Equine Tetanus Study Guide

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.

Etiology

Causative Agent

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

• Morphology: Rod-shaped cells approximately 0.5 x 2.5 micrometers; forms characteristic terminal spores giving a drumstick or tennis racket appearance
• Oxygen requirements: Obligate anaerobe - cannot survive in presence of oxygen; requires anaerobic environment (necrotic tissue, deep wounds)
• Spore characteristics: Extremely hardy; resistant to heat, desiccation, and most disinfectants; can survive in soil for many years
• Distribution: Ubiquitous in soil worldwide (especially cultivated soil) and in the gastrointestinal tract and feces of horses and other animals

Toxins Produced

C. tetani produces two exotoxins:

• Tetanospasmin (Tetanus Toxin): The primary neurotoxin responsible for clinical disease; one of the most potent toxins known; encoded on a plasmid
• Tetanolysin: A hemolysin causing local tissue destruction; facilitates spread of infection but does not contribute to neurological signs

Pathophysiology

Mechanism of Disease

Understanding the pathophysiology of tetanus is essential for both diagnosis and treatment. The disease process occurs in distinct stages:

Step 1: Entry and Germination

C. tetani spores enter the body through wounds (puncture wounds, lacerations, surgical incisions, umbilical stump in foals, or reproductive tract post-partum). The spores require anaerobic conditions to germinate into vegetative bacteria - this occurs in areas of tissue necrosis or deep puncture wounds where oxygen tension is low.

Step 2: Toxin Production and Transport

Vegetative bacteria produce tetanospasmin, which is released when cells lyse. The toxin binds to peripheral motor neuron terminals at neuromuscular junctions and is transported retrograde (backward) via axonal transport to the spinal cord and brainstem within hours.

Step 3: Inhibition of Neurotransmitter Release

Once in the CNS, tetanospasmin crosses synapses and enters inhibitory interneurons. The light chain of the toxin (a zinc-dependent metalloprotease) cleaves synaptobrevin (VAMP), a protein essential for vesicular neurotransmitter release. This blocks release of the inhibitory neurotransmitters glycine and gamma-aminobutyric acid (GABA).

Step 4: Clinical Manifestations

Without inhibitory neurotransmitters, motor neurons become hyperexcitable (disinhibition), leading to sustained, unopposed muscle contraction and exaggerated reflex responses. Shorter nerves (facial, jaw muscles) are affected first, explaining why trismus (lockjaw) is often an early sign. The binding of tetanospasmin to neurons is IRREVERSIBLE - recovery requires regeneration of new nerve terminals, which takes weeks.

"TET = TIGHT" - Tetanus causes tight/spastic muscles (blocks inhibition in CNS) "BOT = BOTTOM (falls down)" - Botulism causes flaccid/weak muscles (blocks ACh at NMJ)

Clinical Signs

Clinical signs typically develop 3 to 21 days after wound contamination (incubation period can range from 1 day to 2 months). Signs are often preceded by a history of wound or surgical procedure within the preceding month, though in up to 50% of cases, no wound is identified ("idiopathic tetanus").

Classic Clinical Findings

Disease Progression

Tetanus typically follows a predictable progression if untreated:

• Early stage: Stiffness, mild gait abnormality, third eyelid prolapse when stimulated, subtle trismus
• Moderate stage: Generalized rigidity, sawhorse stance, difficulty eating/drinking, hyperesthesia to stimuli
• Severe stage: Recumbency, inability to rise, severe spasms, respiratory compromise
• Terminal stage: Respiratory failure due to respiratory muscle paralysis, death

Neonatal Tetanus in Foals

Foals (less than 6 months of age) may present differently and often have a more severe course:

• Often present recumbent and may display seizures
• Umbilical stump is a common site of entry - examine carefully
• Similar survival rates to adults with appropriate treatment
• Foals from unvaccinated mares are at highest risk due to lack of colostral antibodies

Diagnosis

Tetanus is primarily a CLINICAL DIAGNOSIS based on characteristic clinical signs and history. There is no rapid, definitive confirmatory test, and laboratory findings are typically unremarkable.

Diagnostic Criteria

A presumptive diagnosis is made based on:

• Classic clinical signs: stiffness, lockjaw, third eyelid prolapse, sawhorse stance
• History of wound or surgical procedure (though may be absent in 50% of cases)
• Inadequate or unknown vaccination status
• Hyperesthetic response to stimuli (third eyelid flashes with tap to face)

Laboratory Findings

Laboratory tests are NOT typically diagnostic but may include:

• CBC/Chemistry: Usually unremarkable; may show dehydration, metabolic acidosis, or stress leukogram
• Wound culture: Isolation of C. tetani is difficult and often negative; not routinely performed
• Serum tetanus antitoxin: Low or absent antibody titers support diagnosis but take days to receive
• CSF analysis: Typically normal; useful to rule out other neurological conditions

Differential Diagnosis

Treatment

Treatment of tetanus is supportive and symptomatic - there is no way to reverse toxin already bound to neurons. Treatment goals include: eliminating the source of toxin, neutralizing circulating toxin, controlling muscle spasms, and providing supportive care.

Treatment Protocol

Prognosis

Prognosis depends on vaccination status, severity of signs, rapidity of progression, and timing of treatment:

Mortality Rate: Up to 80% in unvaccinated horses; significantly better (around 20-30% in some studies) with early aggressive treatment. Recovery period is 2-6 weeks for survivors because new nerve terminals must regenerate.

Prevention and Vaccination

Tetanus is a PREVENTABLE disease. Vaccination with tetanus toxoid is the most effective prevention strategy and provides near-complete protection. The AAEP classifies tetanus toxoid as a CORE VACCINE for all horses regardless of age, use, or geographic location.

AAEP Vaccination Guidelines

"4-6 RULE" for Tetanus: Most tetanus vaccination intervals involve 4-6 weeks or 4-6 months: - Primary series: 4-6 weeks between doses - Pregnant mares: Booster 4-6 weeks before foaling - Foals from vaccinated mares: Start at 4-6 months - Wound booster: If more than 6 months since last vaccine