Experienced and professional animal trainer provide their insights in answering this question :
A. We can hopefully provide a better answer to your question if you tell us the species of animal you’re asking about. Although loss of appetite typically points to a gastrointestinal problem,. Without knowing whether you have a bird, cat, dog, etc, we can’t help much. Please try your question again, or request a consult.

How to Identify Common Pet Problems ?

Our sources include academic articles, blog posts, and personal essays from experienced pet care professionals :

Less interest in food is a normal sign of aging for dogs, but not eating for more than a day or avoiding drinking their water are both good reasons to call the vet. Whether your senior dog is slowing down or still a puppy at heart, they need the right nutrition.
For a senior dog who needs a little help regaining an interest in food after an illness or due to declining health, your vet might prescribe an appetite stimulant medication. Mirtazapine, which is actually a human antidepressant, can work as an appetite stimulant in dogs and cats.
The Bottom Line. Most dogs experience decreased appetite as they are aging. The most common cause is dental decay and discomfort when chewing hard foods such as kibble. This is easily remedied by soaking the food, switching to wet food or mixing it with soft ingredients such as yoghurt or pumpkin.
A sudden loss of appetite could, however, signal a more serious illness. Check with your vet if your dog refuses to eat for a day or more. If his appetite gradually decrease but diminishes to a dangerously low level, this may also be a sign of something more serious. If you`re concerned, check with your vet.
In addition to being less active, many dogs at this age begin to develop digestive issues, kidney problems and other conditions that can benefit from diet modification. Only your veterinarian can accurately assess your dog`s needs.
Small dogs are considered senior citizens of the canine community when they reach 11-12 years of age. Their medium-sized friends become seniors at 10 years of age. Their larger-sized colleagues are seniors at 8 years of age. And, finally, their giant-breed counterparts are seniors at 7 years old.
If your dog will drink water but won`t eat, it may be due to nausea, stress, or mouth pain. If they are able to keep the water down, that is a good sign.
Firstly, it`s worth saying that loss of appetite is one of the most common signs of illness, so if your dog hasn`t eaten for 24 hours, then it`s best to call your vet, especially if there are any other troubling signs, like diarrhea or lethargy.
The clinical signs of more advanced kidney failure include loss of appetite, lethargy, vomiting, diarrhea, and very bad breath. Occasionally, ulcers will be found in the mouth.
He has lost interest in all or most of his favorite activities, such as going for walks, playing with toys or other pets, eating treats or soliciting attention and petting from family members. He cannot stand on his own or falls down when trying to walk. He has chronic labored breathing or coughing.
Shivering/shaking and lethargy/fatigue are symptoms that indicate your dog is sick and/or in pain. The cause is likely fever, ingesting poison, trauma (a physical injury), a virus (parvo or distemper), or an organ disorder (liver or kidney disease).
The average lifespan for small dog breeds ranges from 10 to 15 years, with some breeds living as long as 18 years. In general, small dogs live longer than their larger counterparts, with the shortest living breeds still exceeding the average lifespan of most large breeds.
For feeding a sick dog that won`t eat, most vets recommend bland chicken with white rice or white meat from a rotisserie chicken. Other options are sweet potatoes, unseasoned pumpkin (not pumpkin pie filling!), bone broth, and meat-based baby food.
The aging profile of dogs varies according to their adult size (often determined by their breed): smaller dogs often live over 15–16 years (sometimes longer than 20 years), medium and large size dogs typically 10 to 20 years, and some giant dog breeds such as mastiffs, often only 7 to 8 years.
Cause of Weight Loss In Senior Dogs That Are a Problem

Eight issues which are commonly the root problem in elderly dogs where weight loss is a symptom. These issues are liver/gallbladder disease, dehydration, dental issues, kidney disease, heart disease, diabetes, cancer, and arthritis.

Even if your dog is considered senior, you`ll still want to give them at least 30 minutes of daily exercise. Because of their age, exercise might need to become shorter but it`s still necessary to make sure they`re getting the required stimulation. Choose low-impact activities. Go on walks, not runs.
Take a new approach to exercise

“In fact,” she says, “I would encourage shorter, but more frequent, exercises over a long exercise duration for most senior dogs—for example, a 10-15-minute walk in the morning, a 10-minute walk in the afternoon, and a 20-30-minute walk in the evening.

Just like senior citizens need more sleep, an older dog sleeps a lot when compared to their younger counterparts. On the higher end of the scale, a senior dog can sleep up to 18-20 hours a day, says Dr. Rossman. She estimates that the lower end is probably around 14-15 hours per day.
White or instant rice is an easily digestible carbohydrate which makes it a good source of energy when your dog has an upset tummy or if you are feeding an older dog.
As your dog gets older, their teeth may become more sensitive, which can make chewing kibble more difficult and even uncomfortable. Switching to a soft food can help to alleviate your pet`s oral discomfort when eating.
Dogs cannot drink milk. Do not give your dog regular quantities of cow`s milk since the fat, calories, and lactose can cause many short-term and long-term health issues, including stomach upset, diarrhea, obesity, and pancreatitis.
Healthy dogs can typically survive up to five days without eating, but only if they have access to drinking water. In extreme situations, dogs have been known to survive without sustenance for much longer, but not without any severe health consequences.
Some of the earliest signs of kidney disease in dogs may include subtle weight loss, urinating/peeing more often and drinking a lot more water. Therefore, if you notice your dog is peeing on the floor or asking to go out more, or if your dog is always thirsty, it`s time to visit your veterinarian.
Symptoms of Kidney Failure in Dogs

Weight loss. Nausea and vomiting. Pale gums. Loss of balance, or stumbling.

Relevant Questions and Answers :

the most relevant questions and answers related to your specific issue

Q. I have a 13 1/2 year old Shih Tzu. How old is he in dog years?
ANSWER : A. It’s used to be that dog years were 7 years to every 1. Now it normally around 5 years to every year as long as your dog is healthy and kept up with vaccines. So he’s about 68ish in dog years.

Read Full Q/A … : Shih Tzu Age

Q. Does an indoor cat need to be vaccinated every year?
ANSWER : A. In practice, I recommend a feline combo vaccine every year, but will generally start administering every 3 years once they have had their kitten vaccines and 2 additional yearly vaccines. Rabies, is required yearly by law, and if kept up to date can be good for up to three years also. Based on the age of your cat I would give a yearly feline combo and rabies, and then boost the combo again next year.

Q. I was wondering what would be the best dog for a nine-year-old 14-year-old and a nine-year-old that cannot walk
ANSWER : A. You should pick a gentle, no agressive, inteligent dog. some breeds are predisposed to this kind of character but you should always asses the single animal and its behaviour. You can choose from family breeds like labs, shephards, retrievers.

Read Full Q/A … : Caring for an Elderly Dog

Q. I just obtained a 6 year old Yorkie and he has what looks like cateraics and he also drinks a whole lot of water and has hair lost like hot spots why
ANSWER : A. I would have her checked over because cataracts and increased thirst can be a sign of diabetes. Cataracts can also be related to ageing but 6 years old is not old enough !

Q. My cat is a male and has been nurtured he is 13years old he has lost his appetite and has lost a large amount of weight. He is a inside cat.
ANSWER : A. If he lost lately approx 25 % of his body mass you should see a vet asap. This huge decrese of weight leads in cats to serious liver disorder . Also primary cause of loss of appetite should be found to aply proper treatment.

Q. I have 2 male cats 1 is 10 year’s old and the other is 1 year old they both have been fix and have all there shorts the 1 year old wants to bit
ANSWER : A. Your question got cut off; please re-post or request a consult if you require advice on a specific query

Q. We have a 3 yr old Weiner dog, she is having pus in her eyes, I took her to the vet he gave me derma vet ointment, used it as the doctor prescribed
ANSWER : A. If the pus really isn’t all that bad, and it’s just some discharge, your pup may benefit from a diet change. It could be that the food you’re feeding just isn’t right for your dog, and that’s okay! Dogs grow and change over time, and now that your dog is fully matured, a diet change may be in order. Try something like Taste of the Wild, maybe a grain free dog food, Orijen, or Ziwipeak. These are all really great food options.

If the pus is really bad, and continues to get worse, see your vet again and let them know what’s going on. Maybe you could try a diet change, and then see if there are any improvements.

Remember, you should always gradually change a dogs diet. By gradually, I mean you put a tiny bit of new kibble in with a bowl of the old kibble. Reduce the old kibble by just a few bits of kibble. Throughout the course of at least two weeks (or as long as you want depending on whether or not you want to finish off the old food) you slowly add more of the new kibble while removing some of the old kibble. This makes the process gradual, and won’t cause any tummy-upset in your dog.

Q. My dog is 14 years old and just this year have we noticed scaly skin and it is all over his body except stomach. What should we do?
ANSWER : A. Scaly skin can be caused by a number of things in dogs ranging from general allergies, external parasites, infection and metabolic disease. If your dog is already on a preventive medication for external parasites, it is a good idea to monitor for any other signs of illness. Signs such as hair loss, scratching and chewing, redness and more can help you figure out the cause. For general allergies, a treatment with an allergy medication appropriate for your dog’s size and preventing licking and chewing of areas can help. If hair loss or other changes in behavior such as increased thirst, changes in weight or appetite and vomiting or diarrhea happen in addition to scaly skin, then having your vet check for a metabolic issue with bloodwork is best.