It's true that onions can be lethal to cats and dogs, but if your pet isn't showing any signs of poisoning, you should probably just put this down to experience and make sure it doesn't happen again.
Keep a close eye on your cat over the next few days. Symptoms of onion poisoning include:
- Diarrhoea
- Vomiting
- Weakness
- Slow movement
- Coughing and wheezing
- Blood in urine
If you notice any of these symptoms, get your cat to the vet as soon as possible. If your cat only ate a small amount of the tuna (and therefore a small amount of onion) then he or she should be okay. Good luck!
