Once upon a time, you had "your vet". They were close by, and they saw your pet from birth until death, dealing with all ailments they may incur. As veterinary medicine has evolved to suit the demand our pets receive as "family care", so has the field of specialists grown.
When asked by some "What's the difference? I mean, why can't my regular vet handle this?" the answer is simple: consider human medicine. Something aches, something is off, something is "wrong". We seek the care of our general practitioner, but when it's revealed to be something significant, they refer us to a specialist. It's the exact, same thing.
Board Certified specialists went through great lengths to become just that, and have a deeper understanding of your animal's problems specific to their specialty. In addition to becoming a doctor, they went through many additional years of residency training and had to pass extremely difficult tests to become board certified. While they may not be cheap to consult with, you get what you pay for, and they are worth every penny.
Common specialities include, but are not limited to: Neurology, Dermatology, Dentistry, Cardiology, Ophthalmology, Internal Medicine, Surgery, Nutrition, and Radiology.
You love your 'regular' doctor, and you trust them. Perhaps you feel like you are betraying them by seeking a specialist, but you are not. Believe me, they'll be happy you did. They want your pet to receive the best care, and there are limits to their expertise as a general doctor. If they refer you to a specialist, or you seek one on your own, they will remain your primary doctor. They will consult with the specialist and become partners with them for the long term care of your pet.
Specialist have more in depth diagnostic tools than your general practioner. They have ultrasound, scopes.. all kinds of fancy tools that your general doctor doesn't have. They are used to running special blood tests to pinpoint certain diseases, and they use the medicines they subscribe *all day long*.
Consider it this way: When we shop for groceries, we buy a lot of things. We do this every day: we pick up cereal, bread, milk, eggs. We know a ton about buying all of these groceries. If I really wanted to just know about eggs.. I mean, everything about eggs from top to bottom, inside and out, I'd ask an egg specialist! Your general practice doctor doesn't know less generally speaking, they are just trained to know more about many things, and your specialist is trained to only focus on one thing. It's all they do, and they are damn good at it.
A consult with a specialist, and diagnostics therein, may cost a lot at first. But what you save in the long run of going to an expert and getting the absolute best diagnosis and best treatment and guidance will save you in the long run.
So when do you go? When it's serious, and you know it. It also depends on how much you're willing to do. I've met owners who are willing emotionally and financially to offer palliative care for diseases, but, for their own reasons, would never consider surgery, extensive diagnostics, or adding a roster of medications to their daily life. Honestly those with this mental approach tend to be those who feel "age is a disease", and consider less treatment because they perceive their pet to be "too old" for much. Remember, age statistics are an average. If your dog's breed lives an average of 10 years, remember some die at 7 and others live to be 15. Assume yours may live longer, and never deprive them of support and care.
Bear in mind one important difference between human and animal medicine: with humans, we strive for longevity. We are afraid of dying, and we want to live as long as possible. In veterinary medicine, these concepts to not apply. Our pets do not understand "life span"; they understand their own unique quality measures. For instance, when a human gets cancer, our goal is to kill it, at any cost. When our dogs or cats get cancer, we only aim to "kill it" if it's within a realistic measure of their quality and the realistic standards of their cancer. Mostly our goal is quality, and time within the measures of their understanding of quality. A dog never asks, "Why now? I'm too young!"
If your pet has cancer and you don't want to do chemotherapy, there are still benefits to consulting with an oncologist: they see this all day long, and are familiar with the best methods of palliative care even if you don't want to be more aggressive. If you don't want to manage your dog's heart disease and/or heart failure for any measurable length of time, there are still benefits from the expert guidance of a cardiologist. If you know you will never take your old dog to surgery, still talk to a boarded surgeon: they may have amazing insight to offer you which will best guide your decision.
"I've been to the vet three times, and Molly still won't eat. They still don't know what's wrong with her", See your internal medicine specialist. "My veterinarian has just told me that Molly, after five years of having a heart murmur and no symptoms, is in heart failure now", See your cardiologist. I just found out Molly has cancer, and I want to give her the best care possible.... so on, and so forth.
If and when you see a specialist, remember they are just that. Don't ask the dermatologist to examine Molly's limp, and don't ask the surgeon to treat Molly's ear infection. Can they do it? Sure.. but that's what your primary doctor is there for, and they deserve your ongoing business for those other issues. In fact, they are your best source for managing those other things. "Well, we're here, and we spent a ton of money.. why won't you give Molly her rabies vaccine?" The answer: the specialist is there to address whatever serious concern you have. Once Molly is well, return to your general practioner and address those issues.
As with all illness, don't wait. Owners find themselves in denial and hope their animals recover from their issues with little to no intervention, whether guided by financial constraint or just a hope that nothing that bad is really happening. It is far better to be the owner who "panicked" and brought their dog to a specialist for a concern right away, then to be the one who waited and now has a pet in unstable condition who will ultimately, now, cost more than what you were afraid to spend in the first place.
Lastly, consider pet insurance. I've seen it cover amazing costs for owners, and therein enabled owners to do what was truly best for their pet. It's easy to put it in the back of your mind because your pet is young, but never forget that young pets get horrible disease also, as much as we don't anticipate it.
