Thank you for the information. I completely understand about the situation with large animal vets, in this area, many are shying away from it. The practice I was previously at covered about 100 mile radius due to the scarcity.
There are four differentials I would have on my list to focus on given the history of weight loss and death.
1) Johne's disease. You mentioned a few have been tested with negative results. All it takes is one positive to have the rest of the herd affected. Does can also pass this thru their colostrum/milk. The typical history is that of weight loss with a good appetite. They tend to go downhill very quickly with seemlingly no other problems than weight loss. Different than cattle, goats usually do not have diarrhea associated with this disease. A quick blood test can be done, though the fecal culture is the best test to determine if Johne's is present. Here is an informative article: http://ohioline.osu.edu/vme-fact/0003.html
2) Parasites. It is great you deworm twice a year with different dewormers, but this may not be enough! Resistance is spreading to normal dewormers. As vets, we recommend running fecal exams to determine if actual deworming is needed and if your current program is working. Also if the goats have a heavy burden of parasites, one deworming may not be enough. A simple fecal exam can look to see how effective your deworming program is. I would collect a few different samples (composite) to have tested. There are alot of parasites that can take goats down quickly! Again, typical history is that of weight loss though usually eating well. Being dairy goats, if they are milking, they can be affected even more so since they are putting all their energy towards milking. Even though they look nice and fat, they can still be harboring parasites.
3) Caseous lymphadenitis (CL) also known as pseudotuberculosis. This is caused by a bacteria. This is less likely but I would keep it in mind. This bacteria can cause abscesses, which generally are seen externally where lymph nodes are (the head, should, flank, and hind legs). However, this bacteria can actually cause internal abscesses you do not see with the eye! The animal will typically lose weight and eventually die. Like I said, less likely, but definitely a possibility. www.goatworld.com/articles/cl/cl.shtml
4) Toxins. Again lower on my list, but to have a few affected, you cannot rule out something toxic they might be getting into, such as toxic plants, etc. One thing I wonder about is copper deficiency. Initial symptoms can be weight loss and weakness, but typically before the animal dies, they begin staggering and seem really wobbly in the hindlegs. With copper deficiency, typically there will be a white ring or depigmentation around the eyes ( may be hard if you have light colored animals). Have you seen any of this?
You mentioned wasting disease. Certainly is present in the United States but only in certain areas. Weight loss is one clinical sign but usually there is behavioral changes (decreased mentation) and listlessness. Here is an article concerning the signs: www.goatworld.com/articles/chronicwasting.shtml
Certainly if you do not want to have a necropsy done, you may want to start with a simple fecal exam. You can drop this off at your vet's. It is simple, quick, and inexpensive. I would collect a few samples. This is the easiest thing to check and will also give you valuable information into your deworming program.
Should this come up negative, I would recommend Johne's testing the next animal that comes up showing clinical signs. Take notes on the affected animal as this may clue in your vet further. If you can take a temperature. Knowing whether the affected animals have a fever can help determine what it might be. Should this animal pass, I would highly recommend a necropsy. It may be expensive but it can give you extremely valuable answers, especially for the sake of the rest of your herd. You may be able to discuss with your vet about collecting a few samples rather than a full necropsy to cut down on expenses. If it is something like CL or Johne's, this is imperative to maintaining your herd as other animals will be affected.