Now that breathing rate is thankfully just above normal (20-30bpm) so its not alarming but if his gums are pale we need to be careful here. Anemia (often from fleas, worms, blood parasites) can increasing breathing effort, cause lethargy and become serious for kittens quite quickly. So, we need to be careful here.
That said, for his other signs these are most supportive of a viral cat flu agent. And while we'd want him seen as soon as possible for his gums (sooner, as an emergency if they pale further),we can try some supportive care just now to try to ease his breathing. To start ,we can use some steam treatment. To do so, you can take him in the bathroom while you run a hot shower. The steam will help loosen and clear discharge from the airway. You can also use a baby nebulizer, but often they don’t like things held up to their faces. So, if you find that to be an issue, you can make a little ‘steam tent’ with him in a carrier, the nebulizer next to that, and a bed sheet over both.
Furthermore, if he is building up mucus that the steam isn't shifting, use a cotton ball moistened with warm water to wipe away crust and mucus. As well, non-medicated saline nasal drops (ie Ocean Mist or Little Noses) can be used. To do so, just tilt the head back and drop 2-3 drops in one nostril. Not a favorite, but it helps. After the drops go down, you can let the head up and wipe away any discharge that gets loosened. Then repeat with the other nostril.
Making sure he is getting food and water is important, as congested cats who can’t smell their food often won’t eat as well as they should. Therefore, if need be, try offering smelly wet foods (since they are high in water). It may help to warm it up a bit in the microwave to help him smell it.
Furthermore, since feline herpes virus is a suspect, we can also start OTC L-lysine. This is a nutritional supplement that can help them recover quicker. This is available over the counter at vets, pet stores, and even online. They come as gels, powders, and as crushable tablets that can be mixed into food. An average cat dose is 500mg a day.
Overall, this does sound highly suspicious of an upper respiratory based infection. The antibiotic may have held off any bacterial involvement but with his still having signs that rings alarm bells of viruses. Therefore, you can try the above to help give him some relief. Of course, if these signs linger or again if those gums pale any more, then we'd want the local vet involved (even if it means politely going in as a walk in urgent appointment). They can confirm our concerns and dispense cat safe decongestants (since human ones are toxic for kitties) +/- further antibiotics to help us nip this in the bud.
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