fibers are not as strong as collagen
  • Other organisms may cause similar infections:
    • Rickettsial organisms
      • E.g. Ehrlichia
    • Spirochates
      • E.g. Leptospirosis

Viral Infections

  • Viral infections tend to reach the CNS by haematogenous spread and via peripheral nerves.
  • There are three hallmark lesions of CNS viral infections:
    1. Neuronal necrosis
    2. Gliosis
    3. Vascular changes
  • Several types of virus may cause inflammation in the CNS.
    Negri bodies, as seen in rabies. Image courtesy of BioMed Archive
    • Neurotropic, e.g.
      • Rabies (rhabdovirus)
      • Aujesky’s disease (herpesvirus)
      • Visna (ovine lentivirus)
    • Endotheliotropic, e.g.
      • Infectious canine hepatitis (canine adenovirus)
      • Classical swine fever (pestivirus)
      • Equine herpesvirus type 1 (herpes)
    • Pantropic
  • Other examples of viruses affecting the CNS:
    • Distemper
    • Parvovirus
    • Parainfluenza
    • Herpes
    • FIP
    • FIV
    • FeLV
    • Pseudorabies
    • Rabies

Prion Disease