Are you filled with dread when ‘Dog looking at owner in a funny way’ pops up on your computer screen as the summary of your next case? Well, occasionally that dog with an abnormal facial expression may have a condition that, with time and effort, can be very rewarding to manage in general practice.
You are presented with a 5 year old, male neutered, 28kg Boxer dog that has developed an abnormal facial expression over the last 5 days. His owner reports that he has since become reluctant to walk and appears stiff on all limbs. General physical examination is unremarkable other than a painful swelling of a hind paw nail bed, with purulent discharge. Neurological examination reveals an anxious but appropriate mentation. Cranial nerve examination reveals increased tone in the masticatory muscles and muscles of facial expression, resulting in erect ears, a wrinkled forehead and trismus. The third eyelids are intermittently protruded and there is mild miosis observed bilaterally. The dog stands with an abnormal posture, showing stiffness and extension of all limbs and an elevated tail head. It has a stiff, stilted gait in all limbs. Paw replacement testing and postural reactions appear within normal limits. Testing of the withdrawal reflexes is difficult to assess given the increased extensor muscle tone in all limbs. The dog appears hyper-responsive, with exacerbation of muscle spasms in response to noise and handling.
What is your neurolocalisation?
The clinical presentation is one of a generalised increase in muscle tone. This is most likely to be explained by increased or uncontrolled firing of the peripheral nerves to the affected muscles.
What are your differential diagnoses?
The most likely clinical diagnosis in this dog is generalised tetanus, however consideration should also be given to other causes of peripheral nerve hyperexcitability, such as electrolyte abnormalities (eg hypocalcaemia), and exposure to certain toxins (eg strychnine).
What is your diagnostic plan?
There is no definitive test widely available for the diagnosis of tetanus in practice, and a diagnosis is often made on the basis of consistent history and clinical signs. Affected animals often have evidence of a previous wound, loss of deciduous teeth, or a site of infection (such as the nail-bed lesion in this dog). However, clinical signs may not be observed for 2 weeks following infection and the absence of a wound at presentation would not exclude a diagnosis of tetanus. You run a haematology and serum biochemistry, which are unremarkable other than elevated creatinine kinase (CK) 2,457 IU/L (0-190 IU/L) and aspartate aminotransferase (AST) 95 IU/L (0-49 IU/L). This is most likely to be the result of sustained muscle contractions and recumbency. A primary myopathy would rarely be associated with the degree of extensor rigidity observed in this dog. Total and ionised calcium levels are within the normal range. You perform conscious thoracic radiographs to assess for the presence of megaoesophagus or aspiration pneumonia, which are not observed in this dog. Other diagnostic tests that could be performed to support your clinical suspicion of tetanus would include anaerobic bacterial cultures from the nail bed lesion and serum antibody titres to tetanus toxin. However, these tests are rarely performed in practice as it is often not possible to culture Clostridium tetani from affected cases, and antibody values must be compared to control animals.
What is tetanus?
Tetanus occurs as a result of infection by the Gram-positive, anaerobic, spore-forming bacterium Clostridium tetani. The bacillus is found ubiquitously in the environment, with spores entering the body via an infected external or surgical wound (such as recent ovariohysterectomy), or via the oral cavity in association with tooth root abscessation or loss of deciduous teeth in young animals. Following entry into an anaerobic environment, the spores producethe potent neurotoxin tetanospasmin. This toxin enters the peripheral nerves, migrates to the neuronal cell body in the spinal cord or brainstem, and binds irreversibly to inhibitory Renshaw cells. This binding results in uncontrolled firing of the lower motor neurons and the sustained muscle spasms typical of this disease. Given sufficient time, recovery can occur following sprouting of new axon terminals.
How would you manage this dog in practice?
Management of tetanus focuses on two main aspects: reducing the production of further toxin, followed by supportive care until recovery can occur.
Wound management - The animal should be carefully examined for potential sites of inoculation and these sites should be thoroughly cleaned and debrided.
Antibiotic therapy to eradicate C.tetani and stop further toxin production - Metronidazole (10mg/kg BID) is now considered to be the drug of choice for the treatment of dogs with tetanus. This may be administered intravenously in the early management of the disease, before being continued as a 2 week course per os. Penicillin derivatives are also widely used, however metronidazole has better tissue penetration and less potential to exacerbate the clinical signs of muscle spasticity.
Tetanus antitoxin to neutralise circulating unbound toxin - Use of tetanus antitoxin is controversial, with little evidence to support its effectiveness in all cases. Care should be taken regarding the potential for hypersensitivity reactions following injection, especially by the intravenous route. Intradermal injection of a small volume of antitoxin prior to administration has been suggested, with the appearance of a wheal at the injection site indicating an increased risk of hypersensitivity reactions. However, some animals without a wheal following intradermal injection may still show a hypersensitivity reaction. For this reason, all animals receiving intravenous antitoxin should be monitored closely for signs of vomiting, tachycardia or tachypnoea, and adrenaline should be readily available in case of an adverse reaction.
Sedation, muscle relaxation and analgesia to manage painful muscle spasms - Benzodiazepines, such as diazepam (0.2-0.5mg/kg IV every 4-6 hours) or midazolam (0.2mg/kg IV every 4-6 hours), are frequently used as first line drugs, either by intermittent bolus dosing or as constant rate infusions. Other agents that are commonly used include: acepromazine (0.005-0.05mg/kg IV every 6-8 hours), butorphanol (0.2-0.4mg/kg IV every 4-6 hours), methadone (0.1-0.3mg/kg every 4-6 hours), propofol (1-2mg/kg IV as required) and phenobarbitone (1-6 mg/kg PO or IM every 12 hours). Multiple drugs are frequently administered to a single patient to achieve the optimal level of relaxation. Care should be taken to avoid excessive sedation and constant monitoring, with the facilities to provide ventilation and intensive care, should ideally be available for more severely affected patients.
Nutrition and fluid therapy - Many dogs with tetanus find unassisted eating and drinking difficult, and are at a high risk for aspiration pneumonia. Mildly affected dogs may be able to eat small “meatballs” of soft food or be syringe-fed in an upright posture. More severely affected animals may require placement of either a ercutaneous endoscopically-placed or a surgicallyplaced gastrostomy tube to provide frequent small meals. Intravenous fluid therapy with balanced crystalloid solutions should be used to maintain adequate hydration and to correct electrolyte abnormalities if present.
Supportive care and nursing - The level of supportive care required for dogs with tetanus will depend on the severity of clinical signs, and can be very intensive in severely affected recumbent dogs. Dogs should be kept in a dark and quiet environment with as little stimulation as possible. They should be turned every 4 hours to minimise the risk of de cubital ulcer formation and be provided with ocular lubrication as required. Hyperthermia can result from the sustained muscle spasms and active cooling should be employed should the rectal temperature exceed 39.5˚C.
Numerous potential complications attributable to both tetanus and prolonged recumbency have been reported in dogs. Therefore owners should be made aware of the potential for both life-threatening complications and the need for prolonged hospitalisation, with its associated costs.
What is the likely prognosis for this dog?
The outcome for dogs with tetanus is dependent on both the intensive nature of treatment available and the severity of clinical signs. It has been estimated that up to 100% of dogs that remain ambulatory may survive. For non-ambulatory dogs with generalised tetanus, published survival rates range from 50-92%. Dogs may initially show progression of their clinical signs over the first 7 days, however signs of recovery are typically seen after 5-12 days, with complete recovery over 4-6 weeks.