Hi there, I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm really sorry to hear that Chivas has been ill. I'll do my best to help, but keep in mind that it's not practically or legally possible for me to diagnose her or offer specific treatment plans without examining her in person first.
While it's possible for antibiotics to cause a decrease in appetite or other GI upsets like vomiting and diarrhea, it usually happens within the first 24-48 hours of dosing and resolves within 24-48 hours after the drug is discontinued. Unless the drug did some permanent damage to an organ system, any GI effects would stop within hours after giving the last dose. The fact that it took 7 days for her appetite to decrease and that it hasn't rebounded since stopping the drug would make me highly suspect that something else is going on. Unfortunately, with a pet as aged as Chivas, the list of potential problems can be quite long. While blood work can evaluate the major organ systems for loss of function, that's not always enough to tell us what the issue actually IS. There are plenty of illnesses that can cause a dog to be deathly ill but have normal blood work. In my experience, unfortunately, when I see older dogs who are sick despite normal blood work, my biggest suspicion is for cancer. I'm especially worried for cancer if they are losing weight as well. Because of the diverse nature of cancerous diseases, there's no one test that can diagnose all types of cancers. In my practice when I have patients who are ill with normal blood work, I move on to X-rays of chest and belly and an abdominal ultrasound +/- sampling of liver, spleen, abdominal lymph nodes, or any other abnormal structures. Sometimes they can get cancers such as lymphoma that are what we call infiltrative. These types of diseases don't cause distinct masses, but rather replace the normal tissues inside the organs. If there's lymphoma in the liver or spleen this can sometimes be diagnosed by ultrasound guided aspiration of those organs. Another thing I'd consider for a dog who is having GI issues like lack of appetite or vomiting would be pancreatitis. The blood tests needed to diagnose pancreatitis aren't included in your standard lab work ups, and pancreatitis can be difficult to diagnose on ultrasound, even by an experienced veterinarian. Pancreatitis would be more treatable than cancer, although treatment is largely supportive with anti-nausea medications, special diets, and IV fluids if disease is more severe. Pancreatitis is usually idiopathic, meaning that we don't know what caused it. It can range in severity from something that they get over on their own with no help to life threatening. When I'm really stumped in general practice, I also offer a referral to a board certified specialist to look over my work and run additional diagnostics. If you're up for that, your vet can likely arrange the referral. In a worst case scenario, if I'm suspicious of "bad disease" and referral or further diagnostics isn't an option, I'll sometimes also consider a trial of prednisone. Prednisone is a "broad spectrum" anti-inflammatory medication that can help with a variety of ill-defined diseases. Unfortunately, it has significant side effects and in some dogs can actually worsen their problem. It's hard to tell if it would help or hurt for sure if we don't really have a diagnosis, but in my hands if it's a choice between euthanasia or prednisone, I try the prednisone at least for a few days. It is a potent appetite stimulant also.
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