Central Nervous System Infectious Inflammation

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
    • Infectious canine distemper (morbillivirus)
    • Infectious bovine rhinotracheitis (bovine herpesvirus type 1)
• Other examples of viruses affecting the CNS:
  • Distemper
  • Parvovirus
  • Parainfluenza
  • Herpes
  • FIP
  • FIV
  • FeLV
  • Pseudorabies
  • Rabies