Why is my 17-year-old cat vomiting, sneezing, hacking, and twitching?

This sounds like she has an upper respiratory infection that is making her sneeze and vomit from the sneezing, coughing and nasal discharge. Generally cats get over colds in a week or so without treatment. Since Mieko is seventeen I’m more concerned about her since she could become dehydrated if she’s not eating and drinking normally due to having a head cold. I do recommend a veterinary exam and treatment. She may need fluids under the skin to keep her hydrated and perhaps medication to stimulate her appetite. Often dogs and cats vomit when they have a cough so I don’t think the vomiting is the main concern but certainly does need to be addressed if it continues. In the meantime you can use a warm damp cloth to keep her nose and eyes clean of discharge and heat up small meals of wet food with added water to keep her eating. I put it in the microwave for eight seconds which brings out the aroma since cats often stop eating when they can’t smell the food. If she’s not had senior blood work recently they may recommend that also to check her kidney and liver function, and check for anemia and infection. Antibiotics are given only when a viral cold turns into a secondary bacterial infection. You are right to be concerned with an elderly cat so please take her in for treatment so she can get over this quickly. Thanks for using Petco Pet Education Center, formerly Petcoach to help you care for Mieko.