Strychnine is used in rat, mole and mouse poison so it is not very unlikely that a cat can be poisoned by strychnine although the use of this poison is deceasing in the US. It is a less common cause of accidental cat poisoning than in the past.
A veterinarian will check for strychnine poisoning by looking for the characteristic response to the slightest stimulation such as tapping the cat or clapping hands which starts a seizure. These are very painful for the cat and last about 60 seconds. The cat “throws his head back, can’t breathe and turns blue”.
That is the classic diagnostic tool for strychnine poisoning in cats. As little noise and handling as possible is essential.
The early signs (two hours after ingesting the poison) are clear because they are so typical. The first signs are agitation and excitability followed by the seizures referred to above.
Treatment (check with your veterinarian): Induce vomiting if cat is showing first signs of poisoning but do not induce vomiting if the cat shows laboured breathing or is having seizures and then see you vet immediately. She will administer valium in the appropriate dose to control seizures and place the cat in a dark, quiet room where she will be disturbed as little as possible.
A misdiagnosis for epilepsy can be made.
Source: Cat Owner’s Home Veterinary Handbook 3rd Edition. This book is for cat owners and highly recommended.