Page 649 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

614 SECTION | VIII Rodenticides

VetBooks.ir Mechanism of Action TABLE 47.1 Acute Toxicity Data for Strychnine
Pharmacologically, because of its bitter taste, strychnine
Route of Administration
LD 50 (mg/kg)
Species
strongly stimulates salivary and gastric secretions. This
increases appetite, and as a result, strychnine has been Rat Oral 2.30
used for a long time to counteract the loss of appetite
Rat i.p. 2.50
associated with illnesses.
Strychnine is a potent convulsant. The convulsant Rat s.c. 1.20
action of strychnine is due to interference with the post- Rat i.v. 0.96
synaptic inhibition that is mediated by the amino acid gly-
Mice Oral 2.00
cine. Glycine is an inhibitory transmitter to motor neurons
Mice i.p. 0.98
and interneurons in the spinal cord. Strychnine acts as a
selective competitive antagonist to block the inhibitory Mice s.c. 0.474
effects of glycine at the glycine receptors. Studies indicate Mice i.v. 0.41
that strychnine and glycine interact with the same recep-
Duck Oral 3.00
tor, but at different sites. It increases reflex excitability in
the spinal cord, which results in a loss of the normal inhi- Pigeon Oral 21.00
bition of spread of motor cell stimulation, so that all mus-
i.p., intraperitoneal; i.v., intravenous; s.c., subcutaneous.
cles contract simultaneously. There is also evidence of an Source: From NIOSH (1983 1984).
increase in brain levels of glutamic acid, an amino acid
that acts as a transmitter for excitatory nerve impulses
that excite muscle contraction. The result of these effects Although strychnine has no direct effects on skeletal
is that skeletal muscles become hyperexcitable. With a muscles, all voluntary muscles contract simultaneously
little sound or touch, uncontrollable convulsions and (as discussed earlier). The increase in muscle tone is
seizures become eminent, followed by suffocation and caused by the central action of strychnine. The most pow-
death. Death occurs due to respiratory failure. erful effects are seen on the muscles of joints. Respiratory
muscles (diaphragm, thoracic, and abdominal) contract,
respiration ceases and, eventually, death ensues due to
Toxicity respiratory failure.
Birds poisoned by strychnine exhibit the signs of
Strychnine has been studied for acute toxicity in many
ataxia, ruffled feathers, wing droop, salivation, tremors,
species, and all species that have been tested have been
and convulsions. Death occurs due to respiratory failure.
found to be sensitive. Strychnine is an extremely toxic
On postmortem, lesions are only observed in the lungs—
substance, with an LD 50 of 2.3 mg/kg in rats, 2 mg/kg in
i.e., pinpoint hemorrhages resulting from death due to
mice, 0.6 mg/kg in rabbits, and 0.5 mg/kg in cats and
asphyxia. Rigor mortis occurs soon after death and per-
dogs. An approximate lethal dose for a dog is 0.75 mg/kg
sists for days. Occasionally, wildlife species are also inad-
body weight. Thus, 5-g bait having 0.3% strychnine could
vertently poisoned by strychnine.
be enough to kill a 20-kg dog. Toxicity data of strychnine
There is no evidence of cumulative toxicity from
for various animal species are given in Table 47.1.
strychnine (Gosselin et al., 1984). From animal studies,
Among animals, poisoning occurs with greatest fre-
there is no evidence that strychnine has potential for
quency in dogs due to accidental ingestion or malicious
reproductive and developmental toxicity or mutagenic
intent. The onset of signs can occur within 15 30 min or,
and carcinogenic activity.
occasionally, 60 min after oral exposure, depending on
whether the stomach is empty or full. The clinical signs are
associated with CNS effects. Onset of signs includes rest- Diagnosis
lessness, anxiety, muscle twitching, and stiffness of the
neck. The poisoned dogs usually show the signs of mydria- Diagnosis of strychnine poisoning is based on (1) history
sis, tonic convulsions, contractions of striated muscles, sei- of exposure to a strychnine bait or the presence of
zures, opisthotonus, and death. The animal becomes cracked corn in the digestive tract; (2) clinical signs
sensitive to touch, sound, noise, or any other sudden change of tetanic convulsions, seizures, hypersensitivity to
in the environment. A minor stimulation can trigger violent external stimuli, and muscle stiffness; and (3) chemical
convulsions. Muscular contractions are easily triggered by identification of strychnine in the stomach content,
external stimuli, accompanied by hypothermia, lactic acido- blood, urine, or visceral organs (liver and kidney).
sis, rhabdomyolysis and, consequently, nephrosis, which Strychnine residue can be detected and quantified using
may result in renal failure (Gupta and Crissman, 2012). gas chromatography (GC) flame ionization detector or