I am sorry to hear that your cats have all gotten an upper respiratory infection. Most upper respiratory infections in cats are viral so antibiotics are not helpful initially. Antibiotics are only helpful if there is a secondary bacterial infection. Even so, most cats will be able to fight this off themselves with proper supportive care. In severe cases, antiviral medications can be helpful and may shorten the course or decrease the severity of the infection (the one I use most often is called Famcyclovir) but this is available by prescription only so you will need to have your cat examined by a vet in order to get a prescription. The medication itself is not usually cost prohibitive and I usually use ½-1 250mg tablet twice daily for 14 days. This can be give for longer if necessary. There are no clinical studies on this, however, the evidence is all anecdotal at this point.
While some people advocate the use of L-lysine for cats with upper respiratory infections a recent study did not show this to be effective in reducing the duration or number of outbreaks, so I do not feel strongly about using it. It will not hurt but it may not help and is added expense and one more thing that you have to get into the cat which may be a challenge if he or she is not eating and especially if she is feral and not easily medicated.
If you can catch her and handle her safely, the best thing you can do to make her comfortable is to clean the eyes with warn water and a soft cloth. If there is nasal discharge you can use saline drops to break up the mucus and help him or her breathe better and you can put them in a steamy bathroom or use a nebulizer to help clear discharge as well. You can use regular saline eye irrigation (or contact lens solution) to clean around the eyes as well as this will not hurt if it gets in the eye. Since cats with upper respiratory infections cannot smell very well, you can heat up the food and offer it to them by hand to encourage them to eat. There is an antibiotic eye ointment that is available over the counter in some pet stores or feed stores called Terramycin. This can be used in the eyes 3-4 times a day to help prevent secondary bacterial infection of the conjunctiva. I am not familiar with the specific product you are referencing but if you can give me an active ingredient, I can see if I can turn up a dose for you. If it is an over the counter product, there should be instructions on the package or in an insert.
I hope this answers your question. If you have additional questions or want to discuss your cat's specific clinical signs in more detail please let me know.
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