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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Tetanus is a disease characterized by muscular spasm, caused by a neurotoxin produced by Clostridium tetani.

Pathophysiology!!navigator!!

  • C. tetani is a Gram-positive, spore-forming bacillus. The spores are widespread in the environment, particularly in soil and mammalian feces
  • They typically gain access to the animal via a wound. The oxygen tension within the wound must be low to allow germination. Concurrent infection with other bacteria and the presence of foreign bodies or necrosis within the wound can help produce a favorable anaerobic tissue environment. Under such conditions, C. tetani organisms proliferate locally
  • Death and lysis of the organisms within the wound result in liberation of tetanospasmin, a neurotoxin responsible for the characteristic clinical signs. Tetanospasmin travels to the CNS via the hemolymphatic system and via peripheral motor nerves. The toxin exerts its effect on presynaptic inhibitory interneurons in the ventral horn of the spinal cord. There it cleaves synaptobrevin, a vesicle-associated membrane protein necessary for release of the neurotransmitters glycine and γ-aminobutyric acid. This results in a loss of motor neuron inhibition, and the subsequent hypertonia and muscular spasm
  • 2 other exotoxins are produced by C. tetani. Tetanolysin is thought to increase local tissue necrosis, promoting proliferation within the wound. Another nonspasmogenic toxin may have a sympathomimetic effect
  • The incubation period is highly variable, but it is usually 1–3 weeks. The spores can survive in tissue and germinate after wound healing if conditions then become favorable. Castration wounds and injection sites have also been associated with the development of tetanus.

Systems Affected!!navigator!!

  • Neuromuscular
  • Secondary effects on other systems (respiratory, skeletal, etc.) depending on the presence of complications

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

  • Horses are exquisitely sensitive to the toxin, and the disease has a worldwide distribution
  • A higher incidence may be associated with poor husbandry
  • There may be a higher incidence in warmer areas

Signalment!!navigator!!

No sex, age, or breed predilections.

Signs!!navigator!!

  • There is usually a history of a wound 1–4 weeks earlier
  • There may be lack of vaccination, although tetanus may occur in the face of vaccination
  • The first signs may be vague (local stiffness, lameness, colic)
  • The progression of signs depends on the extent of the infection, the vaccination status, and the age and size of the horse
  • Generally, the signs progress within 24 h, with the horse beginning to exhibit a stiff/spastic gait
  • Trembling, a raised tail-head, flared nostrils, and erect ears are seen
  • Preferential effects on postural muscles result in the characteristic “sawhorse stance”
  • Retraction of the eyes and protrusion of the third eyelids occur following a stimulus (noise or menace)
  • Spasm of the masseter muscles can cause inability to open the mouth (“lockjaw”)
  • Dysphagia results in accumulation of saliva in the mouth and aspiration of feed material
  • Increased rectal temperature and profuse sweating occur in response to prolonged muscular spasm
  • All signs are exacerbated by stimulation and excitement
  • Recumbency, with difficulty or inability to rise, occurs as the disease progresses. This can be accompanied by severe extensor rigidity
  • Horses may exhibit difficulty urinating and defecating
  • Respiratory failure occurs in fulminant cases

Causes!!navigator!!

Infection of a necrotic wound with C. tetani.

Risk Factors!!navigator!!

Unvaccinated horses that have sustained a contaminated soft tissue wound or penetrating wound to the foot are most at risk.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Laminitis
  • Hypocalcemia
  • Rhabdomyolysis
  • Rabies
  • Myotonia

CBC/Biochemistry/Urinalysis!!navigator!!

  • Nonspecific
  • Hemoconcentration and a stress leukogram may present
  • May see hyperfibrinogenemia and leukocytosis with secondary aspiration pneumonia

Other Laboratory Tests!!navigator!!

Anaerobic culture of C. tetani may be attempted from a wound.

Imaging!!navigator!!

  • No specific diagnostic indications
  • Thoracic radiography or ultrasonography if aspiration pneumonia is suspected
  • Ultrasonography of wound sites may help confirm anaerobic infection

Other Diagnostic Procedures!!navigator!!

No specific diagnostic procedures—diagnosis is made based on clinical signs coupled with the history of a recent wound.

Pathologic Findings!!navigator!!

  • Nonspecific
  • May demonstrate a C. tetani-infected wound
  • Secondary traumatic injury or aspiration pneumonia may be present

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

  • Initial treatment is aimed at neutralizing unbound toxin and preventing further release by eliminating the infection
  • Appropriate nursing care, particularly if the horse is recumbent, is vital to maximize the chances of a successful outcome
  • Fluid therapy may be required to maintain hydration. Nasogastric fluids have the added benefit of hydrating colonic content; however, IV or rectal administration of fluids may be necessary in horses where a nasogastric tube cannot be maintained

Nursing Care!!navigator!!

  • Confine to a quiet, dark stall with deep bedding
  • Minimize auditory stimulation with ear plugs
  • Padded walls and/or a padded helmet to minimize injury
  • Frequent turning of recumbent horses (every 2–4 h)
  • Recumbent horses that are unable to rise may benefit from slinging
  • Manual rectal evacuation and/or urinary catheterization may be necessary

Activity!!navigator!!

Restrict activity as much as possible through confinement and sedation.

Diet!!navigator!!

  • High-quality feed and free-choice water should be made easily accessible
  • If the horse is dysphagic, a nasogastric tube can be placed for the administration of feed, water, and electrolytes. The tube can be left in place to avoid the stress of repeated passage
  • In some cases, the passage of a nasogastric tube is not possible and feeding via esophagostomy or gastrostomy, or parenteral nutrition, may be required

Client Education!!navigator!!

Appropriate tetanus prophylaxis should be discussed.

Surgical Considerations!!navigator!!

  • Debride the wound and maximize exposure to air
  • Esophagotomy or gastrostomy may facilitate feeding in severely dysphagic cases
  • Tracheostomy may be necessary if laryngeal spasm and respiratory obstruction has occurred

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Tetanus antitoxin—100–200 U/kg IV or IM (single dose) will bind circulating toxin
  • Acepromazine—0.05–0.08 mg/kg IV or IM every 3–6 h or as required
  • Phenobarbital 6–12 mg/kg slow IV followed by 6–12 mg/kg PO every 12 h alone or in combination with acepromazine
  • Penicillin G (potassium or sodium)—22 000–44 000 IU/kg IV every 6 h for 7–10 days
  • Consider intrathecal administration of 50 mL TAT (20–30 mL in foals) after removal of an equal amount of cerebrospinal fluid from the atlanto-occipital space (requires general anesthesia), or via lumbosacral puncture in the standing horse. This is thought to be most beneficial early in the disease process. There is evidence of improved survival in human patients when TAT is administered intrathecally
  • Local infiltration of the wound with procaine penicillin and/or tetanus antitoxin (3000–9000 IU). This may help eliminate the infection and neutralize toxin present at the site
  • Vaccination with tetanus toxoid—clinical disease does not result in a sufficient immune response. Use separate injection site for antitoxin

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

  • TAT has been associated with the development of Theiler disease (serum hepatitis)
  • General anesthesia and intrathecal TAT administration can result in significant complications (meningitis, seizures). A significant improvement in outcome has not been definitively demonstrated with this procedure

Possible Interactions!!navigator!!

  • TAT will bind tetanus toxoid. These agents should be administered at different sites
  • Phenothiazine drugs (acepromazine) may potentiate barbiturates, causing more profound CNS depression if used together

Alternative Drugs!!navigator!!

  • Magnesium, administered as MgSO4 via IV constant rate infusion, has many potentially useful effects, including muscle relaxation, and reduces the requirement for other muscle relaxants and sedatives in human tetanus patients. Monitoring of serum Mg levels as well as the ECG for signs of toxicity (widening of the QRS) is recommended during therapy
  • Haloperidol 0.01 mg/kg IM every 7 days for long-acting sedation
  • Diazepam 0.01–0.4 mg/kg IV every 2–4 h
  • Macrolides (in foals only), tetracyclines, and metronidazole are alternatives to penicillin that may also be effective in eliminating vegetative C. tetani at the infection site

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Regular physical examination
  • Serial monitoring of packed cell volume, total protein concentration, and/or urine specific gravity to monitor hydration

Prevention/Avoidance!!navigator!!

  • Initial vaccination with 2 doses of tetanus toxoid 3–4 weeks apart
  • Annual toxoid booster thereafter is the current recommendation (although new evidence suggests horses may have protective antibody titers for at least 3 years after the initial vaccine course)
  • Tetanus toxoid should be administered in the case of a wound if there has not been vaccination within the past 6 months
  • Pregnant mares should be given a toxoid booster 4–6 weeks prior to expected parturition
  • Experimental studies indicate that immunity to tetanus challenge is present 8 days after administration of toxoid in horses

Possible Complications!!navigator!!

  • Myopathy
  • Aspiration pneumonia
  • Trauma (fractures, decubital ulcers)
  • Idiopathic acute hepatic disease (Theiler disease) is a rare complication of TAT administration

Expected Course and Prognosis!!navigator!!

  • Horses that are recumbent and unable to rise have a grave prognosis, particularly if progression has been rapid
  • The presence of dyspnea and dysphagia may also negatively influence survival
  • Horses that retain the ability to stand and ambulate have a fair prognosis
  • The clinical signs may persist for weeks; however, survivors will generally stabilize after 7 days and begin to show improvement after 2 weeks
  • Recovery may take as long as 6 weeks but is usually complete
  • The attitude of the individual horse and the ability to provide ideal nursing care are important factors affecting outcome
  • The overall mortality rate in horses is reported to be 50–80%

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Synonyms!!navigator!!

Lockjaw

Abbreviations!!navigator!!

  • CNS = central nervous system
  • TAT = tetanus antitoxin

Suggested Reading ⬆ ⬇

Green SL, Little CB, Baird JD, et al. Tetanus in the horse: a review of 20 cases (1970–1990). J Vet Intern Med 1994;8:128–132.

Mackay RJ. Tetanus. In: Smith BP, ed. Large Animal Internal Medicine, 5e. St. Louis, MO: Mosby, 2015:996–998.

Morresey PR. Tetanus. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 3e. St. Louis, MO: WB Saunders, 2010:637–641.

Steinman A, Haik R, Elad D, Sutton GA. Intrathecal administration of tetanus antitoxin in three cases of tetanus in horses. Equine Vet Educ 2000;12:237–240.

Author(s) ⬆

Author: Andrew W. van Eps

Consulting Editor: Ashley G. Boyle