| Also known as: | Lockjaw |
Description
Tetanus is a rare disease in dogs, and the disease pathophysiology is similar to that described in other mammalian species. The causal agent of tetanus in the dog is the neurotoxin of the gram-positive bacterium Clostridium tetani. Infection occurs due to initial contamination of a wound by C. tetani. Replication of the bacteria occurs due to the anerobic conditions provided by damamged wound tissues. Tetanospasmin (neurotoxin) produced by the bacteria is transported to distant sites via the circulation and by retrograde axonal transport along peripheral nerves. This causes inhibition of the release of GABA and glycine from interneurons in the spinal cord and brain, resulting in persistent rigidity of striated muscle.
Clinical Signs
Because of their high natural resistance to tetanospasmin (compared to the horse and man), affected dogs may have a longer incubation period and clinical signs may be mild; a change in gait may be the only reported abnormality. The localised form of tetanus that develops in the dog may be difficult to diagnose, particularly if no wound is apparent. Unlike the generalised spastic paralysis described in horses, dogs may present with a wound to a single limb (often a thoracic limb) and unyielding stiffness or rigidity in the surrounding area. The rigidity may spread to the contralateral limb and then begin to advance cranially, becoming generalised as the disease progresses. As reported in horses, dogs may display hypersensitivity to auditory or tactile stimuli. Affected animals have also been reported to display caudal displacement of the ears and wrinkling of the forehead. Prolapse of the nictating membranes, trismus (excess jaw tone) and retracted lips (risus sardonicus) are other clinical features.
Diagnosis
As described above, a history of trauma and unrelenting muscular spasm in a localised area is often described in cases of tetanus in dogs. An obvious wound may help to increase suspicion of tetanus, particularly if the wound is necrotic or infected. In this species, diagnosis is often based on clinical judgement rather than laboratory tests, as these may add little helpful diagnostic information. In occasional cases, C. tetani may be cultured from a wound, but this may be impossible if healing has occurred or a wound is not apparent. An important clinical feature of tetanus in dogs that may aid diagnosis in hindsight is that of slow but complete resolution of muscular spasm over a lengthy period of time (weeks or months).
Other neurological diseases causing limb spasticity may need to investigated; myelography for example will assist in ruling out an extramedullary lesion. A complete blood cell count, serum biochemistry, survey radiographs of the thoracic spine and serology for protozoal diseases should also be performed in order to exclude other neurological differential diagnoses.
Electromyography may be used to help to support a presumptive diagnosis of tetanus. Abnormalities relating to a defect in glycinergic inhibition, including the presence of "doublets" (double discharges of the same motor unit at short intervals) and simultaneous activity in both flexor and extensor muscles.
Treatment
The approach to treatment in the dog is the same as that described for the horse. It is important however to exercise caution in the intravenous administration of equine tetanus antitoxin, as this may cause anaphylaxis if used in dogs. An intradermal test may be performed prior to the administration of the antitoxin in order to prevent anaphylaxis. Metronidazole and amoxcillin-clavulanic acid PO have been used successfully to treat tetanus in the dog. Corticosteroids are contraindicated in localised tetanus and may worsen clinical signs if administered.
Noise and external stimuli should be kept to a minimum, and the dog should be kept in a quiet, darkened kennel. Acetylpromazine has also been reported to reduce sensitivity to external stimuli.
Supportive nursing care such as catheterisation and/or enemas may be required. Bedding should be thick and changed regularly, and the patient should be turned regularly in order to prevent decubital ulcer formation. The dog may require hand feeding and drinking. In the recovery stage, physiotherapy may be beneficial.
Prognosis
The long-term prognosis for dogs with tetanus is good providing treatment and nursing care are provided.
References
- De Risio, L., Zavattiero, S., Venzi, C., Del Bue, M., Poncelet, L. (2006) Focal canine tetanus: diagnostic value of electromyography Journal of Small Animal Practice 2006 May 47(5):278-80
- Matthews, B.R and Forbes, D.C. (1984) Tetanus in a Dog Canadian Veterinary Journal May; 26(5): 159–161
- Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial