Re: Weight gain
TRUBBLE...
Your vet should certainly check her thyroid and also check for any other condition that might cause her to gain this much weight. We've never experienced any uncontrolled weight gain when our girls were spayed, so it's not inevitable.
If she does have skin problems that are not parasite related (demodex or scabies), they may be allergy related. Those can be controlled through diet in many or even most cases, so medications are not always necessary (or helpful). A high quality commercial diet in the right proportions, or even better, a high quality raw diet in the right proportions, along with adequate exercise, can work wonders for weight control and skin/coat condition.
You might also want to get a second opinion on her condition(s) from a holistic vet, if you can find one that is available to you. We frequently get second or even third opinions if something is serious enough, even though we have a great deal of confidence in our primary care vet.
Also, while it's true that Boxers are particularly suceptible to a range of serious conditions, there are things that can be done to reduce the risk for some of these conditions, and tests can determine the likelihood of others. For instance, our Boxer and our Dobermann just returned from hospital yesterday where they had their semi-annual echocardiogram and holter monitor application. The echo will test several features of heart construction / condition as well as blood flow and possible leakage. The holter monitor will measure heart rhythm over a 24-hour period in various stages of rest and exercise. If these conditions are present, they can be monitored more closely and medical intervention is frequently successful in helping regulate a rhythm or blood flow problem. We have first-hand experience with managing a heart condition and it never bothered her at all once we got her on proper medication.
Cancer screening is also possible, although it is less effective than cardio workups in telling you what may be coming up with your Boxer and IIRC, it has to be repeated whenever you want a current update (it's not a one-time for the life of your dog test).
None of this is cheap. Each person has to decide what they can afford to do, but I mention these things to say that there is some medical help available from vet science, both holistic and traditional. Before you get too scared, find out what's really going on and what you can do to correct it.
BTW, some of the less expensive things you can do to help are nutritional. Make sure that their water source is pure... no "additives" like flouride or chlorine and no impurities. Use bottled water if you aren't sure about your domestic water source. Avoid chemicals in the yard or around the house that are not organic or natural. You can make your own soap from things like vegetable oil, olive oil, coconut oil, and Apple Cider Vinegar. Avoid pesticides and use natural fertilizer. Use a natural organic shampoo on the dogs instead of one with an insecticde it. Restrict vaccinations to those that are necessary as shown by tests (titers), rather than having a vaccination "cocktail" administered annually.
Sorry to be so long-winded, but you sounded a little panicked and I just wanted to give you a list of things that you can actually control that can help increase at least the quality of your dog's life, and possibly the length of it also. I lost my first Boxer two years ago to cancer. I still live with three dogs, including a rescue Boxer (normal cardio and no signs of CA so far at 5 YO), but there's not a week goes by that I don't sit and cry for her and beat myself up for not doing more than I did. Since then, I've learned a lot about what I can and can't do to help them. So I know where you're comin' from.
Best wishes to you and yours...