Cats dont get "bugs" but they do get infections. I would suspect an abscessed tooth, an infection or a GI upset or acid reflux. Even though he may not appear underhdrated, he still can be.Add water to the canned food and feed him very slowly.
Cats that eat dry can have bouts of dehyration.Indigestion is caused by inadequate chewing. Vomiting is very common. When acid reflux builds up in the stomach, they begin to vomit. Even if they have always eaten this,, they often become intolerant to corn and gluten which are used as filers in most supermarket cat foods.Vets push science diet because they sell it but it is not a great food either as it too contains corn and gluten.
Lack of fluid in the bowel also leads to vomiting, Ingested hair can also be a factor.Anorexia for any extended time leads to liver damage and causes nausea and vomiting as well.
I would add some canned food for himwith some water added.
Serve the two foods separately - do not mix them - add a half pepsid a/c tablet, crushed up into the wet food one time a day.
Put hairball remedy or even plain vaseline on his paw for him to lick off as well.
If his tummy needs a rest, feed baby food with no onion or garlic for a few days.
If he wont eat, and your cat is not diabetic begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.
Give him droppers of unflavored pedialyte a few times a day to make sure he stays hydrated.
You should have a basic blood panel drawn. 13 is not tha old for a kitty - mine is 18. He can still have many good years to live.
Here are agencies hat can help:
American Animal Hospital Association http://www.aahahelpingpets.org/home " Through the AAHA Helping Pets Fund, veterinary care is possible for sick or injured pets even if they have been abandoned or if their owner is experiencing financial hardship." Angels 4 Animals http://www.angels4animals.org/ "Our services range from financial aid to complete treatment to those pets and pet owners in need." Care Credit http://www.carecredit.com/ A credit card company for health care, including veterinary care. "With a comprehensive range of plan options, for treatment or procedure fees from $1 to over $25,000, we offer a plan and a low monthly payment to fit comfortably into almost every budget." Feline Veterinary Emergency Assistance (FVEAP) http://www.fveap.org/sys-tmpl/door/ "Seniors, People with disabilities, People who have lost their job, Good Samaritans who rescue a cat or kitten - any of these folks may need financial assistance to save a beloved companion." The Feline Veterinary Emergency Assistance Prog ram is a nonprofit 501 (c)(3) organization that provides financial assistance to cat and kitten guardians who are unable to afford veterinary services to save their companions when life-threatening illness or injury strikes. God's Creatures Ministry http://www.all-creatures.org/gcm/help-cf.html "This fund helps pay for veterinarian bills for those who need help." Help-A-Pet http://www.help-a-pet.org/home.html "Our efforts focus on serving the elderly, the disabled, and the working poor." IMOM http://www.imom.org/ "We are dedicated to insure that no companion animal has to be euthanized simply because their caretaker is financially challenged." The Pet Fund http://thepetfund.com/ "The Pet Fund is a registered 501(c)3 nonprofit association that provides financial assistance to owners of domestic animals who need urgent veterinary care."
Please let me know how he is doing. I will always be here for both of you.
I hope he feels better very soon.
Sincerest best wishes,