NAVLE Nervous

Equine Tetanus Study Guide

March 28, 2026 25 min read 16 views
Tetanus (also known as lockjaw) is a potentially fatal neurological disease caused by the neurotoxin tetanospasmin, produced by the anaerobic, spore-forming bacterium Clostridium tetani.

Overview and Clinical Importance

Tetanus (also known as lockjaw) is a potentially fatal neurological disease caused by the neurotoxin tetanospasmin, produced by the anaerobic, spore-forming bacterium Clostridium tetani. Horses are considered the most susceptible domestic species to tetanus, making this a high-yield topic for the NAVLE examination.

The disease is characterized by generalized muscular rigidity, painful spasms, hyperesthesia, and autonomic dysfunction. Without vaccination, the mortality rate in horses is reported to be up to 80%. Tetanus toxoid is classified as a core vaccine by the American Association of Equine Practitioners (AAEP) due to the ubiquitous environmental presence of the organism, severity of clinical disease, and near-complete protection provided by proper vaccination.

High-YieldHorses and humans are the MOST susceptible species to tetanus. This is a high-yield fact for board examinations. Remember that wound size does NOT correlate with tetanus risk - even superficial wounds can result in clinical disease.
Clinical Sign Description and Clinical Significance
Sawhorse Stance All four legs stiff and extended, head and neck extended, tail held rigidly erect - results from extensor muscle rigidity
Third Eyelid Prolapse Nictitating membrane protrudes across the eye; can be elicited by tapping face or sudden stimuli; often first sign noticed by owners
Trismus (Lockjaw) Spasm of masseter muscles causes difficulty opening mouth; leads to inability to prehend and chew food; classic early sign
Erect Ears Ears held rigidly erect and forward due to spasm of auricular muscles
Flared Nostrils Dilated nares due to facial muscle spasm; contributes to anxious facial expression
Risus Sardonicus Sardonic grin - rigid facial expression resembling a forced grimace due to facial muscle spasm
Hyperesthesia Exaggerated response to stimuli (sound, light, touch) triggering violent muscle spasms and possible convulsions
Dysphagia Difficulty swallowing due to pharyngeal muscle spasm; saliva may drip from mouth; risk of aspiration pneumonia
Sweating Profuse sweating common; reflects autonomic nervous system dysfunction and increased metabolic demand

Etiology

Causative Agent

Clostridium tetani is a Gram-positive, anaerobic, spore-forming rod-shaped bacterium. The organism has the following characteristics:

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