I am sorry your cat is gaggng. There can be a few posibilities, neither heat related.
Check inside his mouth for a stuck item, oral sore, loose or broken tooth. Look underongue and on roof.
Rub a drop of syrup or honey into his gums.That should perk him up.
1. he has a sore throat. If so, 2 drops of honey every 4 hours will heal it.
2. he is nauseated. Give him one quarter of a ten mg pepcid a/c (famotidine) in a tablespoon of baby food chicken with no onion or garlic.
WAit a half hour and then offer him food.
IT should be pate canned food or baby food. NO DRY!
Wipe him down with a cool damp cloth.If he is dehydrated give him tuna water drained from the can.
Please let me know how he reacts. I will be here all day.
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