One of the most common things to cause sneezing in a cat is an Upper Respiratory Tract Infection.
URTI's are caused by a virus (usually Rhinotracheitis which is a Herpes virus, or by Calici virus).
Since antibiotics do not kill viruses, only bacteria, they will not help your cat get over this viral infection. Antibiotics are only helpful if he gets a secondary bacterial infection on top of this viral infection.
Fish mox is amoxicillin, and each capsule contains about 250mg of amoxicillin. You can read about the dosing here, but if he were my patient, I would not start him on this as antibiotics do not help with viral infections. So, not only will it not help him, but also it might do harm as it can cause loss of appetite and diarrhea, and in some cats, vomiting.
Rhinotracheitis typically has a very watery sneeze - in fact if the kitty sneezes on a tabletop, you can often see 1-2 feet of wet spray on the table!
Because it is a Herpes virus, if a cat has it, he has it for life.
The cat will mount an immune response to the infection, but this will just let the symptoms resolve, while the infection lies in wait. It is NOT contagious to humans.
But, similar to human herpes virus infections, it can reappear later in life at times of stress. This is similar to human herpes virus infections that cause cold sores (which are not contagious to cats!).
Rhinotracheitis is highly contagious, and you could even bring it home on your shoes or pants. If you cat *is* fully vaccinated, but he still broke with sneezing, it suggests that he had this as a kitten and it is now latent in his body. Some stress in his world (vistors, moving, seeing other cats out the window) may have been enough to drop his immune system to a point where he started sneezing and showing symptoms again.
So, what can you do to help him?
Lots of things!
1. Encourage him to eat.
Good nutrition will give his body the energy it needs to fight this infection. As with us when we have a cold, when the nose gets stuffed up food loses its flavour. So, you can offer him a variety of canned food (particuarly the fish flavours as they are particularly strong smelling) which has much more smell than dry. Also, you can warm the food up a little in the microwave as warm food smells more than cold.
You could try getting some human baby food in meat flavours (check that there are no onions or garlic in the ingredients) and mix that with warm water and offer that, or syringe it in little bits into your cat's mouth. Beech Nut makes a line of baby food that has nothing but meat (beef, chicken, turkey or veal) in it.
Here's a link:
If you cannot find this, you could find another meat baby food - just read the label carefully to be sure there are no onions, onion powder, garlic, or garlic powder in it.
Offer him some canned cat food, and mix it with water to make a slurry if he won't eat it.
Boil a chicken breast and then put it in the blender with water to make a baby-food consistency gruel to offer.
You could pick up nutristat http://www.agri-med.com/site/255063/product/NUTRST-4.25 It is a calorie-dense paste that you can syringe into them to get maximum caloric impact from a given volume of food.
Here is another link to ways to encourage cats to eat:
It has some good suggestions.
I'm afraid you may have to force feed him with a syringe to get him started. The human baby food (or making your own puree with cooked chicken breast in a blender with lots of water) goes through a syringe quite well.
2. Encourage him to drink.
What you can do is try to get some calories into him in a liquid form - that way he is getting nutrition at the same time as fluids.
I suggest opening a can of tuna *in water* and offering the liquid.
Also, you can pick up Clam Juice in most grocery stores (sold in with the V8 or the canned tuna) and mix that with some water.
You could try Lactose Free milk (Lactaid is the Canadian brand). Whiskas makes a tetra pack of "Kitty Milk" that is lactose free milk with flavouring added.
You could get some (onion-free) chicken or beef broth and dilute it 50:50 with water, and offer that.
3. Clean his nose.
You can put a bit of warm water on a washcloth, wring it out, then hold it on her nose (if he'll let you) to soften any dried mucus so it comes off.
4. Steam him.
Take your kitty into the bathroom while you run a hot shower for 15 minutes. The hot, steamy air will help to loosen any nasal secretions so he can sneeze the mucus out and clear the airways. Doing this twice a day would be great! You could offer him some canned food while the shower is on so he didn't get too frightened. Alternatively, you could use a humidifier in a room he is in most.
5. Put drops in his nose.
Just go to the pharmacy and ask for ophthalmic saline, or have a look in the contact lens section. What you want it just sterile saline drops to relieve dry eyes - NO medication in it. The pharmacist should be able to point it out to you. Put one drop in each nostril twice daily. The idea is that you are helping to moisten the area so your cat can sneeze out any congestion in there. Continue for a week.
6. Lysine capsules.
Herpes viruses need an amino acid called arginine to replicate and survive. If you give lysine, it substitutes in for arginine but does NOT allow the virus to replicate! Thus, it can stop the virus and really help the cat's immune system to win. You can pick up Lysine at most health food stores. I usually suggest that cats be given 250mg twice daily for 3 weeks. The capsules with powder in them are easiest to give, as you can simply open it, and mix the powder in with some canned food.
You can read more about it here: http://www.thensome.com/herpes.htm
Depending on which virus your cat has (Herpes or calici) this infection could take 1 - 4 weeks to clear up. If you can do nothing else, giving him the canned food is going to make the biggest difference in how well he does with this. His body just needs that extra energy to fight this. Also, the first ingredient in canned food is water, so it is a way of making sure he is getting a bit more fluid.
I'll give you some links to further information:
If your cat is refusing all food, develops green or bloody nasal discharge, is more lethargic or depressed, is vomiting or having trouble breathing, or in any way getting worse, then please take him to see your veterinarian promptly!
Good luck with him! He is lucky to have someone looking out for him!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.