14 Dec 2020
Lotfi El Bahri DVM, MSc, PhD presents a Case Notes from Vet Times on a four-year-old male German shepherd dog‑cross with muscle tremors and severe hyperaesthesia to external stimuli.
Lotfi El Bahri
Job Title
Figure 1. A strychnine-poisoned dog in an opisthotonos posture.
You are presented with a four-year-old male German shepherd dog‑cross weighing 45kg at your emergency veterinary clinic.
The dog escaped from its leash in the morning. When it returned to the property a few hours later, the owner noted anxiety, then muscle tremors and severe hyperaesthesia to external stimuli (touch, sound), with contraction of all four limbs and neck, in a “sawhorse stance” posture.
In view of the clinical signs, strychnine poisoning is strongly suspected.
What is the toxic dose of strychnine and the mechanism of toxicity?
High oral toxicity. The oral median lethal dose is 0.75mg/kg in dogs and 2mg/kg in cats.
Malicious poisoning occurs frequently. Most baits contain 0.5% strychnine concentrations for destroying gophers, moles and wild carnivores. Secondary poisoning may occur.
Strychnine is a powerful convulsant that acts by competitive antagonism of the amino acid glycine, an inhibitory neurotransmitter, at postsynaptic receptors of motor neurons and interneurons of the spinal cord.
Glycine antagonism results in loss of normal descending inhibitory motor tone, leading to severe extensor skeletal muscle spam.
What are the clinical features of strychnine poisoning in dogs?
Clinical signs appear usually within 15 minutes and 2 hours following ingestion of the toxin.
They include:
Severe seizures can cause rhabdomyolysis (marked increase in creatine kinase) serum levels; the normal values in dogs is between is 46U/L and 467U/L).
The released myoglobin caused by muscle damage can result in acute renal failure. Laboratory results also indicate very high serum lipase activity (development of acute pancreatitis).
Toxicological diagnosis is based on the analysis of bait, vomitus and urine by UV spectrophotometry, gas chromatography or high-performance liquid chromatography coupled with mass spectrometry.
What is the approach to managing strychnine poisoning in dogs?
Strychnine poisoning is a time‑critical, life‑threatening emergency with no antidote.
For controlling seizures, an IV catheter should be placed. Diazepam (0.5mg/kg to 2mg/kg IV bolus) should be administered and repeated, if necessary, within 20 minutes (serum half‑life in dogs is 2.5 hours to 3.2 hours) up to three times in a 24-hour period, or 1mg/kg to 2mg/kg rectally. This is contraindicated in patients with severe liver disease, however.
Alternatively, administer lorazepam (long‑acting benzodiazepine; 0.2mg/kg IV bolus, because of its high affinity for benzodiazepine receptors in the central nervous system) or midazolam (0.2mg/kg to 0.4mg/kg IV); this may be repeated once.
Ketamine is contraindicated because seizure‑like effects have been reported, while valproic acid is not recommended (serum half‑life in dogs is between 1.5 hours and 2 hours).
If seizures persist or recur, phenobarbital can be administered (2mg/kg to 5mg/kg IV bolus) and repeated at 20-minute intervals, up to two times. Do not use in patients with significant liver disease, though.
In case of refractory seizures, administer propofol (3mg/kg to 6mg/kg IV initial bolus) followed by 0.1/kg/min to 0.6mg/kg/min constant rate infusion (CRI). Alternatively, 2% to 2.5% concentrations of isoflurane alone with oxygen can be used. For maintenance, use 1.5% to 1.8% concentrations of isoflurane in oxygen.
Attention to airway and breathing is paramount. Affected animals should be intubated and provided artificial respiration with oxygen during convulsions.
Correction of metabolic acidosis (normal pH lower than 7.33; standardised base excess in dogs lower than -4mmol/L) can be carried out with sodium bicarbonate 8.4% solution: 1ml/Lb bodyweight to 2.5ml/Lb bodyweight CRI, depending on the severity of the acidosis, over a four-hour period.
Hypoglycaemia can be corrected using 5% dextrose: 40mg/kg IV every 24 hours; blood glucose should be monitored.
Hyperthermia can be corrected by external cooling every two to four hours (ice baths should be avoided as these may create hypothermia). This should be stopped when the rectal temperature reaches 39.4°C.
Emetics are contraindicated as they stimulate seizures.
To prevent further absorption of strychnine from the intestinal tract, use activated charcoal (AC; 1g/kg to 4g/kg) mixed with water to make a 20% slurry (1g/5ml water) via nasogastric tube as soon as possible post‑ingestion, and after the airway is secured. This may be repeated at eight-hour intervals for three days (reduces enterohepatic recirculation of strychnine). However, AC admitted orally is contraindicated in convulsing or comatose animals.
Urinary acidification by ammonium chloride is of little value.
Because strychnine is highly lipid soluble (log P values of 2.23), consider lipid emulsion therapy – an initial IV lipid emulsion (ILE) 20% 1.5ml/kg IV bolus over one minute, followed by a CRI of 0.25ml/kg/min for the next 30 to 60 minutes.
In non-responsive patients, an additional intermittent bolus can be given IV slowly, up to 7ml/kg.
If clinical signs do not improve after 24 hours, discontinue ILE.
ILE 20% preparations are isotonic and can be given by a peripheral vein or in a central catheter using aseptic techniques.
Affected animals should be maintained in dark, quiet room. Actions or noises can restart convulsions; cotton earplugs can be used to prevent auditory stimulation.
The patient should be observed for a minimum of four days (long elimination half‑life of strychnine).
