NAVLE Nervous

Canine Tetanus Study Guide

📅 March 28, 2026 ⏱ 25 min read

Tetanus is a severe, life-threatening neurologic disease caused by tetanospasmin, a potent neurotoxin produced by Clostridium tetani, a gram-positive, anaerobic, spore-forming bacillus.

Overview and Clinical Importance

Tetanus is a severe, life-threatening neurologic disease caused by tetanospasmin, a potent neurotoxin produced by Clostridium tetani, a gram-positive, anaerobic, spore-forming bacillus. The disease results in characteristic spastic paralysis due to inhibition of inhibitory neurotransmitter release in the central nervous system.

Dogs are relatively resistant to tetanus compared to horses and humans (approximately 600 times more resistant than horses). The minimal lethal dose of tetanus toxin in dogs is 150 ng/kg compared to 0.2 ng/kg in horses. This relative resistance means vaccination is not routinely recommended in dogs, unlike horses and humans.

For the NAVLE, understanding tetanus is essential because it represents a classic example of clostridial neurotoxicosis and is frequently tested due to its distinctive clinical presentation, the importance of wound management, and the requirement for intensive supportive care.

Form	Description and Clinical Features
Localized Tetanus	More common form in dogs. Muscle rigidity and spasms confined to the area near the wound site. May affect a single limb or muscle group. Often progresses to generalized form if untreated. Better prognosis than generalized tetanus.
Generalized Tetanus	Most common presentation in hospitalized dogs (up to 80% progress to this form). Widespread muscle rigidity affecting entire body. Characteristic facial and limb signs present. Autonomic dysfunction may develop. More guarded prognosis.

Etiology and Pathophysiology

The Causative Agent: Clostridium tetani

Clostridium tetani is a gram-positive, obligate anaerobic, spore-forming bacillus found ubiquitously in soil, dust, and the gastrointestinal tracts of animals and humans. The bacterium has characteristic morphology: vegetative cells appear as slender rods (0.5 × 2.5 μm), while sporulating cells develop terminal, spherical spores giving them a distinctive drumstick or tennis racket appearance.

Spore Characteristics

Extremely resistant to heat, desiccation, and many disinfectants
Can survive boiling for 10-15 minutes (some strains resist boiling for up to 3 hours)
Killed by autoclaving at 121°C for 20 minutes
Persist in soil for months to years

Mechanism of Toxin Action

C. tetani produces two exotoxins: tetanospasmin (the primary neurotoxin responsible for clinical disease) and tetanolysin (a hemolysin of uncertain clinical significance).

Tetanospasmin: Step-by-Step Mechanism

Wound Contamination: Spores enter through wounds (deep puncture wounds, surgical sites, nail injuries, teething).
Germination: In anaerobic conditions (necrotic tissue with low oxygen tension), spores germinate into vegetative bacteria.
Toxin Production: Vegetative cells produce tetanospasmin, which is released upon bacterial cell lysis.
Binding and Uptake: Toxin binds to gangliosides (GT1b and GD1a) at peripheral motor nerve terminals.
Retrograde Transport: Toxin travels retrogradely within axons to neuronal cell bodies in the spinal cord and brainstem (rate: approximately 75-250 mm/day).
Transsynaptic Migration: Toxin crosses synapses to reach inhibitory interneurons (Renshaw cells).
Synaptobrevin Cleavage: The light chain of tetanospasmin cleaves synaptobrevin (VAMP), a SNARE protein essential for neurotransmitter vesicle fusion.
Disinhibition: Blockade of glycine and GABA release results in loss of inhibitory motor neuron control, causing uncontrolled motor neuron firing and sustained muscle contraction.

High-YieldThe binding of tetanospasmin is IRREVERSIBLE. Recovery requires regeneration of new nerve terminals, which explains the prolonged recovery period (weeks to months) even after successful treatment. This is a key concept for NAVLE!

Sign	Description
Risus sardonicus	Sardonic grin - lips drawn back exposing teeth, wrinkled forehead, narrowed palpebral fissures. Creates a characteristic sinister grin appearance.
Trismus	Lockjaw - inability to open mouth due to masseter muscle spasm. Contributes to dysphagia.
Erect ears	Ears held rigidly erect and close together due to auricular muscle spasm.
Third eyelid prolapse	Protrusion of the nictitating membrane (enophthalmos due to retraction of the globe from extraocular muscle spasm).
Miosis	Pupillary constriction from ciliary muscle involvement.

Clinical Presentation

Incubation Period and Onset

The incubation period from wound infection to first clinical signs is typically 5-10 days in dogs, with a range of 3-18 days (up to 3 weeks in some cases). The shorter the incubation period, the more severe the expected disease course. Facial muscles are often affected first due to the shorter distance from wound to CNS via shorter cranial nerves compared to the longer peripheral nerves supplying the limbs.

Forms of Tetanus

Characteristic Clinical Signs

Facial Signs (Often First to Appear)

Body and Limb Signs

Sawhorse stance: All four limbs held in rigid extension, unable to bend legs, characteristic of severe generalized tetanus
Stiff gait: Stilted, wooden movements with difficulty ambulating
Tail extension: Tail held rigidly elevated or extended straight behind (flag tail)
Opisthotonus: Dorsal arching of neck and back in severe cases
Hypersensitivity: Exaggerated response to touch, sound, and light stimuli, which can trigger or worsen muscle spasms

Board Tip - Memory Aid: "TETANUS FACE" - Trismus, Erect ears, Third eyelid prolapse, Altered pupils (miosis), Narrowed palpebral fissures, Unusual grin (risus sardonicus), Stiff body with Sawhorse stance, Facial wrinkling, Autonomic dysfunction, Convulsive spasms, Extreme hypersensitivity

Autonomic Dysfunction (Grade IV Tetanus)

Tachycardia or bradycardia
Hypertension (more common) or hypotension
Cardiac arrhythmias (ventricular arrhythmias, AV block)
Urinary retention
Hyperthermia from sustained muscle contraction

Grade	Clinical Signs	Patient Status
I	Oculofacial signs only: miosis, enophthalmos, risus sardonicus, erect ears, trismus, hypersensitivity to stimuli	Ambulatory
II	Grade I signs PLUS dysphagia, stiff gait, sawhorse stance, extended/rigid tail	Ambulatory (may have difficulty)
III	Grade I or II signs PLUS muscle fasciculations, spasms, or seizures	Recumbent
IV	Grade I, II, or III signs PLUS autonomic dysfunction (arrhythmias, brady-/tachycardia, hypo-/hypertension, apnea, respiratory arrest)	Critical - worst prognosis

Tetanus Severity Classification

The Burkitt Classification System is widely used to grade tetanus severity in dogs and has prognostic significance. Dogs with higher grades (especially III and IV) have significantly worse outcomes.

Test	Expected Findings
CBC	Usually normal; may see stress leukogram or mild inflammatory changes if concurrent wound infection
Creatine Kinase (CK)	Often elevated (5-20x normal) due to sustained muscle contraction and myonecrosis
AST	May be elevated secondary to muscle damage
Serum Calcium	Normal (helps rule out hypocalcemia as cause of muscle spasms)
Wound Culture	C. tetani isolation is difficult (requires strict anaerobic conditions); often negative; other wound flora frequently isolated
EMG	Continuous spontaneous motor nerve activity; simultaneous firing of agonist and antagonist muscles

Diagnosis

Clinical Diagnosis

Tetanus is primarily a CLINICAL DIAGNOSIS based on characteristic clinical signs and history. There is no definitive antemortem laboratory test for tetanus in veterinary practice.

Key Diagnostic Criteria

History: Recent wound (may be 1-3 weeks prior), surgical procedure (especially ovariohysterectomy), or teething in young dogs
Characteristic clinical signs: Combination of facial signs (risus sardonicus, trismus, erect ears) plus generalized rigidity
Intact mentation: Dogs remain alert and aware (distinguishes from primary CNS diseases)
Response to stimuli: Hypersensitivity with exacerbation of muscle spasms

High-YieldA wound may NOT be visible at presentation! The original wound may have healed by the time clinical signs develop. Up to 20% of tetanus cases have no identifiable wound. Always screen paws, digits, and oral cavity carefully, and consider radiographs of injured digits to detect osteomyelitis.

Laboratory Findings

Laboratory testing is generally nonspecific but helps rule out differentials and assess complications:

Differential Diagnosis

Strychnine poisoning is the ONLY condition that truly mimics tetanus. Other differentials can usually be distinguished by history, clinical examination, and response to treatment.

Condition	Similarities	Key Differences
Strychnine toxicosis	Muscle rigidity, opisthotonus, hypersensitivity, convulsions. Also blocks glycine.	Acute onset (within hours), no facial signs, no wound history. Toxicology positive.
Hypocalcemia	Muscle spasms, tremors, tetany	No facial signs (risus sardonicus, trismus). Low ionized calcium. Often postpartum.
Masticatory muscle myositis	Trismus, difficulty opening mouth	Trismus ONLY (no generalized rigidity). Positive 2M antibody test. Muscle atrophy in chronic cases.
Meningoencephalitis	Altered mentation, rigidity possible	Altered mentation (tetanus dogs are alert). CSF abnormalities present.
Metaldehyde toxicosis	Muscle tremors, seizures	Acute onset, hyperthermia. History of snail bait ingestion.

Treatment

Treatment of tetanus is multimodal and intensive, focusing on: (1) eliminating the source of toxin production, (2) neutralizing circulating unbound toxin, (3) controlling muscle spasms and rigidity, and (4) providing supportive care until recovery occurs.

1. Source Control and Wound Management

Thoroughly search for any wound, especially between digits, nail beds, and oral cavity
Radiograph injured paws to detect osteomyelitis
Surgical debridement of necrotic tissue and wound lavage with hydrogen peroxide (increases oxygen tension)
Digit amputation may be required if osteomyelitis is present

2. Antibiotic Therapy

NAVLE TipMetronidazole is now preferred over penicillin G because it penetrates necrotic/vascular-compromised tissue better, and penicillin may have GABA-antagonist properties that could theoretically worsen muscle spasms by enhancing the effects of tetanospasmin.

3. Tetanus Antitoxin (TAT)

Equine-derived tetanus antitoxin neutralizes circulating unbound toxin but cannot affect toxin already bound to neurons. Its use remains controversial due to limited evidence of efficacy and risk of anaphylaxis.

Dosage: 100-1,000 IU/kg IV (after test dose)
Test dose: 0.1-0.2 mL subcutaneously 30 minutes before IV administration to check for anaphylaxis
Local infiltration: 1,000 IU may be injected around wound margins during surgical debridement
Timing: Most beneficial when given early; limited effect once toxin is internalized

4. Muscle Relaxation and Sedation

5. Supportive Care

Environmental management: Dark, quiet room; ear plugs/cotton in ears to reduce auditory stimuli; minimal handling
Nutritional support: PEG tube placement for enteral nutrition in dysphagic patients (placed within 48h in most cases)
Fluid therapy: IV crystalloids to maintain hydration
Analgesia: Opioids (buprenorphine, methadone, fentanyl) for pain from sustained muscle contraction
Temperature monitoring: Active cooling if hyperthermia develops (greater than 39.5°C from muscle activity)
Recumbent patient care: Repositioning every 4 hours, padded bedding to prevent decubital ulcers
Urinary management: Manual bladder expression or indwelling catheter if urinary retention occurs
Respiratory monitoring: Watch for laryngeal spasm and aspiration pneumonia; have intubation equipment ready

Drug	Dosage	Notes
Metronidazole (First Choice)	10-15 mg/kg IV q8h for at least 10-14 days	Excellent anaerobic coverage and tissue penetration. Preferred over penicillin - lower dose muscle relaxants needed. Monitor for neurotoxicity at high/prolonged doses.
Penicillin G (Alternative)	25,000-30,000 IU/kg IV q8h	Effective against C. tetani. May have GABA-antagonist properties which could worsen spasms.

Prognosis and Complications

Prognosis

Reported survival rates in dogs range from 50-92% with intensive care. Prognosis depends on severity grade, development of complications, and quality of supportive care.

Complications

Respiratory: Aspiration pneumonia (most common), laryngeal spasm (life-threatening), respiratory arrest
Cardiovascular: Hypertension, arrhythmias, sudden death from autonomic dysfunction
Gastrointestinal: Hiatal hernia, megaesophagus, reduced GI motility
Metabolic: Hyperthermia, dehydration, malnutrition
Urinary: Urinary retention, urinary tract infection
REM Sleep Behavior Disorder: Reported in over 70% of recovering dogs; transient and self-resolving

Recovery

Median hospitalization time is approximately 8 days but ranges from 0-26 days depending on severity. Clinical improvement typically begins around 11 days after presentation. Full recovery may take 4-6 weeks or longer because recovery requires regeneration of new nerve terminals.

Exam Focus: Natural infection with tetanus does NOT confer protective immunity! Dogs can develop tetanus again because the small amount of toxin that causes disease is often insufficient to stimulate robust antibody production. Vaccination after recovery with tetanus toxoid can induce protective antibody titers.

Drug	Dosage	Notes
Diazepam	0.2-0.5 mg/kg IV q4-6h or CRI	GABA agonist - counteracts toxin effect. First-line muscle relaxant and anxiolytic.
Midazolam	0.2 mg/kg IV q4-6h or CRI	Water-soluble benzodiazepine. Alternative to diazepam.
Acepromazine	0.005-0.05 mg/kg IV q6-8h or CRI	Baseline sedation. Used in most protocols. Provides sedation and mild muscle relaxation.
Methocarbamol	55-100 mg/kg IV q30-60min PRN	Centrally-acting muscle relaxant. Used for severe muscle rigidity.
Dexmedetomidine	1-2 μg/kg/h CRI	Alpha-2 agonist. May help with refractory cases. Monitor for bradycardia.
Magnesium sulfate	Empirical dosing; monitor Mg levels	May reduce muscle relaxant/sedative requirements. Helps control autonomic dysfunction.

Prevention

Dogs are not routinely vaccinated against tetanus due to their relative resistance to the disease. Prevention focuses on proper wound management:

Prompt veterinary attention for all wounds, especially deep puncture wounds
Thorough wound cleaning and debridement to remove necrotic tissue
Prophylactic antibiotics for contaminated wounds
Monitoring for early signs of tetanus in animals with recent wounds

Favorable Prognostic Factors	Poor Prognostic Factors
Localized tetanus Low severity grade (I or II) Longer incubation period Slow disease progression Early treatment initiation Adult dogs	Generalized tetanus with rapid progression High severity grade (III or IV) Short incubation period Respiratory complications Autonomic dysfunction Young dogs (less than 2 years)

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