Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
With all this in mind, our approach does depend on their discharges. If you were seeing snotty green discharges from the nose (rather then clear) then that would be a red flag that secondary bacteria are involved and antibiotics from the vet would be indicated. So, with what you have reported, we really need to consider getting their vet involved to help clear this.
Otherwise, I do want to touch on some supportive care that we can start to ease these signs for them. First, if we have congestion then you can consider steam treating them. To do so, you can take them in the bathroom while you run a hot shower. The steam will help loosen and clear some of the snot congesting them. You can also use a baby nebulizer/humidifier, but often cats don’t like things held up to their faces. And since there are two to wrestle, you can consider placing them in a small room (or take turns with a carrier) with a humidifier to give them that steam room/tent treatment to open their airways.
If you see lots of nasal mucous and congestion and the steam isn't shifting, use a cotton ball moistened with warm water to wipe away crust and mucus. You can also use saline nasal drops (like Ocean Mist but not anything medicated). Tilt the head back and drop two to three drops in one nostril. Cats hate this, 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.
As well, we need to keep a close eye that the they keep eating/drinking. The reason is because congested cats can go off their food if they cannot smell it. Therefore, I am glad to hear that all is normal but if you see any appetite decline, then you can try tempting with smelly wet foods (since they are high in water). It may help to warm it up a bit in the microwave to help them be able to smell it better. And if their appetite wanes, you can start them on a calorically dense diet (ie Hill's A/D, Royal Canin Recovery, or Clinicare Canine/Feline Liquid Diet, wet kitten food). These all will provide more nutrition per bite and can be watered down for syringe feeding if need be.
Finally, since there is a risk of feline herpes virus here, you might consider trying them with L-lysine. This is a nutritional supplement that can help them recover quicker and keep the virus at bay. This is available over the counter at pet stores, vets, health food stores. The animal specific ones tend to come in gels, crushable tablets, or powders that can be added to the food. An average cat dose is 500mg a day and this can just help them against that virus to recover quicker.
Overall, when we see multiple cats in the household fall ill at the same time, we have to consider differentials that can be spread through the cats. And the most common cause is going to be our cat flu infectious agents. Therefore, we want to initiate supportive care and closely monitor them. Furthermore, since their discharges are already snotty, we'd want to consider having your local vet dispense antibiotics against our bacterial concerns, cat safe decongestants (ie Bisolvin) +/- feline friendly anti-inflammatories (ie metacam) to reduce airway inflammation and congestion, and get them over this flu.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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