Special Veterinary Pathology

Strychnine poisoning

STRYCHNOS NUXVOMICA  

• Strychnine poisoning is an uncommon occurrence in farm animals and usually occurs as a result of accidental ingestion of seeds of the plant Strychnos nuxvomica (Loganiaceae) or powdered form of nuxvomica used as a bait to kill dogs, foxes or rats etc.
• In the past, strychnine was used in human and veterinary medicine as an analeptic, nervine tonic and ruminotoric, however, now almost deleted from therapeutics and is sometimes used as a rodenticide. It was introduced as rodenticide in 1690.

Sources of poisoning

• Poisoning generally occurs in dogs and cats due to consumption of seeds of Strychnos nuxvomica, baits containing strychnine kept for foxes, rats, mice etc. or ingestion of birds or rats poisoned with strychnine or in other animals and human beings due to ingestion of strychnine treated seeds of peanuts., wheat etc.

Mechanism of toxicity

• Major site of action of strychnine is the recurrent inhibitory interneurons (Rensha cells) of the reflex arc in the spinal cord and medulla.
• Strychnine is a competitive glycine receptor antagonist and thus interferes with glycine receptors mediated post synaptic inhibition at these sites.
• The motor inhibitory effect of glycine in the reflex arc are prevented leading to uncontrolled excitation of spinal reflex, stimulation of extensor muscles, extensor rigidity and tonic seizures.

Clinical signs

• Clinical signs appear within 10 minutes to two hours of ingestion.
• The initial signs of poisoning are nervousness, restlessness, muscle tremors, stiffness and convulsive motion of certain muscles, hyperirritability and intermittent tonic spasms in response to noise or other external stimuli like touch, bright light i.e. animals are extremely sensitive to external stimuli.
• As the condition progresses, the muscular twitching becomes more pronounced leading to spontaneous and nearly continuous tetanic seizures with marked rigidity.
• Skeletal muscles contract antagonistically i.e. limbs are extended and neck is curved upward and backward (opisthotonus) because the extensors are stronger that the flexor muscles.
• Convulsions are intermittent in the beginning with intervening periods of partial relaxation.
• During the period of relaxation, any slight external stimulus elicits excitation and general tetanic spasms.
• Body gives a typical saw horse stance or posture.
• Pupil are widely dilated.
• Body temperature may be elevated.
• Death results from asphyxia due to prolonged paralysis of respiratory muscles usually within a few hours but may be delayed for as long as 48 hours.
• Poisoned animals are conscious until near death.
• Vomiting is rare and dogs often retain stomach contents in fatal intoxications.

Post-mortem lesions

• There are no specific post-mortem lesions, except petechiae resulting from anoxia due to arrest of respiration during the spasms.
• Venous blood is dark.
• Lungs and cerebral meninges are engorged.
• Stomach may be empty or may contain remnants of last meal.

Diagnosis

• History
• Clinical signs
• Laboratory investigation – myoglobinuria
• Post mortem lesions
• Detection of strychnine in urine sample.
• Detection of strychnine in stomach contents, liver, kidneys or suspected baits.
• Onset of rigor mortis is rapid.