VPT 421: Veterinary Toxicology (2+0)

MECHANISM OF TOXICITY

• Three exotoxins are produced by the vegetative form of C. tetani.
• Transport to the CNS occurs retrograde within nerves and via the bloodstream .
• The toxin binds to gangliosides in the brain stem or spinal cord.
• It blocks inhibitory synaptic input, especially at glycine mediated sites, by binding to the presynaptic membrane and blocking the release of glycine , resulting in spastic paralysis.
• It may also inhibit release of GABA and other amino acid neurotransmitters.
• Tetanus toxin may also cause paralysis by inhibiting the release of acetylcholine at neuromuscular junctions.
• Constant muscular spasticity may be produced and normally innocuous stimuli cause exaggerated responses.
• Death occurs as a result of rigidity of the muscles of respiration and associated asphyxia.

Susceptible species

• Although all species of domestic animals are susceptible to tetanus, most cases appear to occur in the horse.
• Horses are the domestic species most sensitive to tetanus toxin.
• Pigs, cattle and sheep are less sensitive and dogs and cats are fairly resistant but are sometimes affected.
• Poultry are more resistant.
• Generally, tetanus occurs in individual animals although outbreaks have been described in cattle, young pigs and lambs.